Experimental Study of Propagation Stage of Pitting ...

Advanced Materials Research Vol. 38 (2008) pp 238-247 online at http://www.scientific.net © (2008) Trans Tech Publications, Switzerland Online available since 2008/Mar/25

Experimental Study of Propagation Stage of Pitting Corrosion of 20Cr13 Steel A.D. Davydov and V.S. Shaldaev Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia [email protected] Keywords: pitting corrosion, stage of propagation, breakdown of metal passivity Abstract. The initiation and development of pitting corrosion of steel 20Cr13 in the NaCl solutions with various concentrations, temperatures, and pH values are studied under the potentiostatic conditions and at the free-corrosion potential. The pitting and repassivation potentials are determined using the method of cycling voltammetry. In spite of the fact that thus determined pitting potential is more positive than the corrosion potential (the open-circuit potential Eo.c.), the long-term experiments, which were performed at the free-corrosion potential, showed that pitting corrosion takes place without imposing a potential using an external power source. It is concluded that the probability of pitting corrosion of steel should be determined by comparing the corrosion potential (the open-circuit potential) with the repassivation potential Erp. Steel 20Cr13 is prone to the pitting corrosion, because Erp is more negative than Eo.c.. In the potentiostatic experiments, the variation of the depth and diameter of pits and their number with the time and the effect of temperature and electrode rotation on the pit propagation are studied. The results, which were obtained at the free-corrosion potential, are much less reproducible. In this case, in contrast to the potentiostatic conditions, the pit depth increased only slightly and the pit width increased to a larger extent. The effect of concentration, pH value, and temperature of NaCl solutions on the pit propagation is considered. It is concluded that the data on the development of pitting corrosion under the potentiostatic conditions can be hardly extended to the conditions of free corrosion potential. Introduction In the pitting corrosion process, commonly, two stages are recognized: the nucleation and the propagation of pits. The majority of published works are devoted to the investigation of the first stage. The results of experimental studies of the second stage are usually used to determine the corrosion resistance of various materials under various conditions and to check the proposed models of pitting corrosion. The second stage (propagation of pits) is less studied, because its investigation takes very much time, the results are frequently poorly reproducible, and, in many cases, they appear to be essentially different for different metals. But under the potentiostatic conditions, at the potentials above the free-corrosion potential, more stable, reproducible results can be obtained in the short-time experiments. Therefore, the majority of investigations were performed under the potentiostatic conditions. However, the manner, in which such results can be extended to the actual corrosion condition, is not quite clear. In the analysis of pit propagation, several factors should be taken into consideration. On the one hand, a certain area around an acting pit is under the conditions of cathodic protection [1]; therefore, the nucleation of new pits within this area is hampered. On the other hand, the aggressive solution, which flows out of acting pit, can weaken the protective layer adjacent to the pit and thus promote the nucleation of new pits on this area. In many works it was shown that the solution inside the growing pits is characterized by a higher concentration of aggressive anions and a lower pH, which promote the nucleation and propagation of pits [2]. In some cases, the conversion of metastable pits

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Advanced Materials Research Vol. 38

