Corrosion rates increase as the amount of sulfur deposition in the atmosphere increases.
Therefore, a straight line relationship will have a substantial safety margin in milder atmospheres. Milder atmospheres allow the protective films formed on zinc to enable a decrease in corrosion rate with time. The corrosion rate of zinc coatings is approximately linear with time in most aggressive atmospheres. Guidelines for life expectancies of zinc coatings exposed to different atmospheres are shown in Table 1. In the first three classifications, the severity of corrosion is usually related to the amount of sulfur dioxide in the atmosphere whereas in the latter the presence of chloride is an important determinator of corrosion rate. Atmospheres are usually classified subjectively as industrial, urban, rural, or marine. Zinc and zinc alloy coatings have good resistance to atmospheric corrosion. Painted Galvanneal has found widespread applications in automobile bodies and structures.
Its rough surface makes it particularly useful as a substrate for application of paints, while the ~85% zinc content of this intermetallic layer provides good protection to the steel substrate. The Galvanneal coating produced on continuous galvanized sheet is a fully alloyed coating with no zinc overlay. This is often confused with attack of the underlying steel however in many cases this intermetallic layer, which contains in excess of 85% zinc, is capable of protecting the underlying steel for many additional years. Zinc iron alloys are present in a reactive layer formed on coatings prepared by dipping prefabricated steel in nearly pure zinc baths and also in the continuous galvanized sheet product termed ‘Galvanneal.’ When coatings on prefabricated items are exposed for a sufficient time, the pure zinc overlay coating will be removed and expose the underlying zinc–iron reaction layer, causing a red staining. From Prosek, T., Goodwin, F., Thierry, D., Alloying of Zn–Al–Mg coatings for corrosion stability improvement. Effect of alloying with Mg and on reduction in mass loss compared to Zn for model alloys exposed for 6 weeks in ECC1 cyclic corrosion test. Good design will minimize the amount of steel that is exposed to the environment and therefore minimize bi-metallic corrosion.įigure 3. By its very nature, the protection of galvanized steel by zinc can result in bi-metallic corrosion when steel surfaces are exposed, such as at cut edges, drilled holes or when uncoated fasteners are inserted into structural items.
Providing specifications are adhered to, intergranular corrosion of zinc is not a problem however certain impurities, especially in zinc aluminum alloy coatings, can cause intergranular corrosion.
Localized pitting occurs only under special conditions, for example in waters where a calcareous scale may crack locally, exposing a small area of zinc. When protective layers have formed and completely covered the surface of the coating, corrosion proceeds at a greatly reduced rate. Zinc has the ability to form protective layers comprising basic carbonates, oxides or hydrated sulfates depending upon the nature of the environment. Zinc coatings generally corrode as a result of the slow general dissolution of zinc from the surface. Goodwin, in Reference Module in Materials Science and Materials Engineering, 2017 2 Corrosion Characteristics of Zinc 2.1 General Considerations