Coatings pigments, inhibiting

Active pigments (inhibitive pigments) These are anticorrosive pigments with chemical and/or electrochemical action. When water penetrates the coating film, these pigments release soluble passivating ions that help in corrosion protection by promoting the... 

The ion-exchange structure may support anionic or cationic exchange. Williams and McMurray [133] showed that release of organic anions from hydrotalcite pigments in poly(vinyl butyrate) coatings could inhibit filiform corrosion with... 

Pigment Dispersion. AMP is used widely as a pigment dispersant for water-based paints and paper coatings. In small amounts, it efficiently disperses pigments and improves pH stabiUty, viscosity, corrosion inhibition, and odor (13). When AMP is used in conjunction with other surfactants, enhanced performance is obtained with less of these ingredients in the dispersion. 

Application of protective paints consists of surface preparation of steel, priming coat and finishing coats. Wherever possible, steel should be blast-cleaned before painting. Primers thoroughly wet the metal to promote adhesion of finishing paints and carry inhibitive pigments. For example, red lead oxide will minimize the spread of rust on metal surfaces. The total thickness of fmishing coats must be at least 0.125 mm for adequate protection and life. Four coats of paint usually are necessary to achieve this. 

It is usual to define primers in terms of the principal inhibiting pigment e.g. zinc phosphate, zinc dust or zinc chromate, and the topcoats in terms of the binder, e.g. alkyd, chlorinated rubber, etc. This practice can be confusing, however, and lead to the selection of incompatible coatings. 

Improvements in process and quality control made significant contributions to the transition from iron to steel as the major ferrous construction material over a century and a half ago. For most of that time red lead was relied upon, and not without a remarkable degree of success, as the rust-inhibitive pigment in anti-corrosive paints. In the last twenty years, however, there has been a similar dramatic change from such simple paints as red lead to synthetic polymer coatings which have as complex a technology as steel manufacture itself. 

Zinc chromate (ZnCr04), also known as zinc yellow (with the approximate formula 4Zn0K204Cr03-3H20), is used as a pigment because of its excellent corrosion-inhibiting effect both in mixed paints and as a priming coat for steel and aluminum. 

Inhibitive Primers. The traditional primers are red lead and linseed oil, alkyd resins and zinc potassium chromate, proprietary epoxy, alkyd, urethane and latex systems with modified phosphate, borate and molybdate pigments. Chromate-based pigments, although in use in aircraft coatings, automotive primers and coil coating primers, are subject to environmental concern on the toxicity of hexavalent chromium which is likely to result in their replacement by innocuous products. 

Figure 1. Dose-response antigen inhibition curves derived from ELISA (enzyme-linked immunosorbant assay), utilizing an antiserum raised against synthetic pigment-dispersing factor (PDF) of Romalea, Assay conditions antigen coat, 200 fmol Romalea PDF/well primary antibody dilution 1 140,000 secondary antibody (peroxidase-labelled antirabbit IgG), 1 1000 other details as described earlier (63). When no Romalea PDF is present in the assay, the obtained value is defined as 100%.

ISO 1249 - Zinc chromate pigments - Basic zinc potassium chromate pigments and zinc tetrahydroxychromate pigments - specifies the requirements, and corresponding test methods for zinc chromate pigments suitable for use in paints and/or corrosion-inhibiting coatings [5.59]. 

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