Pits, corrosion in stainless steel

Takenori, N., Norio, S., Tatsuo, 1. and Okamoto, G., Electrochemical Test for Pitting Corrosion in Stainless Steels , Hakkaido Daigaku Kogakubu Kenkyu Hokoku, 44, 1 (1967) C.A., 70, 16534h... 

The conditions that lead to chloride-induced pitting corrosion in stainless steels in chemical plants are well known and have been extensively examined for this group of materials (Horn et al. 1977). 

Concerning threshold temperatures for pitting corrosion in stainless steels as dependent on the pitting resistance equivalent. 

Uhlig, H. H. and Gilman, 3. R., Inhibition of Pitting Corrosion of Stainless Steel 18/8 in Iron(III) Chloride Solutions by Nitrates , Z. Physik. Chem.,21/6, 127 (1964) C.A., 61,9231c Fisher, W. R., Pitting Corrosion, Especially of Titanium. 1 Corrosion Studies , Techn. Mill. 

Several methods to determine the resistance to pitting corrosion of stainless steel have been introduced. The methods are divided into two groups. The first group includes determination of pitting potential, CPT, in FeClj... 

It is well known that chloride ions in aqueous or humid systems promote pitting corrosion of stainless steel surfaces. 

In a different application, Williams and coworkers were interested in using the SECM to identify a precursor state to the pitting corrosion of stainless steel (12) and also to elucidate the mechanism by which a pit could be maintained and propagate further (13), once it had been initiated. For these studies, a commercial AFM (Quesant Resolver ) was adapted for use as an SECM. The tip electrode was a Pt-Ir wire electrolytically sharpened and insulated, as for electrochemical STM. A two-electrode mode was used with a Pt counterelectrode of much larger area than the stainless steel working electrode. The potential of the counterelectrode was standardized against an SCE. The probe UME was maintained at the same potential as the Pt counterelectrode. Typical tip-substrate separations of 0.1-0.5 /xm were employed for imaging purposes, with the distance carefully established by ap-... 

B. Baroux, Further insights on the pitting corrosion of stainless steels, in P. Marcus (Ed.), Corrosion Mechanism in Theory and Practice, second ed., Marcel Dekker, New York, 2004. 

Pitting, erevice and stress corrosion in stainless steels are avoided in the absence of oxygen and other oxidizers. 

Differential aeration cells can also lead to localized corrosion at pits (crevice corrosion) in stainless steels, aluminum, nickel, and other passive metals that are exposed to aqueous environments, such as seawater. 

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]. 

If carbon or low alloy steel cannot be safely used, corrosion resistant alloys must be evaluated. The production environment must be dupHcated in alloy corrosion tests in the same manner as for carbon or low alloy steels. Standard corrosion tests modified to use the production environment are conducted for the corrosion resistant alloys with due consideration given the probable failure modes, e.g., crevice and pitting corrosion for stainless steels. On completion of this test sequence, the corrosion resistant alloy or alloys to use for tubulars, wellheads, and facilities construction are specified. 

Crevice corrosion in stainless steels is also only observed in corrosives containing chloride. As with pitting corrosion, crevice corrosion is initiated after a critical potential is exceeded. This threshold potential for crevice corrosion is always at a level further into the negative range than the pitting potential for the same material in the same solution. Crevice corrosion can therefore also occur in a material that is resistant to pitting corrosion in a given medium. 

Corrosion inhibitors for steels are being continuously developed because of the ubiquitous use of steel in construction and its somewhat limited corrosion resistance, especially in the presence of water. A great number of papers are on the effect of corrosion inhibitors, and the overwhelming majority deals with the effect of inhibitors on uniform corrosion. Due to environmental restrictions on common inorganic inhibitors (Freedman, 1986), several studies suggest derivatives of some amino acids as corrosion inhibitors. A survey of a number of different organic compounds commonly used as uniform corrosion inhibitors showed that most compounds hardly affect the pitting corrosion of stainless steel however, one of... 

Pre-modem times, i.e., the 1980s, may be considered a real boom in MIC studies. By the 1980s the impact of stagnant hydrotest conditions on inducing MIC (or more accurately, microbially assisted chloride pitting corrosion) into stainless steel at chloride ion concentrations as low as 200 mg per litre was quite... 

Sulfate-reducing bacteria have also been reported to be responsible for pitting corrosion on stainless steels in aqueous environments. The mechanisms proposed are mostly related to iron and steel. However, a different mechanism has been proposed in which the role of thiosulfate in the microbial pitting of stainless steel has been emphasized [137]. The same authors have also demonstrated clearly that SRB-induced pitting corrosion of stainless steel is unlikely to occur in a uniformly anaerobic SRB medium, whereas it will occur when the anaerobic sites are coupled to an oxygen cathode [136]. 

Pitting corrosion can lead to unexpected catastrophic system failure. The spht tubing in Figure 2.17 was caused by pitting corrosion of stainless steel. 

Qin, R.-J., M. Lu, W.-K. Guo, and L. Niu, Localized electrochemical scanning study on pitting corrosion of stainless steel in acidic chloride solutions, Eushi Kexue Yu Fanghu Jishu, 21, 2009, 230. 

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