acom06_2 oxidation of s35315 in water vapor containing atmospheres under cyclic & isothermal...

7/29/2019 Acom06_2 Oxidation of S35315 in Water Vapor Containing Atmospheres Under Cyclic & Isothermal Conditions

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Dear Reader

The main article of this Acom covers a subject that could be very dearto a Finn or a Swede since hot and humid conditions are by naturalreasons synonymous with a sauna to these people. However, the paperis on more hostile conditions than any sauna lover could stand, steamat up to 1000C.

The article describes how heat resistant stainless steels toleratedifferent moisture contents, which is depending on the type of fuel.It is easy to realize that moisture contents differ between wethousehold waste, coal, oil and gas. If the moisture content is toohigh volatile chromium species evaporate from the steel surface andincrease the mass loss and risk of failure.

In the second paper you can learn more about the by far mostimportant part of a stainless steel, the passive lm.

Enjoy the reading!

Yours sincerelyJan Olsson

Technical editor of Acom

Oxidation of S35315 inwater vapor containingatmospheres undercyclic and isothermalconditions

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Passive Films onStainless Steel RecentNano-Range Research

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acom2 - 2006A corrosion management and applications engineering magazine from Outokumpu Stainless


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Oxidation of S35315 inwater vapor containing

atmospheres under cyclicand isothermal conditions

Dr. Pascale Vangeli

Outokumpu Stainless AB

Avesta Research Center, High Temperature Steels

SE-77422 Avesta, Sweden

Abstract

The effects of air in combination with up to 40% of water and temperatures up to 1000C

on S35315 are reported. The exposure time varied from 20 h to 1000 h in a tube furnace,and up to 200 h in a thermobalance, under cyclic and isothermal conditions. Theresults show that the oxidation behavior of S35315 in water vapor is characterized by aprotective chromium rich oxide scale and no breakaway oxidation was observed for thissteel. The mechanism of oxidation in an atmosphere containing water vapor is discussed,linked to the ability for chromium to diffuse to the surface and some assumptions areproposed regarding the effects of different alloying elements such as Ce, Si, Ni and Cr.

Introduction

Outokumpu 153 MAM1 S30 15 , 253 MA2 S30815 and 353 MA2 S35315 is a

family of heat resistant austenitic stainless steels with increased contents of silicon andnitrogen, and micro alloyed (MA) with rare earth metals (REM, reactive elements, RE).S35315 has a signicantly higher nickel content than the other steels. The alloyingconcept has resulted in characteristic family features: High mechanical strength at elevated temperatures, i.e. creep strength. Excellent isothermal oxidation resistance and, above all, excellent cyclic oxidation

resistance and oxidation resistance under erosive/abrasive conditions.Furthermore, MA steels are optimized to complement each other: 30415 for medium-high to high temperatures and moderately aggressive, mainly

oxidizing atmospheres. This grade is extra resistant against embrittlement after serviceat medium-high temperatures.

30815 for high to very high temperatures and/or rather aggressive, mainly oxidizing

atmospheres. This grade is the work-horse of the family. 35315 for the highest temperatures and/or toughest conditions, which often means

environments that are strongly carburizing, strongly nitriding, or contain somealogens/halides. The alloy is designed primarily for service above 1000C (1830F),

although it has been used at temperatures as low as 600C (1110F) in more aggressiveenvironments.

From the MA family, only S35315 is considered in this study and compared to standardheat resistant grades, such as 310S, 309S and 800H.

It is well known that the presence of water vapor changes the oxidation behavior ofmetals and alloys [1]. Chromia-forming alloys are affected by water vapor in the sensethat the critical amount of chromium to form protective Cr O or Cr,Fe O4 scales

increases [2]. The concept that volatilization of chromium species was responsible for thiseffect was rst proposed by Ebbinghaus, who reviewed the thermodynamic propertiesof volatile chromium-containing species and calculated that CrO3 was the dominantevaporating species in dry oxygen and CrO2 OH 2 in moist atmospheres [3].

1

153 MATM

is a trademark of Outokumpu

2 253 MA and 353 MA are registered

trademarks of Outokumpu

Keywords: austenitic stainless steels,

high temperature corrosion, evaporation,

breakaway oxidation, oxidation, water vapor,

isothermal oxidation, cyclic oxidation.

