Minimizing White Rust in Galvanized Cooling Towers

Galvanized coatings for cooling tower systems have been used in industry since the 1950’s.  The galvanizing process binds a layer of zinc metal to steel.  When properly applied and passivated, the zinc coating works as a non-permeable sacrificial anode to prevent corrosion of the underlying steel structure.  White rust is the oxidation form of this zinc coating, and shows up as white spots and bumps on a galvanized surface.  The rust is thought to have the chemical composition of 3Zn(OH)2∙ZnCO3∙H2O, and is porous and generally non-protective of the steel structure.  More importantly, corrosion of the underlying steel can become concentrated under the white rust bump, quickly developing into pitting corrosion.  Left unchecked, leaks can form in a tower basin in as little as two months in severe cases.

The incidence of white rust damage to towers has increased dramatically in the last 10 to 15 years.  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 those facilities considering the purchase of a new cooling tower, it is very important that you consult with your water treatment representative early in the process.  You need to consider make-up water quality, discharge permits, water use restrictions, and the type of chemicals you are willing to have on your plant site, before you choose the construction materials for your tower.  Most cooling tower OEM’s have specific requirements for water quality in galvanized systems.  If you will not be able to meet those requirements, you need to explore other material of construction options.  Plastic, stainless steel, concrete, wood and fiberglass can all be good options.  Higher up front costs need to be compared with operational and maintenance costs. 

If a galvanized tower makes sense in your application, the start up passivation chemistry is critical to maximizing the useful life of your new tower.  The following list should be considered a guide for successful commissioning of a new tower.  This initial passivation may take a few weeks, or several months, and is generally concluded once a dull gray passivated coating can be seen visually on the galvanized metal.  Always consult with you water treatment representative for specific details:

1. All new systems should be precleaned to remove oils and construction dirt.  However, strong acid or alkaline cleaners should be avoided.  Phosphate and silicate cleaners are recommended.
2. You will need to control the pH / alkalinity of the cooling water between 6.5 – 8.0 during the initial passivation period.  This usually requires pH controlled acid feed or an acid based treatment chemical.  If you do not have pH control, do not feed acid.
3. Soft water will prevent the passivation of the galvanizing.  A minimum of 50 – 100 mg/l of calcium hardness as CaCO3 is required.  If your system is designed for soft water, a hard water bypass will have to be used.   
4. Following the initial sterilization of the new system with oxidizing biocides, you should control free chlorine 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.
5. 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, in order to prevent calcium phosphate deposition. 
6. Lastly, it is highly recommended that the passivation be accomplished under conditions of reduced heat load.  Evaporation from heat load can concentrate corrosive ions, increase pH and 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 makeup water alkalinity and dissolved solids.  

 

Once a successful passivation has been conducted, you have some more latitude in your water chemistry.  Tower pH can be slowly increased, if necessary, but should never 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. 

The Association of Water Technologists has written an excellent paper on white rust.  Read the AWT case study here.