White Rust

Over the last several years the problem of “white rust” in new galvanized cooling towers has reached epidemic proportions. We as water treatment specialists are often looked upon to provide information as to the nature of the problem and what can be done to prevent it. In this short dissertation I will attempt to explain the problem and to provide some insights as to what can be done to minimize it.

The condition that is referred to as white rust is characterized by the formation of voluminous white or grayish white zinc corrosion products on galvanized surfaces. These usually begin as localized tubercle like deposits, which are loosely attached to the surface. Gradually the corrosion spreads to all wetted surfaces. Eventually the entire zinc costing may be compromised. The fact that this type of corrosion can occur very rapidly on new cooling towers leaves many an engineer bewildered and many water treatment specialists scrambling for answers. There is a minimal amount of literature available on the subject. The cooling tower manufacturers are also not very forthcoming with solutions to the problem; however, this information available points to the water chemistry of the system as being the major influence on whether or not white rust will be a problem.

Most cooling water systems today are operated in an alkaline pH range. Generally the pH will fall somewhere between 8, 2-9, 4. The major component of the zinc protective coating will consist of basic zinc carbonate (3 2n (OH) 2, or 2nCO3, H2O) It appears that when this forms in absence of sufficient CO2 or HCO3 that it develops a crystalline structure that is not protective, since the majority of cooling water systems operate above the phenopthalein endpoint. This would be the predominant chemistry that would exist. It is the predominant water chemistry that is the root cause of the problem. White Rust is generally no the result of poor water treatment. There is some indication that phosphate based programs ca give some relief such as would be the case with a low level orthophosphate/phosphonate treatment; however, this is not the predominant technology being utilized. The key to minimizing the problem appears to be in the initial start up of the system. If the basic zinc carbonate protective film can be formed under the proper conditions, then subsequent operation in the alkaline range with little problem is possible. It has been the experience of myself and others that a proper start-up program will minimize the problem. Most new cooling towers are set into place a long time prior to start-up. Often times new systems are filled with water, tested, then allowed to sit during this time; however, you could assume that the higher the pH the greater tendency for this to occur. It is this period prior to start-up, which is critical to the prevention of white rust. What has been successful in preventing white rust from occurring is a simple start up procedure as follows:

Taking the above steps does not guarantee that white rust will not occur; however, taking these steps should help to establish a protective oxide film on the galvanize. If white rust has already occurred, the system should be shut down and this procedure should be performed. If the galvanize coating is already too far-gone, then the surfaces exposed will be mild steel. If the corrosion of steel is being controlled and the tower is kept free of debris. It should survive until a protective coating can be applied.

In summary it is important to properly start up a new system to help prevent against white rust. If controlling pH below 8 is not feasible, then a low level alkaline phosphate program may help to reduce the effects of white rust. The following are a list of technical papers, which contain more detailed information on the subject:

• Johnson Keith M. and Joseph B. Mihelic, “Diagnosing White Rust corrosion in Cooling Tower Systems” paper 361, corrosion ’90, National association of Corrosion Engineers. Houston TX (1990)
• Tool, M. Raymond. “Galvanized Cooling Towers White Rust”. Proceedings of the International Water Conference, 48: 139-146, Engineering Society of Western Pennsylvania. Pittsburgh, PA (1987)
• Johnson, Keith M. and Joseph B. Mihelic, “Update on White Rust Corrosion and Control” Paper #TP91-14, Cooling Tower Institute. Houston, TX.