Pitting corrosion prevention

In most cylindrical carbon—zinc cells, the zinc anode also serves as the container for the cell. The zinc can is made by drawing or extmsion. Mercury [7439-97-6J has traditionally been incorporated in the cell to improve the corrosion resistance of the anode, but the industry is in the process of removing this material because of environmental concerns. Corrosion prevention is especially important in cylindrical cells because of the tendency toward pitting of the zinc can which leads to perforation and electrolyte leakage. Other cell types, such as flat cells, do not suffer as much from this problem. 

Figure 2-11 shows weight loss rate-potential curves for aluminum in neutral saline solution under cathodic protection [36,39]. Aluminum and its alloys are passive in neutral waters but can suffer pitting corrosion in the presence of chloride ions which can be prevented by cathodic protection [10, 40-42]. In alkaline media which arise by cathodic polarization according to Eq. (2-19), the passivating oxide films are soluble ... 

Robinson, F. P. A., Pitting Corrosion Cause, Effect, Detection and Prevention , Corrosion Technol., 7, 237, 266 (1960)... 

Water treatment monitoring and control is often a knife-edge operation and must be tailored to the overall operation of the boiler because waterside and gas-side problems usually are interlinked. Consequently (and as with other types of WT boiler), not only should the utility boiler FW be essentially free of dissolved oxygen to prevent waterside pitting corrosion of the economizer and other boiler components, but also the temperature must be high enough to prevent dewpoint condensation and subsequent acid attack on the gas side of the economizer tubes. 

The relatively thick zinc hydroxide film can effectively prevent underdeposit corrosion and other problems. As a further example, in medium hardness waters, with careful control of the water chemistry, this thick Zn(OH)2 film can help offset risks of chloride-induced pitting corrosion with estuarine makeup water. 

Notes Limit 1000 ppm T. Aik. Prevention of pitting corrosion needs clean metal surfaces ... 

The best protection against stray current is, therefore, provided by concrete. Those methods that can improve the resistance of concrete to carbonation or chloride contamination, which are illustrated in Chapters 11 and 12, are also beneficial with regard to stray-current-induced corrosion. It should be observed that this may not be the same for preventative techniques, since conditions leading to corrosion initiation due to stray current are different, in terms of potential, from those leading to corrosion initiation due to carbonation or chloride contamination. For instance, the use of stainless steel or galvanized-steel bars, which improves the resistance to pitting corrosion in chloride-contaminated concrete (Chapter 15), does not substantially improve the resistance to stray current in chloride-free and non-carbonated concrete [4]. In any case, a high concrete resistivity will reduce the current flow due to stray current. 

The tendency of halides to form metal halide complex is very important in understanding the stabilization of corrosion pit by prevention of the repassivation of a defect site within the passive layer. Among the halides, fluoride forms strong complexes with metals. The resistance of chromium to localized corrosion is because of slow dissolution kinetics of Cr(III) salts. Higher-valence oxides are the best passivators (films) because of their slow rates of dissolution. 

Coatings such as modified epoxies, coal-tar epoxy, or solvent-borne epoxy have been used by many shipbuilders since 1998. The problem with these coatings is that the lifetime of these coatings is such that they can give protection for only 8-10 years, which is less than the design life of the vessels. These coatings are not resistant to the temperatures in double-hulled tankers and the bacteria that causes pitting corrosion on the bottom of tanks. The use of solvent-free epoxy on the top and bottom of a crude tanker should prevent corrosion of the tanker. 

Inorganic inhibitors such as chromate, nitrite, nitrates, and arsenate are very efficient for ferrous alloys. They prevent stainless steel pitting corrosion by forming a monolayer or multilayer passivating films. For chromate, the film is formed by iron oxidation (anodic... 

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