The painting of hot dip galvanized steel

Background

Although hot dip galvanized steel offers protection in most environments, it is occasionally painted to either provide added protection or merely to enhance appearance. An advantage of painting over hot dip galvanized, rather than bare, steel is that no underfilm creep occurs as illustrated below.

Many people have applied "paint" over a galvanized surface only to see it peel off within months. The reasons for these paint failures are easy to explain and easy to avoid. Poor surface preparation or incompatible paints account for the majority of failures. Household alkyd enamels should not be used directly onto galvanized surfaces as the alkyd saponifies, i.e. reacts with the zinc surface to form soaps which cause loss of adhesion of the paint layer. The simple descriptions below provide guidelines to ensure good paint adhesion and longevity.

Surface Preparation

It is often said that a paint coating performs as well as the surface preparation given. This is especially true when over-coating hot dip galvanized steel. Surface preparation involves the production of "water-break" cleanliness, i.e. water should not form droplets on the surface but rather maintain a uniformly wet surface.

The following pointers should be noted:

• If possible, specify hot dip galvanizing without passivation (if not it must be removed to avoid impaired adhesion).
• Common sense should be applied and ideally painting restricted to periods of the day when the relative humidity is less than 50% and temperatures not less than 10°C.
• For household applications, use a proprietary galvanized iron cleaner to remove normal zinc corrosion products and other contaminants. Thorough rinsing is essential, plus, when dry, complete priming within a working shift.
• Where wet storage stain has occurred this should be removed using a weak alkali solution (such as 2% ammonia).
• Again, thorough rinsing is essential, plus, when dry, complete priming within a working shift.
• An old method of cleaning is the use of vinegar (weak acetic acid). This method can provide variable adhesion results as the soluble zinc acetate is often not fully washed off.

For more onerous applications, light sweep blasting is often recommended.

Coating Systems

Clearly, paint selection is dependant upon application and the type of corrosion environment encountered. (see Michell's Plain case history)

References

Specification for the Performance Requirements of Coating Systems (No. HDGASA 02-1990. Code of Practice for Surface preparation and Application of Organic Coatings (No. HDGASA 01-1990.

Specification for abrasive sweep-blasting of hot dip galvanizing.

• Blast pressure - 300kPa maximum.
• Media grade 0.2 to 0.8 mm.
• Media type(<5 mhos hardness) – clean silica and slags, alumina, limestone.
• Angle of blasting to surface 30-60°.
• Distance from surface 300-600mm.
• Nozzle type minimum – 10mm venturi type.
• Grit should not be recycled.

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