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Vol. 8. Issue 1.
Pages 1335-1346 (January - March 2019)
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Vol. 8. Issue 1.
Pages 1335-1346 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.10.004
Open Access
Influence of the cold working induced martensite on the electrochemical behavior of AISI 304 stainless steel surfaces
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Gleidys Monrrabala, Asuncion Bautistaa,
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mbautist@ing.uc3m.es

Corresponding author.
, Susana Guzmana, Cristina Gutierrezb, Francisco Velascoa
a Universidad Carlos III de Madrid, Materials Science and Engineering Department – IAAB, Avda. Universidad 30, 28911 Leganés, Madrid, Spain
b IMDEA Materiales, Tecnogetafe, C/ Eric Kandel 2, 28906 Getafe, Madrid, Spain
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Tables (4)
Table 1. Residual stresses determined in the microstructure of the studied stainless steels with different cold working levels.
Table 2. Microhardness results for the studied AISI 304 stainless steel.
Table 3. Parameters obtained from the fitting of the EIS spectra using an equivalent circuit with 2 time-constants in cascade.
Table 4. Results about the pit frequency and morphology obtained from the characterization of all the pits caused the anodic polarization in a medium with chlorides.
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Abstract

It is clear that the corrosion resistance of carbon steels decreases as cold working amount increases, but for austenitic stainless steels, the relation between cold-working and corrosion performance is not clear. The electrochemical behavior of AISI 304 stainless steel with 3 different cold working amounts is characterized by Mott–Schottky analysis, OCP records, EIS and cyclic polarization curves. An innovative cell with gel electrolyte is used for an easy study of the deformed surfaces without modifying them. After the polarization tests, the influence of the deformation on the amount of pits and on their morphological characteristics is also analyzed. The microstructural changes caused by cold rolling are studied, and the residual stresses are determined by XRD using the sin2ψ method. It is proved that AISI 304 stainless steel decreases its pitting resistance in a medium with chlorides when it is subjected to moderate cold rolling, but heavy thickness reduction causes a subsequent recovery of corrosion resistance. The results obtained suggest that this trend is related to changes in the magnitude and type of the stresses (tensile or compressive) on the surface of the material.

Keywords:
Austenitic stainless steel
Cold working
Pitting
Corrosion
Stress
Martensite
Full Text
1Introduction

Pitting corrosion is one of the most difficult forms of corrosion to be reliably managed [1,2] and one that most often limits the in-service performance of stainless steel components. It is well known that the susceptibility of stainless steels to localized corrosion is clearly influenced by the nature and concentration of alloying elements and impurities [3]. However, the manufacturing process can also influence the stability of the passive layer of a stainless steel component in a given aggressive environment and affects the morphology of the attack [4,5].

Cold working is often used to give stainless steel components their final, desired shape and/or to increase their mechanical properties. The process can sometimes cause unexpected changes in the electrochemical behavior of the material. Surface treatments such as machining and grinding also generate extremely cold deformed layers near the surface that clearly affect the corrosion behavior of the stainless steel components [6].

The detrimental effect of cold working on the corrosion resistance of carbon steels is well known [7]. The same behavior should not be assumed for stainless steels. It is true that some published data seem to verify the suggested trend, but not always. A careful of the published bibliography about the effect of cold forming processes on stainless steel offers varied and apparently contradictory information. Four different trends can be found:

  • A)

    Moderate cold working has been reported as being beneficial for corrosion resistance, becoming often detrimental when the amount of cold working further increases. Improvements in the pitting resistance of Ni-free, high-N stainless steels were found for deformations up to 20% [8]. It has also been published that N-bearing AISI 316L stainless steels in neutral chloride medium increase the length of their passive region in polarization curves when they are cold worked up to reductions of 20%, but further cold working causes a decrease in their pitting resistance [9]. The maximum pitting corrosion resistance has been found at 23% cold deformation for Nb bearing austenitic stainless steel in a 3.5% NaCl solution [10].

  • B)

    A detrimental effect of cold working has also been reported. For example, Kurc et al. have reported, for AISI 304 austenitic stainless steels cold rolled within a range from 10% to 70%, that the corrosion performance in 3.5% NaCl decreases as the deformation of the materials increases [11]. Barbucci et al., studying AISI 304 stainless steel with reductions up to 58% by polarization curves [12], have found that pitting susceptibility increases with cold working in chloride solution tests. It has also been reported, for 316L stainless steel in chloride medium using potentiostatic methods, that the total area of pits and the corrosion current increased monotonously with increasing deformations up to 30% [13]. Luo et al. have also found a detrimental effect of cold working up to 90% on the pitting behavior of duplex stainless steels using polarization curves. They have found that the passive layer becomes less protective due to cold working [14]. Metastable pitting of 304 stainless steel in chloride contaminated concrete pore solutions is favored with strains up to 40% [15].

