Pit initiation process

Three basic characteristics of manganese sulfides are worthy of notice [5b] as far as they can influence the pitting initiation process (a) They are electronic... 

Pitting corrosion always remains a worthy subject for study, particularly with reference to mechanism, and the problem conveniently divides into aspects of initiation and growth. For 6061 alloy in synthetic seawater, given sufficient time, pit initiation and growth will occur at potentials at or slightly above the repassivition potential . In an electrochemical study, it was found that chloride ions attack the passive layer as a chemical reaction partner so that the initiation process becomes one of cooperative chemical and electrochemical effects . 

Three basic mechanisms of pit initiation have been advocated in the literature (see, e.g., Strehblow117) as applying to pitting processes at any passive metal. They are shown schematically in Fig. 23. 

It is much less clear how the adsorption leads to such a dramatic change as a potential decay of several hundred volts, occurring within milliseconds. This short time is difficult to associate with film thinning, as assumed in the adsorption mechanism of pit initiation. It is not only that the mechanism of dissolution changes so much that the current efficiency falls from virtually 100% to virtually zero, but also that the resistance of the oxide decreases by orders of magnitude. The control of the process is, to a great extent, taken over by the events at the O/S interface, judging from the capacitance values measured,115 which approach those typical of the electrochemical double layer (cf. Fig. 22). 

Interface Potential and Pit Initiation. It is generally accepted that pit initiation occurs when the corrosion potential or potentiostatically imposed potential is above a critical value that depends on the alloy and environment. However, there is incomplete understanding as to how these factors (potential, material, and environment) relate to a mechanism, or more probably, several mechanisms, of pit initiation and, in particular, how preexisting flaws of the type previously described in the passive film on aluminum may become activated and/or when potential-driven transport processes may bring aggressive species in the environment to the flaw where they initiate local penetration. In the former case, the time for pit initiation tends to be very short compared with the initiation time on alloys such as stainless steels. Pit initiation is immediately associated with a localized anodic current passing from the metal to the environment driven by a potential difference between the metal/pit environment interface and sites supporting cathodic reactions. The latter may be either the external passive surface if it is a reasonable electron conductor or cathodic sites within the pit. 

Surface films appear to play a major role in the initiation of SCC and may also contribute to HE effects. The main role of the surface film is to localize the damage inflicted on the material by the environment. The damage can be caused by the mechanical breakdown of the passive film by slip step or electrochemical breakdown of the passive film (73). SCC may be related to the nature of the surface film. The SCC of carbon steels is related to the presence of magnetite in environments at 90 °C except when pitting is involved in the crack initiation process, as in nitrate medium or in high-temperature water (115, 116). 

Lin et al. [85] developed scanning electrochemical probes to monitor in situ locaHzed corrosion processes occurring on reinforcing steel in NaCl solutions. Chloride ions were found to preferentially adsorb and accumulate at defective sites with MnS inclusions, resulting in pit initiation and propagation. 

In wet atmospheres, nickel initially forms NiO and (NiOH)2 [35,36]. Nickel sulfates are present as corrosion products on the surface in outdoor exposures [37].Jouenefatmospheric corrosion of nickel in industrial, urban, and rural atmospheres. Nickel corrodes through a pitting corrosion process. The highest corrosion rates were observed in industrial areas. The corrosion products were mainly sulfates, chlorides, and n ligible amounts of nitrates surrounded by carbonate species. The pitting corrosion process occurs in two steps on nickel surfaces exposed to an outdoor atmosphere, as shown in Fig. 10.9 [38]. 

Most AFM studies of dissolution and corrosion processes have focused on technologically relevant materials such as aluminum alloys [25-27], copper [28], and steel [29-32]. For example, in situ electrochemical-AFM was used to investigate the corrosion activity of Al-6061 -T6 (alloyed with Fe) immersed in 0.6 mol dm NaCl, near iron-rich intermetallic (AI3 Fe) inclusion sites [26]. Inclusion sites are thought to be favorable areas for pit initiation and represent an important area of corrosion research. By marking the sample using a photoresist grid, it was... 

M. Elboujdaini, Y.-Z. Wang, R. W. Revie, R. N. Parkins, and M. T. Shehata, Stress corrosion crack initiation processes Pitting and microcrack coalescence. Paper No. 00379, CORROSION/2000, NACE International, Houston, TX, 2000. 

Localized corrosion usually involves change in pH and chloride ion concentration around corrosion pits. Therefore, potentiometric SECM tips can be used to study corrosion processes. When a pH microsensor was used as the SECM tip for the in situ measurements during localized corrosion of stainless steel, it was found that pH decreases at the pit initiation stage and increases during pit growth and repassivation (113). 

A comparison was made between pitting of the carbon steel in the sterile medium and a medium containing a culture of an aerobic heterotrophic bacterium. Within the sterile medium, the OCP stays almost constant at -200 mV (SCE) and shows small potential transients caused by pit initiation and immediate repassivation. Complementary SVET analysis confirms the initiation and repassivation processes. Within the bac-teria-containing medium, the OCP starts at -300 mV (SCE) but drops to values of about -600 mV (SCE) after a few hours. Correspondingly, the SVET analysis shows the formation of a single anodic side which does not repassivate but spreads over a large area of the surface, as seen in Fig. 7-34. By certain control experiments the authors could verify that the bacteria, and not a... 

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