User:Aguntert/Corrosion coupon

Corrosion coupons are the most widely used form of corrosion measurement and monitoring today. They are carefully machined small thin bars of various metals which are inserted into an external "rack" or layout of 1 in. piping to the main circulating loop. A large variety of metal alloys are available in various physical configurations, although for HVAC and process cooling applications, rectangular bars of mild carbon steel and soft copper are the primary materials used.

In most cases, the pressure differential across the suction and discharge of the circulating pump is sufficient to flow a small side stream of water through the external coupon rack. This flow rate is regulated based upon various procedural guidelines in order to ensure uniform conditions between tests. Traveling at a few feet per second, however, it never approaches actual flow conditions within the piping system. Where no flow exists, corrosion coupon testing is, by definition, not even possible.

Typically, the external rack is configured to allow the simultaneous testing of two or four corrosion coupons of similar or different metals as the water navigates the standard zig zag piping configuration. Coupon racks are constructed of either steel or PVC - a choice often dependent upon operating pressure and convenience, and may exist immediately near or far remote to the actual piping system.

A specimen of test material to be used in a corrosion test, usually a metal strip or ring shaped to fit into a testing cell or between joints of drillpipe. Rings, or coupons, are weighed before and after exposure, and weight loss is measured. They are also examined for pits and cracks. Corrosion products are analyzed to define the type of corrosion reaction.