Effect of Tensile Stress on the Passivity Breakdown and Repassivation of AISI 304 Stainless Steel: A Scanning Kelvin Probe and Scanning Electrochemical Microscopy Study

11 The interplay between mechanical stresses and electrochemical reactions may lead to stress 12 corrosion cracking or hydrogen embrittlement for many materials. In this work, the effect of 13 the tensile stress on the electrochemical properties of AISI 304 stainless steel was studied 14 using scanning Kelvin probe (SKP) in air and scanning electrochemical microscopy (SECM) 15 in an aqueous 0.5 M Na 2 SO 4 electrolyte. The measurements were performed under load- and 16 load-free conditions. 17 No influence of the elastic stress on the electrochemical potential of the steel was found. In 18 contrast, the plastic strain induces dislocations and dislocation pile-ups, which emerge to the 19 surface. The formation of new active surfaces is accompanied by an increase in the roughness 20 and a 150-200 mV decrease in the steel potential. After activation, the potential increased due 21 to passivation of the emerging surfaces by a newly grown oxide film, which took place under 22 both the load and load-free conditions and followed a time dependence of 𝞿 = A log t + B. 23 Formation and then passivation of the new surfaces increased and then decreased the 24 reduction current of the mediator in the SECM measurements. The effect of residual stress 25 stored in the steel due to the development of dislocations on the reactivity of the re-passivated 26 surface was investigated.


Introduction
Tensile samples were prepared from a 0.5 mm thick plate that was machined to obtain samples compatible with both the local probe techniques used and the in situ tensile setup. A schematic representation of the samples is shown in Figure 2. This specific shape was selected to obtain stress/strain gradient in the diminution area of the sample. The initial gauge length of the specimen was 10 mm. Local measurements were performed over the gauge section of the sample, which is the location with the maximum stress and strain, and over the surrounding diminution areas of the sample. The mechanical properties were determined by tensile testing of flat samples (Figure 2. a) with a strain rate of 0.0067 s -1 using a hydraulic tensile testing machine (HC25 from Zwick). Figure 2. b shows a representative stress-strainload relationship for a sample. The yield strength (YS) was evaluated at 300 MPa, corresponding to 1.5 kN. The ultimate tensile strength (UTS) was 650 MPa with an experimental load of 3.25 kN. The elongation to failure was in the range of 46-48%.
To evaluate the local plastic strain and stress, calculations were performed by finite elements modeling with ABAQUS software using the tensile properties of the steel. An example of a local strain distribution in a tensile sample for an imposed total displacement of 6 mm is shown in Figure 2. c. In this figure, the indicated values of 12.1 and 43 mm are set to show the spreading of the plastic deformation over the gauge section of the specimen, which was initially 10 mm long. Indeed, some non-negligible plastic deformation was found up to 21.5 mm from the center of the specimen. The deformation in the gauge section was approximately 27% in this case corresponding to a plastic of 0.267. Based on these simulations, the extent of the plastic strain over the sample can be estimated. To compare the experimental data and simulation results, the total elongation of the tensile sample and the applied load were used as entry data for simulations.
To apply the load during the SKP and SECM measurements, a constant load cell was used. After mounting the sample on the setup, a load gauge was installed either in the SKP chamber or on the moving table of the SECM instrument. A detailed description of the setup has previously been reported [25,29]. A load gauge and "Mitutoyo" digital caliper were used to control the load and elongation, respectively.

Scanning Kelvin probe
SKP is a non-invasive technique that measures the contact potential difference between a working electrode (i.e., the AISI 304) and a vibrating reference electrode (CrNi alloy needle). In air, the two surfaces are separated to create a capacitor in which, due to the vibration and the variation of the distance, an AC current is generated. The current amplitude is proportional to the contact potential difference between the two electrodes. The potential of the probe is calibrated, which makes it possible to determine the potential of the working electrode relative to that of the reference. Thus, SKP is able to either determine the surface distribution of the Volta potential or monitor the Volta potential at a single point above the surface. Details and the theory of SKP can be found in the literature [31]. In this study, a height-controlled SKP instrument from Wicinski & Wicinski GbR was used. The reference electrode was a needle with a tip diameter of 100 µm, and the distance to the working electrode surface was approximately 50 µm. Surface contour mapping (topographic profile) was performed simultaneously with the potential mapping. Prior to the measurement, the potential of the probe was calibrated relative to that of a Cu/CuSO 4 electrode, but all potentials are given versus the standard hydrogen electrode (SHE). The measurements were performed in ambient air at 50-60% RH. The first term is the contact potential difference between the bulk alloy and oxide film, which relates to the difference in the corresponding Fermi levels of the electrons in the metal (μ e ) and in the oxide (μ ox ), e-is the elementary charge. The second potential drop ( ) relates to the adsorption of environmental components (molecules of oxygen, water, etc.), which alters the conduction and valence bands in the semiconducting oxide film. The electric charges in the oxide are compensated for by the charges of the adsorbed species (e.g., O 2 ions), creating a potential drop ( ⁄ ).

