Ellipsometric Study of the Formation of Films on Iron in Chromate Solutions

Abstract

Film growth on iron was studied by ellipsometry in aerated or deaerated chromate solutions of different pH's at open-circuit potentials and at various anodic potentials.It was found that various films can be formed depending on the experimental conditions.At high anodic potentials iron oxide is chiefly produced with an index of refraction of 2.6 (1-0.12i).At open-circuit potential the iron and chromium hydroxides with the index of refraction of 1.6-1.7 or a mixture of iron and chromium hydroxides with iron and chromium oxide (with index of refraction of 1.6-1.9 and various absorption coefficients) is formed.The kinetics of the film growth at open-circuit potential is logarithmic, and the kinetics of iron hydroxide formation at constant potentials follow the cubic law.The aim of this work was to improve the understanding of the mechanism of passivation of iron in chromate solutions.Ellipsometry was used for determining optical parameters and film growth kinetics of the film formed both at open-circuit potential and at anodic potentials.These studies included pH's of 3.9, 7, and 12.Chromates belong to the most efficient oxidizing type of corrosion inhibitors.They are effective both in aerated and deaerated solutions; they are also effective in neutral and alkaline solutions as well as in acid solutions.The composition and the growth rate of oxide film on iron formed in chromate solutions has been studied by many authors.During passivation chromates undergo reduction with formation of chromium oxide (1-4).The concentration of chromium in the film can be different, depending on the condition of film formation (oxygen concentration in the solution, solution pH).The amount of reduced chromates during passivation process was determined by Uhlig and King (5), Powers and Hackerman (6), Cohen and Beck (4), Brasher (3), and others.The concentration of oxygen in the solution has an especially large effect on the composition of the film (2, 7, 8).The chromium concentration in the protective layer is decreased as the oxygen in solution is increased or as the surface is exposed to the atmosphere.Films formed in chromate solutions at the open-circuit potential are composed of 7-Fe203 -b Cr~Os and grow according to a logarithmic law (3).Kruger ( 9) who was the first to measure the film growth in chromates using ellipsometry found also that the initially air-formed films grew further when immersed in aerated chromate solutions and followed a logarithmic law.