Painting over a hot dip galvanized surface, commonly known as a “Duplex System,” was introduced into the world by a knighted engineer Jan van Eijnsbergen in the early fifties.
Jan found that when an appropriate paint system, of reasonable thickness, was applied over a hot dip galvanized surface, one achieved a synergistic performance from the sum of the two individual coating systems. His further developments assured that increased factors of a minimum of 1,5 to 2,3 times the sum of the individual coating lives, in varying conditions, would be achieved.
Due to some duplex failures, where the paint peels off prematurely, some people feel that the concept of the system does not work.
Paint, like any mechanically applied coating, has to be applied to a suitably prepared surface and while this is essential for painting onto carbon steel, it also equally applies to painting over hot dip galvanized steel.
In most applications when steel is hot dip galvanized, the process following hot dip galvanizing is a quench bath containing a passivation. This reduces the component temperature and passivates the freshly galvanized component, providing temporary protection against the incidence of white rust. In most instances when a duplex coating is required, the passivation is omitted, particularly if the galvanizer is aware of this requirement. Should this not happen, the passivation must be comprehensively removed prior to painting. A simple test indicating the presence of the passivation can be done.
Preparation of the hot dip galvanized substrate can include a chemical washing procedure, a high pressure water blast or a sweep blast. Sweep blasting, which is done at less than 300kPa using a micro-grit at the correct angle and distance from the surface, removes surface imperfections and zinc ash and will also comprehensively remove all passivation from accessible surfaces. This means that in the case of tubular components the inside of the tube (where it is unlikely to be painted,) will still be passivated and therefore protected against the formation of possible white rust.
Galvatech (Pty) Ltd a member of the Association swears by sweep blasting for duplexing. As Christof Krugmann, current Production Manager, says, “Paint coatings are only as good as the surface preparation prior to painting. If you do not go this route it is almost like plastering a wall without “chipping” it before hand. Correct sweep blasting provides an excellent key for subsequent painting.”
Krugmann also says, “Utilizing sweep blasting as a surface preparation might be a little more costly to start off with, but at the end of the day ensures no unforeseen expenses. Even site work like “touch up’s” are minimized by proper adhesion of the paint on galvanized structures.
Paint is not nearly as prone to chipping, not even when bolts are tightened when structures are put together on site. This observation was made by one of our clients who used to “chemically clean” galvanized steel, instead of sweep blasting before applying paint.”
Galvatech is a Full Member Galvanizer of the Hot Dip Galvanizers Association Southern Africa and as such works within the requirements of the Association’s Code of Practice for preparation of new hot dip galvanized steel for painting – HDGASA 01-1990.