Corrosion Science 73 (2013) 250–261

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Corrosion Science

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New understanding of the effect of hydrostatic pressure on the corrosionof Ni–Cr–Mo–V high strength steel

0010-938X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2013.04.013

⇑ Corresponding author at: Corrosion and Protection Laboratory, Key Laboratoryof Superlight Materials and Surface Technology, Harbin Engineering University,Ministry of Education, Nantong ST 145, Harbin 150001, China. Tel./fax: +86 4518251 9190.

E-mail address: zhangtao@hrbeu.edu.cn (T. Zhang).

Yange Yang, Tao Zhang ⇑, Yawei Shao, Guozhe Meng, Fuhui WangState Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Wencui RD 62, Shenyang 110016, ChinaCorrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology, Harbin Engineering University, Ministry of Education, Nantong ST 145,Harbin 150001, China

a r t i c l e i n f o

Article history:Received 3 October 2012Accepted 6 April 2013Available online 17 April 2013

Keywords:A. Low alloy steelB. SEMB. Weight loss

a b s t r a c t

Corrosion of Ni–Cr–Mo–V high strength steel at different hydrostatic pressures is investigated by scan-ning electron microscopy (SEM) and finite element analysis (FEA). The results indicate that corrosion pitsof Ni–Cr–Mo–V high strength steel originate from inclusions in the steel and high hydrostatic pressuresaccelerate pit growth rate parallel to steel and the coalescence rate of neighbouring pits, which lead to thefast formation of uniform corrosion. Corrosion of Ni–Cr–Mo–V high strength steel under high hydrostaticpressure is the interaction result between electrochemical corrosion and elastic stress.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Exploitation and development of marine resources poses greatchallenges for materials, structures and equipment served indeep-sea environment. Accordingly, knowledge of corrosion per-formance of materials in deep-sea environment is essential formaterial selection, which has caused concern of many countriesincluding Russian [1,2], American [3–6] and Indian [7,8], etc. Dete-rioration of materials in deep sea is the comprehensive result ofhydrostatic pressure, temperature, dissolved oxygen, pH, salinityand sea current. As an important characteristic factor of deep-seaenvironment, effect of hydrostatic pressure on the corrosionbehaviour of material attracted the interest of many researchers[9–13]. Beccaria reported the effect of hydrostatic pressure onthe corrosion of nickel in NaCl solution [9] and aluminium in seawater [11], showing that the corrosion rate and the susceptibilityof pitting of nickel and aluminium increased, attributable to an in-crease in the rate of anodic reaction with cathodic reaction rate re-mained unchanged. Our previous work demonstrated that with theincrease of hydrostatic pressure, pitting corrosion resistance ofFe–20Cr alloy was deteriorated distinguished by the decrease ofcritical pit potential and the increase of passive current density[12]. However, reports mentioned above mainly focused on perfor-mance characterization with little information on the mechanism

research of hydrostatic pressure to material corrosion, which lim-ited further development of deep-sea corrosion theory.

Hydrostatic pressure is a unique mechanical factor of deep-seaenvironment distinguished from general marine environment. Forbetter understanding the effect of hydrostatic pressure on materialcorrosion, it is reasonable to draw lessons from research results ofstress corrosion. It is well known that plastic deformation canaccelerate anodic dissolution of metals [14–17]. Plastic strain andhydrostatic pressure are two important factors that can changethe mechanochemical activity of deformed metals according toGutman0s theory and a kinetic equation for the anodic dissolutionof plastically deformed metal far from equilibrium can be ex-pressed as follow [14]:

ip ¼ iaDee0þ 1

� �exp

DPVm

RTð1Þ

where ip is the anodic dissolution density, ia is anodic current den-sity of unstressed sample, De is plastic deformation magnitude, e0

corresponds to the onset of strain for hardening, P is the sphericalpart of macroscopic stress tensor (i.e. hydrostatic pressure) depend-ing on the applied load and Vm is the molar volume of the substance.The depth of the ocean varied from hundreds to thousands of me-ters corresponding to hydrostatic pressure ranging from several todozens of MPa, showing that material deformation is elastic. Hence,Eq. (1) could be modified to:

ip ¼ ia expDPVm

RTð2Þ

According to Eq. (2), the anodic current density of nickel under300 atm and Ni–Cr–Mo–V steel under 60 atm will only increased

Fig. 1. Schematic diagram of experimental setup: (1) high purity nitrogen vessel, (2) driven gas booster, (3) valve, (4) cooling equipment, (5) temperature monitor, (6)reference electrode, (7) thermocouple, (8) working electrode, (9) counter electrode and (10) pressure meter.