  • C)

    A decrease in the corrosion resistance for moderate cold working, and then, an increase for higher cold working amounts has been published by other authors. Studies about austenitic stainless steels used in biomedical applications in a medium that simulated body fluids have found that cold rolling up to 50% decreases the corrosion resistance, while more severe cold deformation (70%) results in improved localized corrosion resistance [16]. A minimum value for pitting potential (Epit) at 50% cold reduction has also been reported for 304L stainless steel after polarization studies in NaCl solution [17]. However, another work from the same research group [18] suggests a minimum value for this parameter at 70% cold rolling for 304L and 316L steels, and a further increase for 90% cold rolling. Minimum Epit values have been obtained for a deformation of 45% by Barbucci et al. when studying AISI 301 in sulfuric acid in the presence of chlorides [19]. For AISI 304 and 204 grades, a maximum pit initiation frequency has been found at a 20% reduction. Another work points out that the pitting attack can be worse for 304L when deformed between 10% and 25% [20].

  • D)

    A few recent studies have found no influence of cold deformation (up to 50%) on the pitting corrosion behavior of either high-N, Ni-free stainless steels in 0.9% saline solution [21] or 304L stainless steel in contaminated sulfuric acid environment [22]. For newly developed high interstitial FeCrMnMoCN austenitic stainless steels, neither localized nor general corrosion seems to be severely affected by cold rolling [23]. In addition, during the polarization study of two Ni-free stainless steels in 3.5% NaCl, no influence of cold forming has been found for the N-richest alloys [24].

The extremely large amount of cold worked austenitic stainless steel components exposed in service to corrosive environments makes it of undeniable interest to clarify and better understand the effects that their processing can have on their durability. Though some of the apparent disagreement can perhaps be understood, bearing in mind that the results have been obtained using different experimental methods, the large number of contradictions existing in the literature highlights the interest in delving more deeply into a subject of such a clear practical relevance.

Deformation affects different metallurgical variables that are often assumed to be able to modify the corrosion resistance of austenitic stainless steels. During the cold working process, the grains are deformed [14], dislocations are created and slipped [25], and needle like structures [26] and bands are formed. It is often assumed that these microstructural changes can affect the electrochemical behavior of stainless steel in aggressive media [18,27].

Moreover, in austenitic [28] or duplex [14,29,30] stainless steels, strain induced martensite can be formed. The presence of martensite is assumed to decrease the corrosion resistance of austenitic stainless steels [20,31,32]. However, there are results suggesting that strain induced martensite cannot be blamed for changes in Epit of cold worked 304 in 0.1 NaCl and that this change with cold working must be related to the effect of stresses, crystallographic texture or a combination thereof [18].

The specific microstructural changes that take place in a stainless steel due to cold working depend on its alloying composition [31], as well as the conditions of the cold working procedure [33]. For example, similar amounts of cold working produce a different volume fraction of strain induced martensite in the same stainless steel when the processing temperature changes [18,31]. For this reason, the authors believe that the traditional and extensively used approach of linking the corrosion behavior to the percentage of reduction suffered by the material could be slightly misguiding. In this study, the electrochemical behavior of the steels is related to a microstructural change that takes place during the forming process (martensite formation).

This work considers the corrosion behavior of AISI 304 stainless steel, which is a very common grade, less alloyed than other often used austenitic grades. The 304 steel has a low stable microstructure [33], and when it undergoes cold forming, it easily forms martensite [18,26]. This fact makes the evolution of its corrosion behavior with the deformation especially complex.

Furthermore, it has been suggested that the surface of the metal that undergoes substantial cold working can have a corrosion resistance different from that of the bulk material [34]. As the surface of cold worked specimens cannot always be completely flat after the deformation processes carried out in the laboratory, the use of innovative gel electrolytes [35] is of special interest for this study. The main advantage of gel electrolytes consists in the fact that they allow the electrochemical study of irregular, complex surfaces easily and without risk of crevice interferences. The gel electrolyte used in our research is a modification of others initially proposed for corrosion monitoring in studies of metallic heritage conservation [36,37].

2Experimental

The studies were carried out using commercial AISI 304 (EN 1.4301) sheets with 2mm initial thickness. The composition (w/w) of the stainless steel was: 18.66% Cr, 8.06% Ni, 1.82% Mn, 0.30% Si, 0.24% Cu, 1.9% Mo, 0.005% S, 0.048% N, 0.043% C and Fe for balance. Cold working was performed at room temperature by a multi-pass unidirectional cold rolling machine with a thickness reduction of approximately 1.5% at each pass. The specimens were air-cooled between each rolling step.