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The inuence of alloying elements is observed and discussed, for example, silicon isassumed to enhance oxidation resistance in water vapor. Moreover, cyclic conditions,often reecting service conditions, may inuence the behavior of the heat resistant steelsin moist atmospheres. The ability for the steel to maintain its protective oxide scale isessential in environments where an oxide scale is exposed to high stress, such as erosive

abrasive conditions and/or when large temperature variations frequently occur.The present study shows how S35315, compared to other standard heat resistantsteels, is affected by different testing conditions, such as: temperature, water vaporcontent in the atmosphere and testing time under isothermal and cyclic conditions.Effects of the water vapor, alloying elements, and cyclic conditions are discussed.

Experimental

The study is carried out on S35315; some comparison testing is done on 309S, 800Hand 310S. The actual chemical compositions are given in Table 1. All the grades arecommercial and fully austenitic. The rectangular specimens are 2 to 5 mm thick andhave a total surface area ranging from 3 to 6 cm2. The specimens, polished with emerypaper (grade #80 and #180 SiC papers), are washed with de-ionized water inan ultrasonic bath, then with ethanol and nally dried before testing.

Oxidation tests are carried out in synthetic air with 0 and 40% of water in a tube furnace forup to 1000 h and in two commercial thermobalances for up to 200 h in the temperaturerange 600 1200C. All the tests have a gas ow of 100 mlmin-1 (average net velocitiesof 0.25 cms 1).

Tests in thermobalance: The short time tests are conducted in moist atmospherein a thermobalance SETARAM Setsys12, and in dry atmosphere in a thermobalanceSETARAM TG9 . The weight change is continuously monitored. The furnace is heatedto the desired temperature, while the specimen is kept at room temperature. The gas isintroduced in the thermobalance. A typical test starts when the specimen is lowered intothe reaction chamber. A cyclic test has a cycle consisting of 2 hours in the hot furnaceand 10 minutes in the cold zone (room temperature in the laboratory air).

Tests in the tube furnace: The long time tests, up to 1000 h, are carried out in a tubefurnace. A quartz twin walled reactor tube has been made for this study. The air + H Ogas stream is rst heated to the thermodynamic equilibrium before it ows through thespecimens. This set-up enables the samples to be exposed to the reaction gas atequilibrium with minimum contact with the sample holder and with an optimal owpattern. Details of the test facility has been described elsewhere [4]. All the grades aretested at the same time. New specimens are used for each time (20, 168, 336, 672 and1000 h), which means that the samples do not suffer thermal shocks before the end of

the test. The samples are air-cooled after testing.

Morphology:After visual observation, the specimens are mounted in resin and polished, andthe cross sections are examined and analyzed using light optical microscopy, electron probeand scanning electron microscopy. Identication of the oxides is performed using SEM/EDX.

Chemical composition of the grades in wt% Table 1

C Si Mn Cr Ni N other

309S

310S

S35315

800H

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The mass changes are very small for the entire test period, as this grade is high alloyedand the temperature is low. First, a weight gain is observed, up to about 0.02 mgcm- .Then a weight loss occurs, reaching -0.048 mgcm-2 at 668 h. As no spallation has occurred

after cooling, this weight loss is due to evaporation. This is supported by a brownish layerpresent on the SiO furnace tube wall in the cold zone, downstream from the samples.It is well known that chromia forming steels suffer chromium vaporization in the form ofCrO3 g) above 1100C [5]. At 600C, its partial pressure is too low to explain the massloss observed. Asteman et al. has studied the oxidation of 304 in O2/H2O atmospheres at00 900C and showed that a signicant amount of chromium evaporates from the

chromium-rich oxide scale, even at 600C [6]. A brown deposit was observed at the exitof the furnace tube wall and identied as chromium oxide, as observed in this study.

The predominant chromium-containing vapor species in environments containingoxygen and water vapor at temperatures below 1100C has been predictedthermodynamically to be chromic acid CrO2(OH)2 by Ebbinghaus [3]. More recentlythe structure and thermodynamic stability of mixed oxyhydroxides of chromium (g)

were studied in more details [78]. Panas investigated the mechanism of evaporationof chromic acid from Cr2O3(s) and gave the reaction energetics for the formation ofCrO2 OH) [910]. Their calculated energetic values were in good agreement with theirexperiments.

The mass loss can then be attributed to the following reaction (1):

1/2Cr2O3 (s) + 3/4O2(g) + H2OCrO2(OH)2(g) 1

Results and discussion

Results at 600C in air with 10% of water, isothermal oxidation

Results from the isothermal oxidation of S35315 in air + 10% of H2O at 600C for upto 668 h is presented in Figure 1.

Fig. 1 Mass change of S35315 in isothermal exposures in air + 10% of H2O at 600C

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Results at 900 C in air with 10% of water, early stage of isothermal oxidation

Comparative gravimetric curves in dry air and in air + 10% of H O continuously recordedduring isothermal oxidation of S35315 exhibit a higher mass gain for the oxidation indry air than in air + 10% of H O, see Figure 2.

