Localized corrosion prevention

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

Dezincification As either plug dezincification (localized corrosion) or layer dezincification (general corrosion) and refers to zinc (Zn) being selectively leached out of brass. It can be prevented either by reducing the Zn content to below 15% or by the addition of trace amounts of inhibiting elements such as arsenic (As)... 

Design of hydraulic systems to avoid excessive velocities or localized turbulence or lo maintain a velocity high enough to prevent ihe accumulation of corrosion products or other deposits that will promote localized corrosion. 

Under these circumstances, appropriate water management objectives should probably include a requirement to take all measures to prevent or minimize localized corrosion processes occurring that could cause pitting, crevice attack, tuberculation, etc. This will undoubtedly require close attention to maintaining clean waterside metal surfaces, but may also require more tolerance of an acceptable rate of general corrosion, to, say, 4 mpy, or a little higher. 

Prevention of Localized Corrosion. Available data on the various physical and chemical aspects of passivity, including the composition, thickness, structure, growth, and properties of passive layers should be used in the studies of localized corrosion. A good understanding of the surface reactions involved in the formation and composition of passive films, passivation/repassivation, is necessary for the development of highly... 

Stress corrosion cracking is a form of localized corrosion, where the simultaneous presence of tensile stresses and a specific corrosive environment prodnces metal cracks [157, 168]. Stress corrosion cracking generally occnrs only in alloys (e.g., Cn-Zn, Cu-Al, Cu-Si, austenitic stainless steels, titaninm alloys, and zirconinm alloys) and only when the alloy is exposed to a specific environment (e.g., brass in ammonia or a titaninm alloy in chloride solutions). Removal of either the stress on the metal (which must have a surface tensile component) or the corrosive environment will prevent crack initiation or cause the arrest of cracks that have already propagated. Stress corrosion cracking often occurs where the protective passive film breaks down. The continual plastic deformation of the metal at the tip of the crack prevents repassivation of the metal surface and allows for continued localized metal corrosion. 

As mentioned in the foregoing text, localized corrosion of metals starts with a local breakdown of passive films and localized metal dissolution then occurs at the breakdown site. The local film breakdown is caused by the adsorption of aggressive anions such as chloride ions, CP, which are hard bases. No reliable information is available about the acid-base characteristic of the passive film surface. It is however expected that the breakdown of passive films will be prevented, if some anions or molecules are firmly adsorbed expelling chloride ions from the film surface. In literature, though, we have seen almost no reliable studies on this subject. 

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