Stainless steels with three different cold working amounts were considered for the study: the as-received material, and stainless steel specimens that were subjected to a 35% and a 65% thickness reduction during cold rolling. The amount of martensite in each stainless steel was determined through the magnetic induction method, as it was previously done [28]. It is well known that plastic deformation of metastable, paramagnetic austenite leads to its partial transformation into ferromagnetic martensite. The amount of martensite determined for each material was: 0.6% in the as-received stainless steel; 16% in the steel with 35% cold reduction and 44% in the steel with 65% cold reduction. As it will be proven in the paper, the amount of martensite is a key factor for determining the pitting resistance. It is known that the relationship between the percentage of thickness reduction and the amount of induced martensite depends on the specific conditions of the used cold working process. Hence, from this point forward, the studied materials are labeled using their martensite content, as 0.6% M, 16% M and 44% M, instead of using the traditional thickness reduction.

The residual stresses were determined by X-ray diffraction (XRD) using the sin2ψ method. A Panalytical Empirean diffractometer was employed, using Cr Kα radiation with a wavelength 2.2897Å from an X-ray tube operated at 30kV and 55mA. The measurements were carried out in the Bragg–Brentano parafocusing geometry, with a divergence slit of 1° in the incident beam and a vanadium β-filter in the diffracted beam. In order to minimize peak positioning errors, diffraction peaks with 2θ>100° were selected in each case, belonging either to the (022) peak of austenite or the (002) peak of martensite, depending on the rolling condition. Peak shift was determined for positive ψ tilts with sin2ψ ranging from 0 to 0.9 at intervals of 0.1 and 4 different φ (0°, 45°, 90° and 135°), being φ=0° for the rolling direction. In Fig. 1, the angles ψ and φ are defined. A biaxial plane-stress state was assumed in the surface of the rolled sheets and the longitudinal and transverse residual stresses were determined from:

being E/1+ν=162GPa and m the slope in the graph dφ,ψ=f(sin2ψ). Peak positions were corrected for absorption, Lorenz polarization effects and Kα2 peak splitting, using the PANalytical software package stress plus 2.2, and assuming a linear absorption coefficient of 925cm−1.

Fig. 1.

Definition of angles in stress analysis measurements carried out during XRD.

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Metallographically polished and etched sections of the stainless steels were observed by optical microscopy in order to obtain information about the microstructural changes caused by the cold working process. The metallographic specimens were etched with Mi17Fe reagent, which was composed of 10ml HNO3, 100ml HCl and 90ml distilled water, heated to a temperature of approximately 40°C.

The local mechanical properties of the specimens were determined through microhardness measurements. A 500gf load was applied for 10s. Approximately 40–60 microindentations were carried out to characterize each studied cold-rolled steel and to check the uniformity of the deformed specimens.

The electrochemical measurements were carried out using an innovative gel cell designed to test surfaces with complex geometry [35,38]. The (w/w) composition of the gel electrolyte used to carry out the electrochemical studies was 40% glycerol and 0.5% agar. These reagents were dissolved in distilled water and heated to 90°C under magnetic stirring. The mixture was stirred continuously for 10min, and then it was allowed to cure in a controlled environment where the temperature and relative humidity conditions were kept at 20°C and 80%, respectively. More information about the gel manufacturing process can be found in a previous publication [35]. This electrolyte wetted 2cm2 of the surface of the working electrode studied. A stainless steel coiled wire was used as counter electrode and a saturated calomel electrode (SCE) as reference electrode. Fig. 2 shows the set-up used for the electrochemical measurements.

Fig. 2.

Image of the portable cell with gel electrolyte used for the electrochemical measurements.

(0.18MB).

For Mott–Schottky measurements, 1% KClO4 was added to the gel to increase its conductivity. The use of corrosive salts is not advisable for these measurements, as pitting occurrence could interfere in the results. For electrochemical impedance spectroscopy (EIS) and polarization curves, 0.5% NaCl was added to optimize the conductivity of the gel electrolyte, but also to increase its corrosivity and allow to obtain information about the influence of cold working on pitting resistance. The use of gels with different salts can cause small changes in the characteristics of the passive layers [38], but the salts were selected because of their suitability for carrying out the different tests. The aim of the work is to compare and understand the electrochemical behavior of stainless steels with different amounts of strain induced martensite, and it can be achieved using different media.

The electronic properties of passive film were determined employing Mott–Schottky approach. Mott–Schottky plots were obtained sweeping the potential (E) from 0.6V vs. SCE to −1.3V vs. SCE, at a frequency of 1kHz, using a 10mVrmsac signal. A scan rate of 1mVs−1 with steps of 20mV in the cathodic direction was set. Up to 6 measurements were carried out to characterize semiconductivity of the passive layer on each studied stainless steel.

