Tribocorrosion behaviors of 304SS: effect of solution pH

Abstract

The tribocorrosion behavior of 304SS in chloride-containing solutions with different pH (pH 7.2–9.2) was investigated under rubbing conditions of 50 N and 100 R min⁻¹ using a pin-on-disc tribometer with an Al₂O₃ pin mounted on a vertical rotating axis. When combined with rubbing, the corrosion of 304SS was accelerated due to the formation of galvanic couples between the mechanical depassivated areas and the surrounding passivated areas. In addition, the continuous sliding-over processes also make the pitting corrosion to occur more easily due to the affinity of Cl⁻ to metal cations near the active surface. However, OH⁻ has better affinity than Cl⁻; therefore, a high pH solution mitigates the local corrosion with a wear track. It was found that high pH solution exhibits good lubricity, and both the friction coefficient and total material loss reduce evidently with increasing pH. From the calculation results and wear morphologies, we can confirm that pure mechanical and corrosion-accelerated wear, including abrasion and delamination, are among the chief reasons for material loss.

---

1. Introduction

As a familiar material, austenitic stainless steels often exhibit excellent corrosion resistance because of their ability to spontaneously form a protective oxide film on the surface and become passive.^1–3 Such a passive film, however, is often susceptible to localized breakdown when exposed to a halide ion containing solution, notably chloride ions, resulting in accelerated dissolution of the underlying metal.^4–6 The corrosion behavior of austenitic stainless steels are mainly determined by the alloy composition^7–11 and solution chemistry characteristics, which include temperature,^10,11 chlorinity^10,12–15 and aggressive solution classification and concentration,^12,16 etc. Among them, the effects of pH on the corrosion susceptibility of austenitic stainless steels have been investigated in considerable detail by many researchers and is reviewed in a number of articles, reviews and books.^12,14–20

Malik et al. reported that with increasing pH, the corrosion rate of 316L steel decreases in a 1000 ppm Cl⁻ aqueous solution, being highest at pH 4 and lowest at pH 9.¹⁵ Recently, Liu and Wu found that in a 3.5% NaCl solution, the acidity of the electrolyte (pH 0.8–5) influences the passivation performance of highly alloyed austenitic 254SMO stainless steel, in terms of the film's composition, structure, growth-rate and cations dissolution behavior, but has no effect on the pitting resistance of the passive film.²¹ Similarly, earlier studies have also shown that the critical potential is not greatly affected in acid 0.1 mol L⁻¹ NaCl with pH ranging from 1 to 7; in alkaline solutions, pH 7–10, however, the critical potential was reported to shift in the noble direction upon the addition of NaOH to 0.1 M NaCl, which directly corresponds to an increased resistance to pitting; above pH 10, the potential also moves in the active direction as the pH increases, which is due to the formation of CrO₄⁻ instead of reflecting the pitting resistance.^12,22

Recently, special attention has been paid to investigations of the role of pH on the corrosion susceptibility of metals and alloys during various friction and wear processes, including abrasion, fretting and erosion etc. in chloride containing corrosive solutions, because during these processes, the mass loss of materials is often not a simple sum of the losses caused by mechanical wear and corrosion but exceed this sum somewhat.^23–28 Therefore, the influence of pH on corrosion or wear resistance is bound to be reflected by the tribocorrosion behavior of the materials. Licausi et al. found that during sliding friction and wear, i_corr is three times higher in an acidic solution (pH 3) than in a neutral (pH 6) or basic (pH 9) one for both cast and sintered alloys. However, at the OCP, the total degradation of the cast alloy is independent of the pH, while for the sintered material, it increases slightly with pH.²⁹ Despite this, Mathew did not accept these results and proposed the total degradation peaks at pH 6 because at this pH value, the reformed passive film layer is not cohesive and the tribocorrosion products are easily sheared off in the presence of motion.³⁰ Unfortunately, little work has been carried out to investigate the influence of pH on the corrosion, wear, and following tribocorrosion behavior of austenite stainless steels in chloride-containing solutions.

