Solutions for the problems of powder coating over hot dip galvanizing
1. Incomplete curing:
- Polyester powder coating powder is thermosetting resins that cross-link to their final organic form by being maintained at a temperature (typically 180 o C), for about 10 minutes. Curing ovens are designed to provide this time at temperature combination. With hot dip galvanized items, with their heavier section thickness, it is necessary to ensure that sufficient stoving time is allowed to meet the curing specifications. Pre-heating of the heavier work will assist in accelerating the curing process in the curing oven.
- The final stage in the hot dip galvanizing process involves water quenching of the work, frequently in a weak sodium dichromate solution. This process cools the work so that it can be handled and passivates the surface of the galvanized coating to prevent early oxidation of the surface. The presence of a passivating film on the surface of the galvanized coating will interfere with the zinc phosphate or iron phosphate pretreatment, and in many cases, render these pre-treatments ineffective. It is essential that hot dip galvanized items are not quenched after galvanizing. This ensures that the zinc surface is in a highly reactive state to accept the pretreatment applied in the powder coating process.
- Pinholing is caused by the formation of small gas bubbles in the polyester coating during the stoving/curing cycle. These bubbles form small craters on the surface and are unsightly.The main reason for pin holing appears to be that the discrete polyester resin particles in contact with the galvanized steel surface do not fuse at the same time as those on the surface of the polyester powder film, because of the mass of the galvanized steel , and the time taken for it to come up to fusion temperature.
- Specially formulated resins with ‘degassing’ agents have been developed to alleviate this problem by delaying the onset of fusion of the powder. Pre-heating the work in a pre-heat oven prior to powder application allows heavier hot dip galvanized sections to be powder coated and deal with the problem of pin holing when used in conjunction with ‘degassing’ grades of polyester powder coating.