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…