The oxidation kinetic in dry air is slow and fully parabolic to the end of the experiment,indicative of diffusion-limited oxide growth, while in air + 10% of H2O, the curve showsa non-parabolic behavior. As it has been previously seen at 600C with evaporationof chromium species, this non-parabolic behavior results from the combination of aparabolic oxidation and linear evaporation.

Results at 900C in air with 10 and 40% of water, long-term exposure under

isothermal conditions

The long-term behavior of S35315 in air + 10% of H O and air + 40% of H O at 900Cfor up to 1000 h in isothermal conditions is illustrated in Figure 3a. Each pointrepresents the mass change of one single sample. Spallation does not occur duringcooling for any of the samples.

Fig. 2 Mass change of S35315 in isothermal exposuresin air with 0 and 10% of H O at 900C

Fig. 3a Mass change of S35315 in long-term isothermal exposures

in air with 10 and 40% of water at 900C

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Obviously, the simultaneous events of oxidation and evaporation are also observedhere. With 10% of water, the oxidation rate is slow and no global mass loss is observed.In air with 40% of water, a mass gain occurs rst, followed by a slight mass loss, showingmore evaporation than oxidation. Obviously, the more water vapor in the atmosphere,the more evaporation of chromic acid occurs.

According to Asteman 11 , the evaporation rate depends on water vaporconcentration in the atmosphere, gas velocity and temperature. Even with up to 40% ofwater in the atmosphere and test duration up to 1000 h, no breakaway oxidation occurs,as for other heat resistant grade like 309S Figure 3b .

The parabolic oxidation of 309S in dry air becomes non-parabolic in air with 10%of water, as for S35315, showing evaporation of chromium species. However, after only100 h of testing, a catastrophic breakaway oxidation occurs, leading to a very rapidoxidation rate. An extensive spallation occurred when cooling the sample down to roomtemperature at the end of the experiment.

The corresponding micrographs for S35315 tested for 1000 h in air with 40% of waterare shown in Figure 4a with element distribution maps.

Fig. 3b Mass change of 309S in isothermal exposuresin air with 0 and 10% of water at 900C

Fig. 4a SEM cross-section micrograph of S35315 oxidized in air + 40% H2O at 900 C

for 1000 h (a) with corresponding EDS maps for Cr (b), Fe (c), Mn (d) and Si (e)

(a)

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Figure 4a shows that the oxide scale consists of a three-layer structure. At the metal/oxide interface, a thin silicon-rich oxide layer, compact and continuous penetratesseveral micrometers down into the alloy along the steel grain boundaries. This is atypical morphology for this grade. These results are in accordance with Jonsson showinga continuous silica layer at 900C on S35315 with and without water vapor in theatmosphere [12]. Next, the main layer consists of a compact and continuous chromialayer, followed by an outer, thin manganese-rich oxide layer, covering the entire surfaceof the sample. No iron-rich oxide is observed, as breakaway oxidation does not occur onS35315 at 900C for up to 1000 h in air with 40% of water.

(b) Cr (c) e

(d) n (e) Si

Fig. 4b SEM cross-section micrograph of 309S oxidized in air + 10% H O at 900 Cfor 1000 h (a) with corresponding EDS maps for O (b), Cr (c), Fe (d) and Ni (e)

(a)

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Figure 4b shows a cross-section of 309S oxidized in air with 10% of water at 900C for 1000 h.The thick and porous oxide scale consists of a chromium-nickel oxide inner layer and

a iron-rich oxide layer at the outer part of the scale, presenting many cracks and pores.The mechanism for breakaway oxidation in air/H2O has already been discussed in

previous papers 13 .Hultquist et al. ave suggested that hydrogen, originating from water vapor, isincorporated in the oxide and thereby a change of its defect-dependent properties,including metal and oxygen diffusion [1416]. Khanna and Kofstad proposed thatprotons H+) from water vapor affect the diffusion properties of the grain boundaries[17]. However, denite experimental evidence is needed to show that the effect of watervapor on breakaway oxidation is related to hydrogen defects.

inian proposed another mechanism for the breakaway oxidation: pores, microchannelsor microcracks formed in the oxide scale during the initial oxidation stage allow ambientgas molecules to penetrate to the metal/scale interface and react with the chromium-depleted alloy 18 . Since H O molecules can react with the chromium-depleted alloysurface, non-protective iron oxide forms and releases H2. Then, this H reduces chromia

and generate more H2O. Therefore, as soon as the microcracks in the Cr O3 scaleappears, the oxidation is accelerated.