Scanning electrochemical microscopy (SECM)
SECM was used to measure the effect of stress on the local electrochemical reactivity of the steel surface in an aqueous electrolyte (0.5 M Na 2 SO 4 ) in the presence of a redox mediator (either 10 mM K 3 (Fe(CN) 6 ) or 1 mM ferrocene methanol). The measurements were performed in a 4-electrode electrochemical cell (using a Pt-grid counter electrode and a saturated calomel reference electrode). The cell was attached to the surface of the gauge section of the tensile sample using O-rings. The area of the sample exposed to the electrolyte was 0.35 cm 2 , and the volume of the aqueous electrolyte in the cell was approximately 10 cm 3 .
The radius of the platinum microelectrode was 12.5 µm. SECM approach curves were performed to position the Pt microelectrode at a controlled distance from the sample. The electrolyte resistance was simultaneously monitored with the local current. As such, the current variation gives information about the surface reactivity whereas the electrolyte resistance gives information about the topography. The size of the scanned area was 500 x 500 µm 2 at a scan rate of 10 µm/s. This value was selected as a tradeoff value between the experiment duration (about 40 min. per map) and minimizing the risk of crashing the tip into the working electrode surface. All the experiments were performed at least 2 or 3 times at different locations above the sample.

XPS analysis
X-ray photoelectron spectrometry (XPS, Kratos Axis Ultra DLD) was used to investigate the effect of plastic strain on the passive layer of 304L stainless steel. Specific samples were prepared from 20 mm width and 0.5 mm thick strips. An unstrained reference sample and a sample strained at 20% plastic deformation were investigated using angular analysis at 0°, 30° and 60°. High-resolution spectra were obtained for the Fe, Cr, O and C elements. From the relative intensities of the Cr and Fe oxides and their corresponding metallic peaks, the thickness of the passive layer was evaluated by considering a bilayer, with a Cr 2 O 3 inner layer and a Fe 2 O 3 outer layer. Physical data were obtained from the literature [

3D Optical profiling for surface topography visualization
Plastic deformation creates dislocations and dislocation pile-ups. To evaluate the effect of stress on the surface roughness, a light profiler (Veeco/Wyko NT1100) was used. The technique is based on using light interferometry to obtain high-resolution 3D surface images.

