Corrosion can lead to many problems, the most significant being perforation that may result in coolant leakage. Other problems may include reduced heat transfer caused by surface scaling, which occurs when metal reacts with oxygen, chloride, and/or inhibitors in the coolant and precipitates back to the metal surface, creating a layer that acts as a heat transfer barrier. Additionally, concerns include the clogging of particulate filters and damage to mechanical seals.
When copper corrodes, it is more often degraded by general corrosion than by pitting. General corrosion will often attack copper exposed to ammonia, oxygen, or fluids with high sulfur content. Another source of corrosion affecting copper is dissolved salt in the fluid, such as chlorides, sulfates, and bicarbonates.
For aluminum, pitting is the most common form of corrosion. Pitting is usually produced by the presence of halide ions, of which chloride (Cl-) is the most frequently encountered in liquid cooling loops. Pitting of aluminum in halide solutions open to air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential and the naturally occurring protective oxide layer or film is penetrated. This film is stable in aqueous solutions when pH is between about 4.0 and 8.5. The film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface oxide film is rapidly repaired. Aavid strongly recommends an inhibitor when using water with aluminum to maintain a clean heat transfer surface.
Stainless steel is typically used in corrosive environments but, as with aluminum, it is sensitive to high concentrations of chlorides (>100 ppm) in an oxidizing environment. Pitting remains among the most common and damaging forms of corrosion in stainless steel alloys, but it can be prevented by ensuring that the material is exposed to oxygen and protected from chloride wherever possible. Stainless steels high in chromium, and particularly molybdenum and nitrogen, are more resistant to pitting corrosion.