New evaporative cooling systems, typically cooling towers, ammonia condensers and fluid coolers are manufactured with several options for materials of construction, each with their own benefits and draw backs. Hot-dipped galvanized steel (steel that has been coated with zinc) has become the most common choice for construction as it provides structural strength at a relatively low cost and enhanced corrosion resistance over that of uncoated steel. However, the zinc coating is not well-suited for locations that are in constant contact with water, and requires passive oxidation to be protected from corrosion when exposed to cycled cooling tower water. Passivation is the process by which this zinc layer is oxidized slowly under controlled conditions to produce a passive layer of zinc oxide on the exterior surface of the coating, which provides superior resistance to corrosion.
While passivation has always been required on galvanized materials in evaporative coolers, it has increasingly become a more prevalent topic of discussion. Previously, cooling units were manufactured ahead of time in standard sizes, and were left outside until a unit of that size was needed. During this time, the towers would be repeatedly exposed to the elements including rain and condensation, which naturally passivated the galvanized surfaces. Advances in manufacturing techniques have improved the time required to design, build, and assemble each unit. Today, almost all cooling towers begin manufacture at time of order and as a result, most units being installed have not had the opportunity to passivate naturally. Therefore, a passivation program must be implemented as soon as possible after installation before the system sees cycled tower water under load.
White Rust Formation
When an un-passivated galvanized surface is exposed to cycled cooling water, which will typically be oxygen-rich and have an elevated pH, white rust will begin to form. White rust is a corrosion product of zinc and is composed of a mixture of zinc, oxygen, water, and carbonate. White rust can form, remain in place, and not grow in extent or coverage. If this is the case, the main disadvantage is the unwanted appearance. White rust formation will not always cause premature failure or issues with the cooling tower. However, if this corrosion process continues, the carbon steel underneath the zinc coating can become exposed. If this occurs, the zinc will no longer offer protection, and the carbon steel will corrode rapidly, resulting in premature failure. The basin is most susceptible to this mode of failure, as the basin will remain fully wet during system operation. Thus, it is vital that the zinc be properly passivated to ensure maximum protection from corrosion and to extend the life span of the equipment.
When installing a new cooling tower, there are several important factors that must be considered with regards to passivation. First, a stainless steel basin should be considered when purchasing the cooling tower. As mentioned previously, basins are the most susceptible to failure by white rust. While more expensive than a galvanized basin, stainless steel does not require passivation, and cannot fail due to white rust formation. Generally, a stainless steel basin will offer a far superior lifespan as compared to a galvanized basin.
If a galvanized basin is selected, then the following considerations are required to ensure proper passivation to protect your newly acquired asset. First, and most importantly, the local makeup water chemistry will play a critical role. Systems with water supplies that are high in dissolved minerals or pH may require an acid feed system to maintain the proper water chemistry to promote passivation. To promote passivation of the zinc coating, it is recommended to maintain specific water chemistry and have the galvanized surfaces regularly exposed to that water. The general water chemistry guidelines are as follows:
- High hardness levels (>100ppm)
- Neutral pH (~6.5-7.3)
- Low total dissolved solids
- Presence of ortho-phosphate
Secondly, wherever possible systems should be started under no or very low load conditions. If the system will be under little to no load, it may be possible to maintain a passivating condition using the natural water chemistry of the makeup water, with addition of phosphate as required. Under these circumstances, uncycled water can be sprayed intermittently in the tower, causing wet-dry cycles which allow for the most effective promotion of passivation. However, if the system will be under load while passivation is being attempted, it may not be possible, or economical, to maintain uncycled water to passivate naturally through bleed, and acid feed may be required to keep pH close to neutral as required. This can be costly both in material costs for acid and phosphate, as the system will be using a substantial amount of water depending on the system size, and in equipment costs, as acid feed requires specialized chemical feed and control systems to safely inject acid and maintain pH levels. In addition, the reaction rate of white rust formation will be accelerated with increases in temperature. The warmer the system the faster the rate of zinc corrosion. This will typically be of the most concern in condenser and fluid cooler systems as the heat source is in the galvanized tube.
A successful passivation program will require cooperation between the water treater, mechanical contractor and end user to achieve the conditions required to promote passivation.
When the galvanized coating on carbon steel is maintained on cooling tower or other evaporative cooler surfaces, it functions to essentially slow down the rate of corrosion of the underlying steel. This promotes acceptable performance of the steel and maximizes the life span of the equipment while minimizing the overall cost of the material. However, the steel must remain wholly covered with the galvanized zinc coating to maintain this corrosion inhibition. White rust formation on these galvanized surfaces may eventually lead to the coating being compromised, resulting in rapid, local corrosion at the site of the exposed carbon steel beneath. Properly passivating the zinc coating on these surfaces can greatly improve the corrosion resistance of the coating, extending its protection of the steel structure. This process can be repeated on cooling evaporative systems that have already been in service to re-passivate the surface, restoring the passive zinc oxide layer. All galvanized surfaces should be regularly inspected for white rust formation, and the process repeated as needed. Talk to your water treatment specialist for any specific questions you may have.