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into stable ones and stable pit growth are favored by the formation of covers, which prevent the mass exchange between the solution in the pit and the bulk solution ([3, 4] and many other works). The authors of [5, 6] studied the real-time pit growth from the edges of 304 SS foils under various applied potentials and temperatures in 1 M NaCl solution. They concluded that the parabolic time dependence of pit growth into the depth (L ~ √t) takes place and the growth is independent of potential. The lateral pit growth is linear with time and dependent upon potential (the pit width increases approximately linearly with potential). The pit depth independence of potential was explained by the fact that the pit bottom was covered with a layer of salt (FeCl2), and the dependence of pit width on potential was explained by the fact that the pit surface near the laterally advancing pit front was not covered with the salt. The authors of [7] measured the surface concentration of pits n at passive iron in borate buffer solutions with pH values between 6.5 and 9.5 containing chloride concentrations between 0.004 and 0.1 M for 180 s after the end of the induction period at different electrode potentials. The mean surface concentration of pits grows during a short time and attains a steady-state value n0 after a characteristic time, which is several tens seconds depending on the prescribed potential and the chloride concentration (the higher the potential and concentration, the shorter is the characteristic time). Under the experimental conditions used in [7], n0 was from several tens to several thousands pits per 1 cm2 of electrode surface area; n0 increased with potential (with a difference between the potential, which is maintained in the experiment, and the pitting potential) and the concentration of chloride and decreased with increasing pH of solution. Around the acting pit, an area was found where no new pits formed. The radius of this area was several tens µm; the lower was the chloride concentration and the higher was pH of solution, the larger was the radius, and it was independent of potential. In [8] it was shown that for the first 20 s after imposing a potential, the number of pits on 316 stainless steel in 0.05 M NaCl solution grew exponentially with the time. Then, new pits ceased to nucleate, and the total number of pits reached a constant value. According to [8], this is due to the fact that there is the limited number of sites (defects and inclusions) for the pit nucleation on the surface. An analysis of the distribution of distances between the neighboring pits showed that, when the number of pits reached the saturation, an area several µm in width existed around a pit, where other pits were absent. This is in agreement with the results reported in [7]. The authors of [9-11] kept the specimens of different steels in the solutions containing Cl- ions and observed the monotonic increase in the mean penetration depth or mean maximum pit depth for hundreds or even thousands days. For example, according to [11], the mean maximum pit depth on mild steel XS 1006 coupons 3 mm thick reached approximately 1.5 mm after an exposure in sea water for 1500 days. In this work, we study the pitting corrosion of steel 20Cr13. Its composition, wt %: Fe base, 12.45 Cr, 0.20 Ni, 0.32 Mn, 0.205 C, 0.014 P, 0.009 S, 0.41 Si, 0.1 V. We use NaCl solutions that are most aggressive with respect to steels. Results and Discussion Pitting Potential. The pitting process, as is known, arises at a certain potential Epit depending on many factors: the concentration of chloride, the presence of other anions in the solution, the pH value, the temperature, etc. Several methods are used to determine Epit. The method of potentiodynamic curves, which are measured at low potential scan rates, is the most popular method. Let us denote Epit, which is determined by this method, as Epitpd. Figure 1 gives the potentiodynamic curve that was measured at a potential scan rate of 2·10-4 V/s on a specimen of steel 20Cr13 in 0.01mol/l NaCl solution. The potentials in the text and figures are presented against the standard hydrogen electrode. The first curve portion abc was obtained by varying the potential from the open-circuit potential, which was measured prior to the beginning of experiment Eo.c. (it was measured in several minutes after the immersion of specimen into the solution) to the potential marked off by c, which is only slightly higher than potential Epitpd.

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The pitting manifests itself in an abrupt increase in the rate (the anodic current density) of metal dissolution at Epitpd. In the range of somewhat less positive potentials, short-term current spikes are observed in the potentiodynamic curve, which are associated with the formation and healing (repassivation) of so-called metastable pits (Fig. 1).

Fig. 1. Cyclic voltammogram for steel 20Cr13 in 0.01 M NaCl solution An exposure of test specimen at a constant potential in the passive range showed that the passive current ip decreases with time. In the general form, according to the experimental results, the dependence of ip on the time t for 20Cr13 steel can be expressed in the form of equation ip = A + B 1/t. Figure 2 gives the plot of ip on 1/t that was measured in 0.01 M NaCl solution with pH 8 at 200C at a potential of 0.38 V (this potential is lower than Epitpd; therefore, an induction period preceding the onset of passivity breakdown is observed on Fig. 2). After the induction period, the current decay, which is linear on these coordinates, is changed by an abrupt increase in the current corresponding to the pitting process. The current decay, probably, is associated with a decrease in the electric field strength in the anodic oxide film with its thickening under the potentiostatic conditions. The hyperbolic decrease in ip with time (ip ~ 1/t) with the growth of anodic oxide film on various metals was reported by several authors, for example, [12-15]. The typical sharp


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transition in time from the growth of anodic oxide film to the beginning of its breakdown is seen on Fig. 2. Obviously, the variations leading to the pitting nucleation proceed concurrently with the film thickening. This can take place, if the processes leading to the breakdown of steel passivity occur in a small number of most defect film sites, while the passive film grows on the rest surface area.

