Residual oxygen in membrane generated nitrogen, when commingled with potassium or sodium chloride drilling brines for underbalanced drilling (UBD) applications, is known to be very corrosive to drill pipe, casing and tubing. Commercial chemical inhibitors are routinely applied in such applications but supporting laboratory test data in the literature often fail to represent actual downhole environments.The corrosion rate data from case study wells has limited value due to available locations and technology for measurement within drilling process flow loops. The test protocol discussed here and corresponding results provide for more realistic evaluation of inhibitor effectiveness and suggest a mechanism for corrosion inhibition.The tests were conducted in stages intended to first establish the baseline corrosive behavior of low alloy steel in oxygenated brine with and without pH control by means of buffering agents. Subsequent tests focused on the effect of corrosion inhibitor concentrations and other additives. The results of the test program indicated that a combination of conditions resulted in significant inhibition of low alloy steel against oxygen corrosion, and is associated with the formation of surface film containing phosphorus. Corrosion rates as low as 50mpy without pitting were achieved, compared with 900mpy with significant scaling for uninhibited oxygenated brine. This paper describes the development of conditions necessary to create a stable protective film in oxygenated brine without the formation of copious corrosion products known to present problems in UBD operations. The test protocol used herein suggests a method for qualifying the performance of candidate inhibitor formulations for specific UBD design conditions.