Chromium, passive oxide films

Passive films formed in aqueous solutions consist of an oxide or a mixture of oxides, usually in hydrated form. The oxide formed on some metals (e.g., Al, Ti, Ta, Nb) is an electronic insulator, while on other metals the passivating oxide film behaves like a semiconductor. Nickel, chromium, and their alloys with iron (notably the various kinds of stainless steel) can be readily passivated and, in fact, tend to be spontaneously passivated upon contact with water or moist air. It should be noted that passivation does not occur when chloride