Corrosion Mechanisms of Painted Metal
Whitepaper
TASA ID: 4149
The purpose of this article is to provide an introduction to the fundamentals of corrosion and its relevance to painted metal. The intent is to provide a general background on the subject that may be useful to both technical and non-technical personnel who are not familiar with corrosion technology. It is not intended to be an exhaustive dissertation on the subject.
A paint coating imparts two important functions to the underlying substrate:
- The aesthetic function gives the substrate a good appearance.
- The protective function protects the substrate from mechanical and chemical damage.
However, due to routine wear and tear, surface scratches and other defects are generated in the paint film and micro-cracks develop. These micro-cracks eventually lead to macroscopic corrosion damage, which results in the coating losing its aesthetic and protective functions.
Corrosion is a ubiquitous and on-going problem. It causes tens of millions of dollars of damage annually in the U.S. alone, and compromises the safety, environmental, and appearance characteristics of the affected structures.
The Corrosion Battle
The battle of corrosion is a constant fight because of the curse of all corrosion engineers and scientists - thermodynamics! The Third Law of Thermodynamics states, in simplified terms, that the naturally occurring state of matter is its lowest energy state, similar to humans on weekends. Metals ordinarily exist naturally as oxides (e.g. iron oxide, aluminum oxide, etc.) because oxides represent their lowest energy state. However, oxides are mined from the ground, and they are subjected to various unnatural acts - such as refining, casting, rolling, and forming into a myriad of different shapes. These metals are now in an "activated" state, and do not want to stay there. They want to revert to their naturally occurring state - oxides, or rust in the case of steel.
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