Evidence of rust staining is a certain indication that a hot dip galvanized coating has reached the end of its protective life
True or False?
Hot dip galvanized coatings are formed by a reaction between molten zinc and steel. The coating structure consists of a series of iron/zinc alloys which are normally overcoated with relatively pure zinc which is retained on the coating surface when the product is withdrawn from the molten zinc in the galvanizing bath. The silicon content of modern steels has an influence on the extent to which the iron/zinc alloys in a galvanized coating are developed. At reactive levels of silicon, alloy formation can be such that little or no pure zinc remains on the coating surface. These coatings are normally thick hence they provide enhanced corrosion protection. The surface appearance is frequently dull grey, due to the presence of iron/zinc alloys penetrating the coating through to its surface.
When these coatings are exposed to the environment over a period of several years, a degree of surface red rust staining is frequently evident. In the vast majority of cases, the red rust staining emanates from the protective iron/zinc alloys present in the coating which in all respects remains intact.
When assessing the protective properties of a galvanized coating in service, the overall remaining coating thickness should be established. The life of a galvanized coating is more or less proportional to its thickness in a given environment, regardless of its surface appearance.
Overcoating by means of an expensive and unnecessary maintenance paint system is often recommended by individuals who lack the technical expertise or the desire to determine the remaining protective properties of a weathered galvanized coating.
