Effects of crevice geometry on corrosion behavior of 304 stainless steel during crevice corrosion in high temperature pure water

Effects of crevice geometry on corrosion behavior of 304 stainless steel during crevice corrosion in high temperature pure water

Accepted Manuscript Title: Effects of crevice geometry on corrosion behavior of 304 stainless steel during crevice corrosion in high temperature pure ...

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Accepted Manuscript Title: Effects of crevice geometry on corrosion behavior of 304 stainless steel during crevice corrosion in high temperature pure water Author: Dongxu Chen En-Hou Han Xinqiang Wu PII: DOI: Reference:

S0010-938X(16)30200-1 http://dx.doi.org/doi:10.1016/j.corsci.2016.04.049 CS 6758

To appear in: Please cite this article as: Dongxu Chen, En-Hou Han, Xinqiang Wu, Effects of crevice geometry on corrosion behavior of 304 stainless steel during crevice corrosion in high temperature pure water, Corrosion Science http://dx.doi.org/10.1016/j.corsci.2016.04.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effects of crevice geometry on corrosion behavior of 304 stainless steel during crevice corrosion in high temperature pure water

Dongxu Chen, En-Hou Han * , Xinqiang Wu

Key Laboratory of Nuclear Materials and Safety Assessment, Liaoning Key Laboratory for Safety and Assessment Technique of Nuclear Materials, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, 110016, Shenyang, P. R. China.

*Corresponding author: En-Hou Han, Tel.: +86 24 2389 3841; fax: +86 24 2389 4149. E-mail address: [email protected] (E.-H. Han).

Research highlights of the present work are listed as follows: 1, A new device to accurately simulate crevice corrosion in high temperature water was designed. 2, E-pH diagram, distributions of DO and pH at high temperature were calculated. 3, Crevice geometry can affect the development of oxide films within the crevi ce. 4, DO and pH value within crevice solution were strongly affected by crevice geometry.

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Abstract The new device of crevice corrosion in high temperature water was designed. Effects of crevice geometry on corrosion behavior of 304 stainless steel during crevice corrosion in high temperature water have been investigated. Both width and length of the crevice affect the oxidation behavior of 304 SS. Different crevice widths results in different distributions of dissolved oxygen concentration and eventually affect the development of oxides within the crevice. The crevice length mainly influences the pH value within the crevice solution. The influencing mechanisms of the crevice geometry on oxidation behavior in high temperature water during crevice corrosion are also discussed.

Keywords: A. stainless steel; C. corrosion in high temperature water; C. crevice corrosion; C. XPS; C. oxidation.

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1. Introduction Type 304 stainless steel (SS) has been widely used as one of the structural materials in nuclear power industry due to its excellent corrosion resistance. However, it is still prone to localized attack after long-term service [1, 2], typically such as crevice corrosion. Crevice corrosion often occurs in occluded regions, so it is more difficult to detect than general corrosion and there is often a long incubation period before attack begins. But once it is initiated it could accelerate the damage of mat erials within crevice rapidly. Therefore crevice corrosion is one of the most destructive and dangerous forms of aqueous corrosion [3-7]. Crevice corrosion is an undesirable degradation of the structural materials in nuclear power plants (NPPs). Many key parts of NPPs like the tube in steam generator (SG), the structure parts in reactors and the forging parts in control systems may be attacked by crevice corrosion easily [8-10]. All these materials in NPPs may be seriously degraded by the attack of crevice corrosion during their services. The main reasons of crevice corrosion of the structural materials in NPPs are the presence of the geometric crevices. It may result in a restriction of mass transport between the crevice and bulk solutions. Therefore, the ionic concentrations within the crevice become much higher than those in the bulk solution. This makes the solution within the crevice become more aggressive and then damages the passive film. In conclusion, crevice corrosion in NPPs may be one of the most serious material degradations and have received much attention in recent years. Many researchers [4, 5, 11-13] have studied the behavior and mechanisms of crevice corrosion and two mechanisms [14] have been proposed to explain the crevice corrosion, namely, critical crevice solution (CCS) and potential drop mechanism. For the CCS mechanism, the depletion of oxygen within the crevice may finally result in the 3

acidification of crevice solution. This can cause breakdown of the passive film and result in a rapid corrosion of metal. For the potential drop mechanism, the depletion of oxygen may finally result in a potential drop within the crevice and in turn cause the crevice corrosion. Both theories have the same viewpoint that oxygen is depleted within the crevice. Therefore, the dissolved oxygen (DO) concentration within the crevice solution is one of the most important factor that affects the crevice corrosion behavior. It is generally believed that many factors [15], such as the crevice geometry (crevice gap, crevice depth, exterior surface to interior crevice area ratio, number of crevice sites), the bulk solution environment (DO, temperature, pH value), and the alloy composition (major constituents, minor additions, impurities) can influence the process of crevice corrosion. Many researchers have investigated the effects of different factors on crevice corrosion behavior of alloys [16-21]. Some work was focused on the effects of temperature on the crevice corrosion resistance of alloys [16, 20]. The resistance against crevice corrosion is often expressed as a critical temperature, above which the crevice corrosion possibly occurs. Actually, the temperature changes primarily affect the critical breakdown potential of passive films. It was found that both the repassivation potential and the breakthrough potential showed a linear decrease with increasing temperature. Some work was focused on the effects of alloy elements and ions in solution on the crevice corrosion resistance of alloys [18, 21]. It was found that the -

nitrogen element in SS dissolved through crevice corrosion as NO 3 , markedly suppressing the corrosion. It was also found that the presence of sulphate ions suppressed the anodic dissolution reactions and increased the endurance of chloride attack on Alloy 600. However, little work has been done to study the effects of c revice geometry on crevice corrosion of metal, especially in high temperature pressurized water due to the difficulties of simulating tests. It is difficult to construct an accurate crevice device which is suitable for the test in high temperature pressuri zed water environments. So it is quite difficult to clarify the influence of crevice geometry on crevice corrosion in high temperature pressurized water environments. Song [19] studied the geometry scaling for crevice corrosion. It was reported that the main geometry factors affecting crevice corrosion are crevice length ( L) and crevice gap (δ 0 ). The geometry scaling factor should be L 2 /δ 0 . However, the above results are theoretical 4

without the support of experimental evidence. The present work is to develop an exposure testing device of crevice corrosion in high temperature pressurized water which can control the crevice geometry accurately and to investigate the structures, morphologies and compositions of the oxide films within the crevice after exposure tests under different crevice geometry conditions, using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X -ray photoelectron spectroscopy (XPS). The influencing mechanism of crevice geometry on crevice corrosion behavior of 304 SS in high temperature pressurized water is also discussed. 2. Experimental 2.1 Crevice specimen and apparatus of testing loop The new simulating test device of crevice corrosion in high temperature water used in the present work was designed and made (Fig. 1) which could adjust the crevice geometry including crevice gap, crevice depth, exterior surface to interior crevice area ratio accurately. The specimens for the crevice tests consisted of two parts, specimen (4) and crevice former (3), both of which were made fro m 304 SS and were cylindrical. The sizes of specimen and crevice former were different. The specimen and crevice former were fixed with an alumina bolt (1). The alumina bolt used in the present work for two purposes. Firstly, alumina has both excellent mechanical performance and corrosion resistance at high temperature, which are suitable for the experimental studies in high temperature aqueous solution. More importantly, the alumina bolt has excellent insulation performance in high temperature water. As de scribed in the quoted paper, during the crevice corrosion, anodic regions in the crevice couple with cathodic regions on the bold surface through the transportation of charge via electrons through the metal and ions through the solution [14]. Therefore, if a 304 SS bolt was used to fix the two parts of the specimen, the electrons may transport through the bolt. This may affect the crevice corrosion behavior during the tests. In fact, a zirconia bolt, which has a better corrosion resistance than an alumina bolt, can also be used in the present work. Both kinds of bolts used have little influence on the experiment results. The specimen was immovable and the crevice former was rotatable. Firstly, make 5

the crevice former attached with the specimen closely. The c revice width under this condition was 0. Then make the crevice former rotated slowly. The rotating of the crevice former can make it move along with the bolt, therefore, the crevice width changed. In brief, the width of crevice was adjusted accurately through the rotating angle of the crevice former. In order to keep a constant crevice width, a type of 304 SS nut was used to fix the crevice former (2). The alumina bolts used in this study were M6 bolts (whose pitch is 1 mm), so the width of the crevice coul d be calculated through the rotating angle of the crevice former easily. For instance, when the rotating angle of the crevice former was 45 degree, the crevice width can be calculated as 45 o /360 o ×1 mm=0.125 mm. Exposure tests of the above crevice specimens in high temperature pressurized water were carried out in a refreshed autoclave made of 316 SS with a volume of 2 L. The autoclave was also pre-oxidized before the exposure tests. Detailed information on the testing loop and control system has been descri bed in the previous work [22]. 2.2 Materials and testing conditions The composition of 304 SS used in the present work is listed in Table 1. Crevice test specimens were cut from a mill-annealed plate with a thickness of 50 mm. The plate has been solution annealed at a temperature ranging from 1050 to 1150 °C and may be slightly cold worked during fabrication. Fig. 2 shows the detailed sizes of the top and bottom parts of the specimens that have been used in the present work for different crevice widths (Fig. 2a) and crevice lengths (Fig. 2b). The crevice length can be varied by changing the diameter of the crevice former (3). As shown in Fig. 2b, the diameter of specimen (4) was fixed to be 30 mm, when the diameter of crevice former was 14, 18 and 22 mm, the corresponding crevice length was 4, 6 and 8 mm, respectively. The specimens were mechanically abraded with emery paper up to #2000 successively and washed ultrasonically in ethanol before exposure tests. The crevice specimens were exposed to 290 °C and 8 MPa pure water containing 3 ppm O 2 . The specimens were immersed in the autoclave for 150 h and the testing conditions are summarized in Table 2. 2.3 Experimental procedure

