Environmental factors affecting pitting corrosion of type 304 stainless steel investigated by electrochemical noise measurements under potentiostatic control Helmuth Sarmiento Klapper 1, Joachim Goellner 2, Andreas Burkert 1, Andreas Heyn 1,2 1
Federal Institute for Materials Research and Testing – BAM, Unter den Eichen 87, 12205 Berlin, Germany 2
Otto-von-Guericke University Magdeburg Universitaetsplatz 2, 39106 Magdeburg, Germany Corresponding author: Helmuth Sarmiento Klapper Baker Hughes Baker-Hughes-Strasse 1, 29221 Celle, Germany Direct: +49 5141203 6741 Fax: +49 5141 203 254
[email protected]
Abstract Electrochemical noise measurements on anodically polarised type 304 stainless steel surfaces in contact with buffer solutions of neutral pH were performed to study the effect of chloride ions in the nucleation of pitting corrosion. Passive layer stability and susceptibility to pitting corrosion after pickling and passivation at different environmental conditions were also investigated by means of electrochemical current noise measurements under cathodic and anodic polarisation. According to the obtained experimental results pits nucleate independently on the presence of chloride ions. It has been also shown that protectiveness of stainless steel surfaces after pickling strongly depends on the relative humidity of the environment in which the surface is subsequently passivated. Keywords: stainless steel, pitting corrosion, passivity, potentiostatic
1. Introduction Electrochemical noise (EN) is in principle a physicochemical phenomenon related to dynamic equilibrium state of electrochemical processes. It consists basically on fluctuations detectable in current and electrochemical potential produced by the difference between kinetics of the concerned anodic and cathodic partial reactions. The use of electrochemical noise analysis (ENA), it means the acquisition and analysis of EN signals, for the investigation of electrochemical relevant processes, in special those involved in corrosion science, has strongly increased in the last 30 years. This is basically because of ENA's possibility to assess electrochemical information with high sensitivity in real time. Beside these features, ENA do not demand, in contrast to many other electrochemical techniques in corrosion research and monitoring, an excitation signal. It is widely accepted that the acquisition of EN signals allows monitoring localised corrosion of freely corroding metallic materials 1
[1-3]. In this particular case, the electrochemical current noise (ECN) is measured between two nominally identical electrodes by means of a zero resistance ammeter (ZRA). The electrochemical potential noise (EPN) of the electrically coupled electrodes can be monitored using a reference electrode by a high impedance voltmeter. The ECN and EPN signals correspond to the time-varying components filtered from the original current and potential signals using a band-pass, and are subsequently amplified. The benefits by using high-pass filtering in terms of resolution and detectability have been discussed in detail by Goellner and co-workers in references 4-6 . On the other hand, EN measurements under potentiostatic control have been widely used in corrosion research. In the past, topics related to passivity and initiation processes of localised corrosion on stainless steels have been extensively studied using EN measurements under anodic polarisation 7-27 . In this case, a counter electrode is added to the experimental setup to maintain the potential of the working electrode constant by means of a potentiostat, and the ECN is measured with a ZRA. Passivation of stainless steels is associated with the spontaneous formation of a few nanometers thin and semiconducting protective layer on their surface [28-32]. Nevertheless, the passive layer formed on stainless steel surfaces cannot be considered as a rigid film. It behaves like a dynamic system that includes a selfadjustment mechanism characterized by passive layer breakdown and repassivation processes [13, 28, 30]. This mechanism of stochastic nature takes place permanently and contributes to the adjustment of the passive layer, for instance, after changes in environmental conditions such as temperature, pH, flow velocity, concentration of halide ions, etc. Therefore, the stability of the passive layer depends strongly on this repetitive process. It is well known that the chemical composition, crystalline structure, thickness, and electronic properties of the passive film formed on stainless steels depend on their chemical composition as well as on the physicochemical properties of the environment surrounding the surface, and if applied, on the polarisation potential [30-32]. In the past, however, the influence of environmental conditions affecting passivation, passive layer stability and pitting corrosion resistance of stainless steel surfaces has been less considered than the role of metallurgical particularities in the metallic substrate. The role of some environmental factors on passive layer stability and nucleation of pitting corrosion of stainless steel surfaces is, for instance, not completely understood. Consequently, in continuation to the investigations devoted to the oxygen reduction reaction discussed in references [27, 33], electrochemical noise measurements under potentiostatic control were performed to study the effect of chloride ions on pit initiation of type 304 stainless steel. In addition, the role of the relative humidity (rH) on passive layer stability of pickled stainless steel surfaces was investigated by means of ECN measurements under anodic and cathodic polarisation.
