CORROSION RESISTANCE OF

NICI

Table of Contents Page

PART I. INTRODUCTION 3

PART II. CORROSION BY CAUSTIC SODA........................... 4 A. Nickel .................................... _ . . . . . . . . . . . . . . . . . 4

1. Effect of Concentration, Temperature and Carbon Content ...... _ . . . . . . 4 2. Effect of Velocity ............... _ ... __ ..................... _ 6 3. Effect of Aeration .......................................... 6 4. Effect of System Thermal Gradients .............................. 7 5. Effect of Impurities .,. _ ................. _ ............ _ . . . . . . . 7 6. Effect of Stress ............................................. 8 7. Effect of Dissimilar Metal Contact .. _ ...... _ ...... _ ............. '. 8 8. Cathodic Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

B. Nickel-Chromium Alloys (Alloy 600) .. _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 C. NickelCopper Alloys (Alloy 400, Alloy K-500) ....................... _. 10 D. Copper-Nickel Alloys .... _ ...... _ ............. ___ ............ _ ., 11

Copper-Nickel Alloy CA 706 (90-10) Copper-Nickel Alloy CA 710 (80-20) Copper-Nickel Alloy CA 715 (70-30)

E. Iron-Nickel-Chromium Alloys (Alloy BOO) ......... _ .............. _ . .. 13 f. Austenitic Chromium-Nickel Stainless Steels (AISI 300 Series) . . . . . . . . . . .. 13 G. Iron-Base NickelChromium...copper-MoJybdenum AHoys and Nickel-Base Chro-

mium...copper-Molybdenum Alloys .......... _ ....... __ ............. , 15 {Alloy 825. CARPENTER 20Cb-3, HASTELlOY alloy G and cast ACt CN-7M alloys}

H. Nickel-Base Molybdenum or MolybdenumChromium-lron Alloys. . . . . . . . . . . 16 (HASTElLOY alloy C-276, Alloy 625. HASTEllOY alloy B)

I. Cast Irons and Ni-Resists .... _ . _ .... __ .. __ . __ . . . . . . . . . . . . . . . . . . . . 17

PART m. CORROSON BY OTHER ALKAliES.. ............... .... .... 19 A. Caustic Potash (KOH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 19 B. Ammonia and Ammonium Hydroxide ............................... 20 C. Other Alkaline Solutions of Sodium and Potassium Salts ................ 22

PART IV. INDUSTRIAL APPLICATIONS. ... .... . ............... ...... 24 A. Caustic Soda Manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 24 B. Caustic Potash Manufacture ..................................... 28 C. Caustic Soda Storage and Transportation .......................... ". 28 D. Soap Manufacture ....................... _ ......... _ . . . . . . . . . .. 30 E. Pulp and Paper Industry ...... __ ............... _ .. _ . . . . . . . . . . . ... 32

1_ Digesters _............................................ ... 32 2. liquor Heaters ............................................. 33 3. Black liquor Evaporators .................. _ . . . . . . . . . . . . . . . 34 4. Recausticizing ............................................. 34

f. Aluminum Industry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 35 G. Caustic fusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 35 H. Petroleum Refining ............ _ .............. _ . _ . . . . . . . . . . . . .. 36 I. Caustic DescaJing . _ . _ .................... _ . . . . . . . . . . . . . . . . 37 J_ Reclaiming Caustic for Economy and Pollution Control. . . . . . . . . . . . . . 37

PART V. WELDING ............. __ ..... _ . . . . . . . . . . . . . . . . . . . . . . 38 A. fabrication of NickelClad Equipment .. _ ..................... " 38. B. Repair of Equipment in Caustic Service ......... _ ...... _ . . . . . . . . . . .. 39 References .......... _ ......................................... ' 40 Trademarks ..................................... . . ... Inside back cover

Table I

Nominal Compositions of Nickel Alloys in Use or Corrosion Tested in Caustic Solutions

Composition. %

M;aterial Hi Fe Cr Mo Cu C Si Mn

WROUGHT MATERIALS Nickel

Hickel 200 99.5 0.15 -

-0.05 0.06 0.05 0.25

Hickel20t 99.5 0.15 - - 0.05 0.01 0.05 0.20 DURAH.CKEL alloy 30t 94.0 0.15

- -0.15 0.55 0.25 0.25

Nickel-Chromium Alloys .HCOHEL alloy 600 16.0 7.2 15.8 - 0.10 0.04 0.20 0.20 H.MONtC alloy 75 71.4 0.5 20.5

-- 0.10 - -

Nickel-Copper Alloys MONU alloy 400 66.0 1.35

- -ll.5 0.12 0.15 0.90

MON!., al'lI1 K.SO!!:. 65.0 leO -

-29.5 0.15 0.15 0.60

Copper-NiCkel Alloys Copper-Nkke:1 allo, CA 706 10.0 1.25

- -88.0 - - 0.3

CopperNickel alloy CA 710 20.0 0.75 - -

78.0 - -

0.4 CoppefNlekel alloy cA 715 30.0 0.55

- -61.0 - - 0.5

IronNickel-Chromium Alloys 'NCOLO"- alloy 800' 32.0 46.0 20.5

-0.30 O.M 0.35 0.75

Stainless Steels AISI Type :202 5.0 67.0 18.0

- -0.15ma. 1.0 max 8.1

AISI Type 31)2 9.0 70.5 18.0 - -

0.15 max 0.5 1.5 AI5ITy, . 304 9.5 70.0 18.0 - - 0J)8 max 0.5 1.5 AISI TYlle 304l 10.0 69.0 18.0

- -0.03 max 0.5 1.3

AISI tYPe 316 13.0 65.0 17.0 2.0mi .. -

O.OS max 0.5 1.7 AISI tJllO 316l 13.0 65.0 17.0 2.0 min - 0.03 max 0.5 1.8 AISI Type 309 13.5 60.5 23.0 - - 0.20 malt 1.0 max 2.0m3X AISI Type 310 20.0 52.0 25.0

-- 0.25mn 1.0 max 2.0ma.

AISI Type 330 35.0 41.0 1.5.1) - - 0.25 rna. 1.0 rna. 2.0 mal AISI Type 347 11.0 68.0 18.0 - - 0.08mu 1.0 max 2.0mu AISI Type. 438

-Bal 17.0 - - 0.12 max - -

Iron Base NicketChromium-Copper-MoIybxlenum Alloys c;"'Rn'n:~- Stainless No. 20

Corrosion Resistance of Nickel and Nickel-Containing Alloys in Caustic Soda and Other Alkalies

PART I. INTRODUCTION

Caustic soda (sodium hydroxide) is the most widely used and avaBablealkaline material. In the United States almost all of the caustic soda is pro-duced as a co-product in the production of chlorine by the electrolysis of sodium chloride. The elec-trolytic cells used can be divided into two general types: mercury cells and diapllragm cells. With mercury cells,high purity SOt;(. caustic is pro-duced directly, whereas with diaphragm cells. the caustic concentration produced is within the range of 9 to 15 per cent, and has to be further purified and concentrated before sale. A small amount of caustic soda is produced by the lime-soda proce$S which WaS formerly the prime source for this chemical.

Caustic soda is generally marketed in concen-trations of 50 percent, 73 per cent or anhydrous. The chemical industry is the largest consumer of caustie soda, followed by the rayon and film in-dustries. the pulp and paper industry and the aluminum industry.

A large number of alloys can be used for han-dling caustic soda, and selection is based upon such factors as concentration. temperature, im-

3

purities in the caustic, the necessity for product purity. corrosion rate, susceptibility to stress-corrosion cracking (caustic embrittlement) and economics. Caustic soda can be handled in cast iron or steel equipment at low temperatures. if iron contamination is not detrimental to end use. At elevated temperatures, however, iron and steel are subject to caustic embrittlement and high cor-rosionrates. Plant and laboratory tests and oper-ating experience over many years have demon-strated that nickel and nickel alloys are the preferred materials for handling caustic solu-tions in many applications. Nickel can be used for practically an concentrations and temperatures.

In addition to caustic soda. several other im-portant alkalies are discussed in this bulletin. but no attempt has been made to be all-inclusive.

Nominal compositions of alloys referred to in the text are shown in Table L Materials other than nickel-eontaining alloys included in a num-ber of tests are reported for reference purposes.

An corrosion rates are reported as mils pene-tration per year (mpy). (1 mil = 0.001 inch.)

Fig. 1 - These caustic soda evaporator units are a combination of both solid Nickel 200 and steel clad with Nickel 200. Diaphragm cell liquor feeds into the double-effect evaporator: overflow from a settler tank feeds the single'effect evaporator for conc.entration to 50% caustic soda. The system produces 700 tons of salt and delivers 434 tons of NaOH (100%1 per day.

Ph%qraDh by courtesy ot the Swenson Division Of Whiting Corporation.

PART II. CORROSION BY CAUSTIC SODA

A. Nickel 1. Effect of Concentration, Temperature

and Carbon Content Corrosion test results for nickel in commercial caustic soda solutions were obtained by a number of investigators at different times and locations. Typical test data are shown in Table II and these have been incorporated in the isocorrosion chart, Figure 2. Only at high caustic concentration near the boiling point does the corrosion rate exceed one mil per year. This isocorrosion chart is in-tended only as a guide; there are specific condi-tions under which higher or possibly lower cor-rosion rates can prevaiL These conditions are discussed later.

700 r---..,.-----r---r----..,.---"r"> 371

600

500

u... 4oo .. :; "0 Q; Q

~ 100 f-

200

100

316

]60

'13

Table II

Typical Corrosion Test Data for Nickel and High Nickel Alloys in Caustic Soda Solutions

HaOH ;oncen-tration,

%

D-1

4

4

5-10

14

22

34

30"50

49-51

50

50

50

72-73

72

73

73

13

14

15

60 to nearly anbydrnus

Temperature C F

3{l 86

30 86

3{l 86

21-32 70-90

88 190

50-60 12{)-140

65 150

81 178

55-75 131-167 31165 av 149 55-61 131-142 31158 a1l 136 60-70 14~158 av65 al/ 149

150 302

116 273

121 282

95-100 203-212

100-120 212-248 avll0 a1l230 104-116 244-251 av 110 av248 130 266

135 271

15~260 302-500

Less than 0.005 mils per year.