Effect of tensile stress on the potential of 304 stainless steel.
SKP was used to find the effect of stress on the electrochemical potential of steel at the center of the specimen including the gauge section and the surrounding area of the tensile sample as shown in Figure 2.a. The initial potential distribution across the gauge section was constant at ca. 370 ±5 mV vs. SHE. Figure 3 shows the distribution of the potential after application of a 350 MPa tensile load, corresponding to approximately 3% of deformation in the gauge section. As shown in the stress-strain dependence graph (Figure 2.b), this load corresponds to the beginning of the plastic deformation range. The stress decreases the potential at all measured areas (corresponding to a length of 30 mm). However, at the center of the gauge section, the potential was more negative than at the other measurement locations, which is in good agreement with the stress distribution model depicted in Figure 2.c. When the sample was exposed to the load for 5 h, the potential at all points along the sample increased by 20 mV (Figure 3.b). However, removing the load and performing the potential measurement at the rest does not change the potential profile, as shown in Figure 3.c. Loading to yield sets the elastic stress, whereas unloading removes the elastic stress. Because unloading does not change the electrochemical potential, it is assumed that the elastic stress has no influence on the thermodynamics of the electrode. The measurement was repeated 24 h after unloading, and the potential above all the surfaces increased by 10-20 mV (Figure 3.d).
The stress and stain distributions in the sample were modeled using the same deformation of 3%. The plastic strain profile is shown in Figure 3.a. The plastic strain was mainly concentrated at the center of the sample in the gauge section but also spread out from this area. At this elongation, plastic strain existed up to 10 mm from the center of the sample with a steep gradient moving outwards from the gauge section.
A higher load (2.7 kN -540 MPa) was applied to a tensile sample, reaching a deformation of 20% in the gauge section, and the potential distribution presented in Figure 4 shows that the length of the stress-affected area was approximately 35 mm. A similar length was determined using the model presented in Figure 2.c. The initial potential profile was constant at 0.38 V vs. SHE, and due to the strain, the central area and the edges of the affected zone were characterized by lower potentials of 0.2 -0.16 V vs. SHE. Thus, the plastic deformation decreased the potential of AISI 304 steel by about 180-220 mV, which is in agreement with the effect reported for 301LN stainless steel [25,29].
The profile presented in Figure 4.b was measured after unloading and exposure to ambient air. The potential in the gauge section increased to 0.32 V vs. SHE and corresponded to the presence of residual stress, which can be ascribed to the dislocation field. Thus, it is possible to conclude that plastic deformation initially decreased the potential and then subsequently increased the potential. It can be assumed that the decrease in the potential was linked to the   274  275  276   277  278  279  280  281  282  283  284  285  286   287  288  289  290  291  292  293  294   295  296  297  298  299  300  301  302  303  304  305  306  307  308   309  310  311  312  313 emergence of new strain-induced surfaces (dislocations) [25,29]. The increase in potential was related to the passivation of the newly formed metallic surfaces.
The impact of the tensile stress on the potential in the center of the gauge section was measured as a function of the load (Figures 5 and 6). At 220 MPa, corresponding to 0.33 % of strain, in the elastic domain, no effect of the load on the potential was observed. However, further increasing the load in the plastic deformation domain proportionally decreased the in situ potential of the surface measured under loading (Figures 5.a, c and Figure 6.a). The potential in Figures 5.a, c varies over a range from 7 to 10 mV, which is close to the noise level. Thus, the potential was relatively uniformly distributed without forming any ordered structures (taking into account a spatial resolution limit of 70-100 m for the SKP instrument). However, the plastic stress creates dislocations that increase the surface roughness Figures 5.b, d ( Table 2).
The graph presented in Figure 6.a shows that at lower loads, the potential decreased proportionally with the strain, and for larger elongations, the potential approaches a limit value of approximately 0.14 V vs. SHE. The graph contains the data for the maximal and minimal potentials measured in each map. After the profile measurements ( Figure 5), the SKP tip was positioned above the surface to monitor possible changes in the potential (Figure 6.b). The evolution of the potential over 1000 s under a constant applied load shows that the potential slowly increased during exposure in air, which can be ascribed to the passivation of the newly formed surface.
The passivation process in air at 60% RH for a pre-strained sample was monitored for a longer period of time (i.e., 19 h, as shown in Figure 7). The potential variations were modeled using a regression function, = A log (t) + const, which can mimic equation 1 related timedependent growth of the oxide film. In Figure 7, regression line 2 shows that the experimental dependence can be described by a logarithmic function. Coefficient A relates to the rate of passivation, and R is a regression coefficient (Table 3). A small amount of strain shows a low rate of passivation that then increases for higher loads. The probe is localized above a surface containing unaffected passive areas and the newly formed low-potential surfaces of the emerged dislocations. At low loads, the potential is averaged over the active and passive locations. At high loads, all the surfaces in the gauge section are relatively active. In this case, the probe monitors the passivation kinetics, which is nearly logarithmic (Table 3). Thus, the coefficient A depends on the ratio of active to passive surfaces, whereas the rate of passivation for a single dislocation must be determined using high spatial resolution electrochemical measurements.
To evaluate the effect of the elastic stress, the SKP maps were measured for the pre-strained gauge section (20% straining) under loading and load-free conditions (Figure 8.a, b). Unloading does not change the potential. This is in agreement with results obtained for low strains (Figure 3.b, c). Thus, it can be assumed that the potential of the strained surface is mainly determined by the density of the emerging dislocations and by their passivation due to coming into contact with air.