2. Experimental Specimens of type 304 stainless steel (UNS S30400) having different geometries were used for ECN measurements under potentiostatic control. Since stainless steel specimens with small surface areas have been successfully used in the past in similar investigations for assessing nucleation events of pitting corrosion, segments of 10 mm were cut from a wire having diameter of 0.3 mm and embedded in polymeric resin 2
producing specimens with surface area of 0.07 mm2. Prior to ECN measurements the specimens surface were treated by grinding using SiC abrasive papers up to 600 grit. Type 304 stainless steel sheets having dimensions of 200 x 50 x 1.7 mm were also used as specimens for ECN measurements. The sheets were mechanically ground to 240 grit (as delivered condition). The main advantage by using this specimen geometry is that surface treatments can be easily applied and subsequently several measurements can be conducted on the same surface. ECN measurements on sheets of type 304 stainless steel were carried out using an acrylic glass tube with 16 mm diameter as electrochemical cell. Electrochemical cells were fixed on the metallic surface by a protective lacquer. The lacquer was also used to delimitate the tested surface area of 200 mm2 and contributed to avoid crevice corrosion. In the present study some sheets were pickled using HF+HNO3 pickling paste and then adequately rinsed with distilled water. After pickling one sheet was immediately evaluated, other subsequently exposed 48 hours to an environment with 5% rH, which was simulated using a desiccator filled with silica gel, and a third was passivated 48 hours in an environment with 95% rH produced in a climatic chamber. The electrochemical cells were typically fixed on the sheet after pickling but prior to passivation. Another sheet was first treated by grinding to 240 grit and then passivated in air for 15 minutes prior to the electrochemical measurements to simulate a freshly passivated surface. The chemical compositions of specimens used for ECN measurements under potentiostatic control having different geometries are included in Table 1. Table 1. Chemical composition of investigated type 304 stainless steel specimens Geometry Wire Sheet
Surface area [mm2] 0.07 200
Chemical composition [wt%] C Cr Ni S P Mn Si Fe 0.03 17.9 9.3 0.027 0.04 1.17 0.30 Bal. 0.04 18.2 8.3 0.003 0.03 1.32 0.31 Bal.
ECN measurements were carried out using a potentiostat with an integrated ZRA (Jaissle Elektronik GmbH). ECN signals were filtered with a band-pass of 0.1 – 40 Hz. The used sampling rate of 100 Hz was in agreement with the Nyquist-Shannon sampling theorem. A KCl saturated Ag/AgCl reference electrode (200 mVSHE) and a titanium oxide coated titanium mesh were placed in the middle of the measuring cell and used as reference and counter electrode respectively. Specimens embedded in polymeric resin have been tested in a normal glass cell. EN measurements were performed at room temperature (22 2 °C) in aerated (240 µmol/L O2) borate buffer solutions of pH 6.6 with 0.05 M NaCl or without any addition of chloride ions. The open-circuit potential (OCP) was monitored during 10 s prior to polarisation. ECN measurements were taken during 5 to 20 min. The used polarisation potentials of +550 mVSHE and +400 mVSHE have been intentionally selected in the present study for sheets and embedded specimens, respectively, to induce them to undergo pitting corrosion. These potentials were selected based on preliminary experimental results from potentiodynamic polarisation tests and EN investigations reported in references [27, 34]. Additional measurements on cathodic polarised surfaces were performed at -600 mVSHE. The selection of this polarisation potential is based on experimental results from ECN measurements discussed in reference [33], where a clear differentiation among several surface conditions of type 316 Ti stainless steel (UNS S31635) was obtained.
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Three measurements were carried at each experimental condition in separate experiments. For the quantification of the electrochemical activity the standard deviation (SD) of ECN, and the mean value of the current density (i) were calculated over the measuring time. Because of complexity of overlapped transients further analysis was only focused on single transients. They were classified by shape according to the designation introduced by the authors in reference [27]. In addition, the metastable growth rate of nucleated pits was calculated dividing their maximal anodic current amplitude, also called peak current by Williams et al. in reference [19], or transient height in references [23, 24], by the time consumed for reaching it. The metastable growth rate of nucleated pits (κ) has units of nA/s. The current decay also typically called repassivation rate was defined as the maximal current amplitude divided by the time consumed for returning to the original value of current, and has the same units. Correspondingly, the metastable pit growth-to-repassivation-rate-ratio (η) was determined as the quotient of these two rates.