A.e1ation Agitation

Nooe Hone

Hone None

Air agitated Air agitated

Extensive due to filling tank

None None

None due to filling tank

Extensive Mild

None None

None due to lining tank

None due to filling tank

Moderate by lOOgpm flow from pump

None None

None due to filling tank

Moderate due to filling tank

None by rocking of tank

None due to filling tank

None due to filling tank

Not specified by movement of tank car

Not specified due 10 filling tank

None None

Test Period,

days

27

1&2

1&2

124

90

133

37

16

30

135

393

14

183

119

111

52

126

II trips of 7-9 days 35

2

5

Comments

Test coupons removed, cleaned and dried each day fOf lOdays Average of tests run at 8 separate laboratories Average of tests run at 8 separate laboratories Storage lank

First effect of multiple-effect evaporator Storage tank coupons immersed 95% of time Storage tank in which air was bubbled through from bottom Single-effect evaporator_ Rates are average of 3 tests Storage tank coupons fully immersed Storage tank

T ransler piping. at pump discharge

laboratory test on tubing; average of 4 coupons Storage tank

Storage tank

rest tank_ simulating action of tank car Storage tank coupons immersed 95% of time Storage tank coupons lully immersed Coupons in railroad tank car

Storage tank between evaporator and finishing pots. Ammonia Soda Process Concentration in caustic evaporator

Corrosion Rate, mils per year

Hicke! 200

0.01

0.05

0.05 0.15

0_02

nil

0.03

0.09

0.02 0.02

0.07

-

0.3

OJ

0.13

0.05

0.02

0.3

1.6

3.9

Nickel-Copper Alloy

(MONEL alloy 40ll)

0.01

0.16

0.21 O.ll

0.05

0_01

-

0.19

0.03 0.02

0.10

-

0.7

0.3

0.16

0.04

0.10

0.4

1.7

13.4

Nickel-Chromium

Alloy ltNCONEl

alloy 6nO\

nW

-

-

0.05

0.03

0_01

0.03

-

0.02 0.02

0.03

0.25 0.4

0_1

0.14

0.06

0.0\

-

1.3

-

Table llt

laboratory Corrosion Tests in Caustic Solutions at Elevated Temperatures

NaOH Concen-tration,

% Temperature C F

Test Period,

hr Nickel 206

20 110 262 15 nil 40 110 262 15 nil 60 llO 262 15 nil 80 110 262 15 nil 20 115 272 19 nil 40 115 272 19 nil 60 ll5 272 19 nil 80 US 272 19 nil 20 162 355 19 nil 40 162 355 19 nil 60 162 355 19 nil 80 162 355 19 nil 20 149 332 19 40 149 332 19 60 149 332 19 20 132 270 ]9 40 132 270 19 GO 132 270 19 80 132 270 19 20 111 340 19(2 tests} 40 111 340 19 (2 tests) 60 l7l 340 1912 testsl 80 17l 340 19 (2 tests) 20 156 345 20 40 156 345 20 GO 15G 345 20 86 156 345 20 20 127 293 15 40 127 293 15 GO 127 293 15 80 127 293 IS 20 150 334 18 46 150 334 18 GO 152 336 19 20 183 394 15 GO 183 394 15 80 183 394 15

2. Effect of Velocity Velocity has little effect on the corrosion rate of nickel in caustic at temperatures below 500 C (932 F) but at 540 C (1004 F) and above, increas-ing velocity may cause a several-fold increase in the rate of attack. Figure 4 shows the results of two-week laboratory experiments by Gregory, et al., in high temperature molten caustic soda under dynamic conditions.

6

Corrosion Rate, mils per year MONEL alloy 400

nil 3 1

WORTHITE sIs (solution quenched)

4 9 1 nil 25,69 36.28 2,38 nil. nil 14 17 33 1

3. Effect of Aeration

Hi-Resist Type 2

94 6 17 28

Cast ACI CN7M

10 1 12 nil 151 2

Aeration has not been observed to accelerate corrosion in lower concentration caustic soda solutions. However, at high concentrations and temperatures, such as occur when concentrating to anhydrous, precautions should be taken to minimize aeration.

M"lten Coust;c Sodo 720 C (1328 Fl 480

400 >-~ 680 C (1256 Fl E 320 ~ 0

oc 240 .----------c

_Q E (; u

600 C {I I 12 Fl 580 C {1076 Fl

540 C (l004 F)

635 C P 175 Fl 400 C ( 752 FI

500 C I 952 F)

100 200 300 400 500 600 Rotot;on $peed_ rpm

Fig. 4 - Corrosion rate of nickel as a function of rotational speed.'

Table IV Static Corrosion Rates of Nickel and Nickel Alloys

in Molten Caustic Soda

Corrosion Rate, mils per year Temperature

460e 500e 58fl e 680e Alloy (750 f) (932 f) (1076 f) (1256 f)

Hickel20t 0.9 1.3 2.5 37.8 HASTELLOY a!toy e 100.5 HASTELt.OY aUoy D 0.7 2.2 9.9 MONEl. alloy 400 1.8 5.1 17.6 INCONEL aUoy 600 U 2.4 5.1 66.4 OURANICKEI. alloy 30t 1.7 3.2 10.4 40.7 NIMONIC all01 75 1.1 14.3 20.8 47.6

(pitted) Gained weight. Swollen outside surface largely oxide-heavily cor

roded.

4. Effect of System Thermal Gradients In molten caustic soda at temperatures above about 550 C (1022 F), nickel is subject to thermal gradient mass transfer.:; 4;. 7 In this type of at-tack, nickel is dissolved in caustic at a high tem-perature surface and is precipitated at a low temperature surface in a circulating system. Gregory, et at, concluded that the corrosion rate of nickel in molten caustic soda could be ten times as great under dynamic conditions as it was under static conditions because of the solubility-temperature relationship.:;

The mass transfer effect can be inhibited but not prevented by maintaining a hydrogen-con-

7

taining atmosphere in the vicinity where corro-sion is occurring. Forestieri and Lad found that, as a result of the presence of chromite ion (CrO:I- 1 ), mass transfer and cOITosion were essentially eliminated for 50 hours by one per cent addition of 325-mesh chromium powder in a test loop operating at a fluid velocity of 15 fps and 816 C (i500 F) with a temperature difference of either 11 C (20 F) or 22 C (40 F) .>1.9 However, a small mass transfer deposit was obtained after 250 hours, indicating that a single chromium ad-dition would not protect a nickel system in-definitely.

5. Effect of Impurities Chlorates in caustic can increase corrosion rates as indicated in the later section on caustic soda manufacture (page 27). Small amounts of so-dium chlorate are produced in electrolytic dia-phragm cells. The effect of the chlorate on corro-sion rate is not critical unless the chlorate is de-composed. and thermal decomposition does not occur below a temperature of 260 to 290 C (500 to 554 F). If it is intended to operate nickel equip-ment at or above this temperature range. four alternatives are available:

a. Use "rayon grade" caustic which has a speci-fication of 5 ppm maximum chlorate content.

b. Use caustic produced by electrolytic mercury cells or by the lime-soda process, or,

c. Use anhydrous caustic; there are no chlorates in the anhydrous grade .

d. Add reducing agents as discussed in the sec-tion on caustic soda manufacture (page 27) .

The presence of oxidizable sulfur compounds in caustic soda tends to increase the corrosion rate of nickel at elevated temperatures. This is noted particularly with hydrogen sulfide, mer-captans, or sodium sulfide, and to a much lesser extent with partially oxidized compounds such as thiosulfates and sulfites.

The effect of the addition of oxidizable sulfur compounds to caustic soda on the corrosion rate of nickel has been studied in the laboratory with the results shown in Table V. Test 1 was made during the evaporation of a commercial caustic soda solution under 28 inches of vacuum. Sulfur content of the original caustic. calculated as per

Table V

Effect of Oxidizable Sulfur Compounds on Corrosion of Nickel 200 in Caustic Soda

Temperature: 130 C c::: 5 C (266 F c::: 9 Fl.

Corrosion Test Rate, No. Corrosive mils per year

Commercial Sodium Hydroxide being concentrated from 50 to 75% NaOH (Sulfur content at start. calculated as H,S. 0.009%} 1.7

2 75% C.P. Sodium Hydroxide 0.6 3 75% C.P. Sodium Hydroxide plus 0.75% Sodium

Sulfide 22.8 4 75% C.P. Sodium Hydroxide plus 0.75% Sodium

Th~wlf~e ~9 5 75% C.P. Sodium Hydroxide plus 0.75% Sodium

Sulfite 5.2 6 75% C.P. Sodium Hydroxide plus 0.75% Sodium

Su!!ate 0.6

Chemically pure.

cent H;!S in dry caustic, was 0.009 per cent. Test 2 was made in chemically pure caustic soda. Tests 3 through 6 were made in chemically pure caustic to which the various sulfur compounds had been added.

It has been found that the detrimental effect of oxidizable sulfur compounds in caustic can be counteracted by the addition of sufficient sodium peroxide to form sulfates. An excess of peroxide does not seem to be detrimental. as shown in Table VI which compares the resistance of nickel, iron, and copper to fused caustic soda with and without an addition of 5c;. sodium peroxide. lo In each test, 5 grams of the substance were fused for four hours in a laboratory crucible of the given metal and analyzed for metal pickup.

6. Effect of Stress Experience has indicated that Nickel 200 is not subject to stress-corrosion cracking in pure caustic solutions. However, it is subject to stress-corrosion cracking by mercury, and there have been a few cases of cracking of nickel when "upsets" occur in producing plants that utilize mercury cells.

In addition, cracking along precipitated grain boundary jTraphite in Nickel 200 has occurred after caustic soda exposure above 316 C (600 F). As indicated previously. a low-carbon grade

8

(Nickel 201) will circumvent this problem. Applied or residual stresses apparently do not

significantly affect the genera! corrosion rate of nickeL 11

7. Effect of Dissimilar Metal Contact Galvanic corrosion can occur in caustic soda solu-tions if different materials of construction are electrically connected. Whether this effect is aca-demic or critical depends upon the specific condi-tions that exist in a partiCUlar installation. For instance, the data in Table VII illustrate that gray cast iron corrodes from about one and one-half

Table VI

laboratory Tests in fused Caustic Soda with and without Addition of 5% Sodium Peroxide

Temperature Metal Pickup. grams

COHosive

Caustic Soda

Caustic Soda with 5% Sodium Peroxide

SI rangly attacked

C

350 360 400 450 500 550 600

350 400 450

f Nickel Imn

662 .4 680 .01.02 752 Irace.02 .426 842 .01.02 .2.3 932 .005.015 .2.3

1022 .4.43 1112 .i3.3

662 .0024 .024 752 .0135 .025 842 .OBI .Il

Table VII

Galvanic Corrosion of Gray Cast Iron

Conditions: Corrodent: 5% sodium hydroxide. Temperature: 43 C (ll 0 F). Flow: 16 feet per minute. Aeration: Saturated WIth air. Cathode to anode area ratio 2: 1.

Copper

trace .013 .03

Corrosion Rate of Corros ion Rate of Gray Cast Iron. Cathodic Material, mils per year mils per year

In In Cathodic Galvanic Galvanic Material Insulated Couple Insulated Couple

Nickel 200 l.l 1.5 Nickel 200 0.6 ]6 0

to three times its normal rate when connected to Nickel 200 or MONEL alloy 400, under the given set of conditions. However, the normal rate for cast it-on in 5~-;' caustic is so low that these higher corrosion rates are usually tolerable.

At higher caustic concentrations, tempera-tures, and with large cathode to anode r;:ttios. galvanic corrosion becomes more pronounced. The galvanic current curves shown in Figure 5 are from tests made above and close to the upper tube sheets of operating caustic evaporators. The general conclusions to be drawn from these tests are that in concentrated caustic soda solutions. significant galvanic corrosion may occur on cast iron or steel when in contact with nickel or cop-per. In the construction of caustic evaporators. it is desirable, if not actuaHy necessary, to use nickel or nickel-clad steel tube sheets in conjunc-tion with nickel tubes.

70r---~----'----r----.----r----'----r---. u. q

~ b.O ~ 5.0 t Si 4.0 c

~

{; 3.0 " o

" ~ 1.0 .(

GoJvonrc Current Flow Between C05-t 1on end Copper

A"'?oe: c.,;~ !"on Ar~" . C 3QS -;:: ;:. C,~d~ode: Ccpoe r A~e'1 0.*9"4 ;-:: =.

JS

7.0,-----,..-----,----,..-----.--__ -,-____ ,--__ -,-__ --,

~

.~ 0

" ? U " "D

" .(

3.0

2.0

1.0

0 Q S

Golvonic Cv ... enf flow Between Cad Iron ond Niclel

A"\Cd:~: C~S~ h,)~ Af"-e:2 =-.C c.J~a ~ J c. C,+ode: N;d:e~ Are~ ...:. il.24; 5.;)::.

