Protecting your System from Leaks and Performance Degradation
Water and water/glycol solutions are common heat transfer fluids used in cooling systems and recirculating chillers. Although the fluids are the lifeblood for your heat transfer applications, they can also cause corrosion within your systems. This corrosion can result in a reduction in system thermal performance due to scaling on the heat transfer surface, decreased flow due to reduced pipe diameters from corrosion deposits, and ultimately the need for system component replacement due to corrosion damage.
Corrosion is the chemical or electrochemical reaction between materials, usually a metal and its environment, which results in deterioration of the metal and its properties. This article will cover chemical corrosion. (For more information on electrochemical or galvanic corrosion, please see our application note "Avoiding Galvanic Corrosion.") Corrosion of metallic components is an inherent problem for water and water/glycol cooling systems because many metals naturally tend to oxidize in the presence of water. The dissolved oxygen in water accelerates most corrosion processes. In closed loop systems, dissolved oxygen is consumed over time and no longer poses a corrosion risk. For open loop systems, however, continued exposure to air allows oxygen to dissolve into the coolant. Therefore, open loop systems often suffer more corrosion problems compared to closed units.
Corrosion is usually classified as either general or localized. General corrosion is the loss of metal uniformly distributed over an entire surface. It typically does not lead to rapid system failure because the rate of metal loss can be discovered before the metal ruptures. Localized corrosion, on the other hand, is not as predictable. It usually shows up in the form of pitting, which can penetrate the metal very quickly, forming cavities or holes. Another common form of localized corrosion is cavitation, which occurs when pockets of vapor form in a liquid. This process occurs when local pressure near the metal surface falls below the vapor pressure of the liquid. When these vapor bubbles collapse or implode, they generate large amounts of energy. This causes severe pitting to system components (such as pumps), generates a great deal of noise, and results in a decrease in pump efficiency.