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After exposure tests, the specimens were cleaned and dried carefully. The surface appearance was firstly examined by a Leica S6D stereomicroscope and the oxide morphology was examined by an INSPECT F SEM equipped with an EDS. The phase analysis of oxide films were performed using a D/Max 2400 XRD analyzer with Cu K alpha radiation. XPS measurements were performed with ESCALAB250 X -ray photoelectron spectrometer. The detailed parameters of the XPS have been given in the previous work [23]. 3. Results Fig. 3 shows the surface morphologies of the 304 SS with the crevice length of 4 mm and different crevice widths. Obvious crevice corrosion was observed at the crevice width of 125 μm (Fig. 3a). The crevice corrosion became not obvious with increasing crevice width to 250 μm (Fig. 3b). Little crevice corrosion took place at the crevice width of 500 μm (Fig. 3c). These results indicate the above crevice device is suitable for simulating investigation of crevice corrosion in high temperature pressurized water. And also indicates that the crevice width can obviously affect the degree of corrosion within the crevice in 290 o C water. Therefore, a crevice width of 125 μm will be used for further tests under different crevice length conditions. Fig. 4 shows the surface morphologies of the 304 SS with the crevice width of 125 μm and different crevice lengths. Obvious crevice corrosion took place at different crevice lengths of 4 mm (Fig. 4a), 6 mm (Fig. 4b) and 8 mm (Fig. 4c) respectively. It was also found that the crevice surface, crevice mouth region and free surface of the specimen showed different colors. For instance, in Fig. 4a, the color of oxide film on the crevice surface was darker than that on the free surface. Some brown rings could also be observed near the crevice mouth. The above results indicate that the surface oxide films were different at different sites along the crevice length. Fig. 5 shows the SEM morphologies of the corrosion products formed on the deeper position within the crevice (marked (3) as shown in (Fig. 3a)) of 304 SS with the crevice length of 4 mm and the crevice width of 125 and 250 μm respectively. It is found that the corrosion products within the crevice at different crevice widths show different characteristics. Typical spinel oxide particles are observed at the crevi ce width of 125 μm (Fig. 5a). However, many flaky oxide particles are observed at the crevice width of 7

250 μm (Fig. 5b). Fig. 6 shows the SEM morphologies of the corrosion products formed on different positions within the crevice (marked ( 1) to (3) as shown in (Fig. 3a)) of 304 SS with the crevice width of 125 μm and the crevice length of 4, 6 and 8 mm respectively. It is found that the corrosion products within the crevice at different crevice lengths show different characteristics. Different morphologies of oxide particles are observed at different positions within the crevice at the crevice length of 4 mm ( Fig. 6a, 6b and 6c). The largest oxide particles are observed at the crevice mouth ( Fig. 6a), flake oxide particles are observed within the crevice (Fi g. 6b) and typical spinel oxide particles are found at the deeper site within the crevice (Fig. 6 c). However, many spinel oxide particles are observed at all positions of (1), (2) and (3) within crevice at the crevice length of 6 and 8 mm (Fig. 6d to 6i). It is found that the oxide particles at position (2) are spinel structure instead of flaky structure at the crevice length of 6 mm (Fig. 6e). Dissolution of spinel particles are clearly observed at the deeper site within the crevice at the crevice length of 8 mm (Fig. 6i). Fig. 7 shows some special SEM morphologies of the oxide particles formed within the crevice at the crevice width of 125 μm and the crevice length of 8 mm after exposure tests. It is found that at the deeper position within the crevice the spinel oxide particle was dissolved (Fig. 7a). It is also found that at the position near crevice mouth, many small spinel oxide particles were formed on the bulk spinel oxide particle (Fig. 7b). The small spinel oxides on the bulk oxide particle are b elieved to be formed through precipitation. The dissolution of spinel oxide particles released more metallic ions into crevice solution. The ions were aggregated near the crevice mouth as a result of the electrostatics and hydrodynamics. Therefore, the sma ll spinel nuclei grew up through the precipitation of metallic ions from crevice solution. This may be possible evidence of the dissolution-precipitation mechanism [23] of the oxide particles and further detailed work is necessary to clarify this. Fig. 8 shows the XRD patterns of the oxide films formed within the crevice of 304 SS with the crevice length of 4 mm and the crevice width of 125 and 250 μm respectively. The positions of characteristic peaks suggest a hematite structure dominates at various sites. However, the intensity (height and area) of characteristic peaks of hematite oxide at the crevice width of 125 μm (a peak height of 25 and an area of 469 at 2θ=33 o , a peak height of 8 and an area of 83 at 2θ=39 o , a peak height of 10 and 8

