Closed hydronic loops are of critical importance in many applications including industrial process equipment as well as heating, ventilation, and air conditioning (HVAC) systems. In industrial applications, closed loops are typically used to cool essential processes or equipment that would not tolerate the variability typically observed in open cooling towers. The major challenges encountered when operating a closed loop are microbiological growth and corrosion. Molybdate and nitrite have been used for decades to control corrosion in closed loops, along with borate as the most widely used buffer. However, each chemical has its limitations. Molybdate is very costly and performance is highly dependent on chloride/sulfate levels and temperature; nitrite provides very robust performance but at high dose. Additionally, nitrite is known to promote biological growth. Borate compounds, though not banned yet, have been classified as Substances of Very High Concern (SVHC) under the EU REACH regulations. Because of these drawbacks and increasing environmental regulations, new non-toxic corrosion inhibitors are needed that can provide excellent corrosion inhibition, meet strict discharge regulations, and maintain performance in the presence of high microbiological growth. Here we present a next generation non-toxic closed loop corrosion inhibitor that contains no nitrite, heavy metal, P, B, or filming amine. Laboratory feasibility experiments based on jar testing, pilot closed loop testing, and electrochemical testing show excellent corrosion inhibition results (<0.2 mpy) on mild steel in highly corrosive water. Corrosion inhibition results are compared to incumbent corrosion inhibitors based on nitrite, molybdate, and filming amine. Additionally, the biostability of the new corrosion inhibitor is demonstrated through stability experiments in the presence of cooling water microorganisms.
Closed cooling loops are encountered across a variety of industries where cooling is critical to operations and operators cannot tolerate the variability typically seen in open cooling towers. The water inside a closed loop is ideally circulated for a long period of time without need for significant makeup water or deliberate blowdown of the loop. Most industrial plants will contain some amount of closed cooling loops; closed loops can be found in refineries, petrochemicals, steel making, and microelectronics. Because closed cooling loops can run with little maintenance for long periods of time, they can become a neglected part of a customer’s water treatment system. This neglect can lead to significant issues that can negatively impact heat transfer efficiency in the cooling process. The most common issues in closed cooling loops are corrosion and microbial growth.1,2 Scaling in closed loops is not normally a concern as most operators fill their loops with relatively low hardness/alkalinity makeup water. In terms of water quality, the main driver for corrosion in closed loops is chloride concentration. Most closed loops have a chloride concentration less than 50ppm. Typical temperatures encountered in closed loops vary, but in general temperatures are 35 – 80°C. Closed loops are constructed mostly with carbon steel, stainless steel, and copper alloys, however the use of aluminum alloys in closed loops is becoming more prevalent. Closed loop volumes can vary significantly; in HVAC applications volumes can be as low as 10 gallons, whereas some refineries have closed loop volumes of several million gallons.