Open circuit potential (OCP) was monitored in gel with chlorides for 20min before EIS measurements. EIS spectra were acquired at OCP, over a frequency range between 10kHz to 10mHz, with 5 points per decade and using 10mVrms as the applied sinusoidal perturbation. The impedance spectra were fitted using electrical equivalent circuits with Zview software. Each experiment was performed at least 3 times to check reproducibility.

Cyclic anodic polarization curves were performed using a sweep rate of 1.2mVs−1. Their scan direction was reversed when the current density reached 10−4Acm−2. The destructive polarization tests were carried out just after the non-destructive EIS measurements.

After the polarization tests in gel with chlorides, the formed pits were identified and their morphological characteristics were studied using an optoelectronic microscope.

3Results and discussion

Fig. 3 plots the XRD patterns corresponding to the three rolling conditions. 0.6% M corresponds essentially to an austenitic steel. A peak corresponding to the martensitic phase (α′ martensite, BCC phase) is already detectable for the 0.6% M steel. However, it is clear from the intensity of the diffracted beams that the rolling process induces the transformation of austenite to martensite. For the 44% M steel, a single peak corresponding to austenite is detectable in XRD pattern. These results confirm the intense microstructural changes that the stainless steel sheet undergoes when its thickness is reduced from 35% to 65%, using the deformation procedure chosen for this work. These changes have already been suggested by the magnetic characterization of the material, which reveals that, in this case, the martensite amount increases from 16% to 44%. This fact is coherent with results published by other authors showing that the stacking-fault energy of 304L stainless steel decreases with increasing deformation [22].

Fig. 3.

XRD patterns of the studied stainless steels with different cold working levels.

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From XRD results and changing the ψ angle (Fig. 1), a shift of the peaks can be observed and information regarding the stresses in the different metallurgical phases can be obtained. The intensity of the austenitic (022) peak at 2θ=128.03° is sufficient to apply the sin2ψ method for the steel sheets with 0.6% M and 16% M. However, the reduction in the intensity of the austenitic peaks in 44% M is so large that it impedes the determination of residual stresses. For 44% M steel, stress measurements have been carried out from the (002) peak of martensite, 2θ=106.12° instead. The intensity of the martensite peaks for 0.6% M and 16% M does not allow the calculi of stresses in that phase. Table 1 shows the obtained values for the two components of residual stresses, σx and σy (Fig. 1). σx and σy can be identified with rolling and transverse directions of the cold worked stainless steels, respectively. The residual stresses were slightly compressive in both directions for the 0.6% M stainless steel. The cold forming process determines the sliding of the adjacent atoms – a fraction of the interatomic distance – and the shape of the grains and inner stresses of the material are modified. Initially, cold working tends to counterbalance the compressive stresses existing in the sheet. For the 16% M material, whose thickness has been reduced a 35%, the stresses have already become tensile, especially in the rolling direction. In the 44% M stainless steel, the stresses in the martensite are highly compressive. Martensite formation from metastable austenite implies an increase in volume [39]. Previous literature has estimated that the martensitic transformation can cause a volume increase ranging from 2% to 5%, strongly dependent on the C content of the material [39]. This transformation, which is very intense between 16% M and 44% M, explains the changes in the stresses, passing from slightly tensile to compressive. However, when 0.6% M is deformed to 16% M, the effect of the martensite formation, which should increase the compressive stresses, is more limited and becomes masked by the stresses generated by the rolling process in the microstructure.

Table 1.

Residual stresses determined in the microstructure of the studied stainless steels with different cold working levels.

Stainless steel  Measured peak  Measured stresses
(MPa)
    Rolling direction  Transverse direction 
0.6% M  Austenite  −206±26  −241±29 
16% M  Austenite  +283±22  +5±22 
44% M  Martensite  −992±31  −1387±31 

Images of the changes in the microstructure that cold rolling has caused in the studied material can be seen in Fig. 4. Equiaxial austenite (γ) grains can be seen in 0.6% M steel (Fig. 4a). The small amounts of martensite existing in the 0.6% M steel (Fig. 3) make the presence of this phase barely detectable by metallographic observations. The study of steels after cold rolling (16% M and 44% M) shows a structure of elongated austenite (Fig. 4b and c). Moreover, as the thickness reduction by cold rolling increases, α′-martensite becomes more detectable in the microstructure. The precipitation of this phase causes a decrease in the grain size of the steel, confirming the observations reported by other authors [40]. In Fig. 4c, the microstructure is very deformed, with grains very elongated in the rolling direction, and a high presence of martensite.

Fig. 4.

Microstructures corresponding to the longitudinal sections of the studied stainless steels with different cold working levels: (a) 0.6% M; (b) 16% M; (c) 44% M.

