13-9] MOISTURE 709 



use of vapor-phase inhibitor wrap, plastic film barriers, and dry nitrogen 

 atmosphere. 



Chemical corrosion can be prevented by the application of protective 

 coatings of certain metals or organic finishes over the base metal susceptible 

 to the corrosive effects of the atmosphere. The three most common forms 

 of surface protection are anodizing processes, chemical film treatment, and 

 electroplating. Anodizing consists of the formation of an aluminum oxide 

 layer on the surface of aluminum alloys by an electrochemical treatment. 

 Formation of a chromate film on an aluminum surface is an example of 

 chemical film treatment. However, this method of surface protection 

 does not offer protection from electrochemical corrosion. The electroplating 

 method of surface protection is employed when it is necessary to provide a 

 good electrical current path or when soldering is necessary. Surface 

 protection for magnesium parts requires methods similar to those used for 

 aluminum alloys. For steel parts, zinc or cadmium plating is used. Copper 

 is usually protected by nickel or silver plating. 



Electrochemical corrosion can be avoided by preventing contact between 

 dissimilar metals. If it is necessary to use different metals in contact with 

 each other, the following design principles should be followed. At the 

 junction, one of the metals can be plated with a film which is similar to the 

 other adjacent metal. This, in effect, will decrease the electrolytic potential 

 differences. The plating of cadmium or zinc on steel in contact with a 

 material such as aluminum will alleviate the damaging effects of electrolytic 

 corrosion. An inert material as a gasket or washer will act as a mechanical 

 insulator. The use of organic coatings or a zinc chromate primer coat on 

 the contact faces of the metals is satisfactory for most metals except 

 magnesium. The application of a coat of clear lacquer over the joint after 

 assembly is necessary when using an organic coat. The application of 

 corrosion inhibitors to the faces of each of the metals and the sealing of all 

 joints containing dissimilar metals are other forms of protection. Perhaps 

 the best course to follow when it is necessary to have dissimilar metals in 

 contact is to assemble the piece parts in such a way that the larger piece is 

 the anodic (corroded area) whereas the smaller part will be cathodic or 

 protected. This will allow a higher degree of structural safety beyond 

 the above-mentioned protection methods employed in the design. 



The reduction in the damaging effects from corrosion can be accomplished 

 by careful design and use of proper materials and finishes. For example, 

 electrical insulators constructed from ceramic material should have glazed 

 surfaces or the surfaces should be sealed from moisture. Terminal strips and 

 boards should be fabricated from material with low moisture absorption. 

 Techniques employing the principles of impregnating and potting will 

 provide a measure of protection from the effects of moisture and associated 



