How To Control White Rust or Zinc Corrosion in Cooling Towers
Cooling tower operators face increasing pressure to reduce fresh water consumption, limit wastewater discharge, and meet tightening environmental requirements, but doing so creates the need to control the emergence of white rust or zinc corrosion in cooling towers. One of the most effective strategies for achieving these sustainability and water usage goals is to increase the cycles of concentration within the cooling system. Higher cycles reduce the overall water demand within the system, but as a result of this process, the pH and alkalinity rise within the recirculating water. These concentration cycle changes can accelerate white rust or zinc corrosion on galvanized tower components, which can have a detrimental impact on the entire system if not properly addressed and maintained with the help of the experienced and knowledgeable team from ProChemTech. This technical guide explains why white rust forms under high-COC conditions and outlines proven methods to control white rust in cooling towers while still maintaining the water savings and operational efficiencies that higher cycles make possible.
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What is White Rust or Zinc Corrosion?
White rust is the common name applied to zinc corrosion; the products of corrosion typically appear as a white to dirty gray voluminous deposit below the water line on galvanized steel surfaces exposed to the recirculated water. The white color is due to the formation of zinc carbonate, which does not form a corrosion limiting protective film on the base metal. Due to this lack of protective film formation, white rust corrosion will continue until the protective zinc is entirely removed from the underlying steel, which is then subjected to accelerated corrosion (red rust) and premature failure. White rust has been a substantial corrosion problem in many galvanized steel cooling towers, causing equipment failure in as little as three years. White rust corrosion will occur on any galvanized surface exposed to cooling water with a pH greater than 8.2. Rate of corrosion will be determined by a combination of pH, alkalinity, and presence and concentration of any specific corrosion inhibitor(s) for zinc metal.
Passivation
Review of many technical papers and manufacturer’s recommendations in the past ten years shows that a passivation procedure is recommended to control white rust corrosion on new galvanized cooling towers. In general, the passivation procedures recommended fall into two general methods; operate for 4 to 12 weeks at a pH between 7.0 and 8.2 with hardness over 30 mg/l present; or, clean and use an ortho phosphate treatment program. Unfortunately, neither of these methods produces a galvanized surface that will not suffer immediate and long white rust corrosion when the cooling water pH exceeds 8.2.
GalvaGard
ProChemTech has researched the problem and evaluated all the various passivation and treatment recommendations. What we found, in both the laboratory and in the field, are that none of the passivation procedures and treatments recommended prevented white rust when the cooling tower water exceeded a pH of 8.2. Corrosion rates were determined by pH, alkalinity, and the various ions present in the cooling water. It was found that some recommended treatments, such as use of phosphates, phosphonates, and advanced dispersant polymers, actually increased the white rust corrosion rate.
During some routine corrosion rate studies using zinc corrosion coupons, we noted a substantial drop in zinc corrosion rates when a single raw material was changed in a cooling tower treatment formulation.
R&D based on this observation resulted in our development of a new zinc corrosion inhibitor technology, GalvaGard™, in 1994. This unique technology has been used in a wide array of formulations suitable for makeup waters ranging from soft to extremely hard with high pH values and any level of alkalinity.
Initial White Rust Control
We have found that simply operating with a high level, 25 to 50 mg/l, of active GalvaGard for the first week of a new cooling tower provides excellent protection of galvanized surfaces from white rust. The most common method is to operate the cooling tower with a GalvaGard containing cooling water inhibitor formulation at two (2) to four (4) times the normal recommended dosage for a week or two.
Long Term White Rust Control
No passivation procedure, including GalvaGard, is effective for long term control of white rust when the cooling water pH will exceed 8.2. The only effective methods for long term control of white rust are the following.
- Control cycles of concentration so that the cycled pH of the cooling water never exceeds pH 8.2. This option is obviously wasteful of fresh water and thus defeats the objective of minimizing cooling tower water use.
- Incorporate pH control by acid feed to the cooling water to maintain the cooling water below pH 8.2 at all times. Control of cooling water pH by acid addition is at best difficult in practice and must utilize high quality pH control equipment combined with routine calibration of the equipment with on-site verification that the desired pH level is being maintained. Loss of pH control with subsequent over feed of acid can quickly destroy galvanized surfaces, often in less than 24 hours.
- Utilize a treatment program that incorporates GalvaGard either as a component of the scale, corrosion, and deposition inhibitor, or use a separate feed of a GalvaGard concentrate product.
Continuing use of a treatment program that provides a maintenance level of 5 to 25 mg/l active GalvaGard in the cooling water is sufficient to provide excellent control of white rust at cooling water pH values up to 9.8 and alkalinities up to 2,500 mg/l.
Galvanized steel fluid cooler tubes operated since 2007 using a GalvaGard cooling water product with hard, high pH and alkalinity City of Phoenix city water.
ProChemTech International, Inc.
“Innovation in Water Management” Apache Junction, AZ, and Brockway, PA
814-265-0959 www.prochemtech.com 480-983-5385 prochem@prochemtech.com