Corrosion protection conversion coatings

Other important metal finishings to protect against corrosion are conversion coatings such as anodization (especially for aluminium), electroless plating, and electrophoretic painting. The first is done to form a passive layer, and is described in greater detail in Section 16.4. 

Yang X.F., Tallman D.E., Gelling V.J., Bierwagen G.P., Kasten L.S., Berg J. Use of a sol-gel conversion coating for aluminium corrosion protection. Surf. Coat. Technol. 2001 140 44-50... 

Application of passive films can in some cases reduce the probability of stress corrosion cracking. Conversion coatings may help to protect surfaces against initiation of stress corrosion cracking and eventually reduce the requirement for more costly remedies (annealing, shot peening, etc.). 

Cerium compounds have been found effective in reducing the corrosion rate ot aluminum alloys by inhibition ot the cathodic reaction. The cerium can be incorporated in the passive oxide film to provide a protective conversion coating [36,37]. 

A primer On metal, the purposes of a primer are to enhance corrosion protection and to give excellent adhesion. The primer will contain anticorrosive pigments, such as strontium chromate or zinc phosphate, which will slowly release ions that can repair damage or faults in the underlying conversion coating. 

Chemical conversion coatings are applied to previously deposited metal or basis material for increased corrosion protection, lubricity, preparation of the surface for additional coatings, or formulation of a special surface appearance. This operation includes chromating, phosphating, metal coloring, and passivating. 

A related but little studied area of adhesion and corrosion protection involves the chemical effects of metal substrates on coatings and other polymeric materials and conversely of polymeric materials on metals. In the curing of certain air-oxidizing coatings on steel, for example, reduction of ferric to ferrous species in the surface metal oxide, substantial thinning of the oxide, and oxidation of the coating material have been reported to occur in the interfacial... 

In order to obtain maximum corrosion protection for painted metal articles, the metal parts are pretreated with an inorganic conversion coating prior to the painting operation. These zinc or iron phosphate coatings greatly increase both paint adhesion and corrosion protection. Traditionally, a chromic acid post-treatment has been applied to these phosphatized metal surfaces to further enhance corrosion protection. 

A large segment of the metal parts produced by industry are painted for both decorative purposes as well as to increase the corrosion resistance and extend the useful life of the product. To obtain maximum quality from painted metal articles, it is of paramount importance to pretreat the metal parts with a conversion coating process. ( 1,2) Pretreatment processes contribute a significant improvement in corrosion protection and durability to metal articles by ... 

Almost all aluminum structures are painted with organic polymers for corrosion protection. The purpose of incorporating an inhibitor interface (chromate conversion coating) between the substrate and tlie paint film is to ensure protection vAien paints fail to perform( ). It has been generally acc ted that no matter what Icind of paint system, and how well it is applied, it always will have some porosity defects and will degrade with tine during service. 

The mechanism of developing corrosion protective properties in an inorganic coating principally consists of forming insoluble oxides on the netal surface. Additionally, oxides must have certain corrosion inhibition (redox) properties which can protect the nnetal substrate from corrosive species like Cl and 804 . In the case of chromate conversion coating, OCC, the oxides of aluminum and chromium have been responsible for their corrosion inhibitive properties which were derived from their soluble and insoluble portions of the... 

Chromate conversion coatings are used widely on aluminum alloys as a pretreatment for painting, though in some applications, where noncondensing atmospheric exposure is expected, they may be used as the primary means of corrosion protection. Chromate conversion coatings are used on magnesium, cadmium, and zinc, and on galvanized steel to suppress the formation of white rust. 

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