Fig. 2. Cross-sectional sizes and two-dimensional morphologies of corrosion pits at different hydrostatic pressures: (a) 1 atm, (b) 20 atm, and (c) 60 atm.

Y. Yang et al. / Corrosion Science 73 (2013) 250–261 251

Fig. 3. Corrosion morphologies of Ni–Cr–Mo–V high strength steel after different immersion time in 3.5% NaCl solution at 1 atm: (a) 5 h, (b) the magnification of (a); (c and d)7 h; (e) 10 h [13]; (f and g) 20 h.

252 Y. Yang et al. / Corrosion Science 73 (2013) 250–261

by 8% and 2% respectively compared with 1 atm, which is contradic-tory with 73% and 43% obtained from literature [9,13]. Therefore,Eq. (1) cannot entirely describe the corrosion behaviour of materialunder different hydrostatic pressures, which means that there ex-ists a complicated effect between hydrostatic pressure and electro-chemical corrosion.

Deep-sea corrosion has its own unique characteristic. On onehand, materials and structures served in deep-sea environmenthave to suffer from sea water corrosion, i.e. electrochemical

corrosion; on the other hand, once pitting generated on materialsurface, the stress state around pits will change resulting fromhydrostatic pressure and strain induced from pits will promotethe anodic dissolution and thus the further growth of pitting. Soit is expected that corrosion in deep-sea environment is the inter-action between electrochemical corrosion and hydrostaticpressure.

Recently, finite element analysis was widely used to simulatelocalized stress state around corrosion pits and some significant

Fig. 4. Corrosion morphologies of Ni–Cr–Mo–V high strength steel after different immersion time in 3.5% NaCl solution at 20 atm: (a) 5 h; (b and c) 10 h.

Fig. 5. Corrosion morphologies of Ni–Cr–Mo–V high strength steel after different immersion time in 3.5% NaCl solution at 60 atm: (a and b) 5 h; (c, d and e) 10 h [13].

Y. Yang et al. / Corrosion Science 73 (2013) 250–261 253

1 10 100

0.00

0.25

0.50

0.75

1.00

31µm

23.5µm3.5µmP=0.5

Cum

ulat

ive

Prob

abili

ty (n

/(N+1

))

Pit size / µm

Pit size / µm

Pit size / µm

length width depth

1 10 100

0.00

0.25

0.50

0.75

1.00

53µm

26.1µm3.3µmP=0.5

Cum

ulat

ive

Prob

abili

ty (n

/(N+1

))

length width depth

1 10 100

0.00

0.25

0.50

0.75

1.00

3.4µm

74.5µm

Cum

ulat

ive

Prob

abili

ty (n

/(N+1

))

length width depth

P=0.586.7µm

(a)

(b)

(c)

Fig. 6. Statistical distributions of the 3D sizes of corrosion pits of Ni–Cr–Mo–V highstrength steel at different hydrostatic pressures after 5 h immersion in 3.5% NaClsolution: (a) 1 atm, (b) 20 atm, (c) 60 atm.

Table 1Typical 3D sizes of corrosion pits of Ni–Cr–Mo–V high strength steel at differenthydrostatic pressures.

Hydrostatic pressure (atm) Length (lm) Width (lm) Depth (lm)

1 31 23.5 3.520 53 26.1 3.360 86.7 74.5 3.4

1atm 20atm 60atm0.0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

Wei

ght l

oss

/ g.m

-2

Hydrostatic pressure / atm

Fig. 7. Weight loss of Ni–Cr–Mo–V high strength steel after 10 h immersion time in3.5% NaCl solution at three hydrostatic pressures.

