initial stages of local corrosion destruction of pipe steel in solutions imitating underfilm...

ISSN 2070�2051, Protection of Metals and Physical Chemistry of Surfaces, 2011, Vol. 47, No. 7, pp. 856–862. © Pleiades Publishing, Ltd., 2011.Original Russian Text © M.A. Petrunin, L.B. Maksaev, T.A. Yurasova, A.I. Marshakov, published in Korroziya: Materialy, Zashchita, 2010, No. 10, pp. 1–7.

856

INTRODUCTION

At present, stress corrosion cracking (SCC) is oneof the most dangerous types of corrosion attacks, espe�cially for operating gas mains [1–3]. The SCC causesa breakdown of pipelines that often result in seriousecological consequences (oil spills, fires) or significantmaterial losses and sometimes human victims (gas�pipeline explosions). Intensive studies have beenundertaken in the past 30 years [3]; nevertheless, thereis no general opinion in the literature on the factorsthat cause corrosion cracking in underground pipe�lines, nor is there a clear understanding of how thesephenomena occur. A number of competing theories onthe occurrence of cracks in construction steel havebeen proposed [4], but most authors suggest thatcracks may occur due to nonuniform anodic dissolu�tion in areas of the steel surface, which causes theappearance of cracklike defects in the near�surfacelayer known as “stress concentrators” [5–7].

Currently, optical and electrochemical methods ofstudies have been developed that allow one to carry outin situ observations of changes that occur on the metalsurfaces due to corrosion or anodic dissolution [8–10]. These methods, in particular scanning reflecto�metry, allow one to visualize interphase processesand estimate the parameters of local corrosiondefects [11, 12].

The aim of the present paper is to study the initialstages of the local dissolution of the pipe steel in pH�neutral media and demonstrate the possibility of esti�mating the sizes of corrosion defects that may initiatethe SCC of metal.

MATERIALS AND METHODS

Samples of X70 pipe steel with surface areas of7.5 cm2 (Fig. 1) were used in these tests. The sampleswere cut from pipe that had been used for 15 years atthe 6�km mark of the Ukhta�Torzhok�2 gas main,where colonies of corrosion cracks were discovered in2002. The steel has the following composition, %:0.1 C, 1.6 Mg, 0.33 Mg, 0.03 Cr, 0.05 Nb, 0.018 Cu,0.03 Mo, 0.005 V, 0.006 S, 0.03 P, 0.035 Al, and0.01 Ti. The surface of the sample was polished withsandpaper and deoiled using ethanol.

The borate buffer solution (0.4MH3BO3 +0.1MNa2B4O7) with a pH level of 6.7 was used as theworking solution, on the background of which(122 mg/l KCl, 483 mg/l NaHCO3, 181 mg/l CaCl2 ⋅2H2O, 131 mg/l MgSO4 ⋅ 7H2O) electrolyte was pre�

Initial Stages of Local Corrosion Destruction of Pipe Steelin Solutions Imitating Underfilm Electrolyte

M. A. Petrunin, L. B. Makseva, T. A. Yurasova, and A. I. MarshakovFrumkin Institute of Physucal Chemistry and Electrochemistry, Russian Academy of Science,

Leninskii pr. 31, Moscow, 119991 RussiaReceived June 15, 2010

Abstract—Using the method of scanning reflectometry, it was determined that the pipe steel dissolves non�uniformly with a high rate of the appearance of local corrosion defects (pittings) in neutral solution imitatingelectrolyte. The parameters of individual pittings were determined, along with the fact that they propagatemainly at a depth that may cause corrosion cracking under cyclic mechanical stresses. Factors of stress inten�sity that cause local corrosion defects were calculated.

Keywords: local corrosion, pitting, pipe steel, underfilm electrolyte, stress intensity factor

DOI: 10.1134/S2070205111070136

Fig. 1. External view of initial sample.