Depletion of chromium has been described by Evans as Intrinsic Chemical Failure(InCF) when the chromium concentration decreases below the thermodynamic valuefor Cr/Cr2O3 equilibrium, and Mechanical Induced Chemical Failure (MICF) when thedepleted surface is exposed to the oxidizing atmosphere [19].

According to Henry et al., the catastrophic oxidation due to water vapor is the resultof an intrinsic effect on the scale growth mechanism [20]. This breakaway oxidation issuggested to be initiated by the arrival of oxygen- and hydrogen- containing species inthe chromium depleted metal/oxide interface to form non-protective iron oxides.

Asteman et al. showed that in water vapor containing atmospheres the formation of

volatile species, such as CrO2(OH) or CrO2(OH)2, are responsible for the breakdownof the chromia scale [6, 11]. The evaporation of chromium species results in a depletionof chromium contained in the oxide scale. For Asteman, this leads to poorer protectiveproperties of the oxide scale. The resistance of chromia-forming steels is enhanced by thefollowing substrate factors: high chromium concentration, fast diffusion in the bulk, anda high density of steel grain boundaries.

(b) O (c) Cr

(d) Fe (e) Ni

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Astemans work was in accordance with a previous study, additionally suggesting that thetransition protective/non-protective oxide is due to a combination of different events [13].Evaporation of chromium species leads to an increased chromium diffusion and tochromium depletion in the zone below the metal oxide interface. A critical chromiumconcentration, below which the chromia does not re-form, leads to the formation of anon-protective iron-rich oxide scale and the catastrophic breakaway oxidation.

This is also in agreement with what is observed here. S35315 contains 25% of Cr anddoes not show any breakaway and iron-rich oxide formation, whereas 309S with 22.5%of Cr does.

Besides the chromium effect, Pettersson showed recently that nickel alloying plays arole in the oxidation of chromia forming alloys [21]. Her calculations of predicted phaseequilibria for ternary Fe-20Cr-xNi alloys demonstrated two effects of nickel alloying.The rst is a slight increase in chromium activity, promoting chromia formation and thesecond is a doubling of the chromium diffusion coefcient, enhancing the ability of thesteel to repair its protective chromia scale. This was supported by data from Jnsson and

kermark [22, 23].ilicon is known to have a benecial effect on isothermal oxidation kinetics in both

dry and moist atmospheres [24]. On experimental alloys Fe-20Cr-35Ni + either 0.5% or2% of Si, Pettersson has shown that the formation of a silicon oxide sublayer suppressesbreakaway oxidation at 1000C in air with 10% of water [25].

The combination of high Cr, Ni and Si contents plays a preponderant role in theoxidation behavior of S35315, making the alloy very resistant to moist atmospheresand thus to more aggressive atmospheres, such as carburizing or nitriding environments.S35315 has been previously studied, subjected to carburization and nitridation attemperatures ranging from 8501200C. The results showed that S35315 was the mostcorrosion resistant grade in these environments compared to N06601, 309S and 310S [26].

Results at 900C under cyclic conditions in air with 0 , 10 and 35% of water

The study carried out under cyclic conditions gives us information on the adhesion ofthe oxide scales. The curves are recorded for the entire cycle (time of exposure and timeof cooling to room temperature). The weight losses, due to spallation during cooling, arerepresented in the curves by a vertical step downward.

For S35315, in cyclic oxidation testing with a 2-hour hot dwell time at 900C,Figure 5, the same trend as in the isothermal exposures is observed.

The addition of 10% and 35% respectively, water vapor in the atmosphere enhanced theevaporation of chromium species, but did not lead to breakaway oxidation.

Fig. 5 Mass change of S35315 in cyclic exposuresin air with 0, 10 and 35% of water at 900 C

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Results at 1000C under cyclic conditions in air with 0 and 20% of water

The oxidation behavior of S35315 at 1000C in air with 20% of water, Figure 6a, ischaracterized by the same trend as for the other temperatures, with mass loss after 120 h dueto the simultaneous events of oxidation and evaporation; evidence of more evaporationthan oxidation. Spallation does not occur during cooling for the entire test period.