an area of 90 at 2θ=41 o ) is weaker than that at the crevice width of 250 μm (a peak height of 72 and an area of 799 at 2θ=33 o , a peak height of 81 and an area of 881 at 2θ=39 o , a peak height of 13 and an area of 148 at 2 θ=41 o ). Moreover, the intensity of characteristic peaks of spinel oxide at the crevice width of 250 μm (a peak height of 27 and an area of 493 at 2θ=36 o , a peak height of 9 and an area of 102 at 2θ=30 o ) is weaker than that of 125 μm (a peak height of 29 and an area of 538 at 2 θ=36 o , a peak height of 14 and an area of 231 at 2θ=30 o ). Fig. 9 shows the XRD patterns of the oxide films formed within the crevice of 304 SS with the crevice width of 125 μm and the crevice length of 4, 6 and 8 mm respectively. The intensity of characteristic peaks of hematite oxide at the crevice length of 4 mm (a peak height of 25 and an area of 469 at 2 θ=33 o , a peak height of 8 and an area of 83 at 2θ=39 o ) is weaker than those of 6 mm (a peak height of 152 and an area of 1252 at 2θ=33 o , a peak height of 30 and an area of 285 at 2θ=39 o ) and 8 mm (a peak height of 47 and an area of 545 at 2 θ=33 o , a peak height of 44 and an area of 453 at 2θ=39 o ). Moreover, the intensity of characteristic peaks of spinel oxide at the crevice length of 8 mm (a peak height of 25 and an area of 475 at 2 θ=36 o , a peak height of 11 and an area of 132 at 2θ=30 o ) is weaker than that of 6 mm (a peak height of 63 and an area of 997 at 2θ=36 o , a peak height of 22 and an area of 315 at 2θ=30 o ). This is consistent with the previous SEM observation that the dissolution of the spinel oxide particles has been observed at the 8 mm crevice length (Fig. 6 and Fig. 7). Fig. 10 shows the XPS depth profile for oxide films formed within the crevice with the crevice length of 4 mm and the crevice width of 125 and 250 μm respectively. It is found that Fe content in the oxide films at the crevice width of 125 μm is lower than that of 250 μm (Fig. 10a), while Cr content is higher than that of 250 μm (Fig. 10b). From outer to inner layer, Ni content in the oxide films at the crevice width of 125 μm is initially lower than that of 250 μm, while with increasing sputtering time, it become higher than that of 250 μm (Fig. 10c). Fig. 11 shows the XPS depth profile for oxide film formed within the crevice with the crevice width of 125 μm and the crevice length of 4, 6 and 8 mm respectively. It is found that at the crevice length of 8 mm the Fe content in inner layer of the oxide films is higher than others. However, the Fe content decreased obviously in outer layer of the oxide films and become lower than that of 4 mm (Fig. 11a). It is also found that at the crevice length of 6 mm the Cr content in oxide 9

films is higher than others (Fig. 11b). The Ni content is higher th an others in inner layer of the oxide films at the crevice length of 6 mm and lower than others in outer layer of the oxide films (Fig. 11c). 4. Discussion The analyses of SEM morphologies (Fig. 5, Fig. 6), XRD patterns (Fig. 8, Fig. 9) and XPS spectra (Fig. 10, Fig. 11) in the present work suggest that the oxide films formed within the crevice at different crevice widths or lengths after 150 h exposure tests have different morphologies and structures. It is believed that in crevice corrosion, the geometrical parameters such as the crevice width and length may influence the DO concentration distribution, the ion concentration distribution and the potential distribution within the crevice [19]. The crevice geometrical restricted the transport of the oxygen and the ions within the crevice, which may eventually affect the concentration distribution of them. On the other hand, crevice length may affect the IR drop of the electrolyte within the crevice. The potential gradient within the crevice is a function of distance, x, into the crevice [24]. It is generally believed that the mass transport in solution is mainly based on the diffusion, electromigration and liquid convection [25-31]. The liquid convection within an occlusion region can be ignored for this kin d of crevice corrosion. Therefore, this model considers mass transport by diffusion and electromigration. The flux equation is given by

 Ci x zi Di F  Ci RT x

N i = - Di

(1)

where the symbols are explained in the Appendix A. The rate of production or depletion of species i by chemical reaction ( R i ) is given by

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 Ci + t   N  = Ri x i

(2)

Combining equation (1) and (2), the concentrations of aqueous species in a dilute solution filling the crevice are governed by the following mass conservation equation