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The numerous microhardness measurements carried out on the surface of the specimens (Table 2) reveal that the hardness increases as the reduction in thickness does. The increase of the hardness is higher from 0.6% M steel to 16% M steel than from 16% M to 44% M steel. This fast increase in hardness for small deformation amounts has been found previously for other austenitic stainless steels [41]. As martensite is harder than austenite, it could be logical that the precipitation of this new phase could be one of the key factors justifying the evolution of hardness. However, an important contribution of other factors cannot be discarded. A reduction in grain size is usually assumed to be a factor that increases the hardness of the materials. In the case under study, it is obvious that the grains become highly deformed (Fig. 4) and their size seems to be reduced. As the deformation of the steel increases, the number of grain boundaries that the microindentor should cross is clearly much higher. So, the contribution of the grain deformation to hardening cannot be overlooked. On the other hand, stresses are usually considered to be a factor in hardening, but in this case, the obtained data (Table 1) does not reveal any clear correlation between the evolution of stresses and the evolution of hardening (Table 2).

Table 2.

Microhardness results for the studied AISI 304 stainless steel.

Stainless steel  Microhardness
(HV) 
0.6% M  195±
16% M  381±17 
44% M  448±16 

Mott–Schottky analysis allows to obtain information about the electronic structure of the passive layer and its stoichiometry. Mott–Schottky relationship expresses the E dependence of the capacitance of the space charge layer of a semiconductor electrode (C) under a depletion condition [42]. The results of Mott–Schottky analysis have been obtained from plots as those shown in Fig. 5. Using this type of plots, donor (Nd) and acceptor (Na) densities in the semiconductive passive layers can be calculated from regions where a linear relationship between C−2 and E exists [43]. Nd can be obtained from linear regions of the curves with positive slope using the following equation:

where e is the electron charge (−1.602×10−19C), ɛ is the dielectric constant of the passive film, usually taken as 15.6 for oxides formed on stainless steel [44,45], ɛ0 is the vacuum permittivity (8.854×10−14F/cm), k is the Boltzmann constant (1.38×10−23J/K), T is the absolute temperature and EFB is the flat band potential. On the other hand, Na can be calculated from the linear regions with negative slope using the following equation:

Fig. 5.

Examples of Mott–Schottky plots obtained for the stainless steels with different levels of cold working.

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Low values of Nd and Na increase the corrosion resistance of passive films [4,46]. The point defect model (PDM) developed by MacDonald [47] identifies the oxygen and cation vacancies in the passive film with Nd and Na, respectively. The cation vacancies prevent further growth of the passive film, while oxygen vacancies favor the absorption of depassivating ions as chlorides. Na values are considered especially determining for the corrosion behavior [48].

The stainless steel passive layers are usually formed by chromium and iron oxides [49], though hydroxides are also often detected [42]. Chromium oxide is assumed to be a p-type semiconductor while iron oxides have n-type semiconductivity [4,50]. Hence, Na and Nd values can be related to the carrier densities of chromium oxide and iron oxides, respectively. Moreover, a complex structure can be figured out in the high-potential region of the plot, where two positive slopes are clearly defined. Two different donor densities have been calculated (Nd1 and Nd2), taking into account these two slopes. The double slope in the n-region has been already found in the characterization of other stainless steels [51]. The origin of these two slopes has been sometimes attributed to Fe2+ cations in excess in the structure of the oxide that could be located in tetrahedral or octahedral sites [52]. However, other authors have reported results that seem to prove that hydroxides have n-type semiconductivity [53]. Hence, a relationship between the hydroxides often admitted to be comprised in the outer layer of the passive film of stainless steels and this contribution cannot be discarded.

In Fig. 6, the carrier densities obtained from Mott–Schottky analysis for the studied materials are plotted. Mott–Schottky analysis has been previously used to study the influence of the forming process on the protective properties of the passive layers on stainless steel [4,14]. It has been reported that Na and Nd rise with increasing deformation [4,15], but some other authors have found a reduction of carrier densities when working with very deformed 304 [54]. A comparison of results for 0.6% M and 16% M (Fig. 6) can be considered coherent with the first observation about an increase in charge carrier densities of the passive layer with the cold working of the steel. For very high amounts of cold working (44% M), the results of Mott–Schottky analysis show that the carrier densities are lower than for intermediate cold working (16% M), also confirming previous reports [54]. It is true that the results obtained by this technique offer a meaningful dispersion, but the high multiplicity of the tests allows to conclude that cold working initially causes more conductive (less stoichiometric) passive layers. At the same time, very severe, further cold working of AISI 304 stainless steel causes a decrease in the carrier density, that is, a foreseen recovery of the protective properties of the passive layer.

Fig. 6.

Density and type of charge carriers obtained in the passive layers of stainless steels. Results obtained from the analysis of Mott–Schoktty approaches.

