Once rust stains appear on a weathered hot dip galvanized surface you can be sure that the coating has reached the end of its protective life.?

True or False?

With organic coatings, adhesion is achieved by way of a mechanical bond between the coating material and the substrate. Hot dip galvanizing is somewhat different in that adhesion is provided by a diffusion process whereby atoms of zinc and iron combine to form a series of Fe/Zn alloys at the steel zinc interface during the period that the steel is in contact with molten zinc. In most cases, these hard abrasion and corrosion resistant alloys make up at least 50% of the overall thickness of the final coating. Depending on minor variations in steel composition, these alloys can grow profusely to the extent that not much unalloyed and relatively pure zinc is present on the coating surface. Inevitably, the overall thickness of such coatings is greater which enhances their protective life. There is of course a limit to what is desirable since excessively thick iron/zinc alloys can be brittle and prone to mechanical damage.

When these hot dip galvanizing coatings are exposed to the environment, the relatively thin outer zinc layer gradually weathers away thus exposing the alloys of iron and zinc. If these alloys are exposed to moisture and oxygen, a degree of red rust staining can develop and this is frequently assumed to be rust emanating from exposed steel, which is not the case. It is in fact the iron within the coating that causes their surface staining.

Appearance can be misleading. Measuring the remaining coating thickness with an instrument, which operates on the magnetic principle, is the only reliable way to determine the actual condition of the galvanized coating.

While these thicker coatings invariably provide longer corrosion free life, the downside is that they are not as pleasing to the eye, due to the presence of dull grey surface patches. If aesthetics are important as in the case of architectural features, the pre-selection of steel with respect to its silicon and phosphorous content will overcome this problem.

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