 Ci  2Ci = Di + t  x2 zi Di F      Ci  + Ri RT  x   x 

(3)

which the first term on the right-hand side of equation (3) represents the transport of ions by diffusion due to the concentration gradients. The second term represents the transport of charged species due to the electrostatic potential gradients. The third term represents the rate of production or depletion by chemical reaction. The crevice geometry used in the model of this work is schematically shown in Fig. 12, where w represents the crevice width, L represents the crevice length, x represents the distance along crevice length and the crevice mouth is defined as x=0. Because the chemical equilibria between the species are supposed to occur very fast compared to the diffusion phenomena due to concentration and potential gradients. Therefore, it can be thought that at the steady state

 Ci =0 t It is mentioned that the DO distribution within the crevice is the main reason of the different oxidation processes among different crevi ce geometries. Therefore, it is necessary to find the distributions of DO within the crevice at different crevice geometries. For oxygen in solution, the transport process from bulk solution into crevice solution is based on the diffusion driven by the DO concentration gradient and the electromigration process can be ignored. Therefore, equation (3) and (4) yields

 2 CO - DO = RO  x2 2

2

2

11

where the equation (5) gives the relationship between DO concentration ( CO2 ) and crevice distance (x). The principal cathodic reaction in nearly neutral solutions is O 2 + 2H 2 O + 4e - → 4OH -

(6)

The reaction rates can be converted from the respective current densities following Faraday’s law [32]

Ri =

Ii ni F

(7)

Therefore, the reaction rates of oxygen is given by

R O2 =

I O2 4F

×

l A

where l/A is the crevice aperture [25, 27]. The current density of the depletion of oxygen is given by

I O2 = I 0

 η Cx exp  -  Cb  βc  RT is related to the Tafel slope of the cathodic reduction. 1 - α  zF

where β c  = 

Combining equations (5), (8) and (9), the equation for neutral oxygen into crevice is

 2 CO  - η(1 - α)n O F  I0 l =exp  × 2 x DO n O FCb A RT   2

2

2

2

CO2

(10)

From equation (10), it can be found that the distribution of DO within the crevice is mainly affected by the crevice perimeter (l), the crevice cross-sectional area (A) and the DO concentration in bulk solution ( C b ) . The constants and values of parameters used for the modeling are given in Table 3. The diffusion coefficient of species i ( Di ) 290 o C is acquired by linear extrapolation through the Wilke -Chang equation [33]. The calculated results of the distributions of DO within the crevice at different 12

at

crevice widths of 125, 250, 500 and 1000 μm are shown in Fig. 13. It is found that the DO concentration decreased rapidly with increasing distance from the crevice mouth ( x) at the crevice width of 125 μm and reached to about 0.005 M at x=1 mm. It is also shown that the reduction rates of DO concentration at different crevice widths become smaller with increasing crevice width. The previous work of Kim et al. [34] reported that the electrochemical corrosion potential (ECP) of metal decreased with decreasing DO concentration. For 304 SS in 288 °C water, the ECP can decrease to about -0.65 V at the DO concentration of about 1 ppb. Fig. 14 shows the calculated potential-pH (E-pH) diagram of Fe-Cr-Ni alloy in 290 oC

pure water. The possible reactions in the present work are listed in Table 4. The 0

standard free energies ( G T )

of the species involved are calculated by the equation as

follows.

G 0T = G 0298 -  T - 298  × S0298 + T

T

 CpdT - T  298

298

Cp T

dT

(11)

The thermodynamic data are listed in Appendix B. The heat capacity ( C p )

of each

species is acquired by linear extrapolation using the standard free energies at 250 o C and 300 o C [35-38]. The concentration of each ion is considered as 10 -6 M. From the above calculated DO distribution (Fig. 13) and calculated E-pH diagram (Fig. 14), it can be found that at the crevice width of 125 μm, the potential drop is pretty large and may result in formation of different oxide films at different sites within the crevice. Meanwhile, with increasing exposure time, the DO concentration in the crevice solution decreased. The potentials at the same site decreased and multilayered oxide films may be formed. As a result, the stabilized oxides within the crevice finally became Fe 3 O 4 spinel oxide due to the serious potential drop (Fig. 5a). However, at the crevice width of 250 μm, the potential drop is smaller than that of 125 μm. At the initial stage of exposure, hematite oxide Fe 2 O 3 films were formed. With increasing exposure time, the potential decreased and the stabilized oxides within the crevice finally became lamellar FeCr 2 O 4 particles (Fig. 5b). In Fig. 14, it is found that the largest stable region of oxides at the crevice width of 125 μm is FeCr 2 O 4 oxides. However, the largest stable region of oxides at the crevice width of 250 μm is Fe 2 O 3 oxides. This is consistent with the XPS 13