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OCP is the result from the electrochemical equilibrium reached between the tested metal and the medium. It has been proved that the gel electrolyte favors rapid equilibrium (OCP stabilization) for passive austenitic stainless steels, compared to traditional liquid electrolytes [35]. The measurements carried out with gels containing 0.5% NaCl on 0.6% M steel clearly confirm this fact (Fig. 7). Stable OCP values are immediately detected. For the 16% M and 44% M steels, the stability of OCP is slightly more difficult to achieve. OCP shows moderately increasing values during the first minutes of exposure, though the potentials always remain within a range characteristic of stainless steel in a passive state. The somewhat different OCP stabilization trend shown by the rolled stainless steels could perhaps be related to the presence of a meaningful amount of martensite in their microstructure. Moreover, a certain influence of grain deformation (Fig. 4) in this fact cannot be completely discarded.

Fig. 7.

Influence of cold working on the stability of the OCP of the stainless steels during the onset of their exposure in gel media with chlorides.

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At any rate, the most remarkable difference observed among the curves in Fig. 7 is the transients corresponding to metastable pitting. These transients can be easily seen for 16% M steel, especially during the first minutes of exposure, but they never appear for the two other studied stainless steels. Previous literature has also found the highest number of metastable pits for intermediate cold rolling reductions [55]. The transients show a steep fall – corresponding to the breaking of the passive layer – and then, a progressive recovery – that can be identified with the repassivation process. This type of transients for stainless steels is sometimes related to the presence of S inclusions [1], where pits tend to nucleate. However, in this case, the S content of the studied stainless steel is low, and no S-precipitate has been found during the metallurgical study carried out (Fig. 4). A possible explanation can be found in the results from stress analysis performed on the materials. The tensile stresses in the 16% M steel (Table 1) can favor the breaking of the passive layer. The development of metastable pitting has been already related to the presence of tensile stresses in deformed specimens [15]. On the other hand, the compressive stresses detected on the other studied steels could even exert a certain protective effect for the integrity of the passive layer. Manufacturing processes such as shot-peening, which generates compressive stresses on the treated surface, exert a positive effect on the corrosion resistance [48]. Moreover, the tendency to metastable pitting of 16% M in the onset of the exposure and the stable passivity of the 0.6% M and 44% M steels are also coherent with the conclusions drawn for Mott–Schottky analysis about the influence of the cold working on the electronic structure of the passive film (Fig. 6). Results as those shown in Fig. 7 can be in line with the reported fact that the frequency of nucleation of metastable pits in S31803 duplex stainless steel increases up to deformations of 70% but then decreases [14].

On the other hand, the least stable passivity of 16% M steel (Fig. 7) is coherent with the highest carrier density detected using Mott–Schottky analysis (Fig. 6) for the passive layer of this steel. Thus, tensile stresses seem to promote less stoichiometric passive layers in stainless steels that can lead to foreseeably worse corrosion behavior in aggressive environments.

EIS studies provide complete information about the electrochemical behavior of the stainless steels. Measurements carried out using gel electrolyte cells with chlorides have allowed to obtain spectra such as those shown in Fig. 8a. To achieve adequate fitting of the spectra, it has been necessary to use a two time-constant equivalent circuit (Fig. 8b). As it has been previously done to simulate the electrochemical behavior of passive stainless steels, the two time-constants have been placed in cascade [22,56]. The resistive behavior appearing at high frequencies is identified with the resistance of the electrolyte (Re). At medium frequencies, a time constant is defined, though it appears quite overlapped with the low-frequency time constant in the spectra. The medium-frequency time constant comprises a resistance (R1) and a constant phase element (CPE1) in parallel. It can be assumed that this time constant is related to the electrochemical processes on the surface of the passive layer [56]. The low-frequency time-constant also comprises a resistance (R2) and a constant phase element (CPE2) in parallel. This low-frequency time constant has been previously related to the charge transfer process taking place at the metal-passive layer interface [56]. In the equivalent circuit, CPE are used to simulate a non-ideal capacitive behavior, where the n parameter depends on the dispersion of the capacitance with the frequency.

Fig. 8.

(a) Examples of the EIS spectra obtained for the stainless steels with different cold working levels in a gel electrolyte with chlorides; (b) equivalent circuit used to simulate de experimental EIS spectra.

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The mean values obtained for the electrochemical parameters after fitting the EIS spectra are shown in Table 3. It can be seen that the R2 value, which is clearly related to the control of the corrosion rate of the stainless steel in passive state, decreases approximately one order of magnitude from 0.6% M to 16% M. This decrease of the low frequency resistance with cold rolling has also been reported by other authors [14,25]. However, a partial recovery of R2 is detected with additional rolling. The trend observed for R2 is clearly related to that observed with other techniques (Figs. 6 and 7), being this recovery of R2 also reported previously for high strains [16].

Table 3.

