Galvanized coatings for cooling tower systems have been used in the industry since the 1950s. The galvanizing process binds a layer of zinc metal to steel. When properly applied and passivated, the zinc coating works as a nonpermeable sacrificial anode to prevent corrosion of the underlying steel structure. White rust, which is the oxidation form of the zinc coating, shows up as white spots and bumps on a galvanized surface.
White rust is thought to have the chemical composition of 3Zn(OH)2∙ZnCO3∙H2O. It’s porous and generally does not protect the steel structure. More importantly, corrosion of the underlying steel can become concentrated under the white rust bumps, quickly developing into pitting corrosion. Left unchecked, leaks can form in a tower basin in as little as two months in severe cases.
To help companies currently using or considering the purchase of a galvanized cooling tower, U.S. Water Services in St. Michael, MN has prepared the following information on white rust, its causes, control and prevention.
The incidence of white rust damage to towers has increased dramatically in the last 10 to 15 years, according to the water treatment company. More stringent environmental discharge concerns have led to the reduction or elimination of effective chemicals in both the galvanizing process and in water treatment programs. Chromate has been all but eliminated, and molybdate, phosphate and zinc treatments are being restricted in many communities. In addition, as facilities attempt to minimize water volumes used in cooling, the softening of make-up water has become more common, as it allows the cooling system to be run at higher cycles of concentration. All these factors have contributed to an increase in white rust damage to new galvanized tower systems.
For management considering the purchase of a new cooling tower, U.S. Water Services emphasizes the importance of consulting a water treatment representative early in the process because multiple factors need to be considered before site before choosing the tower’s construction materials:
- Make-up water quality
- Discharge permits
- Water-use restrictions
- Type of chemicals management is willing to have on site
Most cooling tower OEMs have specific requirements for water quality in galvanized systems. If a facility will not be able to meet those requirements, management should explore other construction materials options. Plastic, stainless steel, concrete, wood and fiberglass can all be good alternatives. Higher upfront costs need to be compared with operational and maintenance costs.
If a galvanized tower makes sense in your application, the startup passivation chemistry is critical to maximizing the useful life of the new tower. The following list is a guide for successful commissioning of a new tower. Initial passivation may take a few weeks or several months, and generally is concluded once a dull gray passivated coating can be seen visually on the galvanized metal. Always consult with a water treatment representative for specific details.
All new systems should be pre-cleaned to remove oils and construction dirt. However, avoid strong acid or alkaline cleaners. Phosphate and silicate cleaners are recommended.
The pH/alkalinity of the cooling water needs to be controlled between 6.5 to 8.0 during the initial passivation period, which usually requires pH controlled acid feed or an acid-based treatment chemical. If you do not have pH control, do not feed acid.
Soft water prevents passivation of the galvanizing. A minimum of 50 to 100 mg/l of calcium hardness as CaCO3 is required. If the system is designed for soft water, a hard water bypass will have to be used.
Following the initial sterilization of the new system with oxidizing biocides, the free chlorine needs to be controlled below 1.0 mg/l during the passivation process. Spikes of free chlorine above 1.0 mg/l can remove the passivation layer, even if all the other chemistry is maintained correctly.
Stabilized phosphate chemistry is very effective in promoting zinc passivation. The recommended phosphate concentration can range from 20 ppm 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.
Passivation is best accomplished under conditions of reduced heat load because evaporation from heat load can concentrate corrosive ions, and increase pH as well as the potential for fouling. If heat load cannot be avoided during initial passivation, then the risk of white rust will increase, especially for systems with moderate to high make-up water alkalinity and dissolved solids.
Once a successful passivation has been conducted, there is some more latitude with the water chemistry. Tower pH can be increased slowly, if necessary, but never should exceed 9.0. 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. Overfeed of phosphonates and chelating polymers can remove the passivation from the zinc, requiring a new passivation procedure to be run.