depth profile results of the oxide films at different crevice widths (Fig. 10). The development of different oxide films at the same site can also be verified through the XPS depth profile results. In comparison with the crevice width of 250 μm, the contents of Fe, Cr and Ni of oxide films at the crevice width of 125 μm showed obvious differences from inner layer to outer layer of the oxide film. Fig. 15 shows the calculated results of the distributions of DO within the crevice at different crevice lengths of 4, 6 and 8 mm. It is found that the reducing rates of DO concentration at different crevice lengths become larger with increasing crevice length. Increasing crevice length can enhance the potential drop at the same position within the crevice. These different potentials may influence the development of oxide films during crevice corrosion. However, it can be found that the changes of the reducing rates of DO concentration at different crevice lengths are not so obviously as that at different crevice widths (Fig. 13). Therefore, it is believed that the crevice length in this study does not influence the DO distribution as much as the crevice width. Therefore, another effect on pH value within the crevice solution that influenced by crevice length should be considered to explain the different oxidation behaviors (Fig. 6) of 304 SS during crevice corrosion at different crevice lengths. Fig. 16 shows the calculated results of the distributions of pH value within the crevice at different crevice lengths of 4, 6 and 8 mm. Because of the presence of H + , the concentration of H + within the crevice solution is based on the diffusion and the electromigration. According to equation (3), the distribution of H + concentration is given as follows [26, 27].

 CH  2CH z H DH F     = DH + +  CH t  x2 RT  x   x  RH

(12)

The reaction rate of hydrogen is given by

RH =

IH × nHF

l A

(13)

The current density of the depletion of hydrogen is given by 14

IH = I0

 η CH exp  -  Cb  βc 

(1

Combining equation (13) and (14), the equation (12) can be transformed into the following form

ay " by ' cy  d  0

dy |x=4 =0 . dx

where the y represents the pH value. The boundary conditions are x=0,y=5.6; The general solution of equation (15) is

y = c1eδ1x + c 2 e δ2 x

(16)

where δ1 and δ 2 are the two solutions of equation (16), c1 and c 2 are the coefficients. Therefore, the distribution of pH value can be calculated by equation (1 6) and be drawn in Fig. 16. It is shown that the pH value decreased rapidly at the positi on near the crevice mouth. With increasing distance, the pH value eventually decreased to a stable region where the values have little change. It is also shown that the reducing rates of the pH value near the crevice mouth increased with increasing crevice length. Moreover, the stable regions of the pH value decreased with increasing crevice length. According to the expression of water ionization constant [39], the pH of neutral solution at 290 o C is around 5.6. It is found that at the crevice length of 4 m m, the pH value eventually decreased to about 5. However, at the crevice length of 8 mm, the pH value has already decreased to about 4.4 which may result in the dissolution of oxide films. Therefore, the crevice length mainly influences the pH value within the crevice solution. From the E-pH diagram, it is found that the Fe 3 O 4 or FeCr 2 O 4 spinel oxides are unstable when the pH value decreased to about 5 or lower. At the crevice length of 8 mm, the pH value may decrease to less than 4.5. This may eventually result in the dissolution of spinel oxides as shown in Fig. 6i. Some previous work [40-43] has investigated the oxidation behavior of SSs in high temperature pressurized water and the influence of water chemistry. Moreover, the oxidation behavior of 304 SS during crevice corrosion in high temperature pressurized water has been investigated in our previous work. It is believed that the decreased DO 15

within the crevice results in a potential gradient. Such a potential drop affects the development of oxide films. Therefore, at the early stage of crevice corrosion, the crevice length mainly influences the potential drop within the crevice. At the crevice length of 8 mm, the potential can drop to the stable region of Fe 3 O 4 , so more Fe element has been detected in the inner layer (Fig. 11a). Moreover, with the development of crevice corrosion, the pH value within the crevice reduced and may result in the dissolution of some Fe 3 O 4 oxides. Therefore, the percentage of Fe 3 O 4 in outer layer of the oxide films decreased and the content of Fe element at the crevice length of 8 mm became lower than that of 4 mm. At the crevice length of 6 mm, it is found that the content of Cr element is higher than those of 4 and 8 mm (Fig. 11b). This is due to the primary stable region of oxides under this condition is FeCr 2 O 4 as discussed above. The different oxide films within the crevice at different crevice lengths can also be verified through the XPS results. Fig. 17 shows the XPS spectra of Fe 2p3/2, Cr 2p3/2 and Ni 2p3/2 in the oxide films formed within the crevice at different crevice lengths of 4, 6 and 8 mm (named (a), (b) and (c) respectively) of 304 SS. For the Fe 2p3/2 peak decomposition, binding energy (BE) around 710.6 eV and 708.5 eV are the characteristics of Fe 3+ and Fe 2+ respectively, while the BE around 707.4 eV is the characteristic of metallic Fe 0 [44-46]. For the Cr 2p3/2 peak decomposition, the BE around 577.0 eV with a satellite peak around 586.7 eV can be assigned to Cr 3+ , while the BE around 574.2 eV with a satellite peak around 583.5 eV can be assigned to Cr 0 [44-46]. For the Ni 2p3/2 peak decomposition, the BE around 854.9 eV with a satellite peak around 861.0 eV can be assigned to Ni 2+ , while the BE around 852.7 eV with a satellite peak around 858.5 eV can be assigned to Ni 0 [44, 47]. It can be found that the Fe2p3/2 spectrum and the Ni2p3/2 spectrum exhibited the peaks of Fe 0 and Ni 0 respectively in the outer layer of the oxide film at the crevice length of 8 mm (Fig. 17c). This is mainly because the dissolution of unstable Fe 3 O 4 or FeCr 2 O 4 spinel oxide films which may result in a thinner oxide film than those of 4 and 6 mm. Moreover, it can be found that the contents of Fe 2+ and Cr 3+ in inner layer and secondary layer (700 s, 2400 s and 3600 s) at the crevice length of 6 mm are higher than those of 4 mm (Fig. 17a and Fig. 17b). These suggest that the oxide films formed within the crevice at the crevice length of 6 mm are mainly FeCr 2 O 4 . 5. Conclusions 16

