Pitting Attack - A general term given to any depression on the metal surface caused by corrosion. Not usually a concern unless the mpy is high.
Generalized Attack - Uniform corrosion over entire surface of the coupon. The following describes different forms of attack that can be observed on coupons. Pitting corrosion can cause rapid metal failure even if the overall corrosion rate is low. Keep in mind these rates and comments assume general system corrosion. However, Table One gives guidelines that have been published for assessing corrosion in cooling tower systems and closed loops using fresh water make up. Whether a corrosion rate is good or bad is relative to the water used and the operating conditions. To convert corrosion rates expressed in millimeters/year (mm/y), a common metric measurement, to mpy, multiply mm/y times 39.4. 1.0 mil/year translates into 1/1,000th of an inch of metal loss per year. From this weight loss and the dimensions of the coupon, a corrosion rate in mils/year (mpy) is determined. The coupons are then removed and returned to a lab where they are cleaned and re-weighed. Normally, system water is allowed to circulate over the corrosion coupons for about 30 - 90 days. Corrosion coupons, representative of system metals, are inserted into a coupon rack installed on the system to be checked. A common method of determining corrosion rates is by using corrosion coupons, which are uniform-sized, pre-weighed strips of metal.
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