3. Results ECN measurements were performed on stainless steel specimens with a surface of 0.07 mm2. They were anodically polarised at +400 mVSHE in borate buffer solutions of neutral pH with and without the presence of chloride ions. ECN time records of stainless steel type 304 in aerated buffer solutions containing 0.01 M NaCl showed transients related to nucleation processes of pitting corrosion with amplitudes around ±20 nA. These ECN transients had preponderantly shapes characteristic for type I and type II. Typical current and ECN time records of type 304 stainless steel in the buffer solution without chloride ions are presented in Figure 1. The electrochemical current density corresponds to the unfiltered signal. ECN transients were also recognised in the time records obtained in chloride free buffer solutions. Selected portions of the ECN time record are shown in Figure 2. It was observed that the majority of single transients obtained during experiments in buffer solutions without addition of sodium chloride correspond to the type II, which are characterized by a quick increase followed by an immediate but slower decrease of the current. Some transients of the type III were also discerned in these ECN time records. Type II transients typically exhibit η-values larger than 1. In contrast, type III transients exhibit balanced rates, therefore, a symmetric shape and η-values near 1.
Figure 1. Typical time records of type 304 stainless steel at +400 mVSHE in borate buffer solution of pH 6.6 at room temperature a) current density, and b) ECN.
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Figure 2. ECN transients arising from type 304 stainless steel at +400 mVSHE in chloride-free borate buffer solution of pH 6.6 at room temperature. Furthermore, EN measurements under potentiostatic control at +550 mVSHE were conducted on type 304 stainless steel having different surface conditions. Figure 3 presents the time records of current density, which corresponds again to the unfiltered signal. The time records in Figures 3a and 3b correspond to the current density of a pickled surface, and a pickled surface after subsequent passivation in an environment with 95% rH respectively. The time record obtained from a freshly passivated surface was also included as reference in Figure 3c. It was observed that the current density of freshly passivated surfaces increases immediately after polarisation of the specimen from hundreds of µA/cm2 to values larger than 1 mA/cm2. The current densities of pickled and passivated specimens, in contrast, were three orders of magnitude lower, and decreased with time. Surfaces pickled and those pickled and passivated in a lowhumidity atmosphere showed some current spikes (Fig. 3a). The time records of pickled surfaces subsequently passivated in a high-humidity environment, however, exhibit no current spikes (Fig. 3b). Figure 4 presents the corresponding ECN time records. The scale of the ordinate in the ECN time records of Figure 4a and 4b is the same in order to facilitate comparison between them. The magnitude of the ECN transients arising from freshly passivated surfaces is larger than ± 10 µA. Therefore, a ten times larger scale than those used for pickled surfaces was necessary in Figure 4c. Overlapped ECN transients with high amplitudes observed on freshly passivated surfaces are characteristic for pit propagation, which is in line with the aforementioned increase of the current density during polarisation. In addition, the occurrence of pitting corrosion on these surfaces was confirmed by microscopic examinations after electrochemical measurements. Preliminary pickling reduced considerably the electrochemical activity determined on freshly passivated surfaces. The ECN transients arising from the pickled surface shown in Figure 4a can be ascribed to metastable pitting. On the other hand, The ECN time record of type 304 stainless steel surfaces after pickling and passivation in an environment with high humidity showed no transients (Fig. 4b). Electrochemical parameters obtained by statistical analysis of the time records are summarized in Table 2. Pickled surfaces subsequently passivated in a high-humidity atmosphere showed the lowest current densities and correspondingly the lowest standard deviations of ECN, which are one order of magnitude lower than those from the other pickled surfaces. The electrochemical parameters from passivated surfaces were almost three orders of magnitude higher than those determined on pickled and passivated surfaces.
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Figure 3. Typical current density time records of type 304 stainless steel having different surface conditions polarised at +550 mVSHE in borate buffer solution of pH 6.6 + 0.05 M NaCl at room temperature a) pickled, b) pickled and passivated 48 h at 95% rH, and c) passivated.
