Why does painted aluminum corrode faster than bare aluminum?

12. April 2018

Aluminum is commonly used in the marine industry as light weight and long life construction material. If direct contact with other metals, such as such as steel or bronze is avoided, aluminum can last almost forever.

As bare aluminum is not very nice to look at, paint systems are commonly considered to improve the corrosion resistance and improve the visual appearance of aluminum used in a marine environment. But this conclusion about aluminum is misleading. Especially when aluminum is only coated partly the door for corrosion processes such as filiform corrosion or crevice corrosion is wide open! Proper corrosion protection of aluminum by the use of coatings requires extreme care and process control.

The challenge in painting and protecting aluminum is preventing humidity/water to undergo the paint, for example at a paint edge. If that happens, oxygen contained in the water is consumed by oxidizing the aluminum and as a consequence the local pH drops and that initiates the corrosion process under the paint. This process can obviously only happen, when aluminum is painted or the oxygen exchange is otherwise hindered. And this is why painted aluminum corrodes faster than bare aluminum.

That was for example the case, when WRETEC was recently called in, in order to investigate a corrosion problem on an aluminum hull. The paint system was not prepared with greatest care and blisters had formed under the paint after only very short service life (see picture).

The biggest question in this context is the correct repair treatment in this case! So far there is no standardized process available. Therefore WRETEC started together with partners improving that situation and to come up with a general guideline how filiform corrosion can be successfully be repaired.