Fig. 2. The variation of current with time on steel 20Cr13 in 10-2 M NaCl with pH 8 at a potential of 0.38 V. The second portion of potentiodynamic curve cde was measured at the reverse potential scan: from the potential marked off by c to the potential marked off by e, which is called the repassivation potential Erp. The Epitpd, which is measured using the potentiodynamic method, is much more positive than the corrosion potential (the open-circuit potential Eo.c.), which was measured after a short time (several minutes) after immersing the test specimen into the aggressive solution and prior to switching-on the potential scan (Fig. 1). Comparing Eo.c. and Epitpd, one can conclude that the pitting corrosion of steel is impossible at the free-corrosion potential (when no higher potential is applied using an external power source). However, the results of direct experiments, which are reported in detail below, do not support this conclusion. The long-term experiments showed the possibility of pitting corrosion of 20Cr13 steel in the NaCl solutions at the open-circuit potential. The results obtained here support the opinion that, in order to determine the probability of initiation of pitting corrosion, it is advantageous to compare the free-corrosion potential not with the pitting potential, which was determined potentiodynamically, but with the repassivation potential. This comparison shows that the pitting corrosion can take place at the free-corrosion potential, because this potential is more positive than the repassivation potential (Fig. 1). It should be noted that, when Erp is determined, the potential scan should be reversed after reaching sufficiently high degree of pitting development (after reaching sufficiently high anodic current density). Let us indicate some peculiarities of the process that can be seen from the voltammogram. After the potential scan was reversed in point c, the current continued to increase for a certain time (from c to d), in spite of a decrease in the potential, probably, because such high potential as Epitpd is required only for the rapid activation of surface (the breakdown of metal passive state). The activated metal dissolution is possible at the potentials E < Epitpd. These results allow us to consider the potential Erp not as the potential, above which the existing pits do not repassivate, but as the

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potential, above which pits can initiate and propagate. At Epitpd, the pitting starts very quickly, and at Epitpd > E > Erp, the pitting starts after an induction period. In the following sections, we present the results of investigation of pitting development on steel 20Cr13 in time at the free-corrosion potential and at a constant potential, which was maintained with a potentiostat. Pit Propagation under the Potentiostatic Conditions. Steel specimens were kept at a given potential (somewhat higher than Epitpd, in order to exclude the induction period) for certain periods of time. After every experiment, the specimen was removed from the electrochemical cell, and its surface was examined with Linnik microinterferometer that enables one to measure pit dimensions. Figure 3 gives a micrograph of steel 20Cr13 specimen surface after an exposure in 0.1 M NaCl for 10 s at a potential, which is by 20 mV higher than Epitpd. In these experiments, equally probable distribution of pits over the specimen surface is observed. In this case, if there is any effect of interaction between pits, it is not the weakening of the protective film adjacent to the acting pit by the aggressive solution flowing out of the pit. All pits observed formed very quickly and, thereafter, their number did not increase with time. The number of formed pits n depends on the difference ∆E between the potential, which is maintained in the experiment, and Epitpd : log n increases approximately as ∆E, Fig. 4. Thereby, the depth and diameter of pits remained almost constant (an average depth of the deepest pits was 16 µm, an average diameter was 15 µm).

Fig. 3. Micrograph of a fragment of steel specimen after an exposure in 0.1 M NaCl at a potential of 0.49 V for 10 s. The fragment diameter is 1.6 mm.


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2,2

lg n

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0,10

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Fig. 4. Dependence of number of pits per 1 cm2 of specimen surface area on ∆E, the specimen was kept in 0.01 M NaCl for 15 s. Figure 5 gives the measured pit depth for a specimen that was kept in 0.01 M NaCl solution at a potential of 0.5 V (Epitpd = 0.47 V, Fig. 1). The dependence, which is shown with circles, refers to an average depth of several deepest pits Lmax, and the dependence, which is shown with triangles, refers to an average depth of several shallowest pits Lmin. Comparing the results, one can see that, within the first minute, all formed pits grow with identical rates. Then, some pits continue to deepen, whereas other pits cease to grow. This can be the evidence for the fact that some pits repassivate and cease to grow after a certain time after their formation. In these experiments, a fraction of pits, which cease to grow, is approximately 20% of the total amount of pits recorded. The fact that some pits repassivate should be taken into consideration, for example, in the analysis of the plots of pit-depth distribution. 120

L,µm

100 80 60 40 20 0

0

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Fig. 5. The dependences of average depth of the deepest (circles) and shallowest (triangles) pits on the time of exposure in 0.01 M NaCl at a potential of 0.5 V.