322
After 2400 s, the sample was unloaded, and the potential was monitored at the same location 323 (unloaded, 2 nd cycle). Figure 9.a shows that the curves of the first and the second cycles are 324 parallel, indicating similar rates of steel passivation under the elastic stress and at rest. Figure   325 326 327 328 329 9.b compares the passivation curves obtained from two different samples after straining up to 20% in the gauge section. In one sample, the potential was monitored under loading, and in the second sample, the potential was monitored at rest. Both curves approach the potential measured before the stress was applied (Figure 9.b, curve 1). The experimental results show similar rates of passivation without significant influence from the applied elastic stress.

330
The surface of AISI 304 was pre-ground by using 4000 grit emery paper and then rinsed in 331 ethanol. The potential above the steel was monitored 5 min after the surface treatment. Figure   332 10 compares the transient potentials after grinding and after straining at 15%. For the 333 prestrained surface, potential monitoring was carried out under load-free conditions. Grinding   340 The effect of tensile stress on the OCP was measured by a saturated calomel reference  strained at 20% of plastic strain, and the passivation was monitored, as illustrated in Figure   350 11.b. The resulting passivation curve in the electrolyte could be fit with a logarithmic 351 regression with a coefficient A equal to 0.03 (Table 3). The passivation curve in an aqueous 352 electrolyte was also measured after grinding a steel electrode, which gave a similar   359 The effect of the tensile strain on the local electrochemical reactivity was studied in the same   Figure 14.a shows the potential profile after applying 20% elongation using a 376 tensile machine followed by exposure to dry air for two weeks. We assumed that after this  The gauge section (right side) showed a potential that was 30-40 mV lower than that of the 381 reference side.

382
The same sample was slightly ground using 4000 grit emery paper, rinsed and then exposed 383 to dry air for 2 weeks. The potential map presented in Figure 14.b shows that the substrate 384 containing residual stress (dislocations) was covered by an oxide film with a low potential.
Thus, a surface containing dislocations is able to re-passivate, but the oxide film shows  (Table 2). The distribution of the local strain in the gauge section corresponds to the 458 distribution of the potential (Figures 3 and 4). Thus, we can assume that a potential decrease 459 corresponds to the dislocation density and depends on the applied stress. Figure 6.a shows 460 that for a low strain, the potential proportionally decreases, and at higher elongations, the 461 potential approaches the minimum steady-state value. This is in agreement with the effect of 462 the strain on the dislocation density [37]. Thus, at low strains, the density of dislocations 463 proportionally increases, but at high strain, the density is stable due to the interaction and  Table 3). Deviation of the potential (Figure 7) from the basic 475 logarithmic dependence was mainly observed at the beginning of the passivation process.

476
Exposure to air of the pre-ground surface also increased the potential due to passivation 477 ( Figure 10). It is possible that the passivation kinetics can be described by the high-field Mot- where V is the molar volume of the metal, z is the valence of the metal ion, and F is the 501 Faraday constant. Thus, an increase in the external pressure ( > 0) accelerates the metal dissolution rate. However, thermodynamic analysis [40] showed that the change in free energy due to elastic deformation is insufficient to significantly alter the active dissolution 504 rate. Moreover, SKP measurements of the effect of the elastic tensile stress do not show a 505 shift in the electrochemical potential of the AISI 304 electrode (Figures 3 and 8). However, it 506 is worth to note that the effect was studied for well passivating alloy and in passive 507 conditions. Perhaps this point cannot be directly applicable to systems corroding under 508 activation control.

509
For the stress corrosion cracking mechanism, the important question concerns the impact of 510 the elastic stress on the kinetics of passivation and the general reactivity of the strained 511 surface. Figure 9 shows that the kinetics of oxide growth either under loading or at rest are 512 similar. However, the properties of the oxide films formed under different conditions, such as 513 film homogeneity and composition, were not determined in particular work.      Captions of the Figures: Figure 1: Microstructure of the AISI 304L alloy.        a-curves measured under the loaded and unloaded conditions with linear regression lines. b-Potential monitoring of two different samples. Curve 1 was measured before loading, curves 2 and 3 were measured after application of the plastic strain. The measurement was carried out under loading (curve 3) and at rest (curve 2). Figure 10: Monitoring of the electrochemical potential for an unloaded tensile sample after straining (15% elongation) and for a sample after grinding (4000 grit emery paper). Figure 11: Influence of the tensile strain on the OCP monitored after loading in an aqueous 0.5 M Na 2 SO 4 electrolyte. a-two cycles of 10% of plastic strain (ε p ); b-plastic strain of 20%.