Parameters obtained from the fitting of the EIS spectra using an equivalent circuit with 2 time-constants in cascade.

Stainless steel  Results from the fitting of the EIS spectra
  Re
(kΩcm2
R1
(kΩcm2
CPE1−T
(μFcm−2sn−1
n1  R2
(MΩcm2
CPE2−T
(μFcm−2sn−1
n2 
0.6% M  0.14±0.04  0.10±0.06  13±0.88±0.03  6±18±0.86±0.03 
16% M  0.19±0.02  0.06±0.02  8±0.87±0.06  0.7±0.3  23±0.86±0.02 
44% M  0.16±0.04  0.05±0.09  21±0.83±0.03  2.1±0.6  30±0.92±0.03 

EIS, electrochemical impedance spectroscopy; CPE, constant phase element.

Important information about pitting onset probability can be obtained from polarization curves as those in Fig. 9. These curves – obtained after 30min of exposure to the gel with chlorides – exhibit the typical shape of passive systems, as expected for 304 stainless steel specimens exposed to a media with moderate chloride concentration. All the studied materials pit under high anodic overpotentials and they are able to repassivate when the applied anodic potentials decrease. Metastable pitting has been sometimes observed in the passive region of 16% M steels, while the other studied materials exhibit a more stable behavior in their passive region during the anodic polarization. Relevant parameters obtained from the polarization tests are plotted in Fig. 10. In this figure, it can be observed that 16% M specimens tend to define their corrosion potentials (Ecorr) at higher values than the other steels studied. Moreover, their Epit appears at the lowest values. These last two facts render a shorter passive region (EpitEcorr), presenting 16% M steel the highest probability of suffering localized corrosion among studied materials. Other authors have also found minimum EpitEcorr distances for intermediate amounts of cold rolling in their studies using polarization curves [16,25]. Previous literature presents minimum corrosion resistances for amounts of martensite of 12% [17] or 14% [12], which are close to the amount of martensite of the steel with the lowest corrosion resistance in this work (16%), thought the amount of cold working used in the cited articles to obtain those martensite contents was much higher (50% [17] and 58% [12], instead of 35%).

Fig. 9.

Examples of the polarization curves obtained for the studied stainless steels in a gel electrolyte with chlorides.

(0.12MB).
Fig. 10.

Results related to the pitting probability of the stainless steels obtained from the polarization curves.

(0.22MB).

The metastable pitting, which is a phenomenon that 16% M has proved to be highly susceptible to at its OCP (Fig. 7) and under anodic polarizations, decreases the protective properties of the passive layer [57]. The further nucleation of stable pits is favored on the places where other pits have existed and have been repassivated [15]. A trend to increase the metastable pitting in the passive region during the anodic polarization of cold deformed stainless steels has also be found by other authors that consider higher final thickness reductions [21].

The 44% M steel shows the best pitting resistance among the materials studied. This could be related to the highest amount of protective compressive stresses in this material (Table 1). The increase in the length of the passive region for severely deformed stainless steels has also previously been observed by other authors [16–18]. It is well-known that the presence of compressive stresses in the surface of stainless steel hinders the nucleation of stable pits [58] or the development of other forms of localized attack [59,60]. The decrease in the interatomic spacing due to the stresses at the surface can favor the stability of the passive films [61]. Moreover, compressive stresses modify the electrochemical reactivity of the passive layer [62]. The negative effect of tensile stresses generated during cold working has been previously suggested for 304L stainless steels [18].

The Epass is defined in the polarization curves as the value where the reverse swept cross over the forward swept. The values obtained are not apparently modified by the cold rolling process (Fig. 9), as can also be found in the few published articles that have performed cyclic polarization tests [16,18]. The EpitEpass distance is usually considered as a reliable measurement of the repassivation ability of the pits once they have been formed. The results in Fig. 10 inform that there are not large differences among the repassivation ability of the pits, but they suggest that the pits formed in 44% M steel have the highest difficulties to repassivate. The difficulty to repassivate can be often related to the size of the pits. Bigger pits tend to form more powerful concentration cells between their surface and their bottom, with more marked pH gradients. The shape of the pits can also be a determining parameter for the repassivation, as powerful concentration cells can also be easily found in narrow pits.

The negative influence of the martensite formation in the pitting behavior, so often mentioned in the literature [20,32], is not easy to guess from the results regarding the pitting nucleation. It has been said that the martensitic transformation can cause the rupture of the passive film due to higher density of flaws and generated residual stress [11,32]. However, as the formation of martensite generates stresses that are opposite to the stresses generated by cold working, the global effect of the precipitation of large amounts of martensite during severe cold rolling seems to be beneficial from a corrosion point of view. The possible galvanic negative effect caused by the presence of two distinct phases, austenite and martensite [11,30], is undetectable.

