Painted items cause problems in galvanizing, but the reason and the solution isn't always clear to fabricators.
Most of the time involved in galvanizing is taken in cleaning the steel fabrications to be coated. Of the 3-8 hours that can be involved in processing any one item, all but about 30 minutes are cleaning.
Cleaning mainly involves removing rust and millscale. These are oxides of iron, and are removed chemically by immersion in acid, usually hydrochloric acid. This acid is a water based solution, and it will not penetrate grease, oil or paint. As its difficult to fabricate without some oily desposit ending up on the steel, a degreaser is used prior to the acid. But this degreaser doesn't remove paints.
If the steel does not have all the oxides removed, then that uncleaned part will not galvanize, and leave a bare patch in the coating. The coating isn't just a layer of zinc on the surface, galvanizing is an alloying reaction with the steel, and if its not chemically clean, no reaction takes place. A dirty piece of steel immersed in zinc gets hot, (450C) but not coated!
If the paint is not removed, the oxide underneath doesn't get cleaned, and while the paint will burn off in the hot zinc, the steel underneath gets no coating. So afterward someone will ask "Why is that bit not galvanized?". The culprit paint has by then eliminated its evidence, in being burned off.
What constitutes "painted" isn't always obvious! Pipe and pipe elbows are good examples of material that often is supplied by stockists with a coating. Elbows often come with black paint, and pipe often with clear shellac like coating. It must all come off before galvanizing, and the most common method is shotblasting. The cost of this extra work often exceeds the cost of the galvanizing, but is mostly avoidable.
Buy self coloured material !
Don't use previously painted steel thinking you are saving cutting into a new length and being economical! The removal of that paint will cost more that you'll ever save by using old stock.
Painted elbow.
Welding elbows are available in either "painted black" or "self coloured". When welding in a painted elbow, as above, you will incur extra costs in removing that paint unnecessarily.
Self Coloured elbow
A painted pipe spool presented for galvanizing. The paint must be shotblasted off first, costing more than the galvanizing does!
The cost of paint removal to allow galvanizing almost always exceeds the cost of galvanizing!
