Coating Failures

Substrate is the surface over which the coatings are applied. It is the foundation on which coatings are laid and the surface characteristics, therefore, control the life of the coating. The nature of the substrate has a direct bearing on the degree of protection offered by the coating.

Consider for example steel and aluminum over which the coatings are to be applied in an alkaline environment. It is well known that aluminum has a tendency to corrode in an alkaline environment, whereas steel shows a good resistance to corrosion. A coating applied on aluminum would have a shorter coating life compared to steel because of the sensitivity of aluminum to alkalinity. There are many substrates over which coatings are applied, eg. steel, concrete, wood, plastics, and many others. The nature of the surface has therefore a very important bearing on the coating life. Some of the important characteristics of important substrates are summarized below:

1. Stainless steels	It includes all categories of stainless steels, eg. 300 SS, 316 SS, etc. The surface of stainless steels is generally inert because of the tendency of steels to become passive by formation of a thin oxide layer. The passive layer can, however, be damaged by chloride ions and leads to localized corrosion such as pitting. If the surface of the stainless steel is sand-blasted and well-prepared, stainless steel provides a good adhesion to coatings despite its dense and smooth surface.
2. Aluminum	It has a dense and smooth surface. By light sand blasting, the adhesion capability of aluminum substrate is improved. Aluminum has a tendency to corrode in alkaline environments, however, it is resistant to sulfide impurities unlike iron, steel and copper.
3. Copper	Copper is characterized by a protective film of copper oxide (Cu₂O) on the surface which makes it a highly resistant material for transporting of highly corrosive sea water. It is, however, sensitive to corrosion by ammonia and hydrogen sulfide present in the atmosphere. copper substrate needs sandblasting prior to application of coating.
4. Cast iron	Coatings are easily applied over the case iron substrates. All cast irons contain about 2.14 % Carbon. In cast iron, iron around the graphite is attacked leading to graphite corrosion.
5. Wood	wood is very sensitive to moisture, humidity, temperature and dry-wet cycles. However, it is difficult to apply coatings on wooden substrates with a great degree of certainty.
6. Concrete	Concrete substrate is porous, however, it is chemical reactive. The coating must penetrate into the pores to be effective.