Corrosion Behavior of 13% Chromium Steel in Acid Stimulations^✫

In gas production wells, carbon (C) steel tubulars subject to carbon dioxide (CO₂) corrosion have been replaced gradually with corrosion-resistant 13% chromium (Cr) steel of the martensitic type (AISI type 420 [UNS S42000]), eliminating the need for inhibition and possibly extending tubular lifetime. The effect of matrix acid stimulation treatments on 13% Cr steel tubulars was studied. Laboratory tests using electrochemical techniques were conducted in “live” acid, “spent” acid, and artificial production fluids in the presence of CO₂. Tests included simulation of a full acid treatment cycle to study repassivation of the steel. Typical inhibited live acid, based on 15% hydrochloric acid (HCl), was not unacceptably aggressive toward 13% Cr steel. It was shown that severe damage from locally initiated acid attack in back-produced spent acid, with a reduced inhibitor content but still highly acidic, could be avoided by limiting the contact time with the tubular steel. In typical chloride (Cl^–)-containing brines,13% Cr steel repassivated after acidization within some hours. A risk of pitting corrosion existed only at very high Cl^– levels at elevated temperatures.