GENERAL CORROSION ISSUES

PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47, No. 7 2011

INITIAL STAGES OF LOCAL CORROSION DESTRUCTION OF PIPE STEEL 857

pared with an ionic composition similar to the liquidfound under the cover stripped off of the undergroundpipeline [13].

Reflectograms of the diffusive reflection of theelectrode surface were recorded on a standard EpsonPerfection 3200 Photo computer scanner with an opti�cal resolution of 3200 dpi simultaneously with varia�tions in the corrosion potential in samples during test�ing. The potential was measured relative to the silverchloride reference electrode using the APPA109 mul�timeter.

The tests were carried out using a cell based on aglass cylindrical cuvette with a flat, smooth, opticallytransparent bottom (Fig. 2). The sample was placedupside down on the bottom of the cell with a certaingap between the surface and bottom of the cuvette pro�vided by a glass stop. The angle between the metal sur�face and the object glass of the scanner was selected toprovide an image of the sample in white (Fig. 3a). Formore details of the scanner�reflectometric measure�ments, see [10].

Digital processing of the images conventionallyused for detailed studies of changes in the surface mor�phology was employed for visualization of localchanges in surface reflectance of the sample [14, 15].For this purpose, after being input to a PC and digi�tized, the initial color reflectometric image of theprobed area on the metal surface was subjected tomathematical processing according to a unified algo�rithm that involved illumination gain, optimizing thecontrast, and enhancing the sharpness. During thisprocess, the image was distributed to color channels(RGB filtration).

During the digital processing of the image andoptimization of the contrast, the histogram, i.e., theelement�by�element distribution of the image inten�sity over the sample surface in each color channel, wasrewritten. During the process of rewriting, the range ofdeviation of the image intensity was purposefully wid�ened and the corresponding sharpening of the surfacetopogram allowed one to visualize the weakest differ�ences in the intensity and, consequently, morphologi�cal peculiarities of the sample surface throughout theentire image field.

The degree of surface coverage with defects and theaverage size of an individual defect were calculatedusing software for digital image processing developedon the base of RMagick 2.12.0 (Image Magick 6.5.6�8) software. The total number of pittings on the surfaceof the sample was calculated manually after enlargingthe image by a factor of three.

The depth of pittings was estimated based on thevariations in intensity compared to the intensity of thereference defects, the depths of which were measuredusing the ICh02 indicator with zero precision (GOST

577�68). The error in measurement of the defect depthwas 8 μm. 3D visualization of the surface defects wascarried out using software developed on the base ofSurfer 9.0 software.

For long�term corrosion tests, two types of sampleswere used: the first is made of the X70 steel (Fig. 1) andthe second is made of the 08 kp carbonic steel similarin composition with a working area of 100 cm2 andthickness of 0.1 mm. During tests access for oxygen tothe cell was restricted using tight lid to cover the cell.Products of corrosion were removed from the samplesurface by etching the sample in 5% solution of sulphu�ric acid with 1 % aminophorm admixture [16]. The vol�ume of the uniform corrosion was estimated by weight�ing samples of the 08 kp steel before and after tests.

Voltmeter

Corrosion Electrolyte

RE

Metal

Scanner object plate

Sounding light Reflecting light

Polarizator

Scanner

Analyzator

Bar of scanner

P A

Scanning direction

photodetectorslight source

elect�

defects

(refe�rence

rode)

Fig. 2. Set up diagram of scan reflectometry for studyingcorrosion and electrochemical processes in situ.

–0.645

–0.640

–0.635

–0.630

–0.625

–0.620

–0.615

2018161412100

E, V (SCE (silver�chloride electrode))

Time, h8642

–0.610

Fig. 3. Variations in corrosion potential in sample of X70pipe steel during tests in solution of NS4.

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PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 47 No. 7 2011

PETRUNIN et al.

