The effect coupon size for the determination of atmospheric corrosivity

The corrosivity of an atmospheric environment is often determined by exposing metal coupons and measuring mass loss over a certain time, typically one year. It has been shown previously that orientation of the coupon can affect the derived corrosion loss, particularly at severe atmospheric locations. It also has been shown that the season of exposure will influence corrosion loss and that, depending on weather conditions, corrosivity may vary from one year to the next. Typically mild steel has been used as a determining metal but zinc, copper and alloyed steels also have been employed. To date there has been no investigation regarding the size of coupons used for corrosion rate determination. Reported herein are the results of a one-year trial in which four different sized coupons (50 mm x 100 mm to 200 mm x 400 mm) were exposed to the atmosphere at eight different locations, ranging from sub-alpine conditions to a severe marine environment. In almost all locations the smaller coupons showed higher corrosion loss for the same exposure period and conditions. At the more benign sites the difference in corrosion loss was marginal but noticeable. The change in corrosion loss became more obvious as the environment became more aggressive. After only 6 months exposure at the severe marine site the corrosion loss for the 50 mm x 100 mm coupons was more than three times that of the 200 mm x 400 mm coupons. Copper-bearing steel coupons of 50 mm x 100 mm size corroded at a similar rate to the same-sized mild steel coupons in milder environments but at a much higher rate near ocean locations.