The zinc carbonates are not produced when a hot dip galvanized specimen is exposed to a salt spray fog, therefore this testing method does not give an accurate measurement of corrosion protection.
Salt Spary Test, Wikipedia
For galvanized steel, the spray test accelerates the wrong corrosion mechanism. The salt spray attacks the bare zinc. […] Since the zinc carbonate is not formed, the salt spray interacts directly with the zinc metal. The part corrodes very quickly when the salt spray attacks the zinc metal.
Dr. Tom Langill, AGA (American Galvanizers Association)
Short term corrosion test results derived from accelerated or short term corrosion tests (e.g Kesternick Test, ISO 6988, and Salt Spray Tests) should not be used to predict long term corrosion performance of galvanized coatings.
Salt spray tests cannot be used to accurately test zinc coated steel […]. Without a proper wet/dry cycle, the zinc coating cannot form patina layers. The absence of a patina layer allows constant attack of the zinc metal and gives a very low prediction of the zinc coating lifetime.
The [salt spray] test results should not be regarded as a direct guide to the corrosion resistance of the tested metallic materials, […] for quality control purposes, comparison can be made between specimens coated with the same coating.