Paint types zinc silicate

A special type of coating for steel is used where exposure to crude oil and seawater is encountered, as in oil tankers. This involves an inorganic paint containing zinc powder which gives cathodic protection against localized corrosion. This has already been discussed in connection with the uses of lithium silicates in Chapter 2. Some further points are as follows. 

In zinc-rich paints some early patents indicate the importance of a silica component. Colloidal silica was reacted with finely divided zinc to form a colloidal zinc silicate in which the excess zinc metal was suspended (654). A water-insoluble binder for zinc-containing coatings was produced by mixing alkali-stabilized colloidal silica with lithium hydroxide in suitable proportions (655). Evolution of gas from mixtures of colloidal silica with zinc powder is prevented by adding an indigoid compounds (656). Another formulation involves quaternary ammonium polysilicate solution milled with lead oxide as the binder for zinc (657). Some of the problems stem from the impurities in the zinc powder which promote reaction with the medium (658). Adhesion. of paints of this type of steel is improved by adding up to 2% of a styrene-acrylic resin dispersion (659). 

Silicate binders are used in conjunction with zinc powder to give paints of excellent corrosion resistance. The organo-silicates, e.g. ethyl orthosilicate, are most commonly used. The full potential of this type of binder has probably not yet been exploited. 

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