Influence of crevice geometry on oxidation behavior of 304 SS during crevice corrosion in 290 °C water containing 3 ppm DO were investigated using a kind of self-designed high temperature crevice corrosion device. The following conclusions could be drawn based on the present results. The designed crevice corrosion device satisfies the exposure tests of crevice corrosion in high temperature pure water at different crevice geometry sizes. Both crevice width and crevice length can affect the oxidation behavior of 304 SS during crevice corrosion in high temperature pressurized water. Different crevice widths result in different distributions of DO within the crevice and in turn different potential drops, affecting the development of oxides within the crevice eventually. At the crevice width of 125 μm the potential can drop to Fe 3 O 4 spinel region and the largest stable region of oxides is FeCr 2 O 4 . However, at the crevice width of 250 μm the potential can only drop to FeCr 2 O 4 region and the largest stable region of oxides is Fe 2 O 3 . The crevice length has less effect on the DO distribution than the crevice width. The crevice length mainly influences the pH value within the crevice solution. The oxide films formed within the crevice at the crevice length of 6 mm are mainly FeCr 2 O 4 . At the crevice length of 8 mm, the pH value may decrease to less than 4.5, which may eventually result in the dissolution of the spinel oxides. Acknowledgements This study was jointly supported by the Science and Technology Foundation of China (51371174), the Special Funds for the Major State Basic Research Projects (2011CB610501), and the Innovation Fund of Institute of Metal Research, Chinese Academy of Sciences.

Appendix A. Nomenclature Ci:

Concentration of species i

Cb:

Concentration in bulk solution

Di:

Diffusion coefficient of species i

F:

Faraday constant 17

I:

Current density

Ni:

Flux of species i

ni:

Number of electrons transferred for i reduction

R:

Gas constant

Ri:

Reaction rate involving species i

T:

Temperature

t:

Time

x:

Distance from the crevice mouth

L:

Total depth of the crevice

w:

Gap of the crevice

zi:

Charge number of species i

η:

Overpotential

α:

Transfer coefficient

18

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Figure caption: Fig. 1 Schematic diagram of crevice corrosion device in high temperature water.

23

Fig. 2 Specimen geometry. (a) different crevice widths (b) different crevice lengths

24

Fig. 3 Surface morphologies of crevice specimen after 150 h exposure tests in 290 o C water containing 3 ppm DO at different crevice widths. (a) 125 μm (b) 250 μm (c) 500 μm.

25

Fig. 4 Surface morphologies of crevice specimen after 150 h exposure tests in 290 o C water containing 3 ppm DO at different crevice lengths. (a) 4 mm (b) 6 mm (c) 8 mm.

26

Fig. 5 SEM morphologies of corrosion products formed within the crevice after after 150 h exposure tests in 290 o C water containing 3 ppm DO at different crevice widths. (a) 125 μm (b) 250 μm.

27

Fig. 6 SEM morphologies of corrosion products formed within the crevice after after 150 h exposure tests in 290 o C water containing 3 ppm DO at different crevice lengths. (a) site near the crevice mouth and a length of 4 mm (b) site within the crevice and a length of 4 mm (c) deeper site within the crevice and a length of 4 mm (d) site near the crevice mouth and a length of 6 mm (e) site within the crevice and a length of 6 mm (f) deeper site within the crevice and a length of 6 mm (g) site near the crevice mouth and a length of 8 mm (h) site within the crevice and a length of 8 mm (i) deeper site within the crevice and a length of 8 mm.

28

Fig. 7 SEM morphologies of the oxide particles formed within crevice at the crevice width of 125 μm and the crevice length of 8 mm after exposure tests. (a) deeper site within the crevice (b) site near the crevice mouth

29

Fig. 8 Measured XRD patterns of the oxide films formed within the crevice at different crevice widths after 150 h exposure tests in 290 o C water containing 3 ppm DO.