254 Y. Yang et al. / Corrosion Science 73 (2013) 250–261

results were reported [18–21]. The aim of this work is to furtherreveal the correlation between the corrosion and localized stress

state around corrosion pits by scanning electron microscopy(SEM) and finite element analysis (FEA) to provide a new under-standing of the effect of the hydrostatic pressure on the corrosionof materials.

2. Experimental method

2.1. Material and solution

The material used in this study is a hot rolled Ni–Cr–Mo–V highstrength steel plate. The chemical composition of this steel is asfollows (wt.%): C 0.1, Si 0.21, Mn 0.56, Ni 4.48, Cr 0.56, Mo 0.51,V 0.007, S 0.005, P 0.011, Fe balance.

The electrolyte in this work was 3.5% NaCl (wt.%), which wasprepared by analytical grade chemical and deionized water. Allthe experiments in this work were carried out at the temperatureof 25 �C ± 1 �C.

2.2. Experimental equipment

Schematic diagram of the experimental setup is shown in Fig. 1.Different hydrostatic pressures can be got by pushing differentamount of high purity nitrogen into the pressure vessel using dri-ven gas booster and the test temperature is controlled by the cool-ing equipment in Fig. 1.

2.3. Weight loss test

Specimens were cut into 10 � 10 � 2.5 mm3 for weight loss test.Firstly, all the surfaces of the specimens were finished by 2000 gritsilicon carbide, washed using deionized water, dried and weighedby means of an analytical balance (Sartorius CP225D) with a preci-sion of 0.0001 g for the original weight. And then the specimenswere immersed in 3.5% NaCl solution at different hydrostatic pres-sures (1 atm, 20 atm and 60 atm) for 10 h. After test, corrosion prod-ucts of the specimens were removed by ultrasonic cleaning in thehydrochloric acid solution with hexamethylenetetramine as theinhibitor. Finally, the specimens without corrosion products werewashed with deionized water, dried and weighed again for the finalweight. Weight loss of each hydrostatic pressure was performed atleast two times for reproducibility.

2.4. Corrosion morphology observation

In order to understand the corrosion process of Ni–Cr–Mo–Vhigh strength steel at 1 atm, 20 atm and 60 atm directly, surfaceof the corroded steel specimen at each hydrostatic pressure after

Fig. 8. SEM images of corrosion pits of Ni–Cr–Mo–V high strength steel after 30 minimmersion in 3.5% NaCl solution at 1 atm. The pit morphologies around theinclusions are analogues in these images. These pits show various dimensions, butthey all provide an initial site for matrix dissolution around inclusions. The similarfeatures are also observed in Figs. 9 and 10.

Fig. 9. SEM images of corrosion pits of Ni–Cr–Mo–V high strength steel after 30 minimmersion in 3.5% NaCl solution at 20 atm.

Y. Yang et al. / Corrosion Science 73 (2013) 250–261 255

different immersion time was examined by scanning electronmicroscope (SEM). The procedure was as following: Firstly, thespecimen was wet ground to a 2000-grit finish by silicon carbidepaper and polished by diamond spray. And then, the specimenwas immersed in 3.5% NaCl solution for different time. Finally,corrosion product of the specimen was removed by ultrasoniccleaning in the hydrochloric acid solution with hexamethylenetet-ramine as the inhibitor, dried and prepared for SEM observation.

2.5. Measurement of pit sizes

The 3D sizes of a number of random selected corrosion pits onthe surface of Ni–Cr–Mo–V high strength steel at 1 atm, 20 atmand 60 atm after 5 h immersion in 3.5% NaCl solution were mea-sured using confocal scanning laser microscope (OLYMPUSOLS3100). As measurement examples, cross-sectional sizes andtwo-dimensional morphologies of corrosion pits at 1 atm, 20 atmand 60 atm are given in Fig. 2. It can be seen that the two-dimen-sional sizes of pits mouth are much larger than pits depth at bothhydrostatic pressures. Therefore, the 3D shape of the corrosion pitsin this work can be assumed to be semi-ellipsoidal.

Fig. 10. SEM images of corrosion pits of Ni–Cr–Mo–V high strength steel after30 min immersion in 3.5% NaCl solution at 60 atm.

Table 2Chemical composition of inclusions of Ni–Cr–Mo–V high strength steel after 30 minimmersion in 3.5% NaCl solution at 1 atm.

O Al S Mn

1 49.95 50.05 – –2 – – 51.98 48.023 55.2 44.8 – –4 51.03 44.77 2.37 1.835 52.35 47.65 – –

Table 3Chemical composition of inclusions of Ni–Cr–Mo–V high strength steel after 30 minimmersion in 3.5% NaCl solution at 20 atm.

O Al S Mn Fe

1 35.32 29.99 6.38 5.91 22.42 49.49 50.51 – – –3 53.33 39.98 2.14 1.35 3.2

Table 4Chemical composition of inclusions of Ni–Cr–Mo–V high strength steel after 30 minimmersion in 3.5% NaCl solution at 60 atm.

O Al S Mn

1 19.15 6.66 43 31.192 52.92 47.08 – –3 54.55 45.45 – –

256 Y. Yang et al. / Corrosion Science 73 (2013) 250–261

2.6. Finite element analysis

The localized stress distributions around pits at 1 atm, 20 atmand 60 atm were calculated by commercial ABAQUS finite elementsoftware. Dimensions of 3D solid models are 360 � 360 � 35 lm3

in this work. A uniaxial pressure was applied to the top surfaceof the model with loading direction perpendicular to the corrodedsurface of Ni–Cr–Mo–V steel. Boundary condition is that bottomsurface of the solid model is fixed along the loading direction whilethe model can move in the directions perpendicular to the loadingdirection. An elastic and plastic material model describing theproperties of Ni–Cr–Mo–V high strength steel with a Young’s mod-ulus of elasticity (E) 200 GPa and a Poisson’s ratio of 0.25 are used

in the finite element analysis. The output parameter to reveal local-ized stress distributions of corrosion pits for Ni–Cr–Mo–V highstrength steel in this study is hydrostatic pressure, with the unifiedunit kPa.

3. Results

3.1. Corrosion morphologies observation

Corrosion morphologies of Ni–Cr–Mo–V high strength steelafter different immersion time in 3.5% NaCl solution at 1 atm,20 atm and 60 atm are shown in Figs. 3–5, respectively. By com-paring the morphologies changes in the three figures, it is foundthat there exists one thing in common at both hydrostatic pres-sures: corrosion of Ni–Cr–Mo–V high strength steel begins withthe generation of corrosion pits and ends with the formation ofuniform corrosion.

3.2. Statistical results of 3D sizes at different hydrostatic pressures

Statistical distributions of the 3D sizes of pits at three differenthydrostatic pressures are plotted in Fig. 6. In this figure, the cumu-lative probability of geometry parameters (length, width anddepth) is calculated as n/(N + 1) by a mean rank method [22–24],where N is the total number of pits and n is the order in the totalnumber. According to the statistical results, typical length, width,and depth (with cumulative probability P = 0.5) of corrosion pitsof Ni–Cr–Mo–V high strength steel at different hydrostatic pres-sures are shown in Table 1.

0 20 40 60 80 100 120 140 160 1800

2

4

6

8

10

12

14

16

1 atm 20 atm 60 atm

D/2

h

Pit diameter / µm

D/2h=1

Fig. 11. Variation of D/2h at different hydrostatic pressures.

Fig. 12. Stress distributions in and around semi–ellipsoidal pits of Ni–Cr–Mo–V high strpits, and (c) quarter-ellipsoidal pit.

Y. Yang et al. / Corrosion Science 73 (2013) 250–261 257

4. Discussion

4.1. Effect of hydrostatic pressure on the corrosion resistance of Ni–Cr–Mo–V high strength steel

Weight loss of Ni–Cr–Mo–V high strength steel after 10 himmersion time in 3.5% NaCl solution at three hydrostatic pres-sures is shown in Fig. 7. It is noted that the weight loss increasedwith the increase of hydrostatic pressure from 1 atm to 60 atm,which indicates that increasing hydrostatic pressure deterioratesthe corrosion resistance of Ni–Cr–Mo–V high strength steel. The re-sult had been reported in our previous work [13].

