New passivating pastes for stainless steel without nitric acid

Handled stainless steel (SS) structures often need accurate passivating tre atments before use. Until now the most frequently used passivating pastes h ave been based on nitric (fluoridric) acid and, as a result, are responsibl e for consequent environmental problems. This work addresses the SS passiva tion quality of two low environmental impact pastes, based on peroximonosul fate (P2) or sodium perborate (P3). Tests were conducted on various materia ls using electrochemical tests (cyclic voltammetry, anodic potentiostatic t ransients, open-circuit potential (E) decay), Microscopic and Inductively C oupled Plasma (ICP) measurements. The results, when compared with those obt ained 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 efficient as the P1 paste. Moreover, the protection of the tested SS ca n be further enhanced by adding small amounts of citric acid to, in particu lar, the P3 paste.