RESULTS AND DISCUSSION

The potential of the pipe steel in neutral electro�lytes is usually within the limits of the active metal dis�solution, thus mechanism of appearance of pittingcorrosion centers must be different of metal passivitydisturbance [17, 18]. Figure 3 shows changes in pipesteel corrosion potential during tests. It is obvious thatthe potential varies within range of –0,625– –0,640 V(s.c.e.) i.e., actually corresponds to the area of theactive dissolution of the metal. The passive film mustbe absent on the surface under these conditions, how�ever the corrosion of the pipe steel has a clearly pro�nounced local nature (Fig. 4).

Reflectograms of the diffusive reflection of theelectrode surface showed that first local corrosiondefects appear in 10 minutes after immersion of thesample in the electrolyte (Fig. 4b). The further expo�sure of the sample to solution will lead to the increasednumber of defects and sizes of each pitting. Defects arenot defined visually after 10 minutes exposure of thesample to solution, nevertheless the digital processingof the image allows one to estimate sizes of individualdefects in addition to recognizing their presence.

However, we should notice, that such an intensivedissolution of the pipe steel observed under such con�ditions provides a big number of metal corrosion prod�ucts in the electrolyte (Fig. 4c). In this connection adigital processing of the images of the entire studied

surface of the sample is hampered after 300 min oftesting. The quantity processing of reflectograms ofthe entire surface of the sample was carried out for upto 200 min of testing, i.e., the period when corrosionproducts minimally influence the image quality.

Figure 5 shows the results of the digital processingofimages of the entire working surface of the sampleduring corrosion testing. At the initial stage, weobserved a considerably increased coverage of themetal surface with local corrosion defects that coveredmore than 20% of the sample area after 200 min oftesting (Fig. 5a). This value corresponds to a density ofabout 600 defects per 1 cm2 (Fig. 5b). The averageradius of the defect is 106 μm after 200 min of testing(Fig. 5c).

Two segments with different angles are recognizedin the curves shown in Figs. 5a and 5b. The first seg�ment that lies within the first 80 min is characterizedby an insignificant increase of both the surface cover�age degree and the number of local defects. Duringthis period, the average rate of surface coverage withcorrosion defects was 0.2% per minute (Fig. 5a) andmore than one defect appeared in 1 cm2 per minute(Fig. 5b). The average radius of defects increased inthe first 40 min of testing at a rate of 2.6 μm/min; then,the rate decreased by more than 25 times to0.09 μm/min, i.e., the radius of the defect changesinsignificantly with time (Fig. 5c).

Defect 2

Defect 1

(a) (b) (c)

Fig. 4. Kinetics of pipe steel corrosion development in NS4 solution, min: (a) 0; (b) 10; (c) 60; (d) 120; (e) 200; (f) 400.

(d) (e) (f)



If this tendency continues further (after 200 min oftesting), i.e., only the number of defects increases,while their sizes remain unchanged, it will result in theuniform corrosion of metal with 100% surface cover�age due to the merging of neighboring defects. Thefurther localization of corrosion with defects propa�gating mainly in depth with insignificant variations inthe visible area on the surface of the sample theyoccupy is an alternative process to the equal corrosionof pipe steel.

The kinetics of changes in the sizes of individualdefects over time was studied to determine how thecorrosion of pipe steel develops. For this purpose, twopittings formed when samples were immerged in NS4solution, i.e., that were not registered before testing (inFigs. 4b, 4f they were outlined with circles and markedas defect 1 and defect 2), were chosen. Figure 3 showsa 3D image of both defects at different moments intime after the tests had started. It can be seen that thedepths of both defects increases with time and, after400 min of testing, become equal to 170 and 157 μmfor defects 1 and 2, respectively. The surface area cov�ered with defects also increases. The approximateradius of the defect is estimated based on the value ofthe area. Figure 7a shows variations in the depth andradius of an individual defect over time.

The ratio α = h/r, where h and r are the depth andthe radius of the defect, respectively, may be a charac�teristic of the local nature of the corrosion. Thus, theequation α = 1 implies the development of a hemi�spheric defect on the surface that is frequentlyobserved within the range of metal passivity potentials[19]. The case when α < 1 is typical of ulcerous corro�sion, when shallow ulcers of a small area are formed,and, at α → 0, local corrosion becomes uniform; ifdefects propagate mainly in depth, then α > 1.

Variations in α during the growth of each defect isshown in Fig. 7b. Within this period, as one may see,the average value of α is 2.13 and 2.68 for defects 1 and2, respectively, i.e., in both cases, α > 1. This factshows that corrosion defects propagate namely indepth, this may cause pitting to transform into a crackunder mechanical stresses.

The fact that during these tests high rates of localcorrosion were observed is quite explainable, since it isknown that the rate of the uniform corrosion of car�bonic steel in natural electrolytes decreases with timeand, in the initial stages, may be quite high (up to somemm/year) [19]. As a rule, the layer of corrosion prod�ucts and increment of the depth of corrosion ulcers inthe pipeline cause a decrease in the corrosion currentin the ground with time [20].

Continuous corrosion testing carried out in NS4solution showed that the sizes of local defects continueto increase (Figs. 8a, 8b). Thus, in 33 days of testing,the depth of the largest defect reached 600 μm and itsaverage radius was 247 μm (Fig. 8c). The average rateof local corrosion during testing was high, about

6.6 mm/year; however it was much lower than at thebeginning of the test. There are grounds to think thatthe further exposure of steel to solution will result in adecrease in the corrosion rate. The rate of uniformcorrosion defined using the gravimetric method was0.04 mm/year, which corresponds to the range lossesof mass in carbon steel in natural ground [19].

As was described above, the danger of local corro�sion involves not just the perforation of pipeline walls,but also, and to a greater degree, the possibility of the

0.30

0.25

0.20

0.15

0.10

0.05

200100806040200 120 140 160 180

800700600500400300

200100806040200 120 140 160 180

200100

120

100

80

60

40

200100806040200 120 140 160 180

20

220

Degree of surface coverage with defects(a)

Test time, min

Density of corrosion defects defect/cm2

(b)

Average radius of defects, µm(c)

Test time, min

Test time, min

Fig. 5. Development of local corrosion on the surface ofpipe steel in NS4 solution (pH 6.5): (a) degree of coverageof surface with corrosion defects; (b) density of local cor�rosion defects during corrosion tests (manual calculation);(c) average radius of defect.

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PETRUNIN et al.

initiation of corrosion cracking under stress. In the toppoints of hemisphere defects with depths similar tothose of local defects formed during the initial stage ofpipe�steel corrosion, stress intensity factors (SIFs)were calculated to estimate the danger of crackingunder loading. According to one of the numerousapproximations, the value of SFI may be estimatedusing the following formula (1) [21]:

(1)

where a is the depth of the defect and Q is the factordefined by the expression

KiC 1.12σ παQ

��,=

(2)

where σ is load taking into consideration residualstresses, MPa, and σ0 is the yield stress of steel, which,for X70 steel, is approximately 540 MPa.

The value of σ is calculated from the expression (3)as follows [22]:

(3)

where P is a working pressure of the pipeline, d is thediameter of the pipeline, and δ is the thickness of the

Q π2

4�� 0.212 σ

σ0

��⎝ ⎠⎛ ⎞ 2

– ,=

σ P d 2δ–( ),=

200

150

100

50

0

5050100150

10 min200

150

100

50

0

5050100150

60 min

200

150

100

50

0

50100150200

400 min200

150

100

50

0

5050100150

120 min

200

150

100

50

0

5050

100150

10 min200

150

100

50

0

5050

100150

60 min

200

150

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50

0

5050

100150

400 min

200

150

100

50

0

5050

100150

120 min

1000

200

100150

200100

100100

150

100150

(a)

(b)

0

100

Defect 1

Defect 2

Fig. 6. Three�dimension visualization of individual defects at initial stages of pipe steel corrosion in solution: (a) defect 1;(b) defect 2. Digits on the axes indicate dimensions in μm, axes of applicate (Z) represents inverse value of defect depth, μm.



pipeline wall. The samples were cut from a pipe withfollowing parameters: d = 1200 mm and δ = 11.8 mm.Residual (extra) stresses in pipelines that can practi�cally reach high values resulting in σ/σ0 → 1 must betaken into consideration during calculations [22].