4.2. Effect of hydrostatic pressure on the corrosion process of Ni–Cr–Mo–V high strength steel

Corrosion happens at the sites of local heterogeneities preferen-tially on metal surface. In order to understand fully the corrosion

ength steel at 1 atm: (a) semi-ellipsoidal pit, (b) double distributed semi-ellipsoidal

Fig. 13. Stress distributions in and around semi–ellipsoidal pits of Ni–Cr–Mo–V high strength steel at 20 atm: (a) semi-ellipsoidal pit, (b) double distributed semi-ellipsoidalpits, and (c) quarter-ellipsoidal pit.

258 Y. Yang et al. / Corrosion Science 73 (2013) 250–261

process of Ni–Cr–Mo–V high strength steel at different hydrostaticpressures, it is important to make clear the pits initiation step. SEMimages of corrosion pits of Ni–Cr–Mo–V high strength steel after30 min immersion in 3.5% NaCl solution at three different hydro-static pressures are observed in Figs. 8–10. From Fig. 8a–c, it isnoted that there exist an inclusion or two inclusions in the centerof corrosion pit at 1 atm. The same phenomena are found at 20 atmshown in Fig. 9a–c and at 60 atm shown in Fig. 10a–c. The chemicalcompositions of the inclusions are analyzed by energy dispersivespectroscopy (EDS) and are listed in Tables 2–4. It can be seen thatthe inclusions are mainly composed of aluminium oxide and MnSat each hydrostatic pressure. Inclusions in the steel play an impor-tant role on the initiation of corrosion pits. The preferential dis-solved position is located in the interface between the matrixand the inclusion [25,26]. Therefore, it is accepted that corrosionpits of Ni–Cr–Mo–V high strength steel originate from the

preferential dissolution of the interface between aluminium oxide,MnS and the matrix at each hydrostatic pressure.

At 1 atm, the Ni–Cr–Mo–V steel shows different corrosion char-acteristic with the increase of immersion time. After the immer-sion of 5 h, many pits generate on steel surface (Fig. 3a and b).As immersion time increases to 7 h, the neighbouring pits beginto coalesce (Fig. 3c) on one hand and it looks as if the depths ofthe pits increase and the boundaries of corrosion pits become clearon the other hand (Fig. 3d). After 10 h immersion, the shape of cor-rosion pits is less circular compared with that in 5 h and 7 h(Fig. 3e) [13]. With immersion time increasing to 20 h, corrodedzone starts to coalesce with pits around (Fig. 3f) and uniform cor-rosion forms finally as indicated in Fig. 3g.

At 20 atm, surface of specimen is covered by irregular shape pitsafter 5 h immersion (Fig. 4a). As immersion time increases to 10 h,coalescence trace of localized corroded zone with corrosion pits

Fig. 14. Stress distributions in and around semi–ellipsoidal pits of Ni–Cr–Mo–Vhigh strength steel at 60 atm: (a) semi-ellipsoidal pit, (b) double distributed semi-ellipsoidal pits, and (c) quarter-ellipsoidal pit.

Hydrostatic pressure

Hydrostatic pressure

Hydrostatic pressure

Hydrostatic pressure

Fig. 15. Schematic diagram of stress distribution for one elliptical pit.

Y. Yang et al. / Corrosion Science 73 (2013) 250–261 259

around (Fig. 4b) and uniform corrosion characteristic are observed(Fig. 4c).

In case of 60 atm, irregular shape pits generate on steel surface(Fig. 5a) and the neighbouring pits begin to coalesce (Fig. 5b) after5 h immersion. Corrosion morphologies of Ni–Cr–Mo–V steel after10 h are represented in Fig. 5c–e, which had been reported in ourprevious work [13]. With the increasing of immersion time from

5 h to 10 h, trace of coalescence of the neighbouring pits and local-ized corrosion zone are observed in Fig. 5c and d, respectively. Andalso as the dominative corrosion characteristic, uniform corrosionappear after 10 h immersion at in Fig. 5e.

