Crevice corrosion forming paints

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

Moreover, corrosion processes may run on metals covered with varnish -and paint materials in an atmosphere and lead to the formation of numerous tortuous filiform areas of damage. This type of failure, called filiform corrosion, is a specific form of crevice corrosion. The filiform type of corrosion does not, as a rule, lead to metal failure but only impairs its appearance. Filiform... 

This is a special form of filamentary corrosion occurring on metallic surfaces and is related to crevice corrosion also known as underfilm corrosion. This form of corrosion is generally apparent under painted body of some used cars. It appears as a blister under the paint. The filament propagation underfilm may appear split or joined together, as they propagate in direct lines, some of them reflecting because of obstacles such as adhesive parts of the organic film to the substrate that become trapped in a very narrow place (17). 

Crevice corrosion (CC) occurs beneath flange gaskets, nail and screw heads and paint coating edges, in overlap joints, between tubes and tube plates in heat exchangers etc. The same form of corrosion develops beneath deposits of, e.g. corrosion products, dirt, sand, leaves and marine organisms, hence it is called deposit corrosion in such cases. 

On a laboratory scale, it has been demonstrated in recent years that thin films of silanes applied to metals can protect the metal from many forms of corrosion, including uniform corrosion, pitting corrosion, stress corrosion cracking, crevice corrosion and other forms, in addition to providing excellent and durable paint adhesion (see Durability - fundamentals). Such thin films are typically of not more than 300-mn thickness. They can be applied by immersion of the metal into a dilute silane solution, for example, 5% in water or water/alcohol mixtures, as not all silanes dissolve in water. Brushing, wiping or spraying application methods can also be used. 

Filiform Corrosion. Filiform corrosion is a special form of crevice or oxygen cell corrosion occurring beneath organic or metallic coatings on steel, zinc, aluminum, or magnesium. Filiform corrosion normally starts at small, sometimes microscopic, defects in the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. 

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