Aluminum Boilers

Figure 1: Thermal and Physical properties of steel and aluminum

     Most manufacturers of aluminum boilers and heat exchangers require that pH be maintained within 7.0 to 8.5, which is unlike traditional treatment programs that recommend pH be within 8.5 to 10.5. However, this seems counterintuitive seeing that a low pH will help avoid aluminum corrosion, possibly at the expense of steel. The same goes for maintaining an elevated pH where aluminum loss would be likely. Where aluminum components are included within a condensing boiler system, attention should be given to the chemical compounds included in the inhibitor that is chosen.

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     Special consideration should be given to the inhibitor(s) chosen for a closed-loop with mixed metallurgy. Nitrite, although has been proven to protect steel, does little to prevent aluminum corrosion. Nitrate can effectively inhibit corrosion for both metals within the same system. Molybdate has remained a popular treatment chemical for mixed metal systems in the presence of oxygen-like condensing boiler systems. The use of azoles to protect yellow metals like copper has shown success, in addition to protecting carbon steel. Revisiting the low pH range recommended, silicate and molybdate may not provide the best protection as the chemicals have little effect. The addition of nitrite or nitrate is effective but may require additional acid feed to reduce pH.  Research conducted by LaBrosse and Mowbray [1] concluded that a successful treatment program may require that pH be above the recommended aluminum boiler range to fully inhibit all included metallurgy. This however may void the manufacturer’s warranty but may be justified to protect the entire distribution system. Purified tannins have several advantages over hard chemicals as they are renewable, non-toxic, non-flammable, and biodegradable. They form a homogeneous, iron – and/or aluminum-tannate(s) film protecting the metal surface from corrosion (Patrick 2008). Tannins have only recently been granted more attention as a viable treatment option. Personally, I have found that a blend of chemical inhibitors in conjunction with pH control (8.0 – 8.5) provides adequate protection throughout an entire system containing aluminum.

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     Chemically treating a mixed metallurgy condensing boiler system provides a challenging dilemma in that it forces us to incorporate the entirety of the system, rather than the isolated boiler. An understanding of the galvanic series, the effectiveness of different chemical inhibitors, and knowing the thermal and physical properties of metals can all be useful in treating these systems. All of this goes to point out that water treaters can be the unsung heroes when it comes to the longevity of equipment and its components.