Steve,
Regarding recommended thickness of zinc on steel parts, according to ASM Materials Handbook, they suggest somewhere in the range of 0.001 to 0.002 in for severe environmental condition use. This will provide an extended service life over a thinner coating (say 0.0002 to 0.0005 in) The handbook also suggests "zinc plated steel in such locations is usually given a protective coating of paint."

With regard to whether corrosion is measurable/quantifiable, I do remember having read something about that somewhere, but I am unable to prove or suggest any possibilities there.

Best Regards,

Paul Rivet

Hello Paul:

Thank you for you response, i appreciate it.

Steve DeLucia

Yes, Corrosion is measured (for uniform corrosion) as corrosion rate in the unit "mpy (mils per year)" and can be calculated using the following formula

mpy = 534W/DAT where
W= weight loss, mg
D = density of specimen g/cm3
A= area of speciment sq.in
T = exposure time, hr.

For ferrous alloy corrosion rate < 20 mpy is good, 50-200 mpy is poor and > 200 mpy is unacceptable.

As for the thickness of zinc coating according to AS1650 for steel is as follows:
coating mass( gm/m2) eq. thickness (um)
steel over 5 mm thick 600 84
steel between 2-5 mm 450 63
< 2mm thickness 350 49

The rule of thumb is that the thicker the zinc coating, the better protection you have. I agree with Paul that a duplex system (zinc galvanized + paint) is better for such environment. But you have to keep in mind that a special surface preparation and paint is needed or else the paint will not adhere to the zinc coating.

regards,
Surin Tan

Hello Surin Tan:

Thank you very much for taking the time to respond to my question. Have a great weekend.

Steve