Corrosion Control in Closed Loop Systems

Corrosion damage in closed heating and cooling systems is a common problem. As an integral part of heating, ventilation, and air conditioning systems, they circulate hot water in the winter and cold water in the summer. The quality of the water used to fill the closed system affects the performance of the corrosion control program.  Typically, raw water as received from a municipality or private well is used to fill a closed system. However, this may not be the best alternative. Another option is to use softened or demineralized water. Hardness poses a scaling problem in hot water systems. It is best to soften the make-up water if it exceeds 10 ppm total hardness. Overall, the water used in the closed system should be of the best quality available. As a rule, demineralized water is better than softened water which is better than raw water. Some engineers resist the use of demineralized water because of the common belief that it is more aggressive or corrosive than raw or softened water. Although it is true that untreated, oxygenated demineralized water is very corrosive, particularly at hot water temperatures, corrosion inhibitors such as sodium nitrite passivate metal surfaces resulting in a final system water that is noncorrosive.

Chemical Treatment Options
Several chemical treatment methods have been developed for closed loop systems. The selection of one method over another is determined by the water quality, the type of antifreeze used, the system metallurgy, and any environmental or safety issues that must be considered. If the closed loop system is new, it should be chemically cleaned prior to the start of the treatment program. Chemical cleaning removes oil, mill scale, dirt, welding fluxes, and other contaminants that can interfere with the performance of the treatment program. Chemical cleaning is also
recommended for older systems which suffer from corrosion Borate-nitrite formulations provide corrosion protection which is equal to the old chromate products which were abandoned for environmental  reasons. The sodium tetraborate component creates a buffer in the system that stabilizes the pH between 9.0 and 9.5.  A minimum of 900 ppm of sodium nitrite is required for corrosion protection on mild steel. A 1,000 ppm residual of sodium nitrite is recommended in high temperature hot water systems. For those waters that are high in chlorides or sulfates, 1,500 ppm is required. A general recommendation for inhibitor levels is 800 to 1,200 ppm as sodium nitrite. Nitrite functions as a reducing agent in closed systems. As a result, it does not adversely affect the glycol-based antifreezes. Nitrites are an excellent food source for bacteria.  Nitrite reducing bacteria are a potential problem in closed systems. When nitrite reducing bacteria are present, the nitrite level drops while the specific conductance (TDS) remains the same. A reduction in both nitrite and conductance, however, suggests water is being lost from the system with a corresponding loss of chemical treatment. The remedy for nitrite reducing bacteria is to treat the system with a liquid, non-foaming biocide.

Walter Louis Fluid Technologies has both the products and the know-how to get your closed loop system back to the shape it should be in. Please give us a call.