The pits caused by the cyclic polarization tests in the stainless steels were identified after the electrochemical measurements. Their morphological characteristics were evaluated using optoelectronic microscopy. It allows obtaining images of the pits as those shown in Fig. 11, as well as numerous quantitative data about their shape and size. In Table 4, relevant data obtained from the study of the influence of cold rolling on the morphology of the attack are summarized.

Fig. 11.

Images obtained with optoelectronic microscopy from the pits caused by the polarization curves: (a) 0.6% M stainless steel; (b) 16% M stainless steel; (c) 44% stainless steel.

(0.15MB).
Table 4.

Results about the pit frequency and morphology obtained from the characterization of all the pits caused the anodic polarization in a medium with chlorides.

Stainless steel  Number of pits
(cm−2
Average pit parameters
    Mouth surface
(103μm2
Mouth perimeter
(mm) 
Mouth eccentricity  Depth
(μm) 
Volume
(104μm3
0.6% M  2.2±0.3  10±0.35±0.05  0.22±0.03  37±17±
16% M  3.1±1.4  13.1±0.8  0.39±0.03  0.2±0.1  22.0±0.7  15±
44% M  1.7±0.4  20±0.48±0.01  0.19±0.04  31.7±0.2  33±

The 16% M steel exhibits the highest number of pits per cm2, also being the specimens with the highest variability in this parameter. This fact could be related to the lower stoichiometry and protective properties of its passive layer (Fig. 6). The presence of tensile stresses weakens the passive layer, increasing the number of potential failure points under anodic polarization. In the literature, references indicating that the number of pits increases with the cold rolling reductions can be found [63], but it should be borne in mind that the amount of martensite precipitated in many of previous studies is usually lower than the 44% considered in the present work.

The pits always have very round mouths (Fig. 11) and the cold rolling process does not modify this feature (Table 4). Assuming an elliptical shape of the pit mouths, their eccentricity can be calculated from the ratio of the distance between the center of the ellipse and each focus to the length of the semimajor axis. The obtained values are quite close to zero, which would correspond to a circumference. The shape pit mouths can also be related to ellipses where the ratio between their major and minor axis is approximately 1.3.

Smaller pits have been previously associated with cold worked stainless steel [8]. In our study, the smallest pit mouths (mouth surface and perimeter) are found in the 0.6% M (Table 4) and they tend to increase with the cold working amount of the steel. The deepest pits are found for the 0.6% M steel and shallowest for the 16% M steel. The mean volume of destroyed metal in each pit is relatively similar for 0.6% M and 16% M steels, but in 0.6% M steel the attack tends to proceed in less numerous but deeper pits. For the 44% M steel, the mean volume of each pit is the highest. This fact can explain the highest difficulty to repassivate of this steel, drawn from the analysis of the polarization curves (Fig. 10). The design of the cyclic test at a fixed reversal current density obviously contributes to obtain lower sized pits than when the attack is concentrated on a few pits.

The morphology of pits in 0.6% M material, being the deepest and with the smallest mouths (Table 4), seems to be the most dangerous for their ability to cause unexpected in-service catastrophic failures. The morphology of the attack in 16% M steel, localized in numerous but less deep pits, seems to be the least dangerous. However, it should not be forgotten that the pits appear at lower anodic overpotentials in the studied 16% M material than in the other considered stainless steels. Obviously, these types of observations can be influenced by the media and method used for causing the pits.

4Conclusions

The obtained results can contribute to the understanding of the effect of cold working on the corrosion resistance of austenitic stainless steels. The following conclusions can be drawn for the studied cold rolled AISI 304 sheets:

  • 1.

    The amount and type of the stresses (compressive or tensile) seem to be key factors for pit nucleation. Tensile stresses in the surface of the stainless steel favor the onset of pitting attack.

  • 2.

    The strains of the rolling process increase the tensile stresses in the microstructure, which favor the passive layer breakdown. However, cold working causes a good deal of formation of an expansive phase (martensite) that, when present in extremely large amounts in the material, changes the tensile stresses into compressive ones.

  • 3.

    Amounts of cold working that cause the formation of a moderate amount of martensite -promoting tensile stresses- are related to a decrease in the stoichiometry of the protective passive layer and a decrease in the resistance to the pit nucleation (16% M can be considered within this “moderate” range). Heavy cold working -that promotes compressive stresses- is related to a recovery in the stoichiometry of the protective passive layer and the resistance to the pit nucleation (44% M can be considered within this range).

  • 4.

    Cyclic anodic polarization curves in chloride medium generate round-shaped pits in all the studied steels. The attack is distributed is more numerous, smaller pits in the 16% M. The size of pit mouths is the smallest for the 0.6% M and its pits are the deepest.

Conflicts of interest

The authors declare no conflicts of interest.

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