Minimizing Cooling System Corrosion

Thousands of manufacturing and other industrial plants require cooling water for numerous process applications. These cooling systems, many of which utilize cooling towers for primary heat transfer, are critical for plant operation and productivity. However, corrosion and fouling issues require constant attention to prevent productivity losses and sometimes even plant shutdown. New technologies have emerged to greatly assist in the battle against cooling system corrosion.

While comprehensive cooling water treatment programs typically include chemistry to control scale formation, they also must be designed to protect metal surfaces from corrosion. Cooling systems often incorporate multiple metallurgies, but by far the most common material for piping and other equipment such as heat exchanger shells is carbon steel. For many years in the last century, a common cooling tower treatment program utilized sulfuric acid combined with chromate treatment to protect the various metals within systems, and most notably carbon steel. The chemistry inhibits calcium carbonate scaling by reaction of sulfuric acid with bicarbonate ions (HCO3-) to convert the ions to CO2, which escape as gas. A typical pH control range was within or near 6.5 to 7.0. The second compound in the formulation, disodium chromate (Na2Cr2O7), provides chromate ions that react with carbon steel to establish a protective pseudo-stainless steel layer, which can be particularly effective in the oxygensaturated cooling water generated by cooling towers.

Almost universally, chromate treatment was abandoned in the 1980s due to dawning knowledge, including efforts led by Erin Brockovich, of the toxicity of hexavalent chromium (Cr6+). The most common (by far) replacement programs were based on a combination of inorganic and organic phosphate, aka phosphonate, chemistries, with supplemental chemical additives to control both scaling and corrosion. These treatment methods can be quite complex, and for corrosion protection rely on deposition of reaction products to protect metal surfaces. The deposition products are often subject to process variations or upsets that negatively influence program effectiveness. So, the search for better chemistry came along. Another driving factor in this search involved the increasing concern regarding the impact of phosphorus on the environment, and in particular the growing problem of toxic algae blooms in natural bodies of water. Toxic algae blooms have been well-documented in such warm weather locations as Florida and the Gulf of Mexico below New Orleans, but they have occurred in many other areas including Lake Erie.

Modern polymer chemistry has emerged that are serving as an alternative to phosphate-based treatment methods for corrosion control. Original polymers developed for scale control contained carboxylate functional groups (COO-), where the negatively-charged oxygen atoms bind with hardness ions to modify crystal growth. More advanced compounds such as co- and ter-polymers have structures that may include carboxylate, amide (R-CO-NH2), sulfonic acid (SO3H) and other groups for improved reactivity and resistance to degradation. These enhanced polymers have been formulated to help control other deposition, including calcium sulfate, magnesium and calcium silicates, manganese, and calcium fluoride, to name some of the most prominent.

But what about corrosion control? Non-phosphorus treatments methods, similar to those of the preceding phosphate/phosphonate programs, have been designed to operate at an alkaline pH range (7-9), which tends to minimize general corrosion of metals. But even so, corrosion cells can still develop. The key is compound functionality in which the polymer establishes a protective barrier on metal surfaces. A pioneer and still leading product in this regard goes by the name of FlexPro®, which combines a group of chemistries that “interact directly with metal surfaces to form a reactive polyhydroxy starch inhibitor (RPSI) complex that is independent of calcium, pH, or other water chemistry constituents.” The compounds establish a direct protective layer on metal surfaces, unlike the phosphate/ phosphonate programs that rely on deposition of reaction products to form protective barriers, which, as has been noted, can be difficult to control. Full-scale application of the chemistry has proven very effective. In one instance at a large industrial complex in the southeastern U.S., RPSI replaced previous polyphosphate and then zinc chemistry. Carbon steel corrosion rates have been reduced from 0.2-0.25 mm/ yr to 0.0025 to 0.0075 mm/yr. On a secondary note, the change from zinc and then to RPSI was in part influenced by problems with severe algae formation in a clarifier and recycle pond at the plant. The removal of phosphate from the water solved that difficulty. This is but one of many successful applications of this chemistry.

It must be noted that although these new products have improved cooling system corrosion and scale protection, they are not a cure-all for cooling water treatment. Proper attention to the control and minimization of microbiological fouling is required for every system. The author can comment from direct experience that such fouling can occur very rapidly. If microbes are allowed to settle, the deposits will inhibit the performance of any corrosion/scale inhibitor program, and can generate corrosion on their own. Heat exchanger tube failure and cooling tower fill fouling are but two of the numerous problems that microbes can cause. Due diligence is always required in designing and implementing a comprehensive cooling water treatment program.