Inhibitive pigment

Table 12. Chemical Compositions and Analytical Specifications for Chromate Corrosion Inhibiting Pigments...

Zinc yellow became an important corrosion-inhibiting pigment for aircraft during World War II. However, the war production rate of 11,000 t/yr has not since been reached. Now, 2inc yellow is widely used for corrosion inhibition on auto bodies, light metals, and steel, and in combination with red lead and ferric oxide for stmctural steel painting. 

Fig. 8. Slriiclure ol dry primer film with inhibiting pigment showing range of filler volume fractions. From ref. [70].

Fig. 8 shows a primer formulated with a corrosion-inhibiting pigment such as a chromate. As discussed previously, some permeability to moisture is necessary for these pigments to dissolve and be transported to the interface where passivation of the substrate can occur. Optimum performance is generally found at PVC/CPVC Just below 1 [71]. 

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. 

Some pigments exert an inhibitive effect on the corrosion of metal. The mechanisms are complex and not always fully understood. Inhibitive pigments include red lead, zinc phosphate and zinc chromate. 

Zinc phosphate was introduced as an inhibitive pigment by Barraclough and Harrisonand in the early tests vehicles based on drying oils were used. Later it was claimed that it was an elfective inhibitive pigment when used with all paint media in current use. 

It has been shown that paint films are so permeable to water and oxygen that they cannot affect the cathodic reaction, and that the anodic reaction may be modified by certain pigments. There are, however, many types of protective paint which do not contain inhibitive pigments. It is concluded that this class of paint prevents corrosion by virtue of its high ionic resistance, which impedes the movement of ions and thereby reduces the corrosion current to a very small value. 

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. 

Inhibitive Pigment a pigment which retards or prevents the corrosion of metals by chemical and/or electrochemical means, as opposed to a purely barrier action. Red lead and zinc chromate are examples of inhibitive pigments as opposed to red iron oxide which has little or no inhibitive action. 

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. 

Zn chromate is not important commercially, but its complex with K chromate, known as Zinc Yellow, is used as a corrosion inhibiting pigment(Ref 3) especially in aircraft construction (Refs 4 6)... 

Lead chromate pigments are mainly used as corrosion inhibitive pigments. These pigments are of low opacity and can be combined with higher opacity pigments for use in primers. Zinc chromate, also an anti-corrosion pigment, is used in metal pretreatments such as wash primer, which also promotes adhesion to aluminum and steel. 

Rare earth metal compounds Inhibiting pigments in paints. Silica, alumina, iron oxide or tin oxide with adsorbed rare earth metal cation in alkyd resin-based paints [15]... 

Active pigments are anticorrosives with a chemical and/or electrochemical action. They are also described in the literature as inhibitive pigments. These pigments interact chemically, either directly or via intermediates with the metal substrate to reduce the rate of corrosion. Such intermediates can, for example, be formed by reaction with the resin system. 

SAFETY PROFILE Solutions in methanol are sensitive explosives when no moisture is present. When heated to decomposition it emits toxic fumes of Cl" and Pb. Used as a corrosion-inhibiting pigment in primers and paints, and in making storage batteries. See also LEAD COMPOUNDS and PERCHLORATES. 

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