BEST PRACTICES FOR WHITE RUST PREVENTION (2)
The galvanizing has a coating of zinc-iron intermetallic alloy layers on steel with a relatively pure outer layer of zinc. The zinc is anodic (negative) to steel and will provide sacrificial protection to any small areas of steel that may be exposed (i.e., scratches, cut edges, etc.). Additionally, the zinc coating will oxidize and provide a physical barrier in protecting the bulk of the steel surface from any direct contact with the environment. Bear in mind that the galvanize will wear off eventually. So it is safe to expect that a thicker and more durable zinc coating will provide protection for a longer period of time. Another variable is water treatment chemistry, which has changed significantly since the early 1980’s. With water being the universal solvent, the frequency of white rust corrosion is heavily impacted by water treatment. The most critical timing is during the initial start-up operational period. Common sense dictates that we look at both mechanisms and evaluate what can be done on our own systems as well as what has been done in accordance with established “best practices.”
What does white rust look like? It is often identified by the white, gelatinous or waxy deposit. (Note the photo below. – ) This deposit is a zinc-rich oxide, and can be quite similar chemically to the protective zinc oxide typically identified as a dull-gray passive oxide. One critical difference between the two oxides is that the white rust oxide is porous and generally non-protective of the substrate. The passive oxide is dense and non-porous effectively protecting the substrate from exposure to the environment. Best practices suggests as with any metal, that a stable and passive oxide layer is formed and maintained. The same is true for water treatment purposes in boiler treatment or potable water piping installations.
If the oxide noted above is disrupted, repair is crucial. If the oxide layer is constantly disrupted or removed, general corrosion potential will increase or in the case of galvanized steel, depletion of the zinc coating will eventually occur. And if pitting corrosion occurs and is not mitigated, the life expectancy of the condenser will be greatly reduced.
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Best practices for white rust prevention
With the start of the new year, Condor Technologies will be providing a new section for our website. We will bring you the latest news, industry information, and the latest technologies. The first of which will be a multi-part paper regarding white rust prevention.
It may be best to start out our best practices approach with a brand-new condenser that has never seen treatment and just came off the production line. From birth, a discussion of best practices may be most effective in limiting problems associated with corrosion, deposition, fouling, and microbiological growth. Galvanized steel remains the principal material of construction for factory assembled ammonia condensers. This is driven by galvanized steel being the lowest cost to produce. Furthermore, when it is kept in good shape it can offer 20 years or more life expectancy in cooling applications. The critical point is that it must be kept in good shape. If deterioration or the routine maintenance program fails, corrosion and scale will set in motion. They both have a dramatic impact on both heat transfer (deposition/microbio) and lifespan (corrosion.) Ideally, this pushes things away from green and much further into the red, taken literally and figuratively.
In examining the green approach, corrosion is a major concern. If the lifespan of a condenser is cut in half, this creates much more strain on the environment as a whole. A rising trend in cause of death due to corrosion is a mechanism known as white rust. This specific type of corrosion continues to be a prevalent problem that has led to many towers requiring premature replacement. White rust corrosion can reduce life expectancy significantly, in some rare cases failure has occurred within a year or two of startup (not just hearsay, as actual case histories exist in locations stretching from California to Maryland, typically where the make-up water is more alkaline or higher in pH.)
So the question then is asked “Why the increase in cases regarding white rust? What has changed?” Unfortunately, there is no smoking gun that points to the reason for the rise in cases. Yet, many believe that it stems from two different substances: galvanizing process and water treatment. One popular consideration is that there were changes to both galvanizing process and the water chemistry have increased the potential for white rust corrosion. It is true that there have been notable changes to both the galvanizing process and water treatment chemistry. Much of this has been driven in large part by environmental restrictions and regulations as well as cost-reduction initiatives. Consequently, the hunt is on to examine the changing trend in these manufacturing and treatment changes.
Without getting too involved, it is important to look at where the changes to galvanizing process has an impact on the corrosion cell. A piece of bare iron left outside where it is exposed to moisture will rust quickly. This is the tendency for the iron to revert back to its natural state and drive itself back to an oxygen rich state. It will do so even more quickly if the moisture is salt water. The corrosion rate is enhanced by an electrochemical process in which a water droplet becomes a voltaic cell in contact with the metal, fully oxidizing the iron. Consider the sketch (below) of a water droplet, the oxidizing iron feeds electrons at the edge of the droplet to reduce oxygen from the air. The iron surface inside the droplet acts as the anode for the process. It goes back and forth to each cell providing an oxidizing reaction at the cathode and a reducing reaction at the cathode.
We will continue this article in the following weeks…
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