Materials and Corrosion 2009. 60. No. 1: Pitting corrosion on 90 “”1 1.00203.004005.00 005006007,00B,09.00 Figure 10. Chemical compositions of corrosion deposits; (a) site A, (b)site B, (c)site C, and (d)site D Pitting is a kind of localized corrosion that selectively attacks Till date, there are two controversial theories for pit nucleation only small areas of the metal surface but can result in relatively [31]: the initiation of pitting. One illustrates the competitive great damage. There is a wealth of research that had been carried adsorption between CI and 0, and the other describes the out on the detailed mechanisms of pitting [10-18. With the penetration of small-radius Cl- into the oxide film. In detail, the presence of halide ions, particularly chloride ions, susceptible former introduces a process in which the o of the oxide film is tes of the passive oxide film on aluminum or stainless steel substituted by the selectively adsorbed Cl and then the metal surface may be attacked by this kind of aggressive ions such as oxides are consequently converted into soluble M+cl3 sulfide or silicon oxide inclusions [7, precipitates of carbide, complex compounds. As a result, corrosion pits are formed defects on passive film and so on. As for stainless steel, especially with the dissolution of these complex compounds. The latter hose containing manganese and sulfur, MnS inclusions were theory accounts for pitting as the penetration of small-radius Cl always regarded as the most susceptible sites for pitting [19-29). into the passive film, below which the conductive ions produced Pardo et al. [30] divided the pitting process initiated from Mns by Cl cause the cation to move in a disorderly manner and make into three stages. In this paper, a different three-stage mechanism some specific sites of the film maintaining a high current density including initiation, propagation, and termination was proposed If the electric field of the interface between the film and to enrich the mechanisms of pitting reaches a threshold value, pitting occurs Initiation riefly speaking. the initiation stage is a process lusions and formation of corrosion 5 Table 2. Chemical compositions of corrosion deposits(wt%) Element C 0 Br Si Mo C Cr Mn Fe N ite A 16 23 Site B 17.15 /8.212397.69/ Site d18.835.51/2.123.9126029.321.6831.24292 Figure 11. lon chromatograph results of corrosion deposits within the denotes the content lower than 0.5 wt% www.matcorr.com c 2009 WILEY-VCH Verlag GmbH & Co KGaA, WeinheimPitting is a kind of localized corrosion that selectively attacks only small areas of the metal surface but can result in relatively great damage. There is a wealth of research that had been carried out on the detailed mechanisms of pitting [10–18]. With the presence of halide ions, particularly chloride ions, susceptible sites of the passive oxide film on aluminum or stainless steel surface may be attacked by this kind of aggressive ions such as sulfide or silicon oxide inclusions [7], precipitates of carbide, defects on passive film and so on. As for stainless steel, especially those containing manganese and sulfur, MnS inclusions were always regarded as the most susceptible sites for pitting [19–29]. Pardo et al. [30] divided the pitting process initiated from MnS into three stages. In this paper, a different three-stage mechanism including initiation, propagation, and termination was proposed to enrich the mechanisms of pitting.
Initiation Briefly speaking, the initiation stage is a process of dissolution of MnS inclusions and formation of corrosion pits. Till date, there are two controversial theories for pit nucleation [31]: the initiation of pitting. One illustrates the competitive adsorption between Cl and O2, and the other describes the penetration of small-radius Cl into the oxide film. In detail, the former introduces a process in which the O2 of the oxide film is substituted by the selectively adsorbed Cl and then the metal oxides are consequently converted into soluble M3þ½Cl 3 complex compounds. As a result, corrosion pits are formed with the dissolution of these complex compounds. The latter theory accounts for pitting as the penetration of small-radius Cl into the passive film, below which the conductive ions produced by Cl cause the cation to move in a disorderly manner and make some specific sites of the film maintaining a high current density. If the electric field of the interface between the film and solution reaches a threshold value, pitting occurs. Materials and Corrosion 2009, 60, No. 11 Pitting corrosion on 316L pipes 905 Figure 10. Chemical compositions of corrosion deposits; (a) site A, (b) site B, (c) site C, and (d) site D Table 2. Chemical compositions of corrosion deposits (wt%) Element C O Br Si Mo Cl Cr Mn Fe Ni Site A 16.23 1.68 / 1.68 4.33 7.17 19.44 1.76 39.02 6.44 Site B 17.15 2.95 / 2.95 2.76 5.36 11.81 1.05 34.15 5.24 Site C 18.68 14.09 1.90 3.65 / 8.21 2.39 / 7.69 / Site D 18.83 5.51 / 2.12 3.91 2.60 29.32 1.68 31.24 2.92  denotes the content lower than 0.5 wt% Figure 11. Ion chromatograph results of corrosion deposits within the pits www.matcorr.com
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim