Cooling Tower Passivation
As a general rule of thumb, galvanized cooling tower passivation will extend the system lifespan, but how is this procedure performed and what are the working mechanisms?
The necessity to passivate galvanized (zinc coated steel) systems is generally understood by water treatment professionals but may be an unrecognized concept to owners of new cooling towers or maintenance teams. As a consequence, some galvanized systems are placed into service prematurely and develop corrosion attack to the galvanized surface. Without the protective passivation layers, rapid deterioration of the galvanized surface will ensue. This is a corrosion process referred to as “white rust”, where a white corrosion product forms onto the zinc surface.
Passivation provides maximum protection from corrosion on newly installed evaporative condensers and cooling towers that have hot-dipped galvanized steel tube coil surfaces. Hot-dip galvanizing produces a coating of zinc-iron intermetallic alloy layers on steel with the outer layer being purely zinc. The zinc provides cathodic (sacrificial) protection and a physical barrier. Passivation helps prevent the development of corrosion that could result in rapid penetration through the zinc coating to the steel. If the corrosion makes it entirely through the protective coating to the carbon steel condenser tubes, they will rapidly corrode. Note that fouling material, such as scale or white rust on the tubes, can hide corrosion on the surface of the tubes.
Corrosion control of galvanized steel depends on forming and maintaining a stable and passive oxide layer on the surface of the galvanized steel during the installation of the equipment.
Passivation of galvanized systems to prevent white rust formation requires specific operating parameters, water chemistry, and time. If the conditions for natural passivation cannot be met, other chemical passivation methods may be considered. The time allotted for the passivation process may be weeks to months and may vary with the manufacturer of the equipment. A passivation layer formed by one of these other methods such as phosphates or silicates will protect the zinc surface as long as the water chemistry is favorable to the passivation layer. The initial startup and passivation (or re-passivation) process requires due diligence and adherence to operational and water chemistry conditions. Before any passivation process is attempted, the galvanized surfaces must be cleaned of any dirt and debris, oil, greasy films, or any other contaminants that may interfere with the passivation.
Passivation must take place prior to startup and without full heat load. The heat load should be avoided since water evaporation can concentrate corrosive ions and increase the pH. Softened water should be avoided due to the tendency for corrosion (review RSI and LSI scaling tendencies) and calcium fouling potential. Ideally, a neutral pH should be maintained (6.5 – 7.5) and may require the introduction of acid. This will ensure that the alkalinity of the tower water remains below the threshold at which white rust forms. Also, the combination of low pH and phosphate-based chemicals can significantly reduce the time required. Hardness should be maintained within 100 – 300 ppm (as CaCO3). Stabilized phosphate chemistry is very effective in promoting zinc passivation and the recommended phosphate concentration can range from 20 to 200 ppm, depending on water chemistry and the speed with which you are trying to achieve passivation. However, careful hardness and alkalinity control are necessary if high phosphate dosing is desired to prevent calcium phosphate deposition. NOTE: Water chemistry recommendations may vary by major cooling tower manufactures/trade organizations and obtaining these specifications is recommended.
One visual indication that the passivation process is proceeding is observing a dull gray of darkened appearance to the galvanizing. The system should be inspected periodically to note the change in appearance. It is critical that white rust or deposits do not occur during this procedure which would indicate incorrect chemistries. By the end of the passivation period, the galvanizing should have acquired a darkened gray appearance and can be brought online. Once a successful passivation has been conducted, there is also more latitude with the water chemistry. Tower pH can be increased slowly, if necessary, but exceeding 9.0 is not recommended. Soft water is also acceptable, as long as a corrosion-inhibiting chemical program designed for white rust prevention is used. Regardless of the program used, proper control is important. Overfeeding phosphonates and chelating polymers can remove the passivation from the zinc, requiring a new passivation procedure to be run.
Successful passivation of galvanized surfaces relies on the quality of the galvanized coating, maintaining certain water chemistries, operating parameters, acute observation, and time. Investing the time to passivate a galvanized system is an investment in preserving valuable assets.
Jed Kosch
List of Resources:
[1] “Minimizing White Rust in Galvanized Cooling Towers.” Process Cooling, 1 July 2010, www.process-cooling.com/articles/86178-minimizing-white-rust-in-galvanized-cooling-towers.
[2] “Industrial Water Treatment and Maintenance.” Www.tub.com, 25 May 2005, buildingcriteria1.tpub.com/ufc_3_240_13fn/index.htm..
[3] Vaghela, Mehul C. “IMPROVE PERFORMANCE OF COOLING TOWER BY PASSIVATION.” Vol. 4, no. 3, 2018, pp. 2541–2545.