Fig. 5 -- Current measurements between cast

corrosion resistance in caustic soda, as shown in Tables II, IV and XL.

Alloy 600 is commonly used in equipment for the production of anhydrous caustic when sulfur-bearing fuels are used for heating because it is more resistant to sulfidation than nickel.

There have been a few instances of stress-corrosion cracking of Alloy 600 in some strongly alkaline environments. A review of these serviee failures has indicated that they usually occurred in concentrated caustic solutions at high tempera-tures, 190 to 450 C (374 to 842 F). In seven-day laboratory tests, caustic concentration, tempera-ture, and the presence of air were shown to be important variables, as shown in Tables IX and XIV. No stress-corrosion cracki ng occurred if the Alloy 600 U-bend specimens were stress-relieved at 900 C (1650 F) for one hour or 769 C (1400 F) for four hours after bending.

Table IX StressCorrosion Cracking of INCONEl Alloy 600

U-Bend Specimens in Caustic Solutions-Seven-Day Tests

Temperature Over-pressure. Caustic CORcentratiolt, weight %

C f 150 psi Caustic 10 50 90

200 390 Air NaOH OK OK 250 480 Air NaOH stress-cracked 300 570 Air NaO" OK stresscracked stresscracked 200 390 Argon NaO" OK OK 250 480 Argon NaOH OK 300 570 Argon NaOH OK OK OK 200 390 Air KOH OK slight inter

granular penetration

250 480 Air KOH stresscracked 300 570 Air KOH OK OK stress-cracked

Note: Testing performed in autoclaves under static conditions without replenishment of air or argon.

C. Nickel-Copper Alloys Nickel-copper anoys, such as MONEL alloy 400, are practically as resistant to caustic soda as nickel, as shown in Table II.

The corrosion rate of Alloy 400 is higher than nickel at caustic soda concentrations above 75 per cent when concentrating to anhydrous. It is also

10

Fig. 6 - This barge has eight tanks with a capacity of 34,000 barrels .. The tan~s are used to carry fuel oil or as

ph~lt. and a specli;ll 54.00barret tank fabricated of INCONEl alloy 600-clad steel is used to carry 73 010 caustic soda, am moniabasefertilizers, or jet fuels.

higher than nickel at temperatures above the atmospheric boiling point, as shown in Table III. However, it should be noted that even in those cases where AHoy 400 is inferior to nickel, the corrosion rates are still quite low.

There have been a few reports of stress-corro-sion cracking of cold-worked and stressed Alloy 400 in caustic soda. However, the eXact conditions under which most of these failures occurred are not known. It is known that some of the reported failures associated with mercury cell caustic were caused by intergranular attaek by mercury and subsequent loss of ductility.

Laboratory tests have shown that Alloys 400 and K-500 can be susceptible to stress-corrosion cracking unde.r extreme exposure conditions. that is, high stresses in combination with high tem-peratures and concentrated caustic soda can cause cracking. Table X shows the results observed with tensile loaded specimens tested at 300 C (570 F) in condensing steam after being coated with either potassium or sodium hydroxide. Under these exposure conditions, Alloy 400, which had been cold-worked or cold-worked and stress-relieved prior to testing, was susceptible to stress-corrosion cracking~ cold-worked mate-rial that had been annealed at 850 C (1560 F) or 950 C (1740 F) prior to testing was resistant. As with Alloy 400. Alloy K-500 cracked when cold-

Table X

Stress-Corrosion Tests on MONEL Alloy 400 and MONEl Alloy K-500

Alloy Heat Treatment

MONEL alloy 4(1(1 None-as cold-drawn 850 C (1562 fltIti hr/W.O.

MONEL alloy 40(1 Stress relieved 540 C 11004 fl/ z hr

MONEL alloy 40(1 Works anneal 950 C U742 fll Y2 hr

MONEL alloy KS(I(I None-as COld-drawn 870 C U598 f)f5 min/W.O. 580 C (1076 fl/S hf/fC 810 C US98 A/S min/W.O. + 580 C {1016 fl/16 hr/FC

MONEL alloy K500 None-as colddrawn 870 C (1598 fl/5 min/W.O. 580 C n076 AIS hrl FC 870 C (1598 fl/5 min/W.Q. + saocno76 flfl6 hr/fe

Fumace-coaled at about 10 C (18 Fl/nr to 480 C (896 Fl then "if.-coa!ed to room lef'lperature.

NO = Not Determined o == Specimen fractured 1 == Coarse 'cracks visible to naked eye 2 == Fine cracks visible to naked eye 3 == Deep cracks visible under microscope

worked and was resistant in the annealed condi-tion. However. the thermal-hardening treatment at 580 C (1076 F) rendered the alloy very suscep-tible to cracking.

The practical interpretation of these data is difficult because threshold values of stress, caustic soda concentration, and temperature at which stress-corrosion cracking will occur have not been established. With these limitations in mind, it would appear prudent to stress-relieve AHoy 400 in the range of 538 to 566 C (1000 to 1050 F) or anneal it ill the range of 760 to 816 C (1400 to 1500 F) for one to three hours when it is to be used in higher strength caustic at elevated tem-peratures.

D. Copper-Nickel Alloys The corrosion resistance of copper-nickel alloys in can.stic soda solutions is dependent upon the nickel content of the alloy, as illustrated in Fig-ure 7. There are a limited amount of additional data which are shown in Table XI.

11

Yield Applie II Strength, Stress, ton/ S1l in. tonI sq in.

43.8 33.1 12.8 16.3

24.0 20.1

11.4 8.3 52.5 33.1 21.2 10.3 65.5 37.2

44.9 37.2 53.2 33.1 NO 10.3 NO 37.2

NO 37.2

Type and Degree of Cracking

NaO\( KO\(

IIG 41'11 5 5

OIG OIG

5 5 3IG+TG 5 5 5 4TG 5

OrG OIG 4NI 5 OIG

DIG

4 = Shallow cracks visible under microscope 5 = No cracks

TG = Transgranular cracks IG == Intergranula. cracks I'll = Type of cracking nQt jdentified-cracks very

short.

>-0-E '" 12 o

a::

5 8 .;;; l? ~ 4

20 40 60 Per Cent Nidel in Copper.Nickel Alloys

100

Fig. 7 - Results of corrosion tests of copper-nickel alloys in 50% caustic soda evaporator.

Copper-nickel alloy CA 715 (70% Cu-30% Ni) possesses excellent resistance to dilute concen-trations of caustic soda at low temperatures and appears to have useful resistance to caustic soda solutions of up to 73 per cent at the boiling point. However, this resistance does not extend to fused caustic. Alloy CA 715 has been used successfully as evaporator tubes for concentrating to 50 per cent where copper pickup by the caustic could be tolerated.

Copper-nickel alloys CA 706 (90(~ Cu-lO('~ Ni)

Table XI Corrosion of Copper-Nickel Alloys by Caustic Soda Solutions

Nominal HaOM Alloy Composition Concen- Copper- Temperature

tration, Hickel Wt% Wt% % Alloy Cu Hi C F

5 - 60 40 15-20 I 59-68 11 60 40 Hot-Exact temperature

unknown

5 70 30 1520 .1. 59-68 11 70 30 Hot-Exact temperature

unknown 50 70 30 95 203 50 CA 715 70 30 65 149 73 70 30 105 221

60-75 70 30 150-175 302347 60-1Im 70 30 150260 302500

100 70 30 400410 752770

5 80 20 1520 5968 6075 80 20 150-175 302347

CA 710 60100 80 20 150260 302500

100 80 20 400410 752770

50 90 10 95 203 CA 706

73 90 10 105 221

,. Less than 0.1 mit per year.

and CA 710 (80% Cu-20% Ni) have useful resist-ance to caustic soda solutions but their applica-tion is limited to lower concentrations and tem-peratures than AHoy CA 715. Because of the limited data available it is difficult to define limits for these two alloys.

While corrosion of the copper-nickel alloys by caustic solutions may be aggravated by the pres-ence of sulfur compounds, Alloy CA 715 is able to resist attack under some conditions, as shown in Table XII. No data appear to be available on the susceptibility of these alloys to stress-corro-sion cracking in caustic soda solutions.

Test Corrosion Dura- Rate, tion, mils per days year Comments

21 Nil laboratory test in glass bottle. 25 0.5 Diaphragm cell liquor

coupons in distributor box to settlers.

21 Nil laboratory test in glass bottle. 25 4.3 Diaphragm cell liquor

coupons in distributor box to settlers. 67 0.8 Velocity 1.8 ftl sec. Salt saturated. 30 Nil In storage tank.

118 1.2 l/Z 4.4 In evaporator concentrating from 60-75%. 2 21 In evaporator concentrating

from 60% to anhydrous. 1 70 In anhydrous melt.

21 Nil laboratory test in glass bottle. liz 8.1 In evaporator concentrating

from 6075%. 2 28 In evaporator concentrating

from 60% to anhydrous. 1 90 In anhydrous melt.

67 1.8 Velocity 1.8 ft/sec. Salt saturated.

118 2.0

Table XII Corrosion Rate of Copper-Nickel Alloy CA 715

in Alkaline Solutions Containing Sulfur Compounds

Corrosion Duration, Rate,

Conditions of Exposure days mils per year

1. In open tank used to boil 1822 per cent NaGH to release mercaptans at 80 C (175 fl 30

2. In reboiler of caustic stripper, 12 per cent NaDH.3 per cent Na,S. 10 per cent sodium phenolate + 0.7 mg per liter as sodium

25 mercaptides at 124 C (255 Fl 131 3. In 10 per cent sodium sulfide in storage

tank at atmospheric temperature 81 4. In 60 per cent sodium sulfide in flaker

14 feed tank at 171 C (340 f) 28 5. In regenerator reboiler for steam stripping

of mercaptans from solutizer solution 25.2 per cent potassium hydroxide 37.8 per cent potassium isobutyrate

5.5 per cent potassium sulfide 1.9 per cent potassium mercaptides 2.1 per cent potassium carbonate

at 141 C (286 f) 140 15 6. In vapors from solution in item 5 140 12

~ Pitting up to 3 mils depth.

12

(.

E. IronNickelChromium Alloys Based upon data obtained in several test expo-sures and shown in Tables Xln and XL, it appears that INCOLOY alloy 800 approaches INCONEL alloy 600 in resistance to caustic soda. However, Alloy 800 is more susceptible to stress-corrosion crack-ing than Alloy 600, as shown in Table XIV.

There has not been sufficient experimental work on the stre.ss-corrosion cracking of Alloy 800 to determine if stress-relieving in a tempera-ture range which will cause sensitization (pre-cipitation of chromium carbides in a continuous network) renders the alloy more susceptible to this form of attack. Therefore, it would appear prudent to anneal the alloy in the range of 1120 to 1150 C (2050 to 2100 F) or stress-relieve and sta-bilize at 870 C (1600 F) for one to two hours when it is to be used in higher strength caustic soda at elevated temperatures.

Table XIV Laboratory Tests-Results of U-Bend

Specimens in 90% Caustic Soda at 300 C (572 F) Maximum Depth of Cracks, mils

Argon 15 psig 50 psig 150 psig atm. air air air

Material 1 week 1 week 8 weeks 1 week

INeOLOY alloy 800 10 7 120lal 1151bl INCONEL alloy 600 0 0 75 115 Type 304

Stainless Steel 100 110 11 10

(a) Removed at four weeks. (b) Twoweek test. Note: Testing performed in autoclaves under static conditions without

replenishment of air or argon.