It is well known that austenite stainless steels (e.g. 304 and 316 steels) perform excellently as marine engineering materials because of their general and even pitting corrosion resistances. However, tribocorrosion is an important and widespread phenomenon encountered in many practical engineering situations. When they undergo various contact motions, such as impacting, rubbing and rolling, the good anti-corrosion properties will degrade due to destruction of the protective passive film on metal surface.^25,31–34 In addition, the performance of a passive film is also restricted by its formation conditions, such as the chloride concentration, temperature, and pH. Indeed, the pH is changing in different seawater environments and always ranges from 7.5 to 8.6. Therefore, in this study, a wider range of pH values (7.2–9.2) was used to investigate its effect on the tribocorrosion behavior of 304SS in chloride-containing electrolytes.

2. Experiment

2.1. Materials

304 austenitic stainless steel (304SS) in ring form (Φ_out 54 mm, Φ_in 37 mm) was used in the tests, and its composition is listed in Table 1. Prior to the tribocorrosion tests, each sample was ground with SiC grinding papers (from grade 600 to 1500 grit), degreased ultrasonically in acetone, cleaned with distilled water, and dried with N₂. Before being immersed in tribocorrosion cell, all surfaces except the upper-surface were sealed with insulating glue.

Table 1 Chemical composition of 304SS

Elements	Cr	Si	Mo	Ni	Mn	C	P	S	Fe
Content/wt%	18.50	0.59	0.30	8.12	0.88	0.05	0.015	0.028	Bal.

---

2.2. Solution preparation

Analytical reagents and double-distilled water were used to prepare the artificial seawater referenced to ASTM D1141-98. The chlorinity was 19.38, and pH was adjusted by adding 0.1 mol L⁻¹ NaOH when needed. In this work, the pH of the tested solutions was 7.2, 7.7, 8.2, 8.7, and 9.2, which is equivalent to OH⁻ ions concentrations ranging from 1.58 × 10⁻⁷ to 1.58 × 10⁻⁵ mol L⁻¹.

2.3. Tribocorrosion measurement

The tribocorrosion tests were carried out on a modified pin-on-disc tribometer and its schematic view of components was the same as that described elsewhere.³⁵ During the tests, a rotating alumina pin with a diameter of 4 mm slid against a stationary 304SS specimen under the lubrication of 400 mL seawater at different pH. Note that all tests, in the absence or presence of corrosion, were carried out under fixed frictional parameters of 50 N and 100 R min⁻¹. The friction coefficient was measured by an attached strain gauge and recorded using computer acquisition system. The mass loss was obtained by weighing the sample before and after each test. In addition, cathodic protection technology was performed by applying cathodic potential of 0.7 V to the free corrosion potential (E_corr) to eliminate the electrochemical corrosion during rubbing; therefore, it was possible to assess separately the role of corrosion and wear in the total degradation of the material, and to evaluate the synergy between them. After the tribocorrosion tests, the worn surface morphologies of the 304SS samples were examined by scanning electron microscopy (SEM, JEOL 5600, Japan).

The electrochemical characteristics of 304SS with and without rubbing in artificial seawater were studied using a conventional three-electrode cell, and the assembly of electrodes was also described in ref. 35. Potentiodynamic polarization tests were performed at a scanning rate of 2 mV s⁻¹ from −250 mV to +1000 mV with respect to E_corr of the steel. Upon the electrochemical results, several electrochemical parameters, such as the free corrosion potential (E_corr), pitting potential (E_pit), and corrosion current density (i_corr), were calculated and are listed in Table 1. The corrosion rate with (C, mm per year) and without (C₀, mm per year) wear was also calculated according to ASTM G 102-89. All tests were carried out at least three times at room temperature open to the air, and the average values are reported.

2.4. Calculations

The mass loss rate and the synergistic effect between corrosion and wear in the different solutions were calculated according to ASTM G119-09.