According to the corrosion morphologies change with the in-crease of immersion time at 1 atm, 20 atm and 60 atm, the corro-sion process of Ni–Cr–Mo–V steel at each hydrostatic pressuredisplays similar characteristic, which can be summarized as threestages: pits generation, pits coalescence and formation of uniformcorrosion. Nevertheless, hydrostatic pressure seems to acceleratethe formation of uniform corrosion. For example, Ni–Cr–Mo–Vsteel shows uniform corrosion characteristic after 10 h immersionat 20 atm (Fig. 4c) and 60 atm (Fig. 5e). However, the corrosionmorphologies after 10 h immersion at 1 atm is mainly character-ized by irregular shape pits (Fig. 3e) and uniform corrosion appearafter 20 h immersion (Fig. 3g). That is to say hydrostatic pressureaccelerates pit coalescence rate and thus shortens the time to com-plete the three stages of the corrosion process for Ni–Cr–Mo–Vhigh strength steel.

4.3. Effect of hydrostatic pressure on pit geometry and localized stressdistribution of corrosion pits

Average 3D sizes of corrosion pits for Ni–Cr–Mo–V highstrength steel at three hydrostatic pressures after 5 h immersionbased on statistical results are listed in Table 1. With the increaseof hydrostatic pressure from 1 atm to 60 atm, the average length ofcorrosion pits increases from 31 lm to 86.7 lm and width from23.5 lm to 74.5 lm with depth nearly unchanging. In other words,hydrostatic pressure accelerates the pit growth rate parallel to thesteel surface.

Pit geometry can be described according to the following equa-tion [27,21,28]:

D=2h > 1;pit is wide-shallow shape;

D=2h ¼ 1;pit shows semi-spherical shape;

D=2h < 1;pit exhibits narrow-deep shape;

where D is pit mouth diameter along the length direction and h rep-resents pit depth in this work. Variation of D/2h at different hydro-static pressures is shown in Fig. 11. On one hand, with the increaseof hydrostatic pressure, ratio of D/2h ranges from 3 to 15, whichindicates that pit is wide-shallow shape at both hydrostatic pres-sures. On the other hand, the ratio of D/2h increases with increasinghydrostatic pressure, which demonstrates that hydrostatic pressuremakes pit wider in geometry with the pit depth nearly unchangingin Table 1 as the precondition.

Ni-Cr-Mo-V steel

3.5% NaCl solution

pit growth coalescence of pits

Ni-Cr-Mo-V steel

3.5% NaCl solution

corrosion product

formation of uniform corrosion

Ni-Cr-Mo-V steel

3.5% NaCl solution

Ni-Cr-Mo-V steel inclusions

corrosion product 3.5% NaCl solution

(a)

(b)

(c)

(d)

corrosion product

Fig. 16. Schematic diagram of corrosion model of Ni–Cr–Mo–V high strength steel under high hydrostatic pressure.

260 Y. Yang et al. / Corrosion Science 73 (2013) 250–261

Typical 3D sizes of corrosion pits in Table 1 are used in the finiteelement models for simulating stress distribution at three differenthydrostatic pressures. The stress distributions of corrosion pits ofNi–Cr–Mo–V high strength steel at three hydrostatic pressuresare shown in Figs. 12–14. Shapes of corrosion pits are semi-ellip-soidal and stress distribution shows anisotropic characteristic ateach hydrostatic pressure. It is clearly seen that there are obviousstress concentration zones near the edge of semi-ellipsoidal pitmouth at 1 atm (Fig. 12a and c). As for double pits, the stress dis-tributions are similar with single pit and the stress zone links to-gether between two pits (Fig. 12b). At 20 atm, the stressconcentration zone for single pit is also near pit mouth, especiallythe long axis edge of the semi-ellipsoidal pit (Fig. 13a and c). Andalso, the stress distribution of double pits is similar with single pit(Fig. 13b). As the increase of hydrostatic pressure from 1 atm to20 atm, the stress value increases from dozens of kPa to hundredsof kPa. In case of 60 atm, the shape of corrosion pit becomes morecircular than that at 1 atm and 20 atm and the stress value in-creases to thousands of kPa. The stress concentrates on the edgeof corrosion pit (Fig. 14a and c) and there is stress concentrationzone between two neighbouring pits when they are close to eachother (Fig. 14b).

4.4. Correlation between corrosion and stress distribution

Hydrostatic pressure is one of the important factors that canchange the mechanochemical activity of deformed metal [14]and promotes the anodic dissolution of Ni–Cr–Mo–V high strengthsteel [13]. The positive stress value in Figs. 12–14 indicates com-pressive stress and it is elastic stress. The direction of the hydro-static stress is perpendicular to the internal surface of corrosionpits. Schematic diagram of stress distribution for one elliptical pitis shown in Fig. 15. It is noted that the stress distribution producesa tensile stress effect in the two-dimensional surface. It has beenconfirmed that a tensile stress accelerates the corrosion of metals[29,30]. Hence, it is expected that the zones with high stress valueswill be corroded prior to that with low values. After the generationof wide-shallow shape pits on steel surface, corrosion pits willpreferentially propagate along the direction parallel to steel sur-face due to the stress distribution (Figs. 12a, 13a and 14a). Duringpit growth process, anisotropic stress distribution and some otherfactors, such as the inhomogeneity of the steel, and the microstruc-ture of the steel, will result in the change of pit shape. Therefore,some pits are circular shape (Figs. 8–10) and some are irregularshape with jittered boundaries after different immersion time

Y. Yang et al. / Corrosion Science 73 (2013) 250–261 261

(Figs. 3a, 4a and 5a). Considering the stress value near the edge ofcorrosion pits is higher at high hydrostatic pressure, the dissolu-tion rate parallel to steel surface will be increased. Therefore, cor-rosion resistance of Ni–Cr–Mo–V high strength steel is deterioratedand the average length and width of corrosion pits in Table 1 areincreased.

While two pits on the surface of Ni–Cr–Mo–V steel are close toeach other, they will be coalesced resulting from the stress concen-tration as shown in Figs. 3b, 4b and 5b. The stress values of concen-tration zones increases from dozens of kPa to thousands of kPawith increasing hydrostatic pressure and so the coalescence ratewill be promoted. That may be why the time of the formation ofuniform corrosion at 20 atm and 60 atm is shortened.

4.5. Corrosion model of Ni–Cr–Mo–V high strength steel underhydrostatic pressure

According to SEM observation an FEA, a corrosion model of Ni–Cr–Mo–V high strength steel under hydrostatic pressure is pro-posed and schematically depicted in Fig. 5.

(i) When a fresh surface of Ni–Cr–Mo–V high strength steel isexposed to 3.5% NaCl solution (Fig. 16a), pits generally initi-ate at inclusions due to the preferential dissolution of theinterface between inclusions (aluminium oxide, MnS) andthe matrix at each hydrostatic pressure (Fig. 16b). The gen-eration of corrosion pits can be attributed to the effect ofelectrochemical corrosion, which is mainly the effect of Cl�.

(ii) After the generation of wide-shallow shape pits on steel sur-face, the stress state around corrosion pits changes themechanochemical activity of the steel. Stress concentrateson the edge of corrosion pits and corrosion will preferen-tially dissolve along the direction parallel to steel surface.With the increase of hydrostatic pressure, the stress valueon the edge of corrosion pits increases. Therefore, the pitgrowth rate parallel to steel surface and the coalescence rateof neighbouring pits will be increased (Fig. 16c).

(iii) Higher pit growth rate parallel to steel surface deterioratesthe corrosion resistance of Ni–Cr–Mo–V high strength steeland higher coalescence rate of neighbouring pits makes iteasier to form uniform corrosion at high hydrostatic pres-sure (Fig. 16d).

5. Conclusion

Corrosion process of Ni–Cr–Mo–V high strength steel can be di-vided into three stages at each hydrostatic pressure: generation ofwide-shallow shape pits, coalescence of wide-shallow shape pitsand formation of uniform corrosion. Corrosion pits originate fromthe inclusions (aluminium oxide and MnS) in the steel and stressdistribution around corrosion pit plays an important role on thepit growth process. With the increase of hydrostatic pressure, thepit growth rate parallel to steel surface increases and the coales-cence rate of neighbouring pits is promoted. It takes shorter timeto complete the evolution from wide-shallow shape pits to uniformcorrosion at high hydrostatic pressure. Corrosion of Ni–Cr–Mo–Vhigh strength steel under high hydrostatic pressure is in fact theinteraction result between electrochemical corrosion and elasticstress.

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