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Under these experimental conditions, an average diameter of pits, first, increases and, then, is stabilized at approximately 35 µm, Fig. 6. It should be noted that the pit depth continued to grow also after the stabilization of diameter value (compare with Fig. 5). An increase in the temperature to 700C leads to an increase in Lmax by approximately 1.3 times and a decrease in Lmin (the difference between the electrode potential, which is maintained in the experiment, and Epitpd is kept constant).

35

d,µm

30 25 20 15 10

0

100

200

300

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500

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Fig. 6. The dependence of average diameter of pits on the time of exposure of steel specimen in 0.01 M NaCl at a potential of 0.5 V. Some experiments were performed on the rotating disk electrode (1180 rpm); in this case, both Lmax and Lmin appeared to be by 1.5 – 2 times lower than on the identical, but stationary electrode. Development of Pitting Corrosion at the Free-Corrosion Potential. To investigate the pit propagation under the free-corrosion conditions, the specimens 10x10x2 mm in sizes were kept for a certain period of time in the NaCl solutions with various concentrations and pH values at the temperatures of 20 and 900C. Then, the specimens were thoroughly washed with distilled water, dried with filter paper, and their surface was examined with an optical microscope. In some experiments, the corrosion potential was measured during a long period of time in the NaCl solutions with various concentrations. Free corrosion of steel 20Cr13 steel is characterized by rather prolonged variation in the corrosion potential Ecorr. The corrosion potential has a general tendency to shift in the direction of more negative values (Fig. 7), which is associated with gradual development of pitting corrosion.


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Fig. 7. The variation in the corrosion potential (the open-circuit potential Eo.c.) with the time of steel specimen exposure in the NaCl solutions of various concentration, M: (1) 0.001, (2) 0.01, (3) 0.1, (4) 1.0, and (5) 4.0. The details of the variation in Ecor with time depend on the concentration of solution. After the immersion into the solution, the potential of specimen surface was (0.05 ± 0.03) V in all solutions. Then, Ecor changed as a result of interaction between the surface and the solution components: aqueous solvent, which has the passivating effect, and the activating chloride ions. Judging from the experimental results, the passivating action of water prevails in the initial period of time in the solutions with the concentrations of 1 M and smaller, the potential shifts to more positive values. In a certain time (the higher is the concentration of solution, the shorter is the time), Ecorr starts to shift in the direction of more negative values, probably, as a result of pitting development. In the most concentrated 4 M NaCl solution, the potential begins to shift to more negative values almost immediately after the immersion of test specimens into the solution. It should be noted that, at different concentrations of solution, Ecorr tends to the repassivation potential Erp (see Fig. 1). The results of the study of free pitting corrosion differed from the data of potentiostatic experiments by significantly poorer reproducibility (stability) of results. When several specimens were kept in the aggressive solution under similar conditions, the development of corrosion process on them could be essentially different. In the one-month experiments at 200C, as a rule, pits were not detected with the equipment used in this work. During six-month observations we failed to obtain quantitative data on the rate of pit growth due to a wide scatter of results. As a whole, it can be indicated that, under the conditions of free corrosion, in contrast to the potentiostatic conditions, the pit depth increased only slightly, whereas their diameter increased significantly. A typical situation was the formation of corrosion regions with area of several square millimeters and a depth of several micrometers in 3 – 6 months. Along with such rather large corrosion areas, in some cases, several individual pits with significantly smaller sizes were observed on the specimen surface. At 900C, the pitting corrosion developed much more intensively than at 200C: the pit nucleation started significantly earlier (even after a one-week exposure to the NaCl solution), the pit depth reached several tens micrometers.

