258. The reductive dissolution of ferric oxide in acid. Part III. The mechanism of reductive dissolution

The Significance of the Flade Potential

“…Previous models of duplex passivating films proposed by investigators of the Flade potential fall primarily into the following classes: Model (a) as stated by its supporters appears difficult to maintain for passive potential decay in acid electrolytes. Here it has been shown (13)(14)(15) that films of ~, or ~ Fe~O~ formed on iron by exposure to air at different temperatures are destroyed rapidly by reductive dissolution according to the equations…”

“…[3] has not been observed experimentally. A considerable volume of experimental evidence (15,19) shows that reduction of thermal ferric oxide films proceeds completely to soluble ferrous ions rather than to "FeO" (normal or abnormal) whose existence, even in transient form, has never been detected during cathodic reduction of oxide films on iron. Recent precise and successive quantitative cathodic reduction of duplex Fe~OJ FefO~ thermal films on iron (18) at pH = 7.65 has revealed in turn ~-Fe._.O:,, -/-Fe~O.~, and Fe,O, in that order but with no suggestion of any "FeO" compound either normal or abnormal.…”

Potential Studies on Passivity to Corrosion Induced by Pretreatment Processes

“…Most of these studies have dealt with the formation of relatively thick oxide films, circa 2000s or more, formed at 100-600 v (8)(9)(10)(11)(12)(13)(14)(15)(16). The rate of both types of processes are supposed to depend on the kinetics of the movement of ions, usually metallic ions, into and through the film under the influence of an electrostatic field.…”

“…Experimental work and theoretical developments have been concerned primarily with the so called "valve" metals such as tantalum, aluminum, hafnium, niobium, titanium, vanadium, and zirconium. Most of these studies have dealt with the formation of relatively thick oxide films, circa 2000s or more, formed at 100-600 v (8)(9)(10)(11)(12)(13)(14)(15)(16). The work of Guntherschulze and Betz (17) contains an excellent compilation of results up to 1936.…”

“…Although under certain conditions of formation some thick anodic coatings may be porous, there are strong indications that many of them are relatively compact and nonporous and that even the thicker porous coatings have a compact, nonporous "barrier" layer film underneath which may be from 10A to 200A thick (15,19,20).…”