Based on C, and total mass loss (T), the wear rate with corrosion (W, mm per year) can be calculated using the following equation:

W = T − C (1)

By combining with the pure wear rate (W₀, mm per year) and the pure corrosion rate (C₀, mm per year) obtained from the static Tafel curve, the wear increment (ΔW_C, mm per year) and the corrosion increment (ΔC_W, mm per year) can be deduced as follows:

ΔW_C = W − W₀ (2)

ΔC_W = C − C₀ (3)

Therefore, the synergistic effect between mechanical and electrochemical material loss can be described by eqn. (4) and (5):

S = T − W₀ − C₀ (4)

S = ΔW_C + ΔC_W (5)

In addition, three dimensionless factors, i.e., the total synergism factor, corrosion augmentation factor, and wear augmentation factor, are used to depict the synergism degree of corrosion and wear.

The total synergism factor is:

The wear augmentation factor is:

The corrosion augmentation factor is:

3. Results and discussion

3.1. Effect of pH on electrochemical corrosion

Potentiodynamic polarization tests were carried out to examine the influence of pH on the electrochemical and tribo-electrochemical corrosion of 304SS, and the related corrosion parameters are listed in Table 2.

Table 2 Electrochemical parameters for 304SS in different pH seawater

pH	Corrosion					Tribocorrosion
	7.2	7.7	8.2	8.7	9.2	7.2	7.7	8.2	8.7	9.2
Ecorr/VAg/AgCl	−0.234	−0.137	−0.070	−0.001	0.034	−0.730	−0.691	−0.604	−0.598	−0.567
Epit/VAg/AgCl	—	0.136	0.199	0.233	0.265	0.009	0.058	0.096	0.111	0.208
Epit–corr/VAg/AgCl	—	0.273	0.269	0.234	0.231	0.739	0.749	0.700	0.709	0.775
icorr/μA cm^−2	2.30	0.99	0.83	0.76	0.18	347.0	337.31	300.07	227.50	209.35
βc/mV dec^−1	−51.69	−54.15	−54.13	−54.06	−56.75	−63.32	−59.40	−61.26	−56.62	−60.64
βa/mV dec^−1	26.99	48.37	50.78	49.35	48.55	65.04	66.16	63.73	57.99	63.31

---

Fig. 1a shows the typical potentiodynamic polarization curves for 304SS in alkaline, chloride solutions with different pH values. Consistent with the conclusions reported previously,¹² both the free corrosion potential (E_corr) and the pitting potential (E_pit) shift towards the noble direction with increasing pH, indicating that high OH⁻ concentration inhibits the occurrence of corrosion for 304SS.

Fig. 1 Potentiodynamic polarization curves of 304SS obtained in different solutions (a) without and (b) with mechanical rubbing.

Although it is difficult to distinguish the pitting potential for 304SS when polarized in a pH 7.2 solution, pitting did occur, as proven by the thickly dotted pits found on the surface. In addition, between pH 7.7–9.2, E_pit was found to be a function of pH. By linear regression, the following relation was established:

E_pit = 0.084 pH − 0.503 (9)

This equation can be derived on the view that competitive adsorption occurs between OH⁻ and Cl⁻ for the sites on the passive surface. As discussed in detail by Kolotyrkin,^4,36 the initiation of pitting is a process, where Cl⁻ ions constantly adsorb and accumulate on the passive surface of alloys by displacing adsorbed oxygen until a sufficient concentration corresponding to the critical potential is reached. The Cl⁻ ions succeed at the favored sites in destroying the passivity. However, when OH⁻ ions are present, they also tend to adsorb on the passive surface and displace Cl⁻ ions, due to the better affinity of OH⁻ ions for passive surface of 304SS than Cl⁻ ions.^15,36,37 Hence, in the presence of OH⁻ ions, the initiation of pitting requires a shift in the potential towards a more positive value to enable Cl⁻ ions to reach a concentration adequate enough to break the protective passive film locally. The higher the OH⁻ concentration, the more notable the shift in the pitting potential for 304SS in the positive direction.

