As a general rule of thumb, aluminum boilers used in closed systems require special water treatment consideration, but why?
In the recent decade, the use of aluminum in condensing boiler applications has drastically increased and will continue to replace steel. Aluminum, as a construction material, offers competitive benefits in that the heat-transfer capability is roughly three times that of carbon steel due to its density. The metal is non-toxic, has a high strength-to-weight ratio, and retains its strength at varying temperatures. This efficiency provides more compact units with reductions in fuel consumption which helps to make aluminum attractive to new construction or boiler replacement projects. With all these advantages, however, special consideration to water treatment needs to be given.
8% of the earth’s crust is aluminum, making it second only to silicon (28%) followed by iron (around 5%). By itself, aluminum is relatively corrosion-resistant but suffers when in contact with alkaline substances. Since it is not feasible to have a distribution system entirely composed of aluminum, iron, and copper are often used for these tasks. In these multi-metal systems, the aluminum becomes the anode (active) and can suffer from corrosion. Not only that, but impurities if left in circulation from installation, can increase the risk of metal oxidation. Pretreatment for these systems would ideally remove iron and other suspended solids. To avoid scale, softened water would be ideal. Typically, the water provided by a municipality is sufficient for these boiler systems and may need to be softened.
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.
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  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.
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.
List of Resources:
 Labrosse, M., & Mowbray, M. (2014). Total Approach to Treating Closed Loops Containing Aluminum Boilers. Process Heating, February, 36–38.
 de Rooij, D. M. (2004). Corrosion of Aluminum and Aluminum Alloys. Anti-Corrosion Methods and Materials, 51(2). https://doi.org/10.1108/acmm.2004.12851bae.001
 Crow, P. (2008). A Solution for Aluminum Boiler scaling and Corrosion. Active Additives, 1–7.
 Racine, P. (2019, July 17). Can Aluminum Boilers ‘Play Nice’ With Other Metals? – AquaAnalytics. Aqua Analytics. https://aqua-analytics.com/can-aluminum-boilers-play-nice-with-other-metals/
 Corrosion Control - Galvanic Table. (19997–08). EAA1000. Retrieved October 17, 2021, from http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm