Stainless steel surfaces corrosion

Figure 8. An Fe-17Cr stainless steel surface near a handle-shaped inclusion observed by AFM in 0.5M NaCl. (a) Surface at the corrosion potential of -240 mV (SCE). (b) Surface after application of a potential pulse of I s to 650 mV to initiate pitting, then anodically polarized at ISO mV (SCE). The pitting potential is approximately 350 mV. (Reprinted from Ref. 30 by permission of The Institute of Materials. London.)...

Insulating materials exposed to stainless steel surfaces should be free of chlorides to avoid the phenomenon of stress corrosion cracking that can lead to system contamination and the destruction of tanks and critical system components. 

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

Nitric acid is used by the steel industry to remove surface oxides (pickling) of stainless steels, to brighten and clean surfaces after salt-bath descaling and to prepare stainless steel surfaces for corrosion resistance (passivation). 

NACE Publication 6H189, A State-of-the-Art Report of Protective Coatings for Carbon Steel and Autenitic Stainless Steel Surfaces under Thermal and Cementitious Fire Proofing — A Technical Report by the National Association of Corrosion Engineers Task Groups 7-H-31, 1989. 

Localized corrosion of passivating metals initiates at local heterogeneities, such as inclusions and second-phase precipitates as well as grain boundaries, dislocations, flaws, or sites of mechanical damage. In the case of stainless steel surfaces, pit initiation occurs at sites of MnS inclusions. Exclusion of inclusions and precipitates, nonequilibrium... 

Production of differential aeration cell. A scatter of individual barnacles on a stainless steel surface creates oxygen concentration cells. The formation of biofilm generates several critical conditions for corrosion initiation. Uncovered areas will have free access to oxygen and act as cathodes, while the covered zones act as anodes. Underdeposit corrosion (crevice corrosion) or pitting can occur. Depending on the oxidizing capacity of the bacteria and the chloride ion concentration, the corrosion rate can be accelerated. However, the presence of a biofilm does not necessarily mean that there will always be a significant effect on corrosion. (Dexter)5... 

The pipe joint had been leaking steam and water prior to the failure, and chemical analysis of the scale deposits on the clamp surface after the failure confirmed the presence of a number of sodium-based mineral compounds from the leaking steam, including approximately 10% sodium chloride. The presence of high concentrations of moist, hot chloride salts on the highly stressed austenitic stainless steel surface, particularly with concurrent exposure to atmospheric oxygen, created an ideal chloride stress-corrosion cracking (SCC) environment. 

For example, it is not desirable to have a small anode connected to a large cathode as this favors accelerated localized anodic dissolution. Rivets of copper on a steel plate and steel rivets on a copper plate on immersion in seawater for a period of 15 months resulted in the steel plate covered with corrosion products while the steel rivets were corroded completely and disappeared. As copper is more noble than iron, it accelerated the hydrogen reduction reaction for the oxidation of the steel plate. In the case of the copper plate with steel rivets, the steel rivets corroded because of the relatively important cathodic surface of copper. The same reasoning applies to the corrosion of noncoated auto parts in contact with a large stainless steel surface (Fig. 1.6). 

Mechanical preparation techniques such as grinding can introduce significant cold work into the surface layers e.g. the pitting resistance of ground austenitic [3] and ferritic stainless steel surfaces [4] has been shown to be inferior to that of electropolished surfaces. This was attributed to the presence of cold worked surface layers from grinding, although chemical or electrochemical surface treatments can preferentially remove less resistant phases, e.g.inclusions, which would otherwise be responsible for an inferior corrosion performance. ... 

Bleach is often used to decontaminate surfaces. Ordinary household bleach, containing about 5% sodium hypochlorite, is diluted 1 100 to produce a solution of about 500 ppm. This solution is effective against all microbiological organisms, including spores. However, it is corrosive to some surfaces. If it is used in a BSC, it must be followed by a rinse with sterile water to ensure that the stainless steel surfaces do not corrode. Sometimes a detergent is combined with the dilute bleach solution to make a very effective disinfectant. 

NACE Publication 6H189. A state-of-the-art report of protective coatings for carbon steel and autenitic stainless steel surfaces under thermal and cementitious fire proofing. A technical report by the national association of corrosion engineers task groups 7-H-31, 1989. 

In order to improve our basic understanding of the fundamental reactions, the kinetics and the mechanisms of °Co deposition on stainless steel surfaces under BWR operating conditions were studied in detail in loop experiments by Lin and Smith (1988). The results of these investigations showed that the progress of corrosion on the stainless steel surface is the key controlling process for °Co deposition... 

This mechanism means that diffusion of °Co from the water phase into the oxide film may be the first obstacle for °Co species to be overcome. Larger crystals with higher porosity in the film may facilitate the diffusion process, and vice versa. Thus, the characteristics of the oxide film can be expected to play an important role in °Co deposition and incorporation on stainless steel surfaces. A thick oxide film with a high density, as is obtained by pretreatment of the surfaces with oxygen-containing water, is therefore assumed to be effective in reducing °Co deposition since it lowers the diffusion rates of cobalt into the oxide layer and reduces the corrosion rates of the underlying base materials. 

Thus, the corrosion rate seems to be the rate-determining process for Co deposition onto the stainless steel surfaces when cobalt ions are incorporated into the growing oxide layer. Various authors have described Co deposition by either a logarithmic or a parabolic rate law. From the results of their loop experiments, Lin and Smith (1988) derived the equation... 

Tidal—The tidal zone is an environment where metals are alternately submerged in seawater and exposed to the splash/spray zone as the tide fluctuates. In the submerged condition, metals are exposed to well-aerated seawater and biofouling does occur [11,121. A continuous cover of biofouling organisms protects some metal surfaces such as steel, while the presence of biofouling on stainless steel surfaces can accelerate localized corrosion. Steel is influenced by tidal flow, where increased movement due to tidal action causes an increased steel corrosion rate [121. Curve (b) in Fig. 1 shows that steel corrosion at exposed coating defect sites is as severe in the tidal zone as it is in the splash/spray zone. 

Solubility-driven reactions are not the only significant degradation mechanism for steels. Nitrogen in lithium has deleterious effects 24-28], For stainless steel surfaces exposed to molten lithium [25,26], a specific nitrogen-related corrosion product was identified (LigCrNs). This key finding has formed the basis for a much better understanding of the various corrosion processes in lithium/steel systems. Furthermore, there is also definite evidence that carbides can play an important role in the corrosion of steels by lithium [25,29-33]. 

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