Figure 4. Corresponding ECN time records of stainless steel type 304 at +550 mVSHE in borate buffer solution of pH 6.6 + 0.05 M NaCl at room temperature a) pickled, b) pickled and passivated 48 h at 95% rH, and c) passivated. 6
Table 2. Electrochemical parameters of type 304 stainless steel at +550 mVSHE in borate buffer solution of pH 6.6 + 0.05 M NaCl. Surface condition Passivated Pickled Pickled + 48h at 5 % rH Pickled + 48h at 95 % rH
OCP mVSHE 26 / 50 / 37 191 / 219 / 137 239 / 245 / 255 296 / 243 / 265
i A/cm2 103 3.8 / 3.6 / 3.2 2.1 / 2.0 / 2.2 0.8 / 0.9 / 1.0
SD of ECN nA 7509 / 10630 / 8930 123 / 237 / 124 305 / 44 / 106 10 / 12 / 11
Electrochemical investigations performed on pickled stainless steel surfaces under cathodic polarisation (-600 mVSHE) confirmed that the electrochemical activity, in this case related to the oxygen reduction reaction as well as to the passive layer reduction, is influenced by the passivation conditions used after pickling. The results from electrochemical measurements on cathodically polarised surfaces are summarized in Table 3. The lowest SD of ECN was again observed on pickled surfaces subsequently passivated in a high-humidity atmosphere. On the other hand, pickled surfaces exposed to a low-humidity environment had standard deviations of ECN comparable to those observed on surfaces pickled and immediately evaluated. No relevant difference was observed in the cathodic currents (ic) measured on the pickled surfaces when cathodically polarised. Figure 5 includes portions of typical ECN time records obtained on cathodically polarised surfaces of type 304 stainless steel exposed to aerated borate buffer solutions of pH 6.6 containing 0.05 M NaCl. A significant amount of transients with small amplitude characterized these time records. Pickled surfaces, and those subsequently passivated at 5% rH (Fig. 5a) showed in contrast to surfaces passivated at 95% rH (Fig. 5b) additional transients with longer life times. Experimental results from ECN measurements under potentiostatic control showed, in general, very good reproducibility. Table 3. Electrochemical parameters of type 304 stainless steel at -600 mVSHE in borate buffer solution of pH 6.6 + 0.05 M NaCl. Surface condition Pickled Pickled + 48h at 5% rH Pickled + 48h at 95% rH
OCP mVSHE 179 / 192 / 208 274 / 295 / 278 310 / 266 / 280
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ic A/cm2 14 / 14 / 14 20 / 19 / 16 13 / 15 / 17
SD of ECN nA 37 / 39 / 35 37 / 30 / 33 16 / 9 / 14
Figure 5. Portions of typical ECN time records of type 304 stainless steel at -600 mVSHE in borate buffer solution of pH 6.6 + 0.05 M NaCl at room temperature a) pickled and passivated 48 h at 5% rH, and b) pickled and passivated 48 h at 95 rH.
4. Discussion Nucleation of pitting corrosion on passive metals is intrinsically related to passive layer breakdown [28-30]. Pitting corrosion initiates preferentially at local heterogeneities in the metallic substrate such as dislocations, grain boundaries, or inclusions, with a direct access to the surface 12, 20-22, 35-38 . However, pits can also be nucleated at crystalline defects in the passive film 39 . It is widely accepted that certain species, typically halide ions, can induce passivity breakdown by interaction with the passive film. Consequently, many proposed mechanisms describing early stages of localised corrosion on passive metals are based on the presence of chloride ions in the environment surrounding the surface. They consider different particular effects of the chloride ions that lead to the rupture of the protective layer. Nevertheless, the large amount of ECN transients observed in the time record of type 304 stainless steel in the chloride-free borate buffer solution of neutral pH at +400 mVSHE shown in Fig. 1b suggests that nucleation of pitting corrosion i.e. passive layer breakdown occurs independently on the presence of chloride ions in the electrolyte. In a similar way, Suter et al. determined current transients arising from type 304 stainless steel surfaces in Na2SO4 solutions under potentiostatic control [21]. They associated these events with the dissolution of MnS-inclusions leading to initiation of localised corrosion. In their model of passivity breakdown Marcus et al. pointed out that the nanostructure of the passive film e.g. grain boundaries and step edges are more relevant for the nucleation of pitting corrosion on passive metals than the presence of chloride ions at the interface electrolyte/passive layer [32]. Zahavi et al. 40 and Videm 41 demonstrated on aluminium that chlorides inhibit the coalescence and crystallization of passive films by competing with OH - ions for adsorption on surface sites. Thereby, they are mainly responsible for the delay of repassivation. According to Maurice et al. this can be generalised for pitting corrosion on all passive material 32, 39 . In the present study a clear correlation between the probability of appearance of different types of ECN transients and the presence of chloride ions in the test solution was obtained. As shown in Figure 6, 72% of ECN transients arising from anodically polarised stainless steel surfaces exposed to aerated chloride-free buffer solutions of neutral pH correspond to the type II of transients as defined by the authors in reference [27]. They are characterized by quick current increase followed by a slower but immediate decay of current. The amount of this 8
