Greatly increased corrosion losses are well known to occur during the winter months where cooling tower systems are drained down for freeze protection. To a lesser degree it exists for any section of pipe which is drained and left empty for any extended period of time - with the level of corrosion activity being generally dependent upon the openness of the piping system to the atmosphere.

     Our 10 year history of ultrasonically testing pipe has, beyond any doubt, documented corrosion rates of between 2 and 10 times higher than that of piping which has been continuously filled with water. Even piping systems filled with untreated water will corrode at rates lower than those which have been drained and left empty.

     Due to moist and chemical unprotected conditions in the presence of abundant oxygen, roof level failures generally precede problems at basement or mid level piping by decades. See Technical Bulletin # C-3 about increased roof level corrosion activity during an extended drain down.

     This problem is well illustrated by the below wall thickness data taken from a 45 year old, 18 in. extra strong condenser water piping system at a New York City building property. At the left, the basement piping shows a low and even corrosion rate, and a minimum remaining wall thickness far above the 0.375 in. standard grade pipe which would be installed in new installations today. Remaining service life is virtually limitless. See Technical Bulletin # P-14 for another case history of the damage caused by draining down pipe.

     Shown on the right, however, where the roof level pipe has remained drained for 5 months each winter over a period of 45 years, we find significantly higher corrosion activity, and limited service life remaining.

     In the below bar graph comparison, the original pipe wall thickness is shown against current average and minimum thickness values, and against the minimum acceptable safe limit for those pipe conditions. Here again, a dramatic difference is found between both filled and drained pipe - with the filled pipe still approaching original specifications, and the drained roof level piping approaching minimum allowable values.

     While most water treatment contractors will recommend raising the chemical corrosion inhibitor concentration just prior to winter drain down, in reality, such actions provide little, if any, added protection. Higher concentrations of standard water treatment chemicals are still limited by their ability to penetrate existing corrosion deposits, and typically lose their effectiveness over extended periods of time.

     Years of ultrasonic pipe testing has documented that even the more effective and previously long employed chromate chemicals failed to provide adequate protection under extended periods of being empty and open to the atmosphere. In the above example, this New York City building property was heavily treated with chromate corrosion inhibitors for over 30 years - yet shows a significantly higher wall loss at the roof level not likely occurring only since 1985.

     Although not available through most commercial water treatment contractors, extremely effective corrosion inhibitors for winter drain down or extended lay-up exists through some specialty suppliers. The most effective category of corrosion control agent, known as a Vapor Corrosion Inhibitor (VCI), is sprayed, fogged, or otherwise introduced into the empty pipe in the form of a fine and lightweight powder. The pipe is then temporarily closed off with plastic or a plug in order to retain the VCI vapor.

     VCI products protect metal surfaces with a thin, monomolecular barrier film which re-seals itself and self-replenishes as it is used or lost. Its basic method of corrosion protection is shown in the below chemical schematic. VCI products protect the anodic and cathodic areas of the metal in both the water and gaseous phases - thereby doubling the level of protection provided by conventional treatment chemicals.

     VCI chemicals sublime from their powder form directly into a gas. Once in place, that penetrating gas virtually stops corrosion for a period of up to two years. In actual use, VCI's produce almost total cessation of the corrosion process, as shown in a comparison of mild steel coupons below.

     The VCI generated gas also penetrates existing iron oxide deposits to provide protection to even heavily corroded systems. Because the powder and gas is easily water soluble, it also provides superior corrosion protection to water filled areas of the system as well. Environmentally safe, VCI's require no special removal procedures.

     VCI corrosion inhibitors offer significant benefits in solving the winter lay-up problem, and will greatly extend the service life of such piping systems. While most commonly used for piping applications, VCI products are also ideally suited for chillers, tanks, vessels and other HVAC related equipment which may be removed from service temporarily. Some of the many benefits provided are:

• Easy application, no removal necessary
• Cost effective corrosion protection
• Two years of protection against all types of corrosion
• Three phase protection at the molecular level
• Environmentally safe, non-toxic, and safe to handle
• Protects both ferrous and non-ferrous metals
• Versatile chemistry allows for wet or dry application
• Contains no nitrites, phosphates, or silicones
• Does not require regeneration
• Self-replenishing
• Near neutral pH levels

     In addition to temporary piping protection during winter lay-up or drain down, various other VCI products are available to solve the most difficult corrosion problems. Specialized VCI based chemical water treatment programs may also offer significant advantages to the building owner or plant operator having special needs, and are worthwhile to consider.

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