Corrosion rates and electrokinetics of cobalt and cobalt-rich cobalt-iron alloys in sulfuric and hydrochloric acids

Corrosion rates for Co and Co-rich Co-Fe alloys were determined in sulfuric and hydrochloric acids under both deaerated and oxygenated conditions. For the purpose of comparison, corrosion rates of Co cut from metal sheet were also determined in the same environments as those of sintered alloys. Sintered alloys were inferior to solid Co as corrosion resistant materials. It was found that the surface was more corrosion resistant than the interior. Thus after long exposure to the corrosive acid, the samples became porous inside while the surfaces remained substantially intact. Oxygen had a pronounced effect in accelerating the corrosion of Co and Co-rich alloys, although it is known to have little effect on the corrosion of Fe in acids. The activation energy of dissolution of Co was determined by measuring the maximum corrosion rate in H₂SO₄ at four temperatures. The results showed that the energy barrier for the dissolution of Co in deaerated acid is about twice as large as in aerated acid. Corrosion potentials of the alloys were measured in both HCl and H₂SO₄ solutions. Co and Co-Fe alloys, respectively, showed a more active corrosion potential in HCl than in H₂SO₄ solutions. However, the corrosion rate studies revealed that Co and Co-Fe alloys were less corrosion resistant in H₂SO₄ than in HCl corrosion rates of Co-Fe alloys were also measured by using cathodic polarization curves. The corrosion rates obtained by the electrochemical method were throughout a little higher than the rates determined by the weight loss method. Anodic polarization and passivation behavior of Co-Fe alloys were studied in H₂SO₄ and HCl solutions. The reason why the Co anode achieved passivity in H₂SO₄ solutions but not in HCl solutions is discussed as due to the formation of a protective film --Abstract, page ii-iii