In addition, the i_corr also increases apparently with decreasing pH. When the 304SS sample is immersed in a pH 7.2 electrolyte, i_corr increases over one order of magnitude compared to that in a pH 9.2 solution. In neutral or alkaline solutions, the cathodic reaction of electrochemistry corrosion is dominated by the reduction of dissolved oxygen occurring on the passive surface, as shown in reaction (10).

O₂ + 2H₂O + 4e⁻ ⇄ 4OH⁻ (10)

A high OH⁻ concentration is adverse to shifting the reaction equilibrium to the positive direction. In other words, a high OH⁻ concentration may restrain this oxygen reduction reaction, which provides the driving force for the anodic dissolution of 304SS. Therefore, a smaller i_corr for 304SS is displayed as the solution pH increases from 7.2 to 9.2.

In the case of tribocorrosion for 304SS, as shown in Fig. 1b, E_corr, E_pit and i_corr present the same variation trend with increasing solution pH, i.e., E_corr and E_pit become more positive, and i_corr decreases gradually with increasing pH. However, there is an essential difference between the course of the polarization of 304SS during static corrosion and tribocorrosion. The combination of corrosion and rubbing significantly shifts E_corr and E_pit towards a more negative direction with respect to those in the absence of rubbing. Under these conditions, the change in the pitting potential of 304SS with increasing pH follows the law below:

E_pit = 0.090 pH − 0.643 (11)

In addition, a larger difference between E_corr and E_pit (represented symbolically by E_pit–corr and the values are listed in Table 2) was observed than that obtained from static corrosion experiment at the same pH, which was attributed to the constant removal-recovery of the passive film on 304SS during the periodic sliding-over processes.

Sliding contact between 304SS and Al₂O₃ must destroy the completeness of the passive film and produce an electrochemically active surface with a lower equilibrium potential exposed to a corrosive solution. First, this partial destruction of protective film initiates the occurrence of galvanic corrosion, as previously described by Lucas,³⁴ and shifts the E_corr to the negative direction. In addition, the results showing that i_corr increases by even four orders of magnitude compared to that obtained under static corrosion should also be attributed to the establishment of the galvanic couples between the mechanical depassivated areas (anode) and the surrounding passivated areas (cathode). Most importantly, the continuous sliding-over processes also make pitting corrosion take place more easily. As discussed above, the dissolution of active surface is actuated by the rubbing processes in seawater, as described by reactions (12–14).

Fe − 2e⁻ → Fe²⁺ (12)

Cr − 3e⁻ → Cr³⁺ (13)

Ni − 2e⁻ → Ni²⁺ (14)

As a result of these reactions, the electrolyte very near the active surface gains a positive electrical charge relative to the surrounding electrolyte. The anions (Cl⁻, OH⁻, SO₄⁻, etc.) are attached and migrate to the active surface to achieve a charge balance, and some even react with metal cations to form Fe²⁺, Cr³⁺ and Ni²⁺ chlorides at the anode areas or pits, destroying the passivity and preventing its recurrence by dissolved oxygen as follows:^38–41

Fe²⁺ + Cl⁻ + OH⁻ → FeOCl⁻ + H⁺ (15)

Cr³⁺ + Cl⁻ + OH⁻ → CrOCl + H⁺ (16)

Ni²⁺ + Cl⁻ + OH⁻ → NiOCl⁻ + H⁺ (17)

The pH of the electrolyte very near the active surface decreases due to the formation of H⁺, which causes the further acceleration of corrosion of 304SS and even stimulates the pitting corrosion of 304SS by the nucleation of more metastable pits, as demonstrated by Burstein and Sasaki.⁴² However, the survival of these metastable pit below the pitting potential is determined by the perforated cover over the pit mouth by providing an additional diffusion barrier that enables the concentrated pit solution to be maintained.⁴³ However, the continuous rubbing processes must rupture or even remove the perforated cover, and make the process of pit growth display a wide range of amplitudes in a certain scale of potentials under tribocorrosion, evidenced by the larger E_pit–corr values.

