Kinetics of layer formation and corrosion processes of passive iron in acid solutions

✍ Scribed by K.J. Vetter; F. Gorn

• Publisher: Elsevier Science
• Year: 1973
• Tongue: English
• Weight: 579 KB
• Volume: 18
• Category: Article
• ISSN: 0013-4686
• DOI: 10.1016/0013-4686(73)80036-2

✦ Synopsis

The kinetics of the layer formation process and the corrosion process have been determined from the relation between the stationary (i,J and non-stationary (i,) corrosion rates and the rate of layer formation (it) at passive iron in acid solutions. The rates of both processes depend on the poten- tial difference stss at the passive-oxide/electrolyte-solution interface. The fact that i, and & are independent of the electrode potential is explained by a nearly constant composition of the passive oxide in contact with the solution (Fermi level within the band gap). Linear relations, such as log it + = u + @,+/a,+)-log i,, are obtained, which areexplained bychargetransfer hindrance. The apparent charge-transfer coefficients are tz,+ = l-43 and al-= 0.57 for the layer formation and removal reaction, and a,+ = 0.84 for the corrosion reaction. From the pH dependence (pH = O-35-290) of il and i,,,, the reaction orders of the hydrogen ions are found to be approximately vJ + = -1 and Y. = 0. The corrosion cd depends on the sulphate concentration, with a reaction order Y, = /? = 0.16. From this, the kinetics of the oxide formation HzO.aq$OH-.ox + H+.aq (preceding equilibrium), OH-*ox++O*-. OX + H+. aq (rate-determining) and the kinetics of the corrosion process Fe3+.ox + S04"-.aq%FeS04+.

ad (adsorption equilibrium, Ten&in conditions), FeSO,, +-ad+FeSO.,+.a q (rate-determining), with following dissociation of the complex can be deduced. The apparent charge-transfer coefficients are interpreted by a,+ = 1 + a,, ccl-= 1 -Q~ and a,+ = a + 28~~ (aI, cz, true charge-transfer coefficients).