type of transients, however, is reduced to 50% in buffer solutions containing chloride ions. On the other hand, the amount of type III transients has been observed having a slight increment from 19% in chloride-containing buffer solutions to 24% in chloridefree buffer solutions. Type I transients which are characterized by long propagation times, were, in contrast, clearly reduced from 31% to 4% in chloride-free solutions. The significant increment of transients with longer propagation times in chloridecontaining solutions supposes that chloride ions mainly retard pit repassivation. The last stage of EN transients measured on stainless steels at open-circuit potential conditions has been adduced by Berthomé et al. to the discharge of the surface capacitance rather than the repassivation process [42]. Current decay in single transients arising from nucleation events of pitting corrosion on anodically polarised stainless steel surfaces having short life times, however, has been typically related to the repassivation and/or electron-consuming process [19, 23, 24]. For those transients with longer propagation times, nucleation and growth of a protective oxide layer on the nucleated pit bottom seems to be briefly retarded, probably by the formation of a metastable salt film, as proposed by Okada [43]. Since surface capacitance discharge is controlled by the electron-consuming process e.g. the cathodic partial reaction taking place at the passive layer, it can be assumed that chloride ions are principally responsible for the delay of repassivation during early stages of localised corrosion of type 304 stainless steel. Mechanisms of pit nucleation on stainless steels based on the Zener mechanism 37 , on passive layer breakdown due to mechanical stresses as a result of electrostrictive forces 44 , or on the effect of defective sites such as grain boundaries within the passive layer [39], would be, therefore, more accurate describing nucleation processes of pitting corrosion on stainless steel type 304 than those only based on the influence of halide ions in the environment surrounding the passive layer. Furthermore, as shown in Figure 6, most of the measured ECN transients started with a sharp increase of the current in anodic direction. Ilevbare and Burstein adduced the very sharp rise in current of type II transients to an abrupt rupture of the protective film 24 . Nucleation events of pitting corrosion initiate on type 304 stainless steel surfaces with an abrupt breakdown of the passive layer leading to local metallic dissolution. This precludes the hypothesis that pits nucleate by a cathodic reaction as recently proposed for the aluminium-iron alloy 1050 A 38 . After passive layer breakdown the nucleated pit propagates as long as the involved anodic area dissolves completely and/or repassivates.
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Figure 6. Probability of appearance of single transients arising from nucleation events of pitting corrosion on type 304 stainless steel.
Research works that report the appearance of types I and II ECN transients are summarized in Table 4. While Gabrielli et al. associate the shape of ECN transients with the origin of the metallic substrate [14], Baroux and Gorse related transients with small amplitudes showing a rapid increase of current followed by a slow current decay (type II) with pit nucleation events on MnS-free stainless steels 12, 22 . Nevertheless, Suter and Boehni observed the same type of transient on stainless steels containing MnS-inclusions 21, 36 . The results obtained in the present study from ECN measurement on type 304 stainless steel surfaces confirm that different types of ECN transients arising from nucleation events of pitting corrosion on stainless steel surfaces might be recorded depending on the experimental conditions. The authors demonstrated recently that, excluding effects due to signal conditioning, the shape of the ECN transients does not solely depend on the kinetic of the metallic dissolution, but also on the cathodic partial reaction. The cathodic process determines the amount of electrons being measured as EN signal 27 . Consequently, early statements that suggest an exclusively correlation between the shape of EN transients with the chemical composition and particularities in the microstructure of the metallic substrate are hardly probable. It should be, however, noted that excluding the investigations carried out by Baroux and Gorse [12], type II transients have been exclusively detected, as shown in Table 4, on stainless steel specimens having a very small surface area, typically smaller than 0.1 mm2. On the other hand, ECN transients with a slow increase followed by a sharp decrease of the current (Type I), usually related to metastable pitting 10 , have been preferentially observed on larger electrode surfaces i.e. 78 mm2. These transients have been also detected on small surfaces but in combination with environments producing a strong cathodic process like aerated or low-pH solutions. The influence of the anodic-to-cathodic-surface ratio on the shape of ECN transients arising from nucleation of pitting on stainless steels has been discussed by the authors in detail in reference 27 . The calculated η-values from single type II transients measured on type 304 stainless steel surfaces under potentiostatic control in buffer solutions of neutral pH confirm, as shown in Figure 7, that metastable pit growth rates can be till eleven times faster than the corresponding repassivation and/or electron-consumption rates. The absence of a clear statistical 10
correlation between both indicates the stochastic nature of initiation process of pitting corrosion on stainless steels.
Table 4. Research works reporting ECN transients type I and II arising from stainless steel surfaces under anodic polarisation. Transient type
I
II
Material
Surface area [mm2]
Type 302 SS
100
Type 304 SS
0.002
Type 304L SS
500
Types 304, 316, 316L, 317L SS
78
Type 430 SS
78.5
Type 316Ti SS
100
Type 304 SS
0.07 and 100
18Cr-8Ni SS
7.8 x 10-5
Type 430 SS
78.5
Types 303 and 304 SS Fe-Cr and FeCr-Mo alloys
0.018
Electrolyte Deaerated 0.1 M NaCl 1 M NaCl + 0.1 M H2SO4 Deaerated 0.028 M NaCl 0.1M NaCl 0.02 M NaCl of pH 6.6 Aerated buffer sol. of pH 6.6 + 0.01 M NaCl Aerated buffer sol. of pH 6.6 + 0.01 M NaCl Deaerated 0.03 M NaCl + 0.3 M NaClO4 0.02 M NaCl of pH 6.6 1 M NaCl / 1 M Na2SO4
0.031
Type 316 SS
0.002
Type 304 SS
0.07
1 M NaCl Deaerated 0.1 M HCl Deaerated borate buffer solution of pH 6.6 + 0.01 M NaCl
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Polarisation potential
Ref .
420 mVSCE
[10]
100 mVSCE
[17]
200 mVSCE
[19]
250 - 350 mVSCE
[20]
200 mVSCE
[12] [22]
150 mVAg/AgCl
[27]
250 mVAg/AgCl
[27]
-40 mVSCE
[16]
200 mVSCE
[12]
n.a.
[21]
200 mVSCE
[23]
200 mVSCE
[24]
250 mVAg/AgCl
[27]
Figure 7. Correlation between metastable pit growth and repassivation rates of single events from type 304 stainless steel surfaces (0.07 mm2) at +400 mVSHE in aerated buffer solutions of pH 6.6.
Regarding pit nucleation on stainless steel type 304 surfaces after pickling, it was established by ECN measurements under potentiostatic control that the passive layer stability strongly depends on the conditions at which the pickled surface is subsequently passivated. It confirms that the self-adjustment mechanism of the passive film takes place permanently even if pickling has been previously conducted. It was established, for instance, that pickled stainless steel surfaces which were subsequently passivated in a low-humidity environment are prone to metastable pitting when polarised at +550 mVSHE in borate buffer solutions of pH 6 with 0.05 M NaCl. In contrast, pickled surfaces passivated in a high-humidity atmosphere remain passive at the same conditions. Collateral effects related to the pickling process can be neglected as the same procedure (same pickling paste, same pickling time, same rinse time) was applied on all evaluated sheets. Consequently, they exhibited similar OCPs prior to polarisation, and all were clearly less susceptible to pitting corrosion than freshly passivated surfaces. The differences in the susceptibility to metastable pitting corrosion determined between the pickled surfaces can, therefore, be ascribed to the dissimilar passivation conditions i.e. dissimilar cathodic reactions supporting the selfadjustment mechanism of the passive film according to the humidity conditions. Similar results have been recently obtained by the authors using other surface conditions 34 . It was demonstrated in the past that a strong cathodic process e.g. high relative humidity atmosphere or an oxidiser-rich environment offers ideal conditions for the stabilisation of the passive layer. In contrast, a low relative humidity e.g. a weak cathodic process hinders the self-adjustment process of the passive layer formed on stainless steel surfaces [45]. It should also be noted that the amplitude of ECN transients from time records obtained on freshly passivated surfaces (Fig. 4c) clearly decreased with time. This indicates that during stable pit propagation ECN signals can be strongly affected by the cathodic process, as discussed by the authors in reference [27]. In addition, ECN measurements under cathodic polarisation have shown that passivation conditions also affect the grade of inhibition of cathodic reactions on 12
pickled type 304 stainless steel surfaces. Investigations by Le Bozec et al. regarding the oxygen reduction reaction on cathodically polarised type 316 stainless steel (UNS S31603) surfaces established that the surface condition has a strong influence on the kinetic of this cathodic reaction 46 . A clear differentiation between several surface conditions was also obtained by the authors using short-time ECN measurements under cathodic polarisation as discussed in reference 33 . Evaluated surface conditions included pre-reduced, mechanically ground, passivated, and pickled and passivated stainless steel surfaces. The measured ECN time records on type 316 Ti stainless steel in borate buffer solutions showed two types of cathodic transients depending on the surface condition, and on the oxygen concentration. High-frequency transients, for instance, were ascribed to the reduction of the passive layer and were independent on the oxygen concentration in the solution but dependent on the surface condition. On the other hand, low-frequency transients were related to the oxygen reduction reaction. Therefore, low-frequency transients with high amplitude were preferentially observed on pre-reduced surfaces in oxygen-rich solutions. On mechanically ground surfaces, in contrast, transients with low frequency and high amplitude, and transients with high frequency and low amplitude were detected. High-frequency transients with very small amplitudes were selectively detected on passivated surfaces in contact with de-oxygenated solutions. The time record in Figure 5b showing only high-frequency