forming pre-plated galvanized steel

Different results are obtained with "pre-plated" (electroplated or metal-sprayed) steel, and hot-dipped galvanized steel.

If electroplated or metal-sprayed steel is used, invariably at the point of deformation of the steel, whether by cutting, roll-forming, bending, drilling or punching, minute gaps are left between the plated or sprayed layer and the steel.

This enables water and carbon dioxide to lurk in the gaps, causing corrosion quite quickly.

The steel is partially protected by the zinc coating, because the zinc is higher up the electrochemical series than iron, thus the zinc is corroded away because it becomes a "sacrificial anode" in the small batteries thus made.

A different story is told for hot-dipped galvanized steel, because the steel is sand-blasted/acid pickled to remove scale from the rolling mill, then dipped into a bath of molten zinc.

The zinc forms an alloy by dissolving into the exposed surfaces of the steel.

Once again, after the galvanized steel is deformed, whether by cutting, roll-forming, bending, drilling or punching, the minute gaps do not exist.

In this second case, the exposed zinc oxidises, and the zinc/iron alloy also protects the base steel far better.

In all cases, for longevity, it is better to do the formation work of cutting, roll-forming, bending, drilling or punching, before sending the articles off to be hot-dipped galvanized.

The longer the base steel is immersed in the molten zinc determines the length of the non-corrosion period, because the zinc/iron alloy depth increases with that longer immersion.

The galvanizing company weighs the article before and after the hot-dip process, you pay for heavier steel (which absorbs more heat), also more zinc and the energy required to keep 40 tons or more of that zinc molten.

Properly prepared steel articles, hot-dipped galvanized after full forming is done, can survive for over 80 years, in sea water, which is normally quite corrosive.

Galvanized sheet metal has less resistance then regular sheet metal so it forms easier.

Unless your dies are rough you shouldn't have too much trouble with the galvanizing rubbing off during forming.

You shouldn't have any trouble with loosing the galvanizing integrity with a good clean punch either. Where you are most likely to loose the integrity of the galvanizing is if you have to grind the burrs after punching. Grinding or sanding disk will take off the galvanizing quickly.

If you have a problem with losing the galvanizing integrity you can purchase a can of cold galvanizing primer and spray the bare areas. It is dark gray and if you spray over that with silver paint it will blend in pretty good. Don't go cheap on the silver paint, some of the cheap brands will rub off on your hands. This is good for blending in any welding done with the galvanized material.

Thanks to both of you for your input. It was very helpfull.