Electrochemical Study of the Repassivation of Some Chromium-Containing Alloys

Electrochemical studies were performed on 13% Cr steel and 22% Cr duplex steel in order to determine the state of the alloy, either active or passive, in production and stimulation fluid sequences. The efficacy of a passivating solution in formation of a passive surface layer following acid stimulation was also investigated. Potential decay measurements of the 13% Cr specimens suggested that this alloy was active in the following solutions under an atmosphere of CO₂ at 140°F: 5% NaCl/2% acetic acid, 15% HCl/0.5% H-J inhibitor, Navy solution (0.5% NaNO₂, 0.25% Na₂HPO₄, 0.25% NaH₂PO₄), and spent 15% HCl. The 22% Cr electrode was tested under both CO₂ and a mixture of 5% H₂S/95% CO₂ at 140°F and was found to be passive in 5% NaCl/2% acetic acid and the Navy solution, and active in 15% HCl/0.5% H-J inhibitor and spent 15% HCl. Polarization measurements taken on 13% Cr in 5% NaCl/2% acetic acid suggested that the specimen was in an actively corroding state following pretreatment with production and stimulation fluids with or without the introduction of the Navy solution into the sequence. The polarization curves for the 22% Cr duplex steel electrode showed spontaneous passivation characteristics in 5% NaCl/2% acetic acid with or without the addition of a passivating solution into the fluid sequence.