Painting for enhanced chemical resistance

Hot dip galvanizing is recommended to be used within the pH range of 6 to 12 (see Figure 4).

Outside this range, its service life is likely to be unacceptable. This includes exposure to strong acids and alkalis as well as salts of strong acids and weak bases and vice versa. Hot dip galvanizing may also catalyse the deterioration of certain organic chemicals that are exposed to it. This phenomenon is rare and causes no damage to the hot dip galvanizing. 

Just as coatings provide enhanced protection in corrosive atmospheric service environments, judiciously selected paint systems can also protect hot dip galvanizing from aggressive chemicals. Such approaches are usually only taken where the chemical exposure is low or moderate. Otherwise, the safe principle to adopt in extreme exposure situations is the use of a substrate material which is inherently inert. In such situations, stainless steels and plastic composites often find advantage.

In specific chemicals exposure, the recommendations of an expert or an established successful case history should always be sought.

Figure 3: Desalination plant, Sydney during construction - the lower section of the uprights are painted to protect the surface from salt splash
Figure 4: Corrosion rate of zinc with varying pH. Between pH 6 and pH 12, the corrosion rate of zinc is very low.