On the basis of the experimental data reported above, it is clear that rubbing may significantly increase the susceptibilities of general corrosion and pitting corrosion, as well as corrosion rate. We can therefore draw the conclusion that the electrochemical signals are governed by the continuous depassivation caused by rubbing. In addition, the pH of the chloride-containing electrolytes also have distinguished influence on the electrochemical signals.

3.2. Effect of pH on friction and wear

To study the effects of the solution pH on the friction and wear behavior of 304SS, the friction coefficient was determined. As illustrated in Fig. 2a, corrosion significantly increases the friction coefficient, because its occurrence may deteriorate the characteristics of the contact surfaces between the 304SS sample and the Al₂O₃ pin,³⁵ resulting in an increased friction coefficient and accelerated total material loss (Fig. 1b) compared to those under pure mechanical wear. In addition, through linear regression analysis, the relevant equations of the solution pH and the total materials loss in the absence and presence of corrosion were obtained, as shown in Fig. 2b. The corrosion reactions interact with mechanical friction under these conditions to show a positive synergistic effect, and result in accelerated total mass loss. Moreover, a high pH solution performs good lubricity as well as good corrosion inhibition, so the resulting surface deterioration of 304SS is mitigated by increasing the pH, and the corresponding friction coefficient and total material loss show obvious reduction.

Fig. 2 Friction coefficient (a) and total mass loss (b) curves of 304SS in different solutions.

3.3. Morphologies of the worn surfaces

To determine the wear mechanism between 304SS and Al₂O₃ tribocouples lubricated with different solutions, the morphologies of the worn surfaces were examined by scanning electron microscopy. Fig. 3 shows that the roughness of the worn surface decreases with increasing solution pH, which coincides with the change in the friction coefficient mentioned above. This decrease in roughness proves that a different pH will have a great influence on the 304SS surface characteristics, especially roughness. In addition, many grooves, corresponding to the typical abrasion morphology, were observed on the tested surface over the entire range of solution pH. In addition, the more alkaline the solution, the more shallow the grooves due to the better lubrication of the high pH solution. In addition, laminar tearing also appears, showing that delamination wear also prevails. Consequently, the combined action of abrasion and delamination leads to the very high and continuously increased material loss of 304SS in the process of tribocorrosion.

Fig. 3 pH dependence of the worn surface morphologies of 304SS after tribocorrosion in different solutions.

3.4. Synergy between corrosion and wear

To determine the synergistic effect between corrosion and wear in different pH solutions, a large number of computations were performed, and the results are shown in Fig. 4.

Fig. 4 Synergetic contributions of mechanical wear and corrosion to each other and to the total mass loss of 304SS in different solutions.

Fig. 4a displays the synergistic effects between corrosion and wear, which were calculated using eqn (6)–(8). It is evident that corrosion and wear were claimed to interact and accelerate each other, as evidenced by all three dimensionless factors greater than 1, and the augmentation extent of wear to corrosion is much greater. With increasing pH, the acceleration of corrosion due to wear increases; rather, the acceleration of wear by corrosion decreases gradually, as shown in Fig. 4a. The opposite acceleration tendencies can be explained in the following two ways. On one hand, corrosion can accelerate wear in that its occurrence may deteriorate the characteristics of the contact surface between the 304SS sample and Al₂O₃ pin. In this situation, with increasing pH, the absolute corrosion rate decreases apparently in the presence of rubbing in chloride solutions (Fig. 4b), so the corresponding surface deterioration mitigates, and shows a reduced acceleration of wear. On the other hand, the absolute wear rate decreases gradually with increasing pH (Fig. 4c), as a higher alkaline solution has better lubricity and makes the rubbing process run more smoothly as was pointed out above. Consequently, under the same mechanical condition, after each rubbing-over process, the fresh wear track areas that correspond to the damage areas of the regenerated passive film within the wear track are small in a high pH solution. In other words, during the periodic removal-regeneration processes, the completeness of the passive film is in a state of dynamic balance, and the area ratio of the passive surface to a mechanically depassivated surface, corresponding to cathode-to-anode area ratio (A_c/A_a), is a quasi-constant when 304SS experiences tribocorrosion in the same pH solution.

