May 2020

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Water Management: Minimize white rust in galvanized cooling systems

Galvanized coatings for cooling tower systems (cooling towers, evaporator tube bundles, etc.) have been used in the industry since the 1950s due to their relatively low cost and long service life.

Mowbray, M., U.S. Water Services Inc.

Galvanized coatings for cooling tower systems (cooling towers, evaporator tube bundles, etc.) have been used in the industry since the 1950s due to their relatively low cost and long service life. The galvanizing process bonds 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 (FIG. 1), which is the oxidation form of the zinc coating, shows up as white spots and bumps on a galvanized surface.

Fig. 1. White rust shows up as white spots and bumps on a galvanized steel surface.

White rust is a combination of zinc carbonate and zinc hydroxide. It is 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 2 mos, in severe cases.

The incidence of white rust damage to towers has increased dramatically since the late 1980s. More stringent environmental discharge concerns have led to the reduction or elimination of some effective chemicals in both the galvanizing process and in water treatment programs. Chromate is the most effective treatment for passivating galvanized coatings (Fig. 1), but it has been eliminated as a water treatment option and is restricted for galvanized manufacture.

Other traditional treatments, such as molybdate, phosphate and zinc treatments, are also restricted in many communities. As facilities attempt to minimize water volumes used in cooling, the softening of makeup water has become more common, as it allows the cooling system to run at higher cycles of concentration, thereby minimizing water use. In addition, scale inhibition chemistry has become more effective, and if over-applied, can attack metal oxide passivation layers on piping and tower construction. All of these factors have contributed to an increase in white rust damage to new galvanized tower systems.

For plant management considering the purchase of a new cooling tower, it is important to consult a water treatment representative early in the process, as multiple factors must be considered before choosing the tower’s construction materials:

  • Makeup water quality: A little hardness is beneficial to galvanized coatings on steel
  • Discharge permits: Galvanized towers will always contribute some zinc to the water discharge
  • Water use restrictions: Concentrating cooling water to minimize use increases corrosive ions
  • Plant’s onsite chemical preference: Acid is usually needed during passivation.

Most original equipment manufacturers (OEMs) for cooling towers have specific requirements for water quality in galvanized systems. While slight differences exist, the guidelines generally fall into two categories (FIG. 2). If a facility will not be able to meet these requirements, then management should explore other construction materials options. Plastic, stainless steel, concrete, wood and fiberglass can be good alternatives. Higher upfront costs should be evaluated against operational and maintenance costs.

Fig. 2. Common OEM guidelines for water quality in galvanized systems.

Startup passivation chemistry is critical to maximizing the useful life of a new galvanized tower. Initial passivation will generally take 12 wk–14 wk, and is concluded after a dull gray passivated coating can be seen on the galvanized metal. Waiting to start the passivation, even a week after water circulation has been started, will likely result in an incomplete passivation, white rust and eventual leaks. It is critical that the galvanized steel passivation process is started as soon as water is first circulated through the system:

  • All new systems should be pre-cleaned to remove oils and construction dirt. However, avoid strong acid or alkaline cleaners. Phosphate- and surfactant-based cleaners are recommended.
  • During the initial passivation period, the pH of the cooling water must be controlled between 6.5 and 8, which usually requires pH-controlled acid feed or an acid-based treatment chemical. If real-time pH monitoring and control is not available, then acid should not be fed. Acid feed cannot be consistently controlled within safe parameters by daily manual testing. Equipment damage can be severe and can happen within hours.
  • Soft water prevents passivation of the galvanization. A minimum of 50 mg/l–100 mg/l of calcium hardness as calcium carbonate (CaCO3) is required. If the system is designed for soft water, a hard water bypass will need to be installed and regulated during the passivation period.
  • Following the initial sterilization of the new system with biocides, the free chlorine must be controlled below 1 mg/l during the passivation process. Spikes of free chlorine above 1 mg/l can remove the passivation layer, even if the other chemistry is maintained correctly. If a spike of free chlorine above 1 mg/l occurs during the passivation process and lasts more than 4 hr, then the passivation process should be restarted.
  • Stabilized phosphate chemistry is effective in promoting zinc passivation. The recommended phosphate concentration can range from 20 ppm–200 ppm, depending on water chemistry and the desired speed of passivation. However, careful hardness and alkalinity control are necessary if high phosphate dosing is desired, to prevent calcium phosphate deposition. A best practice is to maintain 20 ppm–40 ppm of orthophosphate in the circulating water for approximately 90 d. If the cooling system runs seasonally, it is helpful to run a phosphate passivation program for the entire first season.
  • Passivation is best accomplished under conditions of reduced heat load because evaporation from heat load can concentrate corrosive ions and increase pH, creating 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. Operating at reduced cycles of concentration during the passivation process can increase the likelihood of a successful passivation.

Once a successful passivation has been conducted, more options are available with the water chemistry. Tower pH can be increased slowly, if necessary, but should never exceed 9. Soft water is also acceptable, as long as a corrosion-inhibiting chemical program designed for white rust prevention is implemented. 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. HP

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