Organic coatings functionality

The relative Importance of the barrier function of organic coatings in corrosion protection has been debated for years. It is clear that, if a metallic substrate could be completely Isolated from its environment, no corrosion would occur. The degree to which a protection system bars oxygen, water, and ions from the substrate would seem likely to be a measure of the effectiveness of the system in preventing corrosion. 

Another example is the very slight delamination that occurs when a thin copper layer is overcoated with an organic coating such as a photoresist and the system is made anodic. The rate of disbonding is a function of the applied potential and hence the rate of dissolution of the copper beneath the coating. Anodic delamination occurs very slowly relative to cathodic delamination at equal potential differences from the corrosion potential. 

Webster, D. C., and Crain, A. L. Synthesis and Applications of Cyclic Carbonate Functional Polymers in Thermosetting Coatings, Progress in Organic Coatings (2000) 275-282. 

Within the past three decades tin-free steel (TFS) has been developed. This material consists of a mild steel base—exactly as used for tinplate—but with a coating of chromium-chromium oxide only about 1/30 as thick as an average tinplate coating. The function of this coating is merely to protect the steel base from corrosion prior to fabrication. TFS containers need to be coated on the inside and outside with one of many organic coatings in order to make them at least as corrosion-resistant as uncoated tinplate containers. To date the main usage of TFS has been for can ends. 

Alkyd resins have been defined as the reaction product of a polybasic acid and a polyhydric alcohol. This definition Includes polyester resins of which alkyds are a particular type. The specific definition that has gained wide acceptance is that alkyds are polyesters modified with monobasic fatty acids. In recent years, the term nonoil or oil-free alkyd has come into use to describe polyesters formed by the reaction of polybasic acids with polyhydric alcohols in non-stoichlometric amounts. These products are best described as functional saturated polyesters containing unreacted OH and/or COOH groups, and they are finding rapidly increasing uses in organic coatings. 

These various layers, although with different functions and purposes, have basically the same ingredients, which are common to organic coatings. In general, an organic coating consists of four basic constituents ... 

Fig. 22 Typical potential maps above a delaminating organic coating on iron as measured with SKP as a function of time in humid air. The defect (filled with 0.5 M KCl) is located on the left. The mapping covers the area from the defect border to the intact area.

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