TP25-18 The Impact of Metal Surface Roughness on Corrpsion Monitoring Water Treatment

Many industrial facilities wish to monitor the long-term condition of their recirculating water systems using corrosion coupons. Multiple metallurgies can be installed in combination, and coupons can also be used to monitor various forms of deposition and microbiological activity. But confusion can occur when coupons from different vendors or consultants are installed, as these may have varying properties, specifically roughness, which impact the measured corrosion rate. ASTM 2688-94 provides guidance on the implementation of a side stream coupon monitoring rack. It also provides instructions on the size of the coupon and a target roughness value of approximately 3.15 m. The updated standard document has removed any recommendations on surface finish. Another standard, CTI STD-149(22), notes that the surface finish can affect corrosion measurements and provides a list of common finishes: sand blasted, glass bead blasted, or double disk ground. However, it falls short of specifying a target roughness value. It is noted in STD-149(22) that coupons of different surface finishes cannot be compared to each other, which can occur when coupons from different sources are installed. To better guide the selection of corrosion coupon surface finish, we used a laboratory recirculating cooling system to evaluate coupons with three roughness values under low phosphorus water conditions to determine the impact of surface roughness on the measurement of weight loss corrosion rate. Longer exposure studies were conducted in an industrial operation setting, in a refinery complex, which results are discussed. In addition to helping understand the impact of surface roughness on corrosion measurement, this study will aid in the interpretation of corrosion results by factoring in the impact of the initial surface roughness.