New passivating pastes for stainless steel without nitric acid

Handled stainless steel (SS) structures often need accurate passivating treatments before use. Until now the most frequently used passivating pastes have been based on nitric (¯uoridric) acid and, as a result, are responsible for consequent environmental problems. This work addresses the SS passivation quality of two low environmental impact pastes, based on peroximonosulfate (P2) or sodium perborate (P3). Tests were conducted on various materials using electrochemical tests (cyclic voltammetry, anodic potentiostatic transients, open-circuit potential (E) decay), Microscopic and Inductively Coupled Plasma (ICP) measurements. The results, when compared with those obtained on the same materials by using a conventional nitric acid paste (P1), indicate that the proposed pastes P2 and P3 are time stable, non toxic and as e cient as the P1 paste. Moreover, the protection of the tested SS can be further enhanced by adding small amounts of citric acid to, in particular, the P3 paste.