The presence of water vapor in the atmosphere during cyclic oxidation at 1000C hastwo effects on 310S, Figure 6b. The rst is evaporation as discussed previously. Thesecond is that spallation occurs when cooling down after only 130 h of testing while thisdoes not happen in dry atmospheres at 1000C for 150 h. The water vapor decreases theability of 310S to maintain its protective oxide scale under cyclic conditions. Its presencedecreases the Critical Mass Gain value (CMG, cf. below) of 310S to about 0.8 mgcm-2

A method to describe the adhesion of the oxide growing on different heat resistant

grades has been previously proposed 27 . Cyclic oxidation kinetic curves at differenttemperatures show a break point (more mass loss during cooling than mass gain duringoxidation). When the mass gain reaches a critical level (break point in the curve), thesample loses more weight during cooling (spallation) than it gains during oxidizing.This Critical Mass Gain (CMG) is a characteristic of the alloy and does not depend onthe temperature. The higher value the better.

Fig. 6a Mass change of S35315 in cyclic exposuresin air with 0 and 20% of water at 1000C

g. Mass change of 310S in cyclic exposuresin air with 0 and 20% of water at 1000C

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Results at temperatures above 1000C and up to 1200C

under cyclic conditions in dry air

Previous studies on the MA family have shown that S35315, even at 1100C for up to200 h, suffers no spallation during cooling. S35315 develops an oxide scale that is veryresistant to thermal shocks, Figure 7a 28 .

Fig. 7b Mass change of 310S in cyclic exposures in dry air [27]

Fig. 7a Mass change of S35315 in cyclic exposures in dry air

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Fig. 7c Mass change of 800H in cyclic exposures in dry air

The CMG for S35315 was found to be3.8 mgcm-2, Table 2 [28].

310S has a low oxidation resistanceunder cyclic conditions, Figure 7b. At1100C, the break point (more mass lossduring cooling than mass gain during oxid-ation) is reached very quickly. The CMG indry atmosphere of 310S has been found tobe 1.3 mgcm 27 . This study shows that

even a high alloyed grade as 800H has alow CMG value (1.7 mgcm-2), Figure 7c.

As previously reported in this paper, the oxide scale developed on S35315 is very resistantto severe thermal shocks. This is to be related to the presence of alloying elementssuch as cerium. It is known that the most important effect of reactive elements (RE) isimproving scale adhesion [29]. RE promotes the formation of a stronger metal/oxideinterface [30]. S35315 contains 0.04%Ce added as misch-metal, thereby enhancing itsability to retain its oxide scale. On the other hand, S35315 contains a non-negligibleamount of silicon. A negative aspect of silicon has been reported as increasing the oxidescale spallation under thermal cyclic conditions 25, 3132 . This aspect is not observedin our study.

As spallation did not occur on S35315 during cyclic oxidation in moist atmosphere,the two alloying element effects obviously compensate each other silicon causingspallation and cerium promoting adhesion. Evidently, the cerium effect is stronger.

CMG (mgcm-2)

310S 1.3

S35315 3.8

800H 1.7

CMG (Critical Mass Gain) for

310S, S35315 and 800H in dry

atmospheres [27, 28] Table 2

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Conclusions

The present study shows that S35315, as other standard heat resistant steels, is affectedby the water vapor content in the atmosphere. The non-parabolic kinetic fromisothermal oxidation is the result of the combination of a parabolic oxidation withevaporation of chromic acid CrO OH

From the isothermal results, S35315 does neither show any breakaway oxidationin moist atmosphere up to 40% of water nor iron-rich oxide formation at any testtemperatures, while 309S does so, already after 100 h exposure at 900C.

For S35315, in cyclic oxidation testing with a 2-hour hot dwell time at 900C,the addition of up to 35% water vapor in the atmosphere enhanced the evaporationof chromium species compared to dry air, but did not lead to breakaway oxidation.Even at temperatures as high as 1000C, the oxide scale remains protective while on310S; spallation during cooling occurs after only 130 h of exposure in air with 20% of water.The presence of water vapor decreases the Critical Mass Gain value (CMG) of 310S toabout 0.8 mgcm

he CMG in dry air, characteristic of the alloy, was found to be as high as 3.8 mgcm-2

for S35315, 1.3 mgcm-2 for 310S and 1.7 mgcm-2 for 800H.It has been shown that the composition combination, high Cr, Ni and Si contents

and presence of cerium, plays a dominating role in the oxidation behavior of S35315.Adding Cr, Ni and Si makes the alloy very resistant to moist atmospheres and thus tomore aggressive atmospheres. The oxide scale developed on S35315 is very resistant tosevere thermal shocks, thanks to the presence of cerium in the steel.

Acknowledgements

The author wishes to acknowledge B. Ivarsson and C. Lille at Avesta Research Centre ofOutokumpu for their contribution and helpful discussions. Thanks are also expressedto S. Amy, Faurecia, for his help with the testing. Special thanks are addressed to R.

Lindstrm for performing the sample preparations and thermogravimetric testing.Furthermore, the author wants to thank E. Torsner, Outokumpu, for her contribution tothis paper.