transients with small amplitudes is very similar to those ECN time records measured on pickled and passivated surfaces of type 316Ti stainless steel in oxygenated solutions [33]. It confirms that pickling and passivation in a high-humidity environment leads to a surface condition that strongly inhibits the oxygen reduction reaction. This is due to the formation of a stable chromium-rich passive layer as confirmed in X-ray photoelectron spectroscopy (XPS) examinations conducted by Le Bozec et al. 45 . Schmuki et al. determined also that chromium oxides are very stable and therefore not reductively dissolved at cathodic potentials in neutral solutions. Therefore, they offer fewer current paths needed for the oxygen reduction reaction 47 . It has been also shown in the past that the SD of ECN i.e. the electrochemical activity due to passive film reduction and oxygen reduction reactions decreased with increasing the chemical stability of the passive layer formed on the stainless steel surface [33]. Experimental results summarized in Table 3 showing dissimilarities in the electrochemical behavior of pickled surfaces under cathodic polarisation can then be rationalized in terms of the chemical stability of the passive layer depending on the specific subsequent passivation process. Type 304 stainless steel surfaces that were pickled and passivated in a high-humidity environment exhibit the lowest SD of ECN compared to the other pickled surfaces, indicating that this particular passivation process leads to the most protective passive film. ECN measurements under anodic polarisation corroborated that these surfaces did not undergo pitting corrosion in borate buffer solutions of neutral pH at +550 mVSHE. In contrast, pickled surfaces subsequently passivated in a low-humidity environment were susceptible to metastable pitting. It is well known that the protectiveness of the passive layer depends on chemical composition, structure, number of grain boundaries and the extent of defects in the film 28-32 . Maurice et al. pointed out that structural changes into the passive film occur during ageing under anodic polarisation. In the case of stainless steels in acid aqueous solutions they observed an increase of the crystallinity of the passive layer and the coalescence of Cr2O3 nanocrystals in the inner oxide 48, 49 . The present study has shown that only pickling is not the decisive step for reaching the appropriate corrosion protectiveness of the stainless steel surface. Since the self-adjustment process of the passive film is 13
supported by the inherent strong cathodic process in a high-humidity environment, it reduces significantly the susceptibility to pit nucleation of the type 304 stainless steel surfaces by improving the chemical stability of their passive layer.
5. Conclusions 1. ECN measurements have shown that pitting corrosion initiates on type 304 stainless steel with an abrupt breakdown of the passive layer leading to local metallic dissolution. This was confirmed by the characteristic shape of ECN transients arising from anodically polarised grade 304 stainless steel surfaces in neutral buffer solutions showing preferentially a quick increase followed by a slower decay of the current. After passive layer breakdown the nucleated pit propagates as long as the involved anodic area dissolves completely and/or repassivates. 2. Nucleation of pitting corrosion on type 304 stainless steel surfaces occurs independently on the presence of chloride ions in the electrolyte. Based on the obtained η-values representing the metastable-pit-growth-to-repassivationrates ratio it was observed that chloride ions are mainly responsible for retarding pit repassivation rather than for facilitating pit nucleation on type 304 stainless steel. 3. The present study has shown that only pickling is not the decisive step for reaching the appropriate corrosion protectiveness on type 304 stainless steel surfaces. It was corroborated by ECN measurements under anodic polarisation that the susceptibility to pitting corrosion of type 304 stainless steel surfaces depends strongly on the conditions at which the surface is subsequently passivated. The self-adjustment mechanism of the passive film being supported by the strong cathodic process in a high-humidity atmosphere reduced significantly the susceptibility to pit nucleation of the type 304 stainless steel. In contrast, a subsequent exposition of the pickled surface to a low-humidity environment leads to a less stable passive film, and therefore, more susceptible to metastable pitting corrosion. The differences in the susceptibility to metastable pitting corrosion determined between these pickled surfaces can be ascribed to the dissimilar chemical stability of their passive layers as confirmed by ECN measurements on type 304 stainless steel surfaces under cathodic polarisation. 4. The acquisition and analysis of ECN signals arising from potentiostatically controlled stainless steel surfaces enables the examination of nucleation processes of pitting corrosion processes with high sensitivity. EN measurements under potentiostatic control provided a better control over the investigated electrochemical system and, consequently, better reproducibility of the testing conditions.