Table XIII Plant Tests-Corrosion Rates in Caustic Production Equipment Using Electrolytic Diaphragm Cell Caustic Exposure times vary from 24 to 29 days

Conditions

NaOH NaGI Temperature Concen- Concen Av Max Min tration, tration,

% % G F G F C

10 12 88 190 91 195 82 23 J.8 93 200 104 220 82

35-40 67 116 240 127 2~0 102 50 1015 93 200 104 220 71 72 ? 121 250 124 255 119

F. Austenitic Chromium-Nickel Stainless Steels

F

180 180 215 160 245

Austenitic chromium-nickel stainless steels offer good corrosion resistance to boiling caustic soda solutions up to about 10 per cent concentration, but from 10 to 50 per cent, the temperature for satisfactory service probably would not exceed 93 to 100 C (200 to 212 F). Generally more severe but inconsistent corrosion rates occur in more

Corrosion Rate, mils per year

'" '" ~ ~ ~ '" '" '" ~ ~ = = ~ ~ M ~ '" ~ :s~ :'a; :Sa:; .,. ~ "

~

" !9~ ~~ .e.e ~

'" '" " '"

700 .-----r-------,-----r----r------.-. 3 It

600

500

'-'-. 400

~ ~ " Q. E ~lOO

200

100

. .o..pF=".Jre~ 5tre~~\CO(fOs~O':"I

\ .1\ :""";)-~oher;.: I C'OC';"1 Bou"cio,y \ Bo;';n9 P;i", C~rve

\ 0' 30moy

\. ,

f to ' ....... 50 m py

.-: 1 ::npv AU Grades

lib

2&0

Q)

19

17 S OL-____ ~--L-~----~L-----~----J o 20 &0 ,,-' 100

fig. 8 - Isocorrosion chart for austenitic chromium nickel 'stainless steels in sodium hydroxide,

J. M. Stone observed that Type 304 stainless steel sensitized for one hour at 677 C (1250 F) was not susceptible to intergranular corrosion during 40-week exposures in: 1:!0

1. 10% NaOH at room temperature 2. 10% NaOH boiling at about 102 C (216 F) 3. 50% NaOH at room temperature. and 4. 50% NaOH at 60 C (140 F).

Therefore. post-weld heat treatment of regular (0.08 max) carbon grades or the selection of a low-carbon or stabilized grade of stainless steel does not appeal' to be required for these exposure conditions. However, intergranular corrosion of sensitized Type 304 stainless steel was observed by Agrawal and Staehle in boiling solutions of 20 to 80('; NaOH.I:t

Chromium-nickel stainless steels are subject to stress-corrosion cracking in caustic soda solu-tions at elevated temperatures. Nathorst H re-ported several cases of stress-corrosion cracking of austenitic stainless i3teels caused by alkalies. A comparison of the cracking behavior of Type 304 and Alloys 600 and 800 is given in Table XIV. A stress-corrosion cracking zone based upon these and other known failures reported in the litera-ture is shown in Figure 8. A dashed line was used to indicate the temperature-concentration bound-ary because this zone is probably not completely defined. Agrawal and Staehle have shown that sensitized Type 304 stainless steel is more prone than annealed material to stress-corrosion crack-ing in boiling caustic sodaP A portion of their data is shown in Figure 9. The cracking obtained was predominantly intergranular in the sensi-tized material and predominantly transgranular in the annealed material.

Commercial standard grade 50r;. caustic soda from diaphragm cells can have up to 11.000 ppm chlorides. and commercial 50 c; caustic soda from mercury cells and reagent grade anhydrous caus-tic can have up to 50 ppm chlorides. It has been

Table XV Corrosion of Stainless Steels by Caustic Soda Solutions

HaOH Concel!- Temperature Test Corrosion

AISI tration. Duration. Rate. Type % C F days mils per year Comments

302 20 5060 122140 134

10'

L

~ '" .} j

10

So;\;"9 So:"

140 C (284 F).I' However, the same reference also cites high corrosion rates for alloys of less than 70'; nickel, which would include WORTHITE stainless steel, in a storage tank handling 73'"; caustic soda at temperatures ranging from 120 to 171 C (248 to 340 F). Thus, the 140 C (284 F) ap-plication may be at the upper limit of usefulness for this aHoy.

if these alloys are to be used in conjunction with nickel and high nickel alloy equipment in strong caustic soda solutions at elevated tempera-tures, consideration should be given to electrical insulation between the dissimilar alloys so as to prevent harmful galvanic effects.

H. Nickel-Base Molybdenum or Molybdenum-Chromium-Iron Alloys

Materials such as HASTELLOY alloys Band C-276. INCONEL alloy 625 and cast CHLORIMET alloys 2 and 3 have not been used to any great extent in caustic soda solutions. Ag a result. corrosion data for them are rather mea~er. Tables IV and XVIII show the results of some iabonltory corrOfiion tests. From these data. it is evident that HASTEL-LOY alloy B can be u:~ed in concentrations up to 50 per cent at the boiling point and that the tempera-ture limit for HASTELLOY alloy C-276 would be somewhat less than with Alloy B. Temperature limitations in caustic soda concentrations ~reater

Table XVIII Corrosion of HASTEllOY Alloys Band C

in Caustic Soda Solutions 18

NaOH Temperature Corrosion Rate. mils per year Concen-tration. HASTEllOY HASTELlOY

% C F alloy B aUoyC

5 Room Room Nil Nil 5 66 150 Nil Nil 5 102 215 Nil Nil

to Room Room Nil Nil 10 103 217

I. Cast Irons and Ni-Resists The beneficial etred of nickel additions 011 the corro:,ioll t'esistance of cast irons in moderately concentrated caustic alkali is shown by data in Tables XIX, XX Hnd XX I. It is evident that nickel contents of 20 to 30 per cent pro\'ide vet'y marked improvement in resistance to corrosion as com-pared 'with unalloyed cast iron. It is also apparent that as lo'w as 3 to 5 c ; nickel may improve the corrosion resistance of cast iron in some con-centration ranges,

Table XIX Effect of Nickel Additions on Corrosion Rates

of Cast Irons in 50 to 65% Caustic Soda Temperature: Boiling under 26 in. (mercury) vacuum. Duration: 81 days.

Nickel, %

o o o 3.5 5 15 20 20 (plus 2% Chromiuml 30

Corrosion Rate, mils per year

73 91 86 47 49 30

3.3 S.O 0.4

In practice. the nickel cast irons most widely used with caustic solutions, where minimum con-tamination of the caustic is desired. are the Ni-Resist alloys and their spheroidHl graphite coun-terparts. the ductile Ni-Resist alloys, The corro-sion rates of these alloys fora number of different exposures are shown in Table XXII.

Table XX Corrosion of Nickel Cast Irons in the

Evaporation of Caustic Soda from 37 to 50 Per Cent Average Temperature: 120 C (248 F). Duration: 51 days.

Corrosion Nickel, Chromium, Copper, Silicon. Carbon. Rate.

% % % % % mils per year

28.60 1.71 1.30 2.87 17 28.37 1.50 2.72 18 14.26 2.39 6.08 1.62 3.15 22 19.40 1.42 3.15 24 19.02 2.90 1.22 3.18 28 20.53 1.25 2.91 31

17

Fig. 11 - Moiten sodium hydroxide at an initial tempera ture of 370 C (700 F) is converted to flake caustic by this flaker and breaker. All surfaces exposed to caustic are nickel except for l'li'Resist Type 3 cooling drum.

Table XXI Plant Corrosion Test in 74% Caustic Soda in Storage Tank

Specimens exposed for total of 32 days (20 days in liquid and 12 days in vapor). Corrosion rates based on 20 days exposure to liquid. Temperature:. 125 C (260 n.

Material

MONEl. alloy 400 H.i-Resist Type 3 HiResist Ductile rron Type 02 Hi-Resist TYlIe 2 Type 304 Stainless Steel Mild Steel Cast Iron

Corrosion Rate, mils per year

0.9 2.5 5 6

15 75 76

Copper-free Ni-Resist Type 2 may be used in preference to Xi-Resist Type 1 (6.50:-; copper) where it is desired to keep copper content of the solution at a minimum, The 30 r ;. nickel cast iron (Ni-Resist Type 3), in addition to having some-what g-rcater resistance to corrosion by hot caus-tic solutions than Ni-Resist TypeS 1 and 2, has a low coefticient of expansion, an advantage for expOSUI"C conditions invo!\'ing sudden changes in temperature.

Of the fi\'e basic types of ~i-nesist. Type 3 ap-pear:' to be the best suited to meet the require-ments for caustic sen"ice. !'\i-l1e::;ist Type 3 or Type D3 can be con::;idered as alternate materials

to nickel and the high nickel alloys for caustic soda concentrations up to 73 per cent, but nickel is preferred for higher concentrations.

There have been occasional stress-corrosion cracking failures with the Ni-Resists in high-

chloride aqueous environments. Although these environments did not include caustic soda, it \\"ollld appear a reasonable precaution to stress-reliew these alloys at 677 C (1250 F) for one hour before use in hot caustic soda solutions.

Table XXII Corrosion Rates of the Ni-Resists in Caustic Soda

Corrosion Rate. mils per year NaOH

'" '" '" '" c

~.~~ C> Concen- Temperature Test "'~ ";:;fN '~("') "Vi v tration, Period, "'", "'", "'", "'", .- '" '70.. '70.. '70.. '70.. ~o:: ~

'" % C F Aeration Agitation days - >- .- >. .- >. .- >. =:Ii ..!. >.. '" x>- X..- 201- XI- c2O>- c..>

8.5-9 82 180 None due to 32 plus 1515.5% 2.5 0.8 1.5 15 filling tank NaCI in storage

tank

10 88 190 Moderate due to 279 plus 12');' NaCl 0.2 4 filling tank in storage tank

I

! 14 88 190 None due loevap. 90 lirst eHect of i 8 multiple effect !

evaporator I 23 93 200 Moderate Medium 48 plus 7-8% NaCI 1.2 21 in salt settler

30 85 185 Moderate Moderate 82 plus heavy con- 0.8 0.4 0.1 0.5 6 centratlOn of suspended NaG! in sail settler

35-45 116 240 Moderate Small 24 plus 6-7% NaGI 3.3 49 in salt settler. Intermittent ex- I posure to vapor

49-51 55 149 None due to 30 storage tank I II filling tank 50 55 131 Moderate 1.8 Ips 173 plus heavy con- 0.5 0.2 I

PART III. CORROSION BY OTHER ALKALIES

A. Caustic Potash (KOH) Caustic potash is produced by the electrolysis of muriate (potassium chloride) brine. Several types and concentrations of KOH are available, but 45 and 50 per cent liquid and 85 and 90 per cent solid are most commonly marketed. Above about 50 per cent concentration, caustic potash has a higher boiling point than caustic soda of the same concentration. This differential is espe-cially pronounced at high concentrations. For this reason. the commercial product is usually not concentrated above 90 per cent because of the high temperatures involved.

In general. those materials which are useful in caustic soda are also suitable for caustic potash.