In a galvanic cell, the apparent external current is nil at the OCP.²⁹ Considering the cathodic i_c and anodic i_a current densities, one equation can be written for the case of the tribocorrosion experiments:

i_aA_a = −i_cA_c ⇔ −i_a/i_c = A_c/A_a (18)

where A_a and A_c correspond to the mechanically depassivated areas and the passive areas that include the surrounding passive surface and the regenerated passive surface within the wear track, respectively. Hence, the absolute value of anodic-to-cathodic current density ratio (−i_a/i_c) in the galvanic cell is determined by the cathode-to-anode area ratio (A_c/A_a). As described above, the more alkaline the solution, the smaller the anode area, and the larger the A_c/A_a value. However, as mentioned above, a high OH⁻ concentration will inhibit the occurrence of corrosion for 304SS. Therefore, affected jointly by corrosion inhibition and lubricity, the absolute value of i_a/i_c and subsequent corrosion rate presents a downward trend with increasing pH (Fig. 4b), but the relative increase in the corrosion rate peaks at pH 9.2 (Fig. 4a). More importantly, for the selection of severe mechanical parameters, the changing trends of the total synergism factor and the total material loss rate are remarkably consistent with those of wear (Fig. 4a, c and d), demonstrating that the problem of material loss originates mainly from pure mechanical wear and corrosion-accelerated wear under these conditions.

4. Conclusions

In this study, the pH dependence of the tribocorrosion behavior for 304SS/Al₂O₃ tribocouples in seawater was examined using a pin-on-disc tribometer. Within the range of pH studied (pH 7.2–9.2), corrosion and wear were observed to accelerate each other. However, a high pH solution provided both good lubricity, as well as excellent anti-corrosion and anti-pitting properties. Therefore, with increasing solution pH, the corrosion rate and wear rate decrease. In addition, delamination and abrasion wear mechanisms coexist when 304SS is matched with Al₂O₃ under 50 N and 100 R min⁻¹ conditions, and the abrasion becomes mild with increasing solution pH. In particular, the occurrence of rubbing may shift the pitting potential to a more negative direction, and it also makes 304SS suffer more easily from corrosion.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (grant no. 51405478) and the National Basic Research Program of China (973 Program, grant no. 2014CB643302). The authors also wish to thank the reviewers for their detailed, rigorous and helpful comments.