References

1. P. Kofstad, High Temperature Corrosion, Elsevier Applied Science, New York, 1988

2. D.L. Douglass, P. Kofstad, A. Rahmel, G. C. Wood, Oxid. Met. 45 (5/6), pp.529 620, 1996

3. B. B. Ebbinghaus. Thermodynamics of gas phase chromium species: The chromium

oxides, the chromium oxy-hydroxides and volatility calculations in waste incinerationprocesses. Combustion and ame 93, pp.119137, 1993

4. . Amy. Water Vapour Effect on the Heat Resistant Grades. Diploma work,Outokumpu, 2000

5. C.S.JR Tedmon, Electrochem. Soc. J. 113 (8), pp. 766768, Aug. 1966

6. H. Asteman, J-E. Svensson L-G. Johansson and M. Norell, Oxid. Met. 52 (1/2),pp. 95111, 1999

. . Espelid, K.J. Brve, V.R. Jensen, J. Phys. Chem. A 102 (50), pp. 1041410423,1998

8. .J.R. Johnson, I. Panas, Inorg. Chem., 39 (15), pp. 31813191, 20009. I. Panas, J.E. Svensson, H. Asteman, T.J.R. Johnson, L.G. Johansson, Chem. Phys.

Lett., 383 (5), pp. 549554, 15 January 2004

10. K. Segerdahl, J.E. Svensson, M. Halvarsson, I. Panas, L.G. Johansson, Mater. Highemp. 22 (1/2), pp. 6978, 2005

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11. H. Asteman, K. Segerdahl, J-E. Svensson and L-G. Johansson, Oxid. Met. 54 (1/2),p. 11, 2000

12. . Jonsson, S. Canovic, F. Liu, H. Asteman, J.E. Svensson, L.G. Johansson,M. Halvarsson, submitted to Mater. High Temp.

13. . Amy, P. Vangeli: Water vapour effect on the oxidation of heat resistant grades.Proc. Eurocorr 2001, Ria del Garda, Italy

14. B. N. Tveten: The Role of Hydrogen and Rare Earth Metals in the Formation ofProtective Metal-Oxides. Doctoral Thesis, 2000

15. G. Hultquist, B. Tveten and E. Hrnlund. Oxid Met. 54 (1/2), pp. 313 346, 2000

16. G. Hultquist, B. Tveten, E. Hrnlund, M. Limbck, and R. Haugsrud, Oxid. Met.56 (3/4), pp. 313346, 2001

17. A.S. Khanna, P. Kofstad, Metallurgia Italiana (Italy) 83 (5), pp. 489 492, May 1991

18. . Jinian, Z. Longgjiang, L. Tiefan, Oxid. Met. 48, p. 347, 1997

19. H.E. Evans. A.T. Donaldson and T.C. Gilmour, Oxid. Met. 52, p. 379, 199920. . Henry, A. Galerie, L. Antoni. Trans. Tech. Publications Ltd, Mater. Sci. Forum

Switzerland 3 9-372 Part 1, pp. 3533 0, 2001

21. R.F.A. Jargelius-Pettersson: Proc. in 6th International Conference on the Microscopyof Oxidation MoO , Birmingham, April 2005

22. B. Jnsson: Assessment of the mobility of Cr, Fe and Ni in fcc Cr-Fe-Ni alloys,Z. Metallkunde 8 , pp. 8 92, 1995

23. . kermark: Theoretical analysis of diffusion in the FeCrNi system with fcc phase.Diploma work, KTH Dept Physical Metallurgy, 1991

24. F. H. Stott, F. I. Wei, Mater. Sci. Technol. 5, pp. 1140 1147, 198925. R.F.A. Pettersson, L. Liu, J. Sund, in Proc. Eurocorr 2005, Lisbon, Portugal, 2005

2 . K. Tjokro, D.J. Young, R. Johansson, J.D. Redmond, Corrosion/91, paper no. 548,(Cincinnati, Ohio: NACE), 1991

27. P. Vangeli, B. Ivarsson, Trans Tech Publications Ltd , Materials Science Forum(Switzerland). 369 372, Part 2, pp. 785 792. 2001

28. P. Vangeli, European Federation of Corrosion Publications (UK), 27, pp. 198208,1999

29. D.P. Whittle, J. Stringer, Phil. Trans. R. Soc. Lond. A, 295, pp. 309 329, 1980

30. P.Y. Hou, J. Stringer, Mater. Sci. Eng. A, 202, pp. 110, 199531. R.C. Lobb, J.A. Sasse, H.E. Evans, Mater. Sci. Technol. 5, pp. 828 834, 1989

32. D.L. Douglass, J.S. Armijo, Oxid. Met. 2, pp. 107 231, 1970

Reproduced with permission from NACE International, Houston, TX.All rights reserved. Paper No 04678 presented at CORROSION/2006,Houston, TX. NACE International 2006.

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Introduction

Stainless steels owe their corrosion resistance to an oxide lm, about 10 atom layersthin. The lm forms spontaneously in most environments and adopts its thickness andcomposition to the surroundings. By changing the composition of the steel, it is possible

to inuence the kinetics of lm formation. Using the different alloy elements in astainless steel to form protective lms stable in aggressive environments is one exampleof nano scale engineering that has been around for almost a century.

Passive lms on stainless steel are still subject to a lively interest within the researchworld. The scientists are currently working on problems such as: How fast arecomposition and thickness responding to change in environment? Which mechanismsare controlling lm dissolution and growth? To what extent are the different alloyingelements leaching out into the environment? In recent years, the environmental aspectshave gained in importance, during manufacturing as well as end-use. For many years,the predominant raw material for stainless steel production has been scrap, there is thusa long tradition as a fully recyclable material.

A more detailed picture of the chemistry and composition of the passive lm canbe found in a recent review by Olsson and Landolt 1 . The experiments on metaldissolution presented can be found in a paper by Herting et al. [2].

When is a passive lm formed ?

At room temperature, pure iron has a cubic face centered structure. This means thatevery iron atom has eight nearest neighbors. A simple form of a stainless alloy is formedby replacing part of the iron atoms by chromium. For a random mixture, the likelihoodthat at least 50% of the chromium atoms gets at least one other chromium as nearestneighbor is 1/8. This corresponds to the empirical limit of 11 12% for the chromiumconcentration where the steel becomes stainless. At this concentration, it is possible toform a continuous network of chromium that can keep the metal matrix together duringselective dissolution of iron. The corresponding threshold for forming a chromiumnetwork in the oxide is about 18%. The mathematical background to this is known aspercolation theory and treats the construction of networks in a wide sense. It is usedalso in other elds, for example for the construction of efcient telecommunicationsnetworks.

The protective oxide lm is formed through selective oxidation of the differentalloying elements. Iron and chromium are preferentially oxidized at the metal oxideinterface. Nickel is remaining enriched in its metallic state under the lm. Iron ismigrating almost ten times faster than chromium through the oxide. Thus, the oxide lmwill be enriched in chromium and most of the iron will be dissolved. The cation fractionof chromium in the lm can amount to 80% under favorable conditions. When the steel

is passive, the oxide lm contains a cation fraction of chromium higher than 50%.By a proper choice of alloying elements, for example by adding molybdenum, nitrogenor tungsten, it is possible to direct the passive lm towards stability under variousconditions.

Passive Films on StainlessSteel Recent Nano-Range

ResearchClaes Olsson, Outokumpu, Avesta Research CenterGunilla Herting and Inger Odnevall Wallinder

The Royal Institute of Technology, Department of Corrosion Science

Drottning Kristinas vg 48; SE 100 44 Stockholm, Sweden

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Fig. 1 Passive lm growth on a stainless steel (continuous line) during and aftera change in electrochemical potential (solid). The experimental uncertainty

is given by the dashed lines. The majority of lm growth, about 6 ngstrm,occurs during the sweep and continues only for a few seconds after thesweep has stopped [3].

How fast is the passivation?

The protective oxide lm grows and stabilizes very fast. After an external perturbation,the lm will respond with a thickness and composition change within seconds. After theinitial fast adjustment, the lm continues to develop towards an enhanced crystallineorder for several hours or even days.

he electrochemical potential is an external parameter that can be easily controlled in thelaboratory. Figure 1 shows how a passive lm grows during and after such a potentialchange. The thickness change curve was recorded in the electrolyte by weighing thepassive lm during the experiment with a quartz crystal nanobalance. This equipmentmakes it possible to record mass changes in real time with a weight resolutioncorresponding to % of a metallic monolayer. The thickness change abates alreadya couple of seconds after stopping the potential sweep. This type of experiments areperformed to nd out which processes limit the growth of the oxide lm, or to studyeffects of chloride adsorption.

How is the oxide lm growing?

