In industrial cooling towers made of galvanised steel, passivation is an electrochemical process that slows or prevents possible corrosion of the metal from which the units are made.
Rust that occurs on the cooling towers during the first period of use, called "white rust", is preventable by pre-treading the metal surface of the cooling tower.
More about this topic in this article.
In the air conditioning(HVAC) market,galvanized steel is, to date, the main material used for shop-assembled cooling towers and evaporative condensers. Despite the growing trend in the use of alternative materials for the construction of cooling towers assemblies, such as fiberglass, plastic and stainless steel or hybrids, galvanized steel remains the most common and cost-effective choice to date; moreover, if properly maintained, it ensures 20 or more years of useful life for the systems.
However, it is precisely the lack of proper maintenance, especially during the first few months of use of the equipment, that can lead to a premature onset of corrosion: the so-called "White Rust". This corrosion can occur within a year or two of the start-up of the cooling tower and can significantly reduce the life expectancy of the tower.
In industrial cooling towers built with galvanized steel, white rust is a localized corrosion attack that occurs rapidly and usually appears as a voluminous white deposit. This rapid corrosion can completely remove the zinc in a localized area of the cooling tower resulting in reduced equipment life. Underneath the white deposit, a localized area is usually visible, indicating where the zinc has been attacked, and it is usual to initially appear as a surface crater.
Generally, white rust is more likely to appear on newly commissioned equipment. Therefore, during the initial weeks of commissioning cooling tower, in which the zinc is exposed to water, an inspection activity should be carried out and, if necessary, a pretreatment that will lead to passivation of the surface. It is also true that white rust attack can occur even on old zinc, particularly when the surface is exposed to high pH, the water has very high conductivity, or in cases where the old surface has been damaged externally.
The consistency of corrosion generally varies: when the white rust deposit is kept wet, it is usually waxy to the touch; however, when the corrosion product dries, it usually becomes hard and brittle.
To prevent white rust and thus ensure the longest possible service life for your galvanized steel industrial cooling tower, it is advisable to provide adequate water treatment to ensure passivation of surfaces during the initial stages.
Initially, the zinc coating must be allowed to develop a natural non-porous layer of "basic zinc carbonate." This natural chemical barrier prevents further rapid corrosion of the zinc coating by the environment or due to normal cooling tower operation.
It should be borne in mind that the basic zinc carbonate barrier forms on the galvanised surfaces within eight weeks from the start of operation of the cooling tower with water of neutral pH (6.5-8.0), calcium hardness of 100-300 ppm (as CaCO3), and alkalinity of 100-300 ppm (as CaCO3). It is therefore very important to allow this to form on the galvanised surfaces of the tower in order to resist white rust corrosion as much as possible. (It will now be clear that the initial operation of the cooling tower can significantly influence its service life).
We strongly recommend that initial operations be done with a pH close to neutral and with moderate use of inhibitors for bio-film formation. Many water treatment companies suggest treatment with corrosion inhibitors of the phosphate/polymer/phosphonate type, which have been found to be effective in preventing the formation of white rust on galvanized steel.
Before starting any operation with the new tower, it is also important to verify that your water treatment company has chosen a suitable program foryour galvanized steel cooling tower.
DECSA, as standard for its cooling towers, uses hot-dip galvanized steelZ-725 panels. These sheets, with a coating weight of 725 g/m² and a thickness of 50.5 µm, have a quality among the most available on the market and were specially chosen to limit the phenomenon of white rust.
Realizing that HDGS panels alone may not be the best choice for every type of water or environment, we have thus introduced a variety of material and finish options for our cooling towers, starting from the" DecsaCoating Plus" coating to the completeSS316 execution.
The table below for our TMA-EU cooling tower shows useful information for the most appropriate solution to avoid white rust.
SOURCE: White Rust Prevention, An Industry Update and Guide Paper - 2012- Presented By: Association of Water Technologies (AWT)
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