Abstract A number of techniques including impedance, weight loss coupons, and thin layer activation have been used to study mild steel corrosion in cooling water systems. These techniques have also been applied to the study of the effectiveness of corrosion inhibitors on precorroded mild steel surfaces. The effectiveness of two inhibitors, a low nitrite (LN) and a total organic corrosion inhibitor (TOCI), in preventing corrosion on a mild steel surface that is covered with corrosion products has been investigated. The techniques used were appropriate for studying the corrosion process and indicated that both LN and TOCI were capable of passivating a corroding mild steel surface.