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The character of corrosion process was essentially different in the NaCl solutions with the additions of HCl (pH 2) or NaOH (pH 12). In 0.1 M NaCl solution with pH 12, at the temperatures of 200C and 900C, no pitting corrosion was revealed for 6 months. In the solutions with pH 2, after all of experiments, steel specimens were covered with rather thick black film (it was thicker at 900C). When the film was removed with fine emery paper, etched surface with different numbers of individual pits was observed. It should be noted that the data on the development of pitting corrosion on steel 20Cr13 cannot be extended to other steels. For example, for stainless steel 403 in 1 M NaCl solution, essentially different results were obtained: pitting corrosion did not spread over the whole surface of test specimen, but was localized on the relatively small area. In 96 h, one through pit 380 µm in diameter formed on a specimen 2 mm thick. When the specimen was kept longer (for 120 h) in the chloride solution, 2-3 smaller not through pits formed in the immediate vicinity of the large through pit. Then (in 150-300 h), an area of continuous pitting corrosion formed around these pits. In this case, the character of development of pitting corrosion corresponds to the case of the weakening of the protective layer adjacent to the large acting pit by the aggressive solution flowing out of this pit. In the experiments with thicker (4 mm) specimens of steel 403SS, the formation of through pit was also observed. It should be noted that the above-described catastrophic development of pitting on steel 403 SS was observed only in 1 M NaCl solution. At lower concentration, it was not observed. Summary The potential of pit nucleation of steel 20Cr13 in the NaCl solution was determined from the direct potentiodynamic curve and the repassivation potential was determined from the reverse curve. It is shown that thus determined pitting potential Epitpd is more positive than the corrosion potential (the open-circuit potential Eo.c.), which was measured within a short period after immersing the specimen into the solution. However, the long-term experiments, which were performed at the freecorrosion potential, showed that pitting corrosion takes place without imposing a potential by an external power source. It is concluded that the possibility of pitting corrosion of steel should be determined by comparing the corrosion potential (the open-circuit potential) not with Epitpd , but with the repassivation potential Erp. Steel 20Cr13 is prone to pitting corrosion, because Erp is more negative than Eo.c.. In the potentiostatic experiments in the neutral NaCl solutions, it was shown that, in the initial period of time, all pits grow with identical rates; then, some pits continue to deepen, whereas other pits repassivate. An average diameter of pits, first increases and, then, cease to change. Thereby, the number of pits does not vary with time; however, it increases significantly when the potential shifts in the direction of more positive values. The development of pitting corrosion is accelerated with an increase in the temperature of solution and is retarded by the electrode rotation. The results, which were obtained at the free-corrosion potential, are much less reproducible. Under these conditions, in contrast to the potentiostatic conditions in the neutral NaCl solutions, the pit depth increased only slightly and the pit width increased markedly (to a larger extent). A typical situation was the formation of corrosion regions with area of several square millimeters and depth of several micrometers in 3 – 6 months. At 900C, the pitting corrosion developed much more intensively than at 200C. In the NaCl solutions with pH 12 at the temperatures of 200C and 900C, no pitting corrosion was revealed for 6 months. In the solutions with pH 2, in all cases, steel specimens were covered with black film, under which etched surface with a certain number of individual pits was observed. This work was supported by the Program of Fundamental Research of Russian Academy of Sciences, The Division of Chemical Science and Materials “New Approaches to Improvement of Corrosion and Radiation Resistance of Materials, the Environmental Safety”.


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References [1] I.L. Rosenfeld: Corrosion and Protection of Metals (Metallurgiya, Moscow, 1970). [2] Corrosion Chemistry within Pits, Crevices and Cracks, edited by A. Turnbull. HMSO, London, 1987. [3] H.S. Isaacs, G. Kissel: J. Electrochem. Soc. Vol. 119 (1972), p. B1628. [4] R.P. Frankenthal, H.W. Pickering: J. Electrochem. Soc. Vol. 119 (1972), p. 1304. [5] P. Ernst, R.C. Newman: Corros. Sci. Vol. 44 (2002), p.927. [6] P. Ernst, R.C. Newman: Corros. Sci. Vol. 44 (2002), p. 943. [7] M. Reuter and K.E. Heusler: Electrochim. Acta Vol. 35 (1990), p. 1809. [8] C. Punckt, M. Bolscher, H.H. Rotermund, A.S. Mikhailov, L. Organ, N. Budiansky, J.R.Scully, J.L. Hudson: Science Vol. 305 (2004), p. 1133. [9] J.E. Strutt, J.R. Nicholls, B. Barbier: Corros. Sci. Vol. 25 (1985), p. 305. [10] J.W. Provan, E.S. Rodriguez: Corrosion Vol.45 (1989), p. 178. [11] R.E. Melchers: Corrosion Vol.60 (2004), p. 937. [12] R. Dreiner: J. Electrochem. Soc. Vol. 111 (1964), p. 1350. [13] M.J. Dignam: J. Electrochem. Soc. Vol. 113 (1966), p. 381. [14] L.V. Andreeva, S.M. Ikonopisov, D.D. Tokarev: Soviet Electrochem. Vol. 7 (1971), p. 1698. [15] A.D. Davydov, V.S. Shaldaev, A.N. Malofeeva, L.V. Kasparova: Russian J. Electrochem. Vol. 41 (2005), p. 1163.