Electro- Zinc plating and Trivalent Chromate finsh

I typical zinc/chromate coating can be tested for corrosion resistance in a neutral salt spray chamber.  You can also use one of the accelerated tests, too, but much more common is the neutral.  The first sign of failure will be "white rust" caused by the chromate failure.  A low grade chromate should last 12 to 24 hours.  High-end chromates and/or chromates with topcoats can go over 120 hours to white rust.  The next failure mode is red rust.  The time to red rust is a function of the zinc thickness.  The more zinc, the longer the part should last before significant red rust.  Hundreds of hours is not uncommon.

To perform the test, you will need a salt spray chamber and all the ancillary equipment.  There might be a local shop that already has one in your area if you don't want to purchase a unit.