Nickel 200 and INCONEL aHoy 600 are both suit-able for service in hot caustic potash. as indicated by the data presented in Table XXIII. Negligible data exist for other nickel alloys in caustic potash. Gegner has suggested that because caustic potash is so similar to caustic soda. the corrosion data in

caustic soda of similar concentration and tem-perature can be used to approximate corrosion resistance in caustic potash. Iff

Under extreme conditions, some nickel alloys are subject to stress-corrosion cracking in caustic potash solutions. However, the information pre-sented in Tables IX and X suggests that stress corrosion cracking of Alloy 600, Alloy 400 and Alloy K-500 is not quite as severe with caustic potash as with caustic soda.

The beneficial effect of nickel in cast iron ex-posed to caustic potash is shown in Table XXIV. The reductions in corrosion rates are similar to those obtained in caustic soda solutions.

Table XXV shows the results of laboratory cor-rosion tests of several Ni-Resist alloys in hot. con-centrated caustic potash. Lower corrosion rates would be expected with a decrease in either tem-perature or caustic potash concentration. Ni-Resist Type 3 appears to have equivalent, and sometimes superior. corrosion resistance in com-parigon to Types 1 and 2.

Table XXIII

KO" COllcen-tration.

%

13

30

47

50

50

50 10 10

Temperature C F

30 85

113 236

139 281

28 82

150 300

.

150 300 150 300 150 300

Aeration

None

None

None

None

None

None None None

Corrosion Tests in Caustic Potash Solutions

Agitation

due to filling tank

Boiling

Boiling

due to lilting lank

2\.61pm

3481pm' 21.6fpm 3481pm"

Test Period. days

207

26

26

207

7

35 7

35

Comments

storage tank impurities-K Jeo J 3 gpl. KCf 170 gpl. KClO .. 0.1% laboratory test-saturated with KCf. 0.05% KClO, laboratory test-saturated with KG\. 0.18% KClO, storage tank impurilies-K,CO. 0.3%. KGI 0.75%. KGto, 0.10% laboratory test-UBend specimen showed no cracking laboratory test laboratory test laboratory test

nil-less than 0.05 mils pel'" year. l-liQuid V-Vapor ." Specimens m~ved at th!s veloc11y fof'" 8 hr each working day and at zero ft per mtn overn1ght and duong

weekends. Th,s was equlvatent to ten 24 hour days at the high velocity rate.

19

Corrosion Rate. mils per year

Nickel 200

nil'

l. 0.2 V.0.3 l. 0.1 V.O.3

nil

nil

nil 0.4 1.6

INCONEl. alloy 600

nil

l. 0.1 V.O.1 l. 0.4 V.O.1

nil

0.5

0.5 0.7 5.7

MONEl. aUoy 400

nil

nil

Table XX'V Effect of Nickel in Cast Iron

on Corrosion by Caustic Potash Concentration: 950 g KOH per liter. Temperature: 400 C (750 F).

Nickel Content of Alloy Iron, %

o 3 6.5

12.4

Table XXV

Corrosion Rate, mils per year

21-30 3.0 2.0 0.4

Corrosion of Ni-Resists in Caustic Potash

Hi-Resist ClIrrosjon Rate, Type Exposure mils per year

1 68-hour test in 81 % KOlt at 225 C (437 f) 30 2 68-hour test in 81% KOH at 225 C (437 f) 20 3 68-hour test in 81 % KOH at 225 C (437 f) 10 2 36-hour test in 92% KOlt at 268 C 1516 f) 10 3 3S-hour test in 92% KOlt at 268 C (516 f) 10

B. Ammonia and Ammonium Hydroxide

Most of the nickel-base alloys, with the exception of the nickel-copper alloys and nickel itself, resist all concentrations of ammonium hydroxide up to the boiling point.

Among the nickel-containing alloys, the aus-tenitic stainless steels are most frequently em-ployed in ammonia and ammonium hydroxide solutions. Austenitic stainless steels exhibit good resistance to all concentrations of ammonia and ammonium hydroxide up to the boiling point. Tables XXVI through XXIX show the results of plant corrosion tests in ammonia- and ammonium hydroxide-containing process streams.

Stone .determined that Type 304 stainless steel, which had been sensitized at 677 C (1250 F) for one hour, was not subject to intergranular corro-sion during a 40-week exposure in 28% NH40H at room temperature.12 However, this resistance does not extend to elevated temperatures in com-mercial solutions, as shown in Table XXVI.

20

Considerable amounts of Types 316 and 316L stainless steels are used in the ammonia-soda process for the production of soda ash (Na2C03 ). The main reaction involves the carbonation of an ammoniated brine to form sodium bicarbonate and ammonium chloride. The ammonia is recov-

Table XXVI Plant Corrosion Test in Ammonia

Surge Vessel of Urea Manufacturing Plant Solution: 22% NH, 71 % H,O, 7% CO, and trace of

NH.NO, . Temperature: 66C (150 F). Test Period: 300 days. Aeration: None. Agitation: Moderate. Location: Uquid phase at bottom of aqua ammonia surge

vessel.

Material

INCOlOY alloy 825 Type 347 Stainless Steel Type 316 Stainless Steel Type 304 Stainless Steet Type 316 Stainless Steel

Table XXVIII Plant Corrosion Test in Ammonia-Carbon

Dioxide Gas Stream in a Metal Refining Plant Gas: 26% NH J 14% CO,. balance water vapor. Temperatur-e: 66 ro 93 C (150 to 200 F); Average 82 C

(l80 F). Test Period: 65 days. Aeration: Moderate. Agitation; 25 to 40 fps gas flow. Location: NH1CO~ stripping still overhead line.

Material

Type 202 Stainless Steel Type 304 Stainless Steel Type 316 Stainless Steel INCOlOY alloy 825 INCOLOY alloy 800 INCONEl alloy 600 Type 410 Stainless Steel Type 502 Stainless Steel Mild Steet

Table XXIX

Corrosion Rate. mils per year

33" >32" Mild Steet >73" >71"

Specimen pitted in crevice beneath insulating wasber. Specimens cQrroded away ..

Nickel is not attacked by anhydrous ammonia, but is resistant to ammonium hydroxide solutions in concentrations only up to about one per cent. Aeration may induce passivity in concentrations under 10 per cent, but even in the presence of air, more concentrated solutions are highly corrosive to nickel. The corrosion data shown in Table XXXI were obtained in room-temperature laboratory tests ()f 48-hour duration in one normal ammo-nium hydroxide, following a previous 48-hour ex-

Table XXXI Corrosionof Nickel 200 in One Normal

Ammonium Hydroxide (1.7% NHa)

Test Condition

Total Immersion Quiet Air"'Agitated

Alternate Immersion Conti!luous lntermittent

Spray {4 to 30 Daysl

Corrosion Rate, milspllryear

0.8

posure. The re~ults of 20-hour tests in highly agitated ammonium hydroxide solutions at room tempelature are shown in Table XXXII. Typical corrosion rates for Nickel 200 in several indus-trial exposures are also given in Table3 XXVII, XXIX and XXX.

Table XXXU Corrosion of Nickel 200 in

Ammonium Hydroxide Solutions

NH.OH Cancentration, %

1.1 12.9 20.2 27.1

CCIfI"lISion Rate, mils per year

o 560 370 180

Tests run to agitated solution at room temperature for 20 hours.

Nickel-copper alloys. such as Alloy 400, are re-sistant to anhydrous ammonia and are slightly more resistant than commercially pure nickel in ammonium hydroxide solutions. as shown in Table XXXIII. However, their usefulness is restricted to dilute solutions up to about 3

Table XXXV Corrosion T e.sts During Dissolving of

Silicates in Caustic Soda Location: Test specimens suspended near bottom of kettle. Temperature: 77 C (170 f). Test Period; 32 days.

Material

Nickel 200 Ni:Resist Type 3 Ni:Resist Type 2 Nickel Cast Iron (3% Ni) Cast Iron Mild Steel

Table XXXVI

COffoswn Rate, mils per year

0.1 0.2 0.5 8

33 41

Conosion Tests in Phosphate Hydrator Composition: 50% solution of sodit.t1l1 tripolyphosphate and

sodit.tm tetrapotyphosphate. Average Temperature: 74 C (165 F}. Test Period: 60 hours. Aeration: Extensive. Agitation: Considerable.

Material

Type 3114 Stainless Steel Type 316 Stainless Steel MONEt. alloy 400 Mild Steel

Corroswn Rate, mils pet year

0.1 0:4 0.7

133

10:t sodium sulfide. the corrosion rates are quite Jow. as shown in Table XXXVII. The most severe service conditions are encountered in hot. concen-trated solutions. The results of two plant corro-sion tests in direct-fired evaporators which con-

Table XXXVII Plant Corrosion Test in a

Sodium Sulfide Storage Tank Solution: 10% Na2S. Aeration: Open tank. Temperature: Atmospheric. Agitation: Only due to filling tank. Test Period: 81 days.

Matl!rial

. Nickel lO!l MQNEL alloy 400 tNCONEL alloy 600 KASrELLOY all.oy B HAsrULOY atloy C Type 304 Stainless Steel Type 316 Stainless Steel ILLlUM G DURIMEr 20 Copper-Nickel alloy CA 715

Corrosiolt Rate. mils per year

PART IV. INDUSTRIAL APPLICATIONS

A. Caustic Soda Manufacture Service records, often dating back for 20 to 30 years, have demonstrated the satisfactory service of nickel and nickel alloys in caustic soda manu-facture. In one plant, nickel centrifugal pumps handling 50% caustic soda from mercury cells are 27 years old and are still in operation. In another plant, nickel evaporators continue to give good service after 30 years' use. Nickel 200, low-car-bon Nickel 201, Alloy 600, Alloy 400 and their cast counterparts are "standard" materials of construction, either solid or as a cladding for

equipment such as evaporators, heat exchanger tubing, pumps, crystallizers, valves, fittings, etc., used in the concentration and handling of caustic soda. Corrosion test data cited earlier in this bul-letin were obtained largely in caustic soda manu-facturing processes.

A comparison of the corrosiveness of caustic soda produced from mercury cells with that pro-duced by diaphragm cells was made by Committee T5A-3D of the National Association of Corrosion Engineers. Data obtained in this survey are shown in Table XL. It appears that there is not a

Fig. 13 - Triple-effect evaporators for the concentration of diaphragm cell liquor to 50% caustic soda_ All threeevapora-tors are constructed entirely of Nickel 200 and Nickel 200-clad steel.

Photograph courtesy of Blaw-Knox Company_

24

Table Xl "Round Robin" Test Program by Four Caustic Soda Producers-Comparison of

Corrosiveness of Diaphragm Cell vs. Mercury Cell Caustic-Conducted by NACE Committee TSA-3D

Average Temperature Company

1 2 3 Material Corredent C f C f C F

Nickel 200 50% NaOH-Oiaphragm Cell 35 95 29 85 88 190 Hickel 200 50% NaaHt>iaphragm Cell 40 104 - - - -Hickel 200 50% NaOHMercury Ceil 38 100 105 221 82 180 Nickel 200 50% NaOHMercury Cell 37 98 45 113

- -

Nickel 20:0 50% NaOHMercury Cell - - Ambient - -Nickel 20:0 73% NaOMHapluagm Cell 119 246 - - 99 210 Nickel 200 73% NaOH-Oiaphragm Cell 125 257 - - - -Nickel 200 73~4 NaOHMercury Cell 114236 - - - -INCONEL alloy tioo: 50% NaOK-Diaphragm Cell 35 95 29 85 88 190 INCONEL alloy 601} 50% HaOMDiaphragm Cel! 40 104 - - - -INCONEL alloy 600 50% NaOHMercury Cell 38 100 105 221 82 180 INCONEL alloy 600 50~{' NaOHMercury Cell 37 98 45 113

--

INCONEl alley 600 50~{' NaOH-Mercury Cell - -

Ambient - -

INCONEL alley 600 73% NaOH-Diaphragm Cell 119 246 - - 99210 INCONEl alloy 6{}0 73% NaOHDiaphragm Cell 125 257 - - - -INCONEL alley 600: 73% NaOHMercury Cell 114 236 - - - -MONEL alloy 400: 50% NaOHOiaphraglll Cen 35 95 29 85 88190 MONR alloy 400 50% N~OH'Diaphragm Cell 40 104 - - - -MONEL aliDY 41111 50% NaOH;Mercury Cell 38 100 105 221 82180 MONEL aUoy 400 50% NaOHMercury Cell 37 98 45 113 -

-

MONEL alloy 400 50% NaOH-Mercury Cell - -

Ambient - -MONEL alloy 400 73kNaoHOlaphraglll Cell 119246

- -99 210

MONEL alley 400 73% NaOH-Diaphraglll Cell 125.257. - ,...,... -

,....

