Initiation of pitting

Table 1.20 Minimum concentration of chloride ions for initiation of pitting...

Pitting is a form of localized corrosion that may perforate the metal. There is much debate about the initiation of pits but, once started, the anode is situated in the pit while the cathode is usually on the surrounding surface. 

It is found that Type 316, containing molybdenum, is more resistant to the initiation of pits. However, once a pit is started, the rate of penetration may be of the same order as for a molybdenum-free alloy. 

The initiation of pitting corrosion starts, then, with some kind of local irregularity containing metallic inclusions and continues with the penetration of Cl- (or other aggressive ion) into the protecting layer at this point. Until the work of Brown... 

The process of pitting may be divided into initiation and growth. Initiation of pitting may be caused by setting up of local cathodes due to impurities as described above. This causes the local cell potential to rise above the pitting potential. Thus, local weak spots in the oxide film break down and a local anodic area develops. Oxide films may be weakened by the... 

Since the initiation of pitting is the localized penetration of the passive film, understanding of this step requires information on the structure of passive films and the mechanisms whereby they can be destroyed locally. Understanding of either of these is complicated by the thinness of the films and the question of the passive film structure when formed by and existing in the aqueous environment as compared with its structure when removed from this environment. The latter is necessary for the use of most of the surface analysis techniques applicable to structure evaluation. As a consequence, specific conclusions as to the structure are frequently inferred rather than more directly established. 

K. OsozawaandN. Okato, Effect of Alloying Elements, Especially Nitrogen, on Initiation of Pitting in Stainless Steel, Passivity and Its Breakdown in Iron and Iron-Base Alloys, R.W. Staehle and H. Okada, Ed., National Association of Corrosion Engineers, 1976, p 135-139... 

Initiation of pitting corrosion takes place when the chloride content at the surface of the reinforcement reaches a threshold value (or critical chloride content). A certain time is required from the breakdown of the passive film and the formation of the first pit, according to the mechanism of corrosion described above. From a practical point of view, the initiation time can be considered as the time when the reinforcement, in concrete that contains substantial moisture and oxygen, is characterized by an averaged sustained corrosion rate higher than 2 mA/m [8], The chloride threshold of a specific structure can be defined as the chloride content required to reach this condition of corrosion. 

The chloride threshold for the initiation of pitting corrosion for a given structure depends on numerous factors [1, 8-10]. Major factors have been identified in the pH of concrete, i. e. the concentration of hydroxyl ions in the pore solution, the potential of the steel and the presence of voids at the steel/concrete interface. 

The great variability of the [Cr]/[OH j threshold is, first of all, a consequence of the stochastic nature of the initiation of pitting corrosion it appears that the chloride threshold can only be defined on a statistical basis. 

The technique is based on the fact that the chloride threshold increases as the potential of steel decreases. In practice, application of very low current densities (< 2 mA/m ) can bring the potential to values in which steel operates in conditions of imperfect passivity so that initiation of pitting is suppressed even if high levels of chlorides, penetrating through the cover concrete, build up at the surface of the steel. 

Monel 400, a nickel alloy containing 66.5% nickel, 31.5% copper and 1.25% iron, has a marked tendency for the initiation of pitting in chloride-containing environments where the passive film can be disturbed. Under stagnant conditions chlorides penetrate the passive film at weak points and cause pitting attack. Sulfides can cause either a modification of the oxide layer, as described for copper, or breakdown of the oxide film of nickel alloys. Pit initiation and propagation depend on depth of exposure, temperature and presence of surface deposits. Little and coworkers [30] reported selective dealloying of nickel in Monel 400 in the presence of SRB from an estuarine environment. 

When the breakdown potential is determined by cyclic potentiodynamic polarization methods, polarization of the sample at potentials below and above the breakdown potential gives information on the initiation and propagation of pits at different levels. Another method involves initiation of pits above the pitting or breakdown potential and then shifts to lower values above or below the protection potential. It is assumed that at imposed values below the protection potential the current is expected to decrease until complete repassivation. 

G. Eklund, Initiation of pitting at sulfide inclusions in stainless steel, J. Electrochem. Soc. 121 (1974) 467-473. 

B. Lin, R. Hu, C. Ye, Y. Li, C. Lin, A study on the initiation of pitting corrosion in carbon steel in chloride-containing media using scanning electrochemical probes, Electrochim. Acta 55 (2010)... 

A material such as stainless steel has a relatively high resistance to initiation of pitting. Therefore, rather few pits are formed. But when a pit has been formed, this may grow very fast. The high corrosion rate in the pit is promoted by large cathodic areas and a thin oxide film that has considerable electrical conductance, i.e. the cathodic current for each pit is allowed to be large. For such a material, it is important to avoid pit initiation completely. 

Initiation of pitting in titanium is more pronounced in Br and T than in CT solutions. The critical pitting potential in 1M Br and 1M T solutions at room temperature are 0.9 and 1.8 V, respectively [23]. Mansfeld [24], following earlier Russian investigations, showed that titanium loses passivity in anhydrous 1N HCl in CH3OH, but that passive behavior as indicated by polarization curves is restored when >0.6% H2O is present, and that the pitting potential becomes increasingly noble as more H2O is added. 

Inclusions or intermetallic precipitates can facilitate pit initiation and growth in different ways. Figure 7.54 shows the effect of inclusions that are anodic, cathodic or inert with respect to the base metal. Inert inclusions (a) do not electrochemically interact with the base alloy, but they can still play a role in the initiation of pitting if... 

Williams, D.E., Westcott, C. and Fleischmann, M. (1984) Stochastic models of the initiation of pitting corrosion on stainless steels. Journal of Electroanalytical Chemistry, 180, 549. 

A correct interpretation of the observed fluctuations requires an understanding of what processes are driving them. Generally speaking, for corrosion reactions this is not thermal fluctuations, so interpretation of the behavior in terms of Johnson noise in a simple resistor is not correct. The first approach to a rigorous analysis was made by Wilhams, Westcott and Fleischmann, in the case of the initiation of pitting corrosion of stainless steel [37-40]. 

Stewart, J. and Williams, D.E. (1992) The initiation of pitting corrosion on austenitic stainless steel on the role and importance of sulphide inclusions. Corrosion Science, 33, 457 74. 

Williams, D.E., Kilburn, M.R., Cliff, J. and Waterhouse, G.I.N. (2010) Composition changes around sulphide inclusions in stainless steels, and implications for the initiation of pitting corrosion. Corrosion Science, 52, 3702-3716. 

Urquidi, M. and Macdonald, D.D. (1985) Solute-vacancy interaction model and the effect of minor alloying elements on the initiation of pitting corrosion. Journal of The Electrochemical Society, 132, 555-558. 

The effect of the dissolved oxygen level on corrosion is dependent on the metal. For metals that form passive films like stainless steel and aluminum, a high oxygen content is favorable in that it helps to delay the initiation of pitting on... 

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