The driving force for lm growth is the difference in generalized potential between

the oxidizing medium, frequently a water-based solution or the atmosphere, and theunderlying metal. This potential drives ion migration through the lm. The passive layeris normally so thin that the growth is limited by a reaction at either of the lm interfaces;one such reaction is the formation of cation vacancies in the oxide lm at the metal/oxide interface. The alternative is rate control by dissolution of cations or incorporationof oxygen at the outer lm interface. The thickness equilibrium is so sharp that a 10%decrease in lm thickness would lead to a tenfold increase in migration rates. Thus, it isnot necessary to introduce cracks or other defects all the way down to the metal to createa local instability it is sufcient to replace a couple of oxygen atoms with chloride tolocally increase the ion migration velocities through the lm.

What type and how much material will dissolve?If one studies a common steel grade: 18Cr-9Ni (EN 1.4301, AISI 304), one nds thatthe predominating specie dissolving from the sample is iron. On a freshly ground surface,the passive lm has not yet stabilized. While studying such a surface during its initialpassivation process, one will initially see a somewhat higher rate of dissolution that abates

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with time. During this exposure time, the passive lm is built up with water and oxygenand reaches a maximum thickness. This lm stabilization leads to lower dissolution.Figure 2a shows a clear enrichment of chromium in the steel surface during exposureto rain water and a simultaneous reduction of the amount of dissolved metal. Duringcontinued exposure, the dissolution rates will decrease until they reach very low levels

at steady state conditions, cf. Fig. 2b. A passive lm weighs about 520 ngnm1

cm-2

The dissolution rates indicated in Fig 2b. are thus less than 10 % of an oxide monolayer.

Practical consequences

The dissolution rates of different cations are even for the worst case at least one orderof magnitude below todays hygienic limits. Previously, dissolution rates from stainless

steels have been estimated by using values from the pure constituting alloy elements.This gives a severe overestimate of the amount of iron and nickel dissolved.The most common delivery surface is cold rolled and pickled. For this nal treatment,

the material has already obtained a passivated surface during the nal pickling stage.Evidently, this results in an increased stability of the material even for the rst contactwith a chemical product. For a bright annealed surface, the response is slightly different.

Fig. 2a Dissolution of chromium during an initial passivation process on a groundplate used for kitchen sinks.

Fig. 2b Dissolution rates of chromium from a ground stainless steel. The dissolutiondecreases with time and reaches a very low rate after a couple of hours

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Concluding remark

Passive lms on stainless steels is one example of nano-engineering that has been aroundfor almost a century. The lms are not at all passive. They constantly adopting tochanges in the environment. To better describe their exible nature and rapid adjustmentto changes in the environment, they are rather to be described as activelms.

References

1. C.-O. A. Olsson and D. Landolt, Electrochim. Acta (2003)

2. G. Herting, I. Odnevall Wallinder and C. Leygraf, J. Electrochem.oc. 152 B23(2005)

3. C.-O. A. Olsson and D. Landolt, J. Electrochem. Soc. 147 (11) 4093 (2000)

The Authors

Claes Olsson is associate professor in engineering physics at Uppsala University andemployed at the Outokumpu Avesta Research Center. He has worked extensively withreal time measurements of passive lm growth on stainless steels and valve metals.

Gunilla Herting. M.Sc. is working on nishing her PhD, focussed on studies of metal

dissolution from stainless steels and related materials, at the department of corrosionscience at the Royal Institute of Technology.

Inger Odnevall Wallinder is associate professor in Corrosion Science at the RoyalInstitute of Technology and has predominantly been working with atmosphericcorrosion. She has identied a number of new corrosion products. Her present researchis focussed on environmental- and health aspects of metal dissolution from differenttypes of surfaces.

For this case, the passive lm will undergo a compositional change, whereas thethickness is less affected. The nal oxide on a bright annealed material is formed duringthe cooling phase after the furnace and will thus have a higher iron fraction than thestandard pickled surface. Another type of delivery surface is a ground or brushed nish.For this case, the oxide lm will have a composition that closely corresponds to thebulk composition of the alloy. This surface remains until the rst contact with a liquid,at which point the lm will be reshufed with dissolution rates corresponding to thereactivity of the liquid. The result will be the same for all the above surface nishes: aprotective lm enriched in chromium with a thickness corresponding to the surroundingmedium. This adjustment process is so fast that highly sensitive measurement equipmentis necessary to unveil it. The amount of dissolved metal is so low that it is only recentlythat the analytical methods, e.g. ICP-AES/MS, have reached detection limits where thepresence of the different alloying elements can be quantied with the necessary accuracy.

The dissolved quantities that have been indicated are all at least one order ofmagnitude lower than the hygienic limits valid today. This is true for the initiallyvery short formation phase of the passive lm. The dissolution rates will decreaseapproximately logarithmically with time. After the short intial passivation phase, the

metal dissolution rates will thus decrease with several orders of magnitude.

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