A comparison between corrosion performances of titanium grade 2 and 7 in strong reducing acids

• Published in: Materials and corrosion Vol. 72; no. 9; pp. 1506 - 1517
• Main Authors: Casanova, Luca, Gruarin, Matteo, Pedeferri, MariaPia, Ormellese, Marco
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.09.2021
• Subjects: acid; Cathodic polarization; corrosion; Corrosion rate; Corrosion resistance; Corrosion tests; EIS; Electrochemical analysis; Electrochemical impedance spectroscopy; Electrode polarization; Immersion tests (corrosion); Linear polarization; Morphology; Optical microscopy; Oxalic acid; Palladium; Sulfuric acid; Titanium; titanium grade 2; titanium grade 7; Weight loss
• ISSN: 0947-5117; 1521-4176
• DOI: 10.1002/maco.202112392

Titanium has a good corrosion resistance, owing to a thin, compact oxide layer that is formed spontaneously when the metal is exposed to the environment. However, even titanium can be subject to corrosion, for example, when exposed to reducing strong acids. This paper aims to analyze the differences in corrosion resistance of commercially pure titanium (Ti grade 2) and the most resistant Ti–Pd alloy (Ti grade 7, ∼0.2% Pd) in a strong reducing acid environment. Immersion tests, with results collected in the form of corrosion rates, are provided in the first part of the present work. Corrosion morphology is investigated using optical microscopy and laser profilometry. Then, an electrochemical characterization of both metals has been performed by means of cathodic polarization tests, electrochemical impedance spectroscopy, and linear polarization resistance. Weight loss data, electrochemical tests, and corrosion morphology are in good agreement: H2SO4, HCl, and oxalic acid are the most aggressive chemicals for commercially pure titanium. In such conditions, the use of Ti grade 7 appears mandatory. This study provides a detailed electrochemical and corrosion resistance comparison between the behavior of commercially pure titanium (Ti grade 2) and the most corrosion‐resistant alloy (titanium grade 7) in several strong reducing acids like HCl, H2SO4, H3PO4, oxalic, and sulfamic acid. The addition of Pd enhances the kinetics of oxygen reduction reaction, while promoting high oxide stability far from the corrosion potential in the cathodic direction.