Depassivation and repassivation of stainless steels by stepwise pH change

Immersion tests with different stainless steels have been performed, while the pH was stepwise decreased and then increased again. During 8.5-day exposure, the depassivation and repassivation pH values as a function of pitting resistance equivalent number were determined. There is always a gap between both pH values (depassivation and repassivation), indicating that for every steel, there are conditions where an existing passive layer can be maintained but cannot be rebuilt after depassivation. In such environments, the passive layer is thicker, consisting mainly of molybdenum and iron rich oxides, while chromium is dissolved. Usually, depending on conditions, the passive layer is more chromium-rich, especially the inner layer. This is relevant, for example, for acidizing jobs in oil and gas industry, proving that repassivation after acidizing will happen promptly, when the pH is increased again.

Automated summary

Immersion tests were conducted on different stainless steels with pH values stepwise decreased and then increased. It was found that there is a gap between depassivation and repassivation pH values for each steel, indicating that in certain conditions, the passive layer cannot be rebuilt after depassivation. In acidizing jobs in the oil and gas industry, it is crucial to maintain a chromium-rich passive layer for prompt repassivation after acidizing.