Alloys of aluminum are being implemented in heat transfer applications more frequently due to aluminum’s very high conductance of heat. However, aluminum is more expensive and highly reactive when compared to standard materials used in these applications, such as steel.
The use of these alloys can provide improvements to heat transfer efficiency for hot water boilers and heat exchangers. However, it is crucial to have a proper water treatment program. It should be tailored to minimize corrosion of aluminum while maintaining protection of other metals within the system.
When implementing aluminum alloys, you should balance the potential savings through reduced utilities consumption against the increased risk of system damage and premature failure due to the reactive nature of aluminum alloys.
A critical factor to protecting aluminum within a water system is maintaining the pH within the stable range for aluminum.
Aluminum forms a naturally protective barrier of aluminum oxide on its surface. However, this oxide film is only stable in a pH range of approximately 4.0 to 8.5. Outside of this range, the oxide film can begin to degrade. Once the film degrades, even in a small area, the aluminum exposed beneath will corrode much more rapidly. This can result in pitting.
KEY INSIGHT: Pitting is the formation of small, deep holes or cavities on a surface. Most commonly a type of localized corrosion on metals.
This pitting is much more aggressive under alkaline conditions. This is typical of most water systems which have been treated with a standard corrosion inhibitor blend. This happens because metals commonly found in water systems are more stable.
Therefore, to maintain protection of the entire system, the pH must be controlled within the stable range for aluminum. It must also maintain adequate level of corrosion inhibition for any other materials in use.
As with any water system, corrosion inhibitors are required to minimize corrosion rates on the metals within the system. However, when systems contain aluminum, the inhibitor must be selected with careful consideration to ensure that all materials are protected.
Any inhibitor must be buffered to near neutral pH. Additional buffering solution may be required to bring the pH within a stable range depending on the raw water’s pH. That figure is typically pH 5.0-8.0.
There are many corrosion inhibitor chemistries that will provide adequate protection from corrosion for steel and other metallurgies. During these chemical processes, the pH of the water will be maintained within the acceptable range. This will minimize corrosion of aluminum.
There are additional considerations if the water system also contains copper, brass, or any other yellow metal. The protection of these materials is also critical to maintaining corrosion resistance of aluminum. Any corrosion of these materials may result in copper ions being released into the water. When contacting any aluminum surface, will result in severe pitting. Therefore, appropriate inhibitors must be present to prevent the corrosion of these materials to ensure protection of both the yellow metals and the aluminum.
Aluminum is a highly reactive metal, being one of the more anodic metals on the galvanic series. Because of this, it is very important to maintain galvanic separation between aluminum within the system and any other metal. This is particularly for those more cathodic.
If aluminum is directly connected to other metals, a galvanic cell will form, which will result in rapid corrosion. Aluminum, acting as the anode, will experience significant degradation and loss of material, eventually leading to failure.
Take steps to minimize or eliminate the use of copper and brass alloys in a system which contains aluminum. At the very least, ensure that these metals are galvanically separated from the aluminum. This can be achieved using dielectric fittings or other separation methods.
The decision to implement aluminum boilers, or other heat exchange equipment containing aluminum alloys within a water system, must be made in a thoughtful way. While aluminum may provide potential savings through improved heat transfer efficiency, this must be balanced with the requirements of the metallurgy throughout the system.
Take care to control water chemistry to ensure any efficiency gains are not negated by damage to other system components. If you have aluminum components within any of your water systems, consult your water treatment professional. They will ensure that your treatment program provides the right protection for your system.
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