Codes and Standards

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VI. PIGMENTS IN ANTICORROSlVE FINISH COATS

Pigments for finish coats must also be chosen with care. Some common types are discussed in a separate chapter. Alkaline pigments (calcium carbonate, ultramarine blue, etc.) must not be used in acidic environments, and alkalis will attack alkali-sensitive pigments (chrome yellow, iron blue, etc.). Metallic pigments may be attacked at either extreme of pH, but, used in flake form in neutral environments, reduce the moisture and oxygen transmission. Aluminum and stainless steel flakes are also ideal for high temperature coatings. Other inert finishes may be employed with nickel titanate, chromium green oxide, titanium dioxide, iron oxide (brown) and a range of calcined pigments based on mixed inorganic oxides.

If both the environment and the chemistry of the pigment are understood, design error can be avoided, and pigmentation can assist the polymer in providing the necessary protection against the environment. Thus, pigments may actually improve the ultraviolet resistance of the vehicle, its resistance to microbiological and biological agents such as marine foutants and mildew, and its physical properties.

Inert extenders may be used to advantage in finish coats to adjust the p.v.c./c.p.v.c, ratios, at tess cost than prime hiding pigments. Pyrogenic and ultrafine silicas may also be used as thixotropes and flatting agents.

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VII. CONCLUSION:

The design of effective anticorrosive coatings for steel structures is a complex discipline that borrows extensively from both corrosion science and coatings technology. Unhappily, all too often there is too little communication between these parent technologies and fewer attempts by scientists and engineers of either practice to "cross the interface" and assess the same problem from the other's perspective.

If we are to serve our respective disciplines to mutual benefit in our avowed war on corrosion, a more intensive effort to appreciate the problems involved on both sides of the interface is required of us all -- corrosion engineer and paint chemist alike. A desire to foster such effort has been the driving force behind the presentation of this chapter.

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