Studies of anodic dissolution and film growth on iron in acid chloride solutions

✍ Scribed by T.A. Oftedal

• Publisher: Elsevier Science
• Year: 1973
• Tongue: English
• Weight: 650 KB
• Volume: 18
• Category: Article
• ISSN: 0013-4686
• DOI: 10.1016/0013-4686(73)80042-8

✦ Synopsis

The rate of dissolution of Armco Fe has been measured as a function of time for deaerated solutions of constant ionic strengtb (HCl + NaCl).

For pH < 1.5, no appreciable change of the measured polarisation resistance was observed. For 1.5 -=I pH < 3 the polarisation resistance gave straight lines when plotted against time. The slope of the lines is represented by n = 2~OJ(Co,-.10'4 -45) Slcm'/min, where Co,. is the concentration of the bulk.

The Tafel slopes were 64-70 mV when 0 < pH < 1.2. The experimental data are explained by two very similar reaction mechanisms. The theory presented involves the formation of two surface complexes denoted by FeOH Fe.H20 and Fe0 Fe.H,O. It is suggested that the complex FeOH Fe.H,O serves as nuclei for film formation when pH z 1.5. This would explain the change of the reaction mechanism at pH = 1.5 from a 60 mV mechanism when pH < 1.5 to a 30 mV mechanism when pH > 1.5.

The theory is supported by data from the literature. It gives a reasonable explanation of the nonstationary 60 mV Tafel slopes. The mechanisms are in harmony with a hyperbolic relation between anodic current density and time. The logarithmic law of film growth may be derived from the same principles.