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[40] J. Zahavi, M. Metzger, Electron microscope study of breakdown and repair of anodic films of Al, J. Electrochem. Soc. 119 (1972) 1479-1485. [41] K. Videm, The electrochemistry of uniform corrosion and pitting of aluminium, Kjeller Report KR-149, Institute for Energy Technology, 1974 [42] G. Berthomé, B. Malki, B. Baroux, Pitting transients analysis of stainless steels at the open circuit potential, Corros. Sci. 48 (2006) 2432-2441. [43] T. Okada, Considerations of the stability of pit repassivation during pitting corrosion of passive metals, J. Electrochem. Soc. 131 (1984) 1026-1032. [44] N. Sato, Anodic breakdown of passive films on metals, J. Electrochem. Soc. 129 (1982) 255-260. [45] H.S. Klapper, J. Goellner, A. Heyn, A. Burkert, Relevance of the cathodic process on the passivation of stainless steels – an approximation to the origin of the rouging phenomenon, Mater. Corros. 63 (2012) 54-58. [46] N. Le Bozec, C. Compere, M. L'Her, A. Laouenan, D. Costa, P. Marcus, Influence of stainless steel surface treatment on the oxygen reduction reaction in seawater, Corros. Sci. 43 (2001) 765-786. [47] P. Schmuki, S.Virtanen, H.S. Isaacs, M.P. Ryan, A.J. Davenport, H. Boehni, T. Stenberg, Electrochemical behavior of Cr 2O3/Fe2O3 artificial passive films by in situ XANES, J. Electrochem. Soc. 145 (1998) 791-801. [48] V. Maurice, W. Yang, P. Marcus, XPS and STM study of passive films formed on Fe‐22Cr (110) single‐crystal surfaces, J. Electrochem. Soc. 143 (1996) 1182-1200. [49] V. Maurice, W. Yang, P. Marcus, X‐Ray photoelectron spectroscopy and scanning tunneling microscopy study of passive films formed on (100) Fe‐18Cr‐13Ni single‐crystal surfaces, J. Electrochem. Soc. 145 (1998) 909-920. List of Tables Table 1. Chemical composition of investigated type 304 stainless steel specimens Table 2. Electrochemical parameters of type 304 stainless steel at +550 mVSHE in borate buffer solution of pH 6.6 + 0.05 M NaCl. Table 3. Electrochemical parameters of type 304 stainless steel at -600 mVSHE in borate buffer solution of pH 6.6 + 0.05 M NaCl. Table 4. Research works reporting ECN transients type I and II arising from stainless steel surfaces under anodic polarisation.
List of Figures Figure 1. Typical time records of type 304 stainless steel at +400 mVSHE in borate buffer solution of pH 6.6 at room temperature a) current density, and b) ECN. Figure 2. ECN transients arising from type 304 stainless steel at +400 mVSHE in chloride-free borate buffer solution of pH 6.6 at room temperature. Figure 3. Typical current density time records of type 304 stainless steel having different surface conditions polarised at +550 mVSHE in borate buffer solution of pH 17
6.6 + 0.05 M NaCl at room temperature a) pickled, b) pickled and passivated 48 h in 95% rH, and c) passivated. Figure 4. Corresponding ECN time records of stainless steel type 304 at +550 mVSHE in borate buffer solution of pH 6.6 + 0.05 M NaCl at room temperature a) pickled, b) pickled and passivated 48 h in 95% rH, and c) passivated. Figure 5. Portions of typical ECN time records of type 304 stainless steel at -600 mVSHE in borate buffer solution of pH 6.6 + 0.05 M NaCl at room temperature a) pickled and passivated 48 h at 5% rH, and b) pickled and passivated 48 h at 95 rH. Figure 6. Probability of appearance of single transients arising from nucleation events of pitting corrosion on type 304 stainless steel. Figure 7. Correlation between metastable pit growth and repassivation rates of single events from type 304 stainless steel surfaces (0.07 mm2) at +400 mVSHE in aerated buffer solutions of pH 6.6.Figure 7.
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