References

1. K. Asami and K. Hashimoto, Langmuir, 1987, 3, 897.
2. S. Mischler, S. Debaud and D. Landol, J. Electrochem. Soc., 1998, 145, 750.
3. E. E. Stansbury and R. A. Buchanan, Fundamentals of electrochemical corrosion, ASM International, Materials Park, OH, 2000, p. 186.
4. J. M. Kolotyrkin, Corrosion, 1963, 19, 261.
5. H. Boehni, Langmuir, 1987, 3, 924.
6. A. P. Bond and E. A. Lizlovs, J. Electrochem. Soc., 1968, 11, 1130.
7. R. F. A. Jargelius-Pettersson, Corros. Sci., 1999, 41, 1639.
8. S. Song, W. Song and Z. Fang, Corros. Sci., 1990, 31, 395.
9. M. A. Streicher, J. Electrochem. Soc., 1956, 103, 375.
10. J. H. Wang, C. C. Su and Z. Szklarska-Smialowska, Corrosion, 1988, 44, 732.
11. R. J. Brigham and E. W. Tozer, Corrosion, 1973, 29, 33.
12. H. P. Leckie and H. H. Uhlig, J. Electrochem. Soc., 1966, 113, 1262.
13. H. H. Strehblow and B. Titze, Corros. Sci., 1977, 17, 461.
14. A. Alavi and R. A. Cottis, Corros. Sci., 1987, 27, 443.
15. A. U. Malik, P. C. Mayan Kutty, N. A. Siddiqi, I. N. Andijani and S. Ahmed, Corros. Sci., 1992, 33, 1809.
16. I. L. Rosenfeld and I. S. Danilov, Corros. Sci., 1967, 7, 129.
17. J. R. Galvele, J. Electrochem. Soc., 1976, 123, 464.
18. P. C. Pistorius and G. T. Burstein, Corros. Sci., 1994, 36, 525.
19. Y. Tsutsumi, A. Nishikata and T. Tsuru, Corros. Sci., 2007, 49, 1394.
20. M. G. Alvarez and J. R. Galvele, Pitting Corrosion, in Shreir's Corrosion, ed T. J. A. Richardson, Elsevier Ltd., Amsterdam, 2010, p. 780.
21. C. T. Liu and J. K. Wu, Corros. Sci., 2007, 49, 2198.
22. H. H. Uhlig and M. C. Morrill, Ind. Eng. Chem., 1941, 33, 875.
23. D. Landolt, J. Phys. D: Appl. Phys., 2007, 39, 3121.
24. E. Mahdi, A. Rauf and E. O. Eltai, Corros. Sci., 2014, 83, 48.
25. A. Berradja, F. Bratu, L. Benea, G. Willems and J. P. Celis, Wear, 2006, 261, 987.
26. R. C. C. Silva, R. P. Nogueira and I. N. Bastos, Electrochim. Acta, 2011, 56, 8839.
27. P. Jemmely, S. Mischler and D. Landolt, Tribol. Int., 1999, 32, 295.
28. G. T. Burstein and K. Sasaki, Wear, 2000, 240, 80.
29. M. P. Licausi, A. Igual Muñoz and V. Amigó Borrás, J. Phys. D: Appl. Phys., 2013, 46, 404003.
30. M. T. Mathew, S. Abbey, N. J. Hallab, D. J. Hall, C. Sukotjo and M. A. Wimmer, J. Biomed. Mater. Res., Part B, 2012, 100, 1662.
31. S. Mischler, A. Spiegel and D. Landolt, Wear, 1999, 225–229, 1078.
32. A. Berradja, D. Déforge, R. P. Nogueira, P. Ponthiaux, F. Wenger and J. P. Celis, J. Phys. D: Appl. Phys., 2006, 39, 3184.
33. Y. Sun and V. Rana, Mater. Chem. Phys., 2011, 129, 138.
34. L. C. Lucas, R. A. Buchanan and J. E. Lemons, J. Biomed. Mater. Res., 1981, 15, 731.
35. Y. Zhang, X. Y. Yin, J. Z. Wang and F. Y. Yan, RSC Adv., 2014, 4, 55752.
36. J. M. Kolotyrkin, J. Electrochem. Soc., 1961, 108, 209.
37. K. V. S. Ramana, T. Anita, S. Mandal, S. Kaliappan, H. Shaikh, P. V. Sivaprasad, R. K. Dayal and H. S. Khatak, Mater. Des., 2009, 30, 3770.
38. S. Asakura and K. Nobe, J. Electrochem. Soc., 1971, 118, 13.
39. S. Asakura and K. Nobe, J. Electrochem. Soc., 1971, 118, 19.
40. Y. T. Ma, Y. Li and F. H. Wang, Corros. Sci., 2009, 51, 997.
41. H. H. Uhlig and J. R. Gilman, Corrosion, 1964, 20, 289t.
42. G. T. Burstein and K. Sasaki, Corros. Sci., 2000, 42, 841.
43. G. T. Burstein, P. C. Pistorius and S. P. Mattin, Corros. Sci., 1993, 35, 57.

---