MONEL aUoy 400 13% HIlOaMercury Cell 114236 - - - -INCOLOY alloy 800 50% NIlOHOiaphragm Cell 35 95 29 85 88 190. INCOLOY alloy 800 50%: HIlOHtliaphragm Cell 40 104

--

- -

INCOLOY alloy 800 50% NaOHMercury Celt 38100 105 221 82180 INCOLOY alloy 800 5{)%NaOH.Mercury Cell 37 98 45 113 -INCOLOY alloy 800 50~i. NaOH.Mercury Cell

- -Ambient

--

IHCOLOY alloy 800 73% NaOIH)iaphragm Cell 119246 - - 99.210 IHCOLOY alloy 800 73% NaOH-Diaphragm Cell 125251

- - - -

INCOLOY alloy 800 73% NaoNMercury Cell U4.236 - - -

~

CARPENfER alloy 20 Cb3 50'!';' NaOHDiaphragm Cell 35 95 29 85 88 190 CARPEtffER alloy 20 Cb-3 50'!" NaOHOiaphragm Cell 40 104

- - - -

CARPENfER alloy 20 Cb3 50% NaOK-Mercury Cell 38100 105 221 82 1811 CAftPEtffER alloy 20 Cb~ 50% NaOH,Mercury Cell 37 98. 45.113

-~

CARPENTER alloy 20 Ch3 50% NaON-Mercury Cell - -

Ambient - -CARPENTER allDY 20 (:b3 73% NaOHOiapllragm Cell U9246

- -210

CARPENTER alley 20 tb3 73% NaOHOiilplnagm Cell 125 257 - - - -

CARPENTER alllly20 (:b3 73% NaOHMercury Cell H4236 -

- - -

ACI tN-7M 50% NaOHOiaphragm Cell 35 95 29" 85 8'8 190 ACt CN7M 50% NaOHDiaphragm Cell 40 104

- - - -

ACI CtOM 50?{. NaOHMer(;ury Cell 38 100 105 221 82 180 ACl eN7M

-504 NaOHMen:ury Cell 37 98 45 il3 - -

ACt CN-1M 50% NaOHMeJ.eury Cell - -

Ambient - -ACI CN-1M 73'l~NaOH-Dlaplmlgm Cell 119 246 - - 99210 ACI CN-7M 73'}~ NlIOH,OillPhraCm Cell 125 257

- -- -

ACI CN"7M 13% NaOH.MetClifY Cell 114 236 - - - -

NiRe.sist Type 3 50% NaOHDiapbragm Cell 35 95 29 85 88 190 Ni-Resist Type 3 50% NaOHDiaphragm Cell 40 104 - - - -Hi-Resist Type 3 50% NaOHMercury Cell 38 100 105 221 82 180

4

C F

54 130 Ambient 60 140

Ambient Ambient -

-

- -

- -

54 130 Ambient 60 140

Ambient Ambient -

-

--

- -

54 130 Ambient 60140

Ambient Ambient - -

- -

- -

54130 Ambient 60 140 Ambient Ambienl - -

- -

-

54 130 Ambient 60 140

Ambient Ambient - -

--

- -

54130 Ambient 60 140 Ambient Ambient - -

- -

--'

54 130 Ambient 60 140

1

Table XL (Cont'd.) "Round Robin" Test Program by Four Caustic Soda Producers-Comparison of

Corrosiveness of Diaphragm Cell vs. Mercury Cell Caustic-Conducted by NACE Committee T5A-3D

Average Tempe,ature Company 2 3 4

Material Corrodent C f C f C f C f

HiResist Type 3 50% NaOM.Mercury Cell 37 98 45 113 - - Ambient Ni:Resist Type 3 50% NaOJ1,Mercury Cell

- -Ambient

- -Ambient

Ni:Resist Type 3 73% NaOH-Oiaphragm Cell 119 241> - - 99 2W - -Hi:Resist Type 3 73% NaOM-Diaphragm Cell 125 257

-- - - - -

Hi:Resist Type 3 73% NaOHMercury Cell 114 236 - - - - - -Type 3.16 Stainless Steel 50% NaOH~iapllragm Cell 35 95 29 85 88 190 54 130 Type 3Ui Stainless Steel 50% NaOH~iaphragm Cell 40 104 - - - - Ambient Type 316 Stainless Steel 50% NaOHMercury Cell 38 100 105 221 82 180 60 140 Type 316 Stainless Steel 50% NaOHMercuI}' Celt 31 98 45 113

-- Ambient

Type 316 Stainless .Steet 5~% NaOHMercury Cell --

Ambient - - Ambient Type 316 Stainless Steel 73% NaOH~.iapllragm Cell 119 246

-- 99 210 - -

Type 3l6.stainless Steet 13% NaOH-Oiapbragm Cell 125 257 -- -

- - -

Type :116 Stainless Steel 13% NaOH'Mercury Cell 114236 - - - - -. -Type 304 Stainless Steel 50% NaOHDiapllragm Cell 35 95 29 85 88190 54 130 Type 304Stainle$$.Sieei 50% NaOlH)iallhragm Cell 40 104 - - - - Ambient Type ~04 Stainle$~ Steel 50% NaOn-Mercury Cell 38 100 105 221 82 180 .60 140

Type 304 Stainless Steel 50% NaOHMercury Cell 37 98 45 113 - -

Ambient 1ype304 StaiRtessSteel 50% NaGH.Mercury Cell - - Ambient - - Ambient Type 304 Stainles$.steel 73~ NaOH-Oiapbragm Cell 119246 -

-99 210 - -

Type 304 Stainless Steel 73% NaOH-Oiaphragm Cell 125 257 - - - - - -Type 304 Stainless Steel 73% NaOHMercury Cell 114 236

- - - - - -

Ductile Cast Iren 50% NaOllDiaphragm Celt 35 95 29 85 88 190 54130 Ductile Cast Iron 50% NaOli-Diaphragm Cell 40 104

- -- -

Ambient Ductile Cast Iron 50% NaOHMercury Cell 38 100 105 221 82 180 60 140

Dllctil~cast Iron 50% NaOliMercury Ce:t 37 98 45113 - -

Ambient Ductile Cast IllIn 50% NaOJt..Men:l.lry tet!

- -Ambient

- -Ambient

D1Ictile Cast Iren 73% NaOH~iapllfagm Cell 119246 - -

99 210 - -Ductile Cast illln 73% NaOH~iaph[agm Celt 125 251

- - - -- -

Ductile cast Iron 13% NaOH.MercuI)' Celt 114236 - - - -

- -

Gray Cast Iron 50% NaOH-Oiaphragm Cell 35 95 29 85 88 190 54 130 Gray Cast Iroll 50% Na(lli-Diaphragm Cell 40 104

- -- -

Ambi'lnt Gray Cast Iron 50% NaQli-Mercury Cell 38 100 105 221 82 100 60140 Gray Cast Iron 50% NaOHMercury Cell 31 98 45 113

- -Ambient

Gray Cast Iron 50% NaOli-Mercury Cell - -

Ambient - -

Ambient Gray Cast Iron 73% NaOli-Dia.phragm CeU H9246

--

99 2It} - -

Gray Cast Iron 73% NaOn-Diaphragm Cell 125 257 - - - - - -

Gray Cast Iron 73% NaOK-MereuI}' Cell 114 236 - -

- -- -

MildSteeJ 50% HaOH-Oiapbragm Cell 35 95 29 85 88 190 54130 Mild Steel 50% NaOH~iallhragmCeIi 4D 104 - -

-- Iynbient

Mild Steel 50% NaOHMercury Cell 38 100 105 221 82 180 60 140

Mild Steel 50% NaOHMercury Cell 37 98 45 113 - - Ambient Mild Steel - 50% NaOM-Mercury Cell - - Ambient - Ambient -Mild Steel 13% NaOHl)iapilragm Cell 119 246 - - 99210 - -Mild Steel 73% NaOfl..Diapbragm Cell 125 251 - - - - - -

Mild Steel 73% NaOMMercury Cell 114 236 - - - - - -

(1) Pitted to a maximum dept" Qt 1 mHo (2) Pitted to a maximum depth of 4 mils. (3) Pitted t a maximum depth of 5 mils. (4) Stress-corrosion crack through

(5) Pitted to a ma.ximlJm depth Qt 3 mils. (6) Mercury droplets in tao". 2 rates shown

are for the duplicate speCimens

one of the identifying punch marks. (not averaged): specimen with high rate showed stress-acce1erated local attack.

26

1

0.2

0.3 2.3 1.2

great deal of difference in the corrosiveness of the caustic produced by these two types of cells, al,'ld that other variables such as temperature and con-centration are more important in influencing cor-rosion rates.

Prior to about 1946, the concentration of 50% or 73 % caustic soda to anhydrous was carried out entirely in direct-tired caustic pots in a batch operation. These pots Were usually constructed of gray cast iron. Nicl

Alloy 600 has also been used extensively for producing anhydrous caustic soda and is the preferred material of construction where the heating is accomplished with any media in which there is a po:.;sibility of sulfur compounds being present. Alloy 600 for this service should be stress-relieved or annealed as indicated in the dis-cussion of nickel-chromium alloys in Part II of this bulletin.

Fig. 15 - Tubes fabricated from Nickel 200 afe used in the inclined heat exchanger in front of the Nickel 200-dad

evaporator which prodl.l

Tt'an;;fer of materia! to and from storage tanks usually requires pumps. Table XLIII shovv's the results of a plant test in which Xickel 200, MONEL alloy 400 and INCONEL alloy 600 corrosion coupons wet'e subject to turbulent flow just downstream of a pump handling 50('~ caustic soda. Similar cor-rosion resistance would be expected fl'O!1l the cast counterparts of these wrought materials (ACI CZ-IOO, M-35 and CY-40). Pumps cast from ACI CZ-IOOhave given over 25 years service as previ-ollsly noted.

Table XLIII field Test in 50% Caustic Soda

Just Downstream of a Pump Temperature: 60 to 70 C (140 to 158 f);

AI/erage 65 C (149 f). Test Period: 393 days. Aeration: Moderate. flow; 100 gpm in 3inch pipe.

Material

Nidle1200 lNCON~L alloy 600 MONEL aUoy 400

_ ..

