Galvanic corrosion coating

Initially, knowledge of the process is required. It is assumed that the component is free m defects, e.g. porosity, as this will affect surface integrity, and free from residual stresses caused by any previous manufacturing process. There is also a risk in the reduction of component fatigue life associated with some surface coating processes. The compatibility between mating surfaces in service must also be addressed because of possible galvanic corrosion failure... 

A detailed discussion of galvanic corrosion between dissimilar metals in contact in a corrosive environment has been given in Section 1.7, but in the case of coating discontinuities the effect of the anode/cathode area relationship and the nature of any corrosion products formed at small discontinuities may modify any choice made on strict considerations of general galvanic corrosion theory based on the potentials of the coating and substrate in the environment under consideration. 

Most of the published data on galvanic corrosion concern solid metal couples rather than bimetallic coating systems, and it is important to bear... 

The coating is, in addition, useful in preventing galvanic corrosion . Plated on steel which is to be used in contact with aluminium alloys, it protects the steel and does not stimulate the corrosion of the light alloy and is itself not consumed as rapidly as a 100% zinc coating. 

The test assembly used originally by Subcommittee Vlll of ASTM Committee B-3 in its comprehensive studies of atmospheric galvanic corrosion had the disadvantage that it depended on paint coatings to confine corrosion to the surfaces in actual contact with each other. In interpreting the results, it was frequently difficult to decide how much corrosion was due to galvanic action and how much to a variable amount of normal corrosion through failure of the paint system. 

The basic objective of the conversion coating process is to provide a corrosion-resistant film that is integrally bonded chemically and physically to the base metal and that provides a smooth and chemically inert surface for subsequent application of a variety of paint films. The conversion coating processes effectively render the surface of the basis material electrically neutral and immune to galvanic corrosion. Conversion coating on basis material coils does not involve the use of applied electric current to coat the basis material. The coating mechanisms are chemical reactions that occur between solution and basis material.1-4... 

The most common overcoats, however, are sputtered carbons. Their role in disk corrosion has been described in contradicting ways. Whereas Garrison [141] clearly observed that carbon, like Rh, can enhance galvanic corrosion, Smallen et al. [131] believe that carbon decreases corrosion by preventing lateral growth of corrosion products. Results of similar tests are sometimes contradictory Nagao et al. [145] have shown an improvement of the corrosion resistance of carbon-coated CoCr alloys on T/H test (with either SOz gas or NaCl mist), whereas Black [146] finds that pyrolitic carbon over a CoCrMo alloy results in elevated corrosion rates. 

Today about one-third of all the zinc metal is used for the process known as galvanization. This process provides a protective coating of zinc on other metals. A thin layer of zinc oxidizes in air, thus providing a galvanic corrosion protection to the iron or steel item that it coats. Several processes are used to galvanize other metals. One is the hot dip method wherein the outer surface of the item to be galvanized is pickled and then immersed into a molten zinc bath. A... 

The type of conversion process will depend on the substrate, the nature of the oxide layer on its surface, and the type of adhesive or sealant used. The formation of a noncon-ductive coating on a metal surface will also minimize the effect of galvanic corrosion. 

Berger, D.M., Electrochemical and galvanic corrosion of coated steel surfaces, in Corrosion Source Book, NACE, Houston, TX, USA, pp. 11-14, 1984. 

Metallic coatings are either more noble than the substrate, as in chromium plate on mild steel, or are base metals which corrode more easily than the substrate. In the former case, the underlying metal is protected by a continuous impervious film of the noble metal which is itself resistant to attack. This method is adequate provided the surface coating contains no holes or flaws and remains intact. Penetration of this layer by the corrosive agent leads to galvanic corrosion at the interface of the two metals. If the object is coated with a more base metal, then protection is by both the physical barrier of the metal film and by cathodic protection at any subsequent defects (Fig. 19). 

Nevertheless, there were reliable reports of corrosion occurring with molybdenum disulphide in films and in greases. Several such reports arose from the US Army , originating with a salt fog test of a missile launcher in which all parts coated with solid film lubricant rusted badly. Subsequent reports described galvanic corrosion of various metals with molybdenum disulphide in moist atmospheres. 

The three metals commonly used in pharmaceutical packaging are tin, aluminum, and steel. Because of its susceptibility to oxidation and corrosion, steel must be galvanized or coated by an epoxy before use, so its application is primarily reserved for drums of bulk material where very high strength is required. Metals can also be formed into pressure cylinders for the containment of gaseous product. ... 

In the traditional enamelling methods, the adhesion is aided by the ground coat enamel and by adhesive oxides which arc responsible for galvanic corrosion of the metal and thus for coarsening the surface. More recent advances in technology allow direct application of the cover coat enamel onto the metal surface. This method is possible in the case of titania enamels and requires steel of special composition, either alloyed with titanium or carbon-free. Another possibility is based on the deposition of Ni on the metal surface. It is also necessary to adjust the slip composition and the coating thickness suitably the fired enamel thickness is only 0.1 —0.2 mm, compared with the usual 0.4 mm. 

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