Galvanizing in contact with chemicals

Galvanized coatings are highly resistant to attack over a wide pH range, particularly in moderately alkaline solutions.

Unprotected galvanized coatings should not be used with acid solutions below pH 6 or alkaline solutions above pH 12.5.

At intermediate values between these limits a protective film is formed on the zinc surface and the coating corrodes very slowly. Since this range covers most types of water and all but the strongest alkalis, galvanized coatings have wide application for storing and conveying liquids. Most organic liquids, other than those acid, attack zinc only slightly and galvanized coatings are suitable for storage tanks and equipment for handling a wide range of organic chemicals, including motor fuels, creosotes, phenols and esters.

Galvanized coatings are used in refrigeration equipment circulating brine solutions treated with sodium dichromate inhibitor.

Table 6 Compatibility of galvanized coatings with various media

Sewage treatment

Galvanized coatings perform extremely well by comparison with other protective coatings for steel in the severely corrosive conditions prevailing in most sewage treatment operations. As a result, galvanized steel is used extensively for structures in sewage treatment plants throughout the world.

Figure 11: Galvanized steel stucture for Sydney desalination plant with an additional duplex coating in the splash zone to provide additional durability
Figure 13: In the range pH 6 to pH 12.5 the zinc coating forms a stable protective film and corrosion rate is low.