Most metals used in process cooling loops tend to deteriorate over time due to corrosion, which manifests itself as pits, cracks or more widespread surface degradation. Corrosion usually results from chemical or electrochemical actions that break down the protective oxides characteristic of most metallic surfaces. Exposure to certain liquid, gaseous or solid agents such as water, water vapor, acids, bases, ammonia, salts and heavy metal ions also can induce corrosion. Some metals, such as iron, are more prone to corrode than others, such as aluminum. However, when untreated water is used in a cooling water loop, corrosion can even attack aluminum cold plates and heat exchangers.
Pure aluminum is corrosion resistant due to its natural tendency to form a thin, protective, hydrated aluminum oxide film on surfaces exposed to air. Comprising a thin inner barrier layer and a thicker, more permeable outer layer, this oxide film measures only about 2.5 nm in thickness on new metal and thickens gradually with age.
When unoxidized aluminum is immersed in pure water, it will form a white hydroxide film, which remains more or less constant in thickness once equilibrium is reached. The equilibrium thickness of the layer depends on temperature. The film is stable in natural water with a pH in the neutral range from 4.5 to 8.5. However, water with a lower pH (more acidic) might attack some aluminum alloys, and water with higher pH (more basic) will attack all aluminum alloys. Aluminum’s resistance to corrosion in natural fresh and tap waters also varies depending on the content of dissolved solids, gases, and colloidal or suspended matter. For instance, the combination of carbonate, chloride and copper can cause some supply waters to be more corrosive.
The hydroxide layer reduces the thermal performance of the cold plate or heat exchanger components by increasing pressure drop, which reduces the flow, and by creating a thermally insulating layer.
Minimizing CorrosionAdding a prescribed amount of ethylene glycol (antifreeze) to the water used in cooling loops can help alleviate corrosion on aluminum cold plates and heat exchangers. Usually a solution of 25 percent ethylene glycol to 75 percent water is sufficient to prevent aluminum corrosion.
Commonly used commercial antifreezes include ethylene glycol (an environmentally hazardous substance) and propylene glycol (less toxic and more environmentally acceptable than ethylene glycol). Ethylene glycol is slightly sweet and odorless while propylene glycol is tasteless and almost odorless. Both have a somewhat syrupy consistency. In their pure states at ambient conditions, these glycols are clear and colorless. Manufacturers add coloring agents to differentiate antifreezes according to type, heat transfer capabilities and other properties, and to facilitate leak detection.
To protect against corrosion, most commercial-grade ethylene and propylene glycols contain a blend of corrosion inhibitors (typically six to twelve, depending on the supplier). These additives protect metal surfaces by applying a combination of physical and electrochemical barriers that reduce the effects of corrosion, thereby allowing the cooling loop to provide years of leak-free cooling. PC
This article was provided by Lytron Inc., Woburn, Mass., a manufacturer of cold plates, cooling systems and heat exchangers. For more information, call (781) 933-7300, e-mail email@example.com or visit www.lytron.com.