The use of aluminum boilers in closed loops has increased in recent years. There are many benefits to using aluminum as a construction material. As compared to carbon steel, aluminum has a much lower density and much higher ability to transfer heat (thermal conductivity). Aluminum transfers heat at roughly three times the rate of carbon steel. As a result, aluminum boilers require much less surface area for heat transfer and a smaller overall footprint to provide the same water heating capacity as a steel boiler. In commercial and industrial applications, floor space is at a premium and the footprint of unit operations is a significant design consideration. Even though carbon steel is less expensive than aluminum, the higher heat transfer efficiency and smaller design makes aluminum an attractive choice for heat exchange (table 1).
While the improvement in boiler efficiency is beneficial, the integration of aluminum into closed-loop systems has resulted in a significant challenge to the water treatment industry. With the addition of aluminum heat exchangers and boilers to closed systems, there is an introduction of a strongly anodic metal. In the presence of iron and copper, which are less anodic, aluminum provides the potential to form galvanic corrosion cells with both free copper and free iron. In these multi-metal systems, it becomes increasingly important to control corrosion of all metal surfaces.
More significantly, manufacturers of aluminum heat exchangers and boilers specify that they be treated to maintain pH ranges that are lower than those typically found in closed loops containing carbon steel. Commercial manufacturers of aluminum boilers instruct clients to operate the fluid in their boilers at a pH range of 7.0 to 8.5. This pH range is designed to reduce corrosion of the aluminum boiler components. However, this is usually at the cost of increased carbon steel corrosion. Traditionally, hot water closed loops operate in the pH range of 8.5 to 10.5. A higher pH range favors lower carbon steel corrosion rates but at the cost of higher aluminum corrosion. Because most aluminum boilers are connected to carbon steel piping as well as other metals such as copper, it is critical to find a total approach that minimizes corrosion throughout the entire boiler system and in all the different materials of construction.
Pretreatment. To preserve the longevity of the closed-loop system, it is necessary that the makeup water for the system first be treated to reduce the suspended solids and certain dissolved ions. Most municipalities provide water that is low enough in suspended solids to meet the requirements for boiler makeup water, but pretreatment is generally required to reduce the calcium and magnesium hardness of the makeup water. Ion exchange units such as water softeners can be used to greatly reduce the potential for hardness deposit formation on boiler heat exchangers. However, softeners have the added benefit of removing ferrous iron from makeup water sources. If left in the system, ferrous iron can lead to galvanic corrosion cells. This issue can be especially damaging in an aluminum boiler, making proper softener operation and maintenance critical.
There are many chemical treatment options for closed loops. Non-glycol based chemical treatments to protect carbon steel often include nitrite, nitrate, silicate, molybdate or combinations therein. Chemical compounds from the azole family are often added to protect yellow metals such as copper and brass. Aluminum is normally protected by a combination of nitrate, silicate and pH control. Some common treatment chemistries are shown below.
Nitrite. Nitrite prevents carbon steel corrosion by oxidizing the steel surface to create ferric oxide (Fe2O3). Nitrite is a very common treatment option in closed hot loops for steel but has little effect on preventing aluminum corrosion.
Nitrate. Nitrate can be an effective adjunct in the prevention of aluminum corrosion. Typically, nitrate is combined with nitrite to provide both carbon steel and aluminum corrosion protection.
Silicate. Silicates are regarded as carbon steel corrosion inhibitors but are generally considered to behave as a passivation agent to all metals if used as an adjunct. Hence, silicates can be used both as an adjunct for aluminum and copper corrosion inhibition and as a stand-alone aluminum corrosion inhibitor.
Molybdate. Molybdate can be a carbon steel corrosion inhibitor in the presence of oxygen. Molybdate also can be used as an aluminum corrosion inhibitor in closed-loop systems.
Azole. Azole chemicals are effective yellow metal (e.g., copper) corrosion inhibitors. The surface reaction creates a cuprous azole passivation layer. Azoles also prevent galvanic corrosion between carbon steel and copper by reacting with cuprous ions in the bulk water.
Low pH Range
Manufacturers of aluminum boilers advise their customers to operate their closed-loop system at a pH of 7.0 to 8.5, in most cases, voiding a warranty if the system is operated outside of this range. Corrosion inhibitors specific to aluminum within this specified pH range include silicate and molybdate. A nitrite/nitrate combination might work, but a combined product has a high pH, which raises the system pH. A secondary acid feed would be required to maintain a lower pH.
We ran trials at our lab, to determine which combinations of the available chemistries best prevented corrosion in soft water in this manufacturer-recommended pH range. Corrosion coupon results showed that a silicate program alone had unacceptable aluminum corrosion. The target is 2.0 mils per year (mpy) or less. Adding molybdate to the chemical treatment program helps the aluminum corrosion problem and the aluminum coupon from the trial appears very clean. However, the carbon steel coupon (which is not shown) had a corrosion rate of 12.2 mpy, which is unacceptably high. At a pH below 8.5, while it is possible to minimize aluminum corrosion rates, it is difficult to control carbon steel corrosion.
High pH Range
Next we ran trials using the same silicate/molybdate combination chemical treatment as before but at a higher average pH. Our results indicated that successful corrosion inhibition of a multi-metal system, including aluminum, is possible at these pH ranges. A side-by-side comparison at different pH shows the main concern is carbon steel corrosion. However, a chemical treatment program can be used successfully at higher pH to inhibit all metals present in the system.
Despite the challenges presented by combination aluminum, carbon steel and copper in the same closed system, there is a successful total approach that can greatly protect the integrity of the system. It starts with a smart pretreatment choice for the source water and ends with a carefully selected chemical treatment program, even if it means operating outside the aluminum boiler manufacturer warranty. Remember, the aluminum boiler warranty does not cover the rest of your system.