Pin Holes in Paint and Coating Films

Our experience in paint and coating film failures has included surfaces on a rather large variety of objects ranging from metal containers of 5-gallon capacity to the outer hull of a Canadian Armed Forces, naval destroyer.

Paint films and paint coatings are often used for protection of the surface that they cover. In many cases the covering must be perfect to be effective. Tiny pinholes expose the substrate to the conditions, which the paint film is being used to isolate from the base metal. Chemical reactions or corrosions in the base metal cause the formation of chemical compounds, which occupy more volume than did the original metal elements. The end result is a swelling and further disruption of the paint film. If the paint film is designed to keep a chemically aggressive, low viscosity liquid from attacking the material underneath any defects in the paint film integrity can be a source of potential catastrophic failure.

Imperfections in films can arise from many sources. Virtually every paint manufacturer specifies that surfaces being covered must be clean, dry and free of oils or greases. In the majority of film failures investigated the primary cause of failure is improper surface preparation. The next most common reason for coating failure is improper application in the form of too thin or too thick a film. In ferrous metal applications failure to remove oils and greases or rust caused separation of the film from the substrate. In painted aluminum and aluminum alloy coating failures, the root causes have been directly traced to failures to properly clean and chemically prepare the aluminum surface. Aluminum surfaces are best anodized where possible.

The least common cause of coating failure has been traced to improper manufacturing practices on the part of the coatings producer.

When an aggressive medium must be kept separate from a reactive base metal the system designers should be aware of the difficulty of producing a perfect film every time it is required.