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Safeguarding Aluminum From Water Corrosion

Many Lytron customers, who use aluminum cold plates and heat exchangers, question why we discourage using untreated water in their cooling loop. The following discussion addresses a major concern: aluminum corrosion and how to prevent it.

Understanding Corrosion

Over time, most metals tend to deteriorate 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 - for example, water, water vapor, acids, bases, ammonia, salts, and heavy metal ions - can induce corrosion.

Depending on their relative position on the periodic chart of chemical elements and their electromotive properties (ability to produce an electric current and thereby enter into destructive cathodic reactions), some metals (such as iron) are more prone to corrode than others (such as aluminum).

Natural Corrosion Resistance of Aluminum

Pure aluminum is corrosion-resistant due to its natural tendency to form a very thin, protective, hydrated aluminum oxide film on surfaces exposed to air. Comprised of 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.Aluminum with white hydroxide layer

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 (see figure 1). 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) may 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 varies also 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.

This layer reduces the components thermal performance by increasing pressure drop, which reduces the flow, and by creating a thermally insulating layer. In an installed cold plate or aluminum heat exchanger, corrosion based on the water properties may also be unavoidable.

An Effective Approach to Minimize Aluminum Corrosion

We strongly recommend adding a prescribed amount of ethylene glycol (antifreeze) to the water used in cold plates to help alleviate aluminum corrosion. Usually a solution of 25% ethylene glycol to 75% water is sufficient to prevent aluminum corrosion.

Commonly employed 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; 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.

You want your cooling loop to provide years of leak free cooling. Using ethylene glycol or propylene glycol to reduce aluminum corrosion ensures this.