Stress intensity factors calculated for the threedefects presented in the Figs. 6a, 6b, and 8 using for�mula (1) are shown in the table.

As can be seen from the table, on the steel surface,defects with SIF of about 10 MPa m1/ 2 have areformed after 400 min of testing. After 33 days of expo�sure of pipe steel to solution imitating underfilm elec�trolyte, the value of SIF at the top point of corrosiondefects must be above 17 MPa m1/2.

The values of SIF presented in the table are consid�erably lower than KC ≈ 85–90 MPa m1/2, which resultsin the growth of the crack in the air [21, 22]. Howeverit is important to consider that the value of critical SIFin the aggressive corrosion media (KSCC) must bemuch lower than KC both as a result of metal hydrogencharging [21] and due to the local dissolution of the tipof the crack [22].

Moreover, values of SIF for which the growth of thecrack is observed may be much lower than the thresh�old values of KSCC under the conditions of cyclic load�ing. Thus, during low frequency cyclic tests with a highratio between the maximum and minimum load (R =0.85) it was observed that the crack transformed anddeveloped from the defect with a depth of less than1 mm [23]. Consequently, the SIF values obtained(table) may be close to the critical values in aggressivecorrosion media and the corrosion defects that appearon the surface of pipe steel may cause the initiation ofcorrosion cracking under the conditions of use forunderground pipelines.

Thus, scanning reflectometry allows one to detectlocal corrosion defects at earlier stages of develop�ment, as well as to define the sizes; growth kinetics;and, consequently, estimate the danger of the destruc�tion of a construction by local corrosion.

160140120100

80604020

4504003503002502000

Defect size, µm (a)

Test time, min

(b)

15010050

180200 1

2

3

4

3.0

2.5

2.0

1.5

1.0

0.5

4504003002000 Test time, min

100

Fig. 7. Development of individual local corrosion defectson the surface of pipe steel during tests in NS4 solution:(a) variation in depth (1, 3) and radius of defect (2, 4),lines 1 and 2 correspond to defect 1, lines 3 and 4 corre�spond to the defect 2; (b) variation in α, i.e., ratio betweendepth and radius of defect, line 1 corresponds to defect 1,line 2 corresponds to defect 2.

(a) (b) (c)

200 µm

Fig. 8. Sample of pipe steel with removed corrosion prod�ucts after 33 days exposure to solution of NS4 (a), fivefold increased fragment of surface (b) 50 times increased frag�ment of surface with the largest local corrosion defect (c).

Stress intensity factors calculated by (1)

The defect number The depth, µm

The stress inten�sity factor, MPa

m1/2

1 (Fig. 6a) 171 9.32

2 (Fig. 6b) 157 8.91

3 (Fig. 8c) 600 17.48

α

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PETRUNIN et al.

CONCLUSIONS

(1) Initial stages of pipe steel corrosion in NS4solution imitating electrolyte under a stripped�off iso�lating cover of the underground pipelines were studied.It was shown that pipe steel was locally dissolved at ahigh rate and a large number of corrosion defectsappeared on the surface.

(2) The densities and average radii and sizes ofindividual corrosion defects were determined. It wasshown that defects grew mainly in depth, which afterseveral hours, was over 100 μm and reached 600 μmafter 33 days of corrosion testing.

(3) The possibility of estimating the danger of localcorrosion (initiation of corrosion cracking of under�ground pipelines under stress in the first place) basedon the data obtained using scanning reflectometry ofthe surface of corrosive pipe steel was demonstrated.

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initial stages of local corrosion destruction of pipe steel in solutions imitating underfilm...

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