30

Fig. 9 Measured XRD patterns of the oxide films formed within the crevice at different crevice lengths after 150 h exposure tests in 290 o C water containing 3 ppm DO.

31

Fig. 10 Measured XPS depth profile for oxide film formed within the crevice at different crevice widths after 150 h exposure tests in 290 o C water containing 3 ppm DO. (a) Fe (b) Cr (c) Ni

.

32

Fig. 11 Measured XPS depth profile for oxide film formed within the crevice at different crevice lengths after 150 h exposure tests in 290 o C water containing 3 ppm DO. (a) Fe (b) Cr (c) Ni.

33

Fig. 12 Schematic diagram of crevice geometry used in the model.

34

Fig. 13 Calculated results of the distributions of DO within the crevice at different crevice widths after exposure in 290 o C water containing 3 ppm DO.

35

Fig. 14 Calculated E-pH diagram for the ternary system of Fe -Cr-Ni alloy in 290 o C pure water. The thermodynamic datas are listed in Appendix B. The standard states used in the present work are molar.

36

Fig. 15 Calculated results of the distributions of DO within the crevice at different crevice lengths after exposure in 290 o C water containing 3 ppm DO.

37

Fig. 16 Calculated results of the distributions of pH value within the crevice at different crevice lengths after exposure in 290 o C water containing 3 ppm DO.

38

Fig. 17 Measured XPS spectra of Fe 2p3/2, Cr 2p3/2 and Ni 2p3/2 in the oxide films formed within the crevice of 304 SS at different crevice lengths after 150 h exposure tests. (a) 4 mm (b) 6 mm (c) 8 mm.

39

40

41

Table caption: Table 1 Compositions of 304 SS used in the present work (wt.%). C

Si

Mn

S

P

Cu

Co

B

Ni

Cr

Fe

0.035

0.66

1.88

0.005

0.023

1.00

0.06

0.0018

9.27

18.65

Bal.

Table 2 Experimental conditions. No.

Width (μm)

Length (mm)

DO (ppb (by weight))

Temperature (°C)

1

125

4

3000

290

2

250

4

3000

290

3

500

4

3000

290

4

125

4

3000

290

5

125

6

3000

290

6

125

8

3000

290

Table 3 Constants and values of parameters used for the modeling Parameter

Value

Units

T

563.15

K

F

96485

C/mol

48,49

R

8.3143

J/mol·k

48,49

D 02

2.44×10 -9

m 2 /s

48

DH+

11.8×10 -9

m 2 /s

48

I

'10 -2

A/m2

10,25

E

-0.6

V

10,25

nH+

4

48,50

nH+

1

48,49

η

0.9

α

0.5

V

Source

25,32,51 51

42

Table 4 The possible reactions of Fe-Cr-Ni-H 2 O system in this investigation. No.

Reaction

(1)

Fe 2+ + 2e → Fe

(2)

Fe 2 O 3 + 6H + + 2e → 2Fe 2+ + 3H 2 O

(3)

FeCr 2 O 4 + 2H + → Fe 2+ + Cr 2 O 3 + H 2 O

(4)

Fe 2 O 3 + 2Cr 2 O 3 + 2H + + 2e → 2FeCr 2 O 4 + H2O

(5)

NiFe 2 O4 + 2H + → Fe 2 O 3 + Ni 2+ + H 2 O

(6)

NiFe 2 O 4 + 2Cr 2 O 3 + 2H + + 2e → 2FeCr 2 O 4 + NiO + H 2 O 3FeCr 2 O 4 + 4H + + 4e → Fe 3 O 4 + 6CrO +

(7)

2H 2 O FeCr 2 O 4 + 6H + + 2e → 2CrOH 2+ + Fe +

(8)

H2O Fe 3 O 4 + 6H + + 4e → 2FeOH + + Fe + 2H 2 O

(9)

43

Table B.1 Thermodynamic data of used species. Species

∆G0(298.15)

∆S0(298.15) [J

[kJ mol-1]

(mol K)-1]

∆G0(563.15)

Source

[kJ mol-1]

H2O

-237.2

70.1

-264

52,53

H+

0

-22.2

13.2

52,53

Fe

0

27.3

-10.6

53,54

Fe 2+

-78.9

-182.1

-10.1

52

FeOH +

-467

34.3

-430

52,54

Fe 2 O 3

-741

90

-779.5

53,54

Fe 3 O 4

-1015.5

146.4

-1076.3

52,53

FeCr 2 O 4

-1339.4

152.2

-1398

53,54

CrO

-350.7

44.8

-368

53,54

Cr 2 O 3

-1050

82.3

-1068.8

52,53

CrOH 2+

-773.6

88.96

-770.2

52

Ni +

-45.6

-173.3

-28.1

54

NiO

-216

38

-233.8

53,54

NiFe 2 O 4

-974.6

125.9

-1030.4

53,54

44