Cooling Water Corrosion Inhibition
Corrosion inhibition is designed to prevent metal loss that can lead to critical system failures in heat exchangers, recirculating water piping, and process cooling equipment.
Corrosion is caused by metals attempting to return to their natural state. It can be present in many forms, including uniform metal loss, localized or pitting, bi-metallic, galvanic, underdeposit, and microbiological-induced corrosion (MIC). The process starts when surface irregularities, stresses, or compositional differences result in the formation of a corrosion cell (anode and cathode). Once started, corrosion at the anode causes metal to be released into the system or redeposited locally. Pitting is particularly problematic because the local loss of metal can result in thru-wall perforation of piping and tubing. Corrosion results in efficiency loss as corrosion products precipitate on critical heat transfer devices and insulate the metals.
ChemTreat has a comprehensive portfolio of organic, inorganic, and non-phosphate corrosion inhibition products engineered to passivate metals by reducing the corrosion potential associated with the corrosion cell’s anode and cathode. Chemicals that form protective films at the anode include chromate, orthophosphate, nitrite, silicate, and molybdate. Chemicals that form films on the cathode include calcium carbonate, polyphosphate, zinc, phosphonate, and azoles. We offer a selection of both anodic and cathodic inhibitors.
ChemTreat’s cooling water corrosion inhibitor treatments fall into several classes:
Zinc is a cathodic inhibitor. The addition of low levels of zinc to traditional high-level chromate programs lowers chromate levels and reduces corrosion rates. The movement away from chromate results in the development of several new classes of corrosion inhibitors. One inhibitor group is a combination of zinc with phosphates and polyphosphates. Combining zinc and phosphonate prevents unwanted zinc deposits. Corrosion control chemistry relies on cathodic inhibition by zinc, coupled with anodic inhibition by orthophosphate. The orthophosphate source is usually the slow reversion of phosphonate, with the majority of the remaining phosphonate functioning to control iron and mineral scale deposition. Adding zinc to aggressive cooling waters (systems low in hardness, alkalinity, and TDS) provides a true corrosion inhibition mechanism. With the water treatment industry moving away from heavy metals and toward higher-pH programs, zinc can play an important role where it can be tolerated. Even permitted levels of zinc as low as 1.0 ppm in the outfall can calculate to several parts allowable in a cooling tower where the blowdown is diluted by other plant flows. In all cases, the HEDP/zinc products are designed for aggressive waters where chromate is not permitted but low levels of zinc are acceptable.
When corrosion inhibition is needed for aggressive waters and zinc is not acceptable, adding phosphate treatment chemicals to open recirculating cooling water systems can significantly lower mild steel corrosion rates. The phosphate source comes from an orthophosphate such as phosphoric acid and/or inorganic polyphosphate such as TKPP. ChemTreat’s inorganic phosphate treatment chemicals incorporate polymer and copolymer dispersants either as a blend or separately to prevent the buildup of calcium phosphate and calcium carbonate deposits in critical heat exchangers and lines. The inorganic orthophosphate program (phosphate functions as an anodic inhibitor) can be stabilized with the dispersant polymers to prevent calcium phosphate buildup. Some cathodic corrosion control can also be accomplished by the poly-inorganic (polyphosphate) component. The supplemental polymers aid in controlling deposition of silt and other suspended solids. Most of ChemTreat’s stabilized inorganic phosphate treatment chemicals contain yellow metal inhibitors such as tolyltriazole or BZT and may also contain phosphonates, polymers, and copolymers as threshold inhibitors and mineral scale dispersants.
All-Organic Corrosion Inhibitors
ChemTreat’s all-organic corrosion inhibitors for open recirculating cooling water systems generally contain an organic phosphate and may also contain polymeric dispersants, polyacrylate, and/or polymaleic anhydrides, copolymers, and a copper inhibitor. Sodium molybdate may also be included in these treatment programs for testing and control. Where possible, the phosphonate control test may be used as a primary or backup control test. These inhibitor programs do not contain inorganic phosphate (orthophosphate or polyphosphate).
All-organic treatment programs are frequently used in systems without pH control, which have a tendency to form scale as opposed to corrosion. As a result, attention to mineral scale control is critical. At the lower end of the pH and LSI ranges, the various combinations of phosphonates, polymers, and dispersants in a one-drum combination product are normally satisfactory for system corrosion and scale inhibition. Supplemental ChemTreat dispersant polymers or blends will be required as the hardness, alkalinity, pH, cycles, and severity of heat load severity are increased.
All-organic programs are not normally used in aggressive waters. These conditions may cause increased mild steel corrosion, requiring supplemental dispersants or sequestrants and providing no substantial improvement in corrosion control. In these systems, we recommend using one of the zinc/HEDP or stabilized inorganic phosphate programs; however, an all-organic program can sometimes be used in these systems if the water can be cycled up enough to make the tower water noncorrosive (LSI of +1 or greater).