Corrosion Rate. mils per year

a.OJ 0.03 0.10

fig. 17 ~ This barge carries 50%. caustic soda from a mercury cell plant to storage facilities'along the Tennessee. Ohio and Mi Rivers. Four cylindrical tanks have a total capacit liquid tons. To insure a long sef\lice life for the ge and to protect product purity. all cargo piping is either solid Nickel 200 or Nickel 2oo-clad steel with Nickel 200 fittings: The cargo fromal! four tanks empties into a Ni,ckeI2oo-clad steel well from which ,it ispl.lmped to on-shore facilities_' .' . '

The use of nickel;..cla

tained above the freezing point. Tables XLIV and XLV give the results of tests in transportation and storage facilities.

It is common practice to load and unload cars of 73% caustic through Nickel 200 heat exhang-ers, pumps and piping.

Table XLIV Field Test in Tank Car Transporting

74% Caustic Soda Temperature: 130 C (265 F). Test Period: 11 trips of 79 days. Aeration: None. Agitation: By movement of tank cars.

Material

Nit;lle1200 INCOlOY alloy 825 MONEL alloy 400 CARPENTER alloy 20 Type 316 Stainless Steel

Corrosion Rate, mils per year

0.3 0.3 0.4 0.9 8.4

Fig. 19 - 1300 feet of transfer pipe with a rolled and welded internal lining of Nicke1 200 carries 50% caustic soda from a marine terminal to a Nickel 2ooc1ad storage tank. Nickel 200-clad tank cars are in the background.

Table XLV Field Test in Storage Tank for 73% Caustic Soda

Temperature: 116 C (240 F). Test Period: 183 days. Aeration: None. Agitation: None except for filling of tank.

-Material

Nickel 200 I NCONEl alloy 600 MONEL alloy 400 Zirconium Titanium Mild Steel

Corrosion Rate, mils per year

0.3 0.4 0.7 L4 4.7

Destroyed during test

30

D. Soap Manufacture Soaps are made by the reaction, called "saponi-fication,"between alkali and fatty oils (gly_ cerides) and fatty acids of animal or vegetable origin, or a mixture of both. The largest produc-tion, and the most familiar, is "hard" soap made with caustic soda as the saponifier. Caustic potash produces a "soft" or liquid soap.

In certain high grade soaps, it is necessary to avoid contamination by such metals as iron and copper in order to obtain a high quality product. Therefore, pure caustic must be used in combina-tion with corrosion-resistant equipment. The matter of iron contamination is particularly sig-nificant in soap-boiling kettles because the soap spends so much time there. particularly in the fuH-boiled process. This is especially significant in the upper parts of the kettles where corrosion rates are highest. Table XLVI shows the results of one plant corrosion test in a soap-boiling kettle. Some of the earliest applications of ~orrosion-resistant materials were in the construction of soap kettles.

Table XLVI Plant Corrosion Test in SoapBoiling Kettle

Specimens immersed near the top of the settling cone duro ing saponification and graining. Temperature: 70 to 100 C (160 to 212 F). Test Period: 106 days.

Material

Hit;ke1200 MONEL alloy 400 .NCOffEl. alley 600 HiResist Type 1 Mild Steel Cast Iron

Corrosion Rate. mils per year

Much of the corrosion test work in soap plants has been concerned with the treatment of spent soap lye and recovery of glycerine. since these processes represent particularly corrosive condi-tions. The pH of the solution during acid treat-ment is usually 4 to 4.5 and sometimes as low as 3, due to the presence of hydrochloric or sulfuric acids. Agitation of the mixture with air, a com-mon practice, tends to increase the corrosion rate of steel. The results of six tests during acid treat-

ment in four different soap plants are shown in Table XLVII.

Alloy 400 and Nickel 200 or steel clad with these materials are used for both acid-treating and caustic-treating tanks because of their resist-ance in both environments. Austenitic chromium-nickel stainless steel and Alloy 600 are also used but pitting is possible with these alloys under certain conditions as shown in Table XLVII. Ni-Resist Type 3 is used instead of Types 1 or 2 for

Table XLVII Plant Corrosion Tests in Acid Treatment of Spent Soap lye

Test 1: Immersed in acid treating tank in mixture of 13% NaCI and 4.5% glycerine to which is added 150 Ib of 28% HCI and 75 Ib of 17% aluminum sulfate per 30,000 Ib soap lye. Temperature: 1 to 82 C (30 to 180 F). Test Period: 167 days. Plant L

Test 2: Immersed halfway down in acid treating tank in mixture of 18% NaCI plus glyce.rine to which is added 0.5% solution of aluminum chloride. Aerated. Average tempera ture: 74 C (160 F). Test Period: 65 days. Plant 2.

Test 3: Immersed in trough of filter in acid treated filtrate from Test 2. Aerated. Average temperature: 71 C (160 F}. Test Period: 65 days. Plant 2.

Test 4: Immersed halfway down in acidtreating tank in mixture of 8 to 10% NaCI and 4.5% glycerine made acid to pH 4.5 with HCI and ferric chloride. Air agitation. Temperature: 21 to 71 C (70 to 160 F). Test period: 28 days. Plant 3.

Test 5: In acid treating tank in spent soap lye made acid to pH 4.5 with HCI and ferric chlo ride, and aluminum sulfate. Agitated wiUl1iIir. Temperature: 54 to 79 C (130 to 114 F). Test Period: 45 days. Plant 3. a. Immersed in bottom of tank near air inlet. b. In vapor in top of tank. .

Test 6: Immersed halfway down in solution containing 13 to .16% NaCI plus Na2S0. and 10 to 12% glycerine made acid to pH 4.5 with sulfuric acid and ferric chloride. Aerated. Temperature: 32 to 100 C (90 to 212 fl. Average 85 C (180 Fl. Test Period: 105 days. P\ant4.

C.onosion llate.lIllJs per year Test 1 Test 2 Test 3 . TesH Test Sa Test Sh TestS

Material Plant 1 Plant 2 Plant! Plant 3 Plal1t3 Plant 3 fitaltt4

MOHEL alloy 400 .9 0.3 4.8 2.9 5.6 4.4 16.0 Hickel 200 1.1 0.9 3.7 L8 5.1 4.7 10.0

IHCOHEL aUoy SOO .6

the construction of soap lye filters and filter plates to withstand possible therma' shock when the hot solutions enter a cold filter process. Pumps of iron-base nickel-chromium-copper-molybdenum alloys such as WORTHITE or DURIMET 20 have given good performance handling both acid- and alkali-treated soap lye. Austenitic chromium-nickel stainless steels, usually Type 304, have been used to advantage for "finishing and packaging" equipment.

Although a considerable amount of the world's soap is still produced batch-wise, efforts to reduce the 4 to 11 days required with the fun-boiled kettle method have resulted in a number of continuous processes for soap manufacture. In one such process, blended fats with zinc oxide catalyst are reacted countercurrently with water in a 65-foot-high. Type 316 stainless steel hydrolyzing tower maintained at 282 to 260 C (450 to 5(,)0 F) and 600-700 psi. Fatty acids are continuously drawn off the top and crude glycerol off the bottom of the column. The fatty acids arevacuum-distiUed and then neutraiized in a high-speed mixer with a caustic soda solution containing salt. thus produc-ing soap in about four hours.24

In other continuous processes which usually utilize centrifuges. the corrosives encountered are much the same and considerable quantities of austenitic chromium-nickel stainless steel. Alloy 400 and Nickel 200 are utilized. Tables XLVIII and XLIX indicate the corrosion rates eneoun-

Table XLVIII Plant Corrosion TestinfourthStage lye Tank

Immersed in tank containing soap lye with 2% MaOH and 11% MaC!.. Temperature: 88 to 96C (190 to 205 F). Test Period: 102 days.

Corrosion Rate, Material mils per year

Nickel zno Nil o MONEL alloy 400 Nil INcolfEl aUoy 600 Nit Type 316 Stainless Steel Nil Type 341 Stainless Steel Nil Type 304 Stainless Steel 0.1 HiResist Type 1 0.1 Mild Steel 1.0 Cast Iron 3.0

Less than 0.05 mils pel' year.

Maximum Depth of Pitting, mils

None None None None None None None

7 22

tered in a lye tank and a centrifuge in one of these processes.

Table XLIX Plant Corrosion Test in Third-Stage Centrifuge

Specimens located at soap discharge. Mixture contained 15% NaOH and 11 % NaC!. Low aeration, flow 350 gallons per hour through 4inch pipe. . Temperature: 91 to 96 C (195 to 205 F). Test Period: 102 days.

32

C orrusion Rate, Maximum Depth Material mils pef year of Pitting, mils

Nickel ZOO Nil o None MONEL alloy 400 Nil None I/'ICONE.L alloy 600 Nil None Type 304 Stainless Steel 0.1 None Type 316 Stainless Steel 0.1 None Type 341 Stainless Steel 0.1 None Hi-Resist Type 1 0.4 None Mild Steel 10 Perforated Cast Iron 12 55

Less than 0.05 mils per year.

E. Pulp and Paper Industry Over a million tons of caustic soda are used annually in the pulp and paper industry, prin-cipally for the extraction of alkali-soluble impuri-ties in multistage bleaching and for pH control. Small amounts are used for preimpregnation of wood chips and for the production of soda pulp.

More than two-thirds of aU paper is pX'Qduced by the Kraft process. Digestion of certain soluble portions of wood chips is accomplished by a hot alkaline liquor consisting of a mixture of dilute caustic soda and sodium sulfide with a total alka-linity of about 3 per cent. The following are prin-cipal areas where carbon steel may corrode at an excessive rate and nickel-containing alloys (usually austenitic chromium-nickel stainless steels) can be used to advantage.

1. Digesters Batch-type Kraft digesters are commonly built of carbon steel with a corrosion allowance in excess of one inch. Until recent years this resulted in a service life of about 15 years, but with the increasingly severe conditions imposed. by modern pulping methods, service life was reduced to about 7 to 9 years. Weld overlays employing A WS E310,

or A WS E310-Mo. have been employed to extend the service life of corroded steel batch digesters. Table L indicates the excellent corrosion resist-ance of stainless steel and several other nickel alloys in one Kraft digester.

Table l Plant Corrosion Test in a Sulfate Process,

Alkaline, Wood Pulp Digester Temperature: 177 C (350 f). Test Period: 586 days. Aeration: None. Agitation: Violent boiling during cook.

Top--Vapors in the top of the digester. Occa sional splashing of chips, pulp and cooking liquors. Bottom-Liquid and slurry on bottom screen of digester.

Specime(ls: Combination of stress and general COrrosion. Strips were stressed beyond the yield point by bolting down over a fulcrum. Some specimens were welded or contained weld overlays as noted. No stress corrosion cracking occurred.

Material Condition

INCONEL alloy 600 Plate. asreceived CARPENTR alloy 20 Welded INCoNEL alloy 600 Welded INCOlOY alloy 825 Plat.e. asreceived INCOlOY alloy 825 Welded Type 316 Stainless Steel Plate, asreceived Type 316t. StainJess steel Plate. asreceived Type 31Sl Stainless Steef Welded Type 316 Stainless Steel Welded AViS Eltflllveriay on Steel Weld Overlay

Note: A dash indicates no coupon was exposed.

Corrosion Rate, mils per year Top Bottom

0.02 0.21 0.03 0.09 0.03 0.23 0.03 0.09 0.03 0.04 0.15 0.05 0.06 0.17 0.06 0.15 0.05 0.17

There are a few Kraft digesters that utilize a duplex process in which the charge is initially acid (pH 4) and later alkaline. Table LI shows corro-sion rates in this process.

There are several hundred continuous digesters operating on wood chips in the United States. These are constructed primarily from carbon steel with high corrosion rate areas lined or clad with Type 316L stainless steeL These high corro-sion rate areas include the upper section. where fresh, hot alkaline liquor is injected, and the bottom section in the area of the blow valve. Internal accessories such as scrapers and chip screens are usually fabricated from Type 316L stainless steeL Construction of digesters with clad Type 316L stainless steel would allow for con-

Table LI Plant Corrosion Test in a Digester Utilizing

a Duplex Sulfate Process Temperature: 100 to 171 C (212 to 340 f). Cycle: Chips steamed for one hour, temperature rises from

100 C to 118 C (212 to 244 f). Acid liquor removed. Alkaline liquor containing 82 gpl NaOH and 25 gpl Na,S added. Charge brought to 171 C (340 f). cooked for total of 5 hours.

location: In vapor. Test Period: 731 days.

33

Material

Type. 316 Stainless Steel Type 341 Stainless Steel CARPENTER alloy 20 INCj)NELalloy Spo MONEL alloy 400 Titanium Mild Steel

Corrosion Rate, mils per year

0.1 0.1 0.1 OJ

23 55

107

Fig. 20 ~ This top separator on a KAMYR continuous di gester separates the chips from the flushing liquor. With the exception of the drive mechanism. this separator is can structed of Type 304 stainless steel.

siderably reduced wall thickness and much lower maintenance costs.

2. liquor Heaters Shell and tube heat exchangers are used to heat the digester liquor prior to its introduction into

both batch and continuous digesters., Results of a corrosion test in such a heater are shown in Table LII. These data may indicate a lower than actual corrosion rate for carbon steel, since the heat exchanger tube walls are at a temperature higher than the liquor in which the test specimens were exposed. The liquor contains a large proportion of fresh caustic and sulfide in addition to some black liquor recovered from a previous digester cook. Experience over many years has proven the ade-quacy of annealed Type 304 stainless steel for this service. The use of "as-welded" tubes has some-times resulted in failure by intergranular cor-rosion immediately adjacent to the weld. This type of corrosion has not been observed when tubes are used that have been made in compliance with ASTM A 249. This specification caUs for welded, drawn, quench-annealed tubing. Tubes of this type have been known to last in excess of.10 years, but service life is dependent on specific operating conditions. In a few instances, the Type 304 stain-less steel tubes have been subject to failure by chloride stress-corrosion crackng. Alloy 600 and Alloy 20 have been successfully employed to resist this type of attack.

3. Black liquor Evaporators To permit recovery of chemical values in the digester liquor when chip cooking is complete, it is necessary to concentrate the liquor, together with the chip wash water. This is required to raise the solids content to more than 50 per cent. which will permit burning in the recovery furnace.

Kraft liquor vacuum evaporators are multiple units usually consisting of one or more sets of six long tube vertical effects connected in series. Corrosive conditions on the tubes are somewhat less severe than in digester liquor heaters since the vacuum operation results in lower boiling temperatures. The first effect operates at the highest temperature of about 135 C (275 F). Tem-peratures decrease in each succeeding effect. It has been customary to use Type 304 stainless steel for tubes in the first effect and often in the second effect. A number of installations have used Type 304 stainless steel tubes in all effects, re-sulting in less frequent downtime fer cleaning, long service Hfe, and maintenance of high heat transfer rates.

34

Table lit Corrosion Test in Kraft Pulping

Exposed 68 Days in Head of liquor Heater Flow rate of 2400 gpm at temperature of 173 C (344 F).

Material

Type 304 Stainless Steel Type 316 Stainless Steel INCONEL alloy 600 MONEL alloy 400 Nickel 200 Mild Steel Cast Iron

Corrosion Rate, mils per year

0.5 0.8 0.9

38 57 95

342

Vapor domes in the hottest effects are often clad with Type 804Lor Type 316L stainless steel, since carbon steel in this area may corrode at a rate exceeding 100 mils per year. There are also instal-lations where sta.inless-elad steel has been used for the entire evaporator body. Advantages of such construction.inelude less carry..over of corrosion products and less fouling of the evaporator tubes by these products.

Defiectorpla.tes and auxiliary piping are usu-any made of solid Type 804L stainless steeL For valves and pumps, Ni-Resist Type 2, CF-8 and CF -8M castings are used.

4. Recausticizing As part of the operation to regenerate chemi-cals reclaimed from the recovery furnace, sodium carbonate is treated calcium hydroxide (milk of lime) to produce sodium hydroxide.

Table lin Corrosion Test in Kraft Pulping

Exposed 68 days in green Uquor. 175 to ~5 gpl as Na2 CO, in flow bQx.from recovery furnace to claSSIfiers. Temperature: 66 to 99 C.(150to 210 F). Some aeration and agitation.

Material

tN(:ONEL alloy 600 Type 302 Stain1ess Steel Type 309 Stainless Steel Type 310 Stainless Steel Type 316 Stainless Steet Nickel 200 MONEL alloy 400 Mild Steel Cast Iron

Corrosion Rate, mils per year

Carbon steel, with a corrosion allowance. has been used for most of the equipment. As shown in Table LIII, fairly high rates can occur on car-bon steeL Light gauge Type 304 stainless steel is an economic selection for troublesome areas.

F. Aluminum Indu~try Despite extensive use of caustic soda by the aluminum illdustry for the extractioRofhydrated alumina from bauxite in the Bayer process, rela-

Fig. 21 - This llfoot long. 16inch diameter pipe has been electmplated with nickel to yield a 30-mil thick deposit on the inner diameter and about 2 mils on the outer diameter. Sections like this are welded toget:he( to form piping used in bauxite refining in the aluminum industry. Lengths of greater than 11 feet can also be plated.

Photo by courtesy of Plating Engineering Company. Milwaukee. Wise.

35

tively smallam{llwts of nickel and nickel altoys have been utiJized in these plants.

Alloy 400 tubes have been successfully em-ployed for digester preheaters. and Alloy 400 insert ferrules have been used to overcome the inlet end corrosion in other steel preheater tubes. Relatively thick (30 mils minimum) electroplated nickel {lll steel has been used to advantage for piping and digester preheater channels. Nickel weld-overlays haVe proven \Iseful on pump cas-ings, and cast nickel (ACI CZ-IOO) has given good service as pump impellers. valve bodies and for other instrumentation.

However. the present practice with bauxite digesters is to use thick-walled carbon steel at low stress levels. Some cases of stress-corrosion cracking (If steel have occurred in plants handling caustic soda solutions in the Bayer extraction process.z::>

The recent trend awaYI;r;om ores high in gibbs-ite C{lntent toward the use of Ores relatively high in boehmite c()ntent has necessitated digester operation at higher pressures and temperatures. This increases the pos!;;ibHity of caustic embrittle-ment of steeL Thus. nickel or nickel-clad steel should be given consideration for the processing of these higher boehmite bauxites.

G. Caustic FuSions Nickel 200 and Nickel 201 are useful as mate-rials of construction for vessels for the caustic fusi{ln of organic comp{lunds. Where tempera-tures exceed 316 C (600 F). the low.-carb{ln Nickel 201 is preferred to preclude grain bound-ary precipitation of carbon which greatly reduces duetility. For those reactions where sulfur com-pounds are present at temperatures over 250 to 300 C (482 to 572 F). either in the process or the heating medium, nickel may be attacked inter-granularly and Alloy 600 is preferred.

One process for the production of resorcinol has involved the caustic fusion of benzene meta disulfonic acid at 325 C (617 F). Equipment for this production has been made of wrought Alloy 600 and ACI CY -40 castings. Both alloys should be stress-relieved as indicated in the section on nickel-chromium alloys (Part II B).

H. Petroleum Refining Cau~tic soda or, occasionally, caustic potash or sodium carbonate is used in petroleum refining for acid neutralization and the removal of unde-sirables such as mercaptans and hydrogen sulfide. Aqueous solutions may range from 2 to 50 per cent.

For many of the applications where temper-ature and concentration are low, the corrosive conditions are mild enough that steel can be used. Where the corrosive conditions are more aggres-sive, Nickel 200, AHoy 400 or Alloy 600 are used. Very often Alloy 400 is used because it appears to have a greater tolerance for the impurities present in the process.

Fig. 22 - Nickelcopper alloy 400 was used for the walls of the caustic stripper towers. reboiler tube bundles and hot caustic lines in this refinery. After 10 years of service. the Alloy 400 components continue to withstand the corrosive mineral acids. sulfur compounds and hot caustic soda in the fluid hydroformer and caustic regenerating equipment.

In the t'egeneration of caustic solutions, it is common practice to use Alloy 400 in critical por-tions of the system where steel is unsuitable. These locations include the regenerator reboiler, preheaters and piping for handling hot caustic solutions and sometimes for the bottom sections of the regenerator towers. These components may be either solid or clad. ACI CZ-IOO, ACI M-35 ductile Ni-Resists and WORTHITE stainless steei have been used for valves and pumps. The results of plant corrosion tests in the reboilers of caustic regenerator units are shown in Table LIV.

36

Table ltV Plant Corrosion Tests in

Caustic Regeneration Units Test A-In open tank used to boil 18 to 22% caustic soda

plus merca pta ns and cresolates for regeneration of caustic solution. Test specimens were immersed in solution above heating coils. Test Period: 30 days. Temperature 38 to 104 C (100 to 220 F). Average 80 C (175 F).

Test a-Just ab~ve reboiler inlet below bottom tray of reo generating tower. Solution 18% caustic soda for tified with naphthenic acid. cresols and phenols to 22 to 28 Be. Solution also contained 0.040/0 mer captan sulfur. Test Period: 660 days. Temperature: 21 to 116 C (70 to 240 f). Average 107 C (225 F).

Test C-At bottom of stripping tower 18 inches above reo boiler tubes. Solution 7 % caustic soda with trace of mercaptans. Test Period: 354 days. Temperature: 121 to 149 C (250 to 300 F). Average 135 C (275 Fl.

Test o-In vapor sectj~n of caustic soda regeneration unit. Solution entenng contained' 13.2% caustic soda. 0.37% sulfide sulfur and 0.80% mercaptide sulfur. Test Period: 55 days. Temperature: 150 C (300 F).

Corrosion Rate. mils per year Material TestA TestS Test C Test D

INCONEl alloy 600

In view of its good resistance to caustic alkalies containing hydrogen sulfide and mercaptans, Al-loy 600 is also a useful material for evaporator tubes or other parts of regenerator systems. Alloy 600, rather than Nickel 200 or AHoy 400, should be used in this service where metal temperatures in excess of about 250 to 300 C (482 to 572 F) are encountered, since Nickel 200 and Alloy 400 are subject to sulfidation at higher temperatures.

A caustic stripper, at a major Louisiana refin-ery, constructed of MONEL alloy 400, exhibited no detectable metal loss after more than 4 l'2 years' service handling up to 45

PART V. WELDING

A. Fabrication of Nickel-Clad Equipment

In the welding of nickel-clad equipment, a cer-tain amount of iron dilution of the nickel weld deposit occurs. Special precautions are usually taken in order to minimize this dilution. With special precautions, the nick~l welds in a nickel-clad tank for a chemical tanker were limited to an iron content of 0.35-3.29%.26 Gegner has sug-gested that considerably more iron than this can be tolerated.27

Although iron-contaminated nickel weld metal and nickel-iron alloys are not severely attacked in 73% caustic soda at 121 C (250 F). as shown by data in Tables LV and LVI, nickel-iron alloys can be the anode in an electrolytic cell with nickel, as shown in Table LVII. Note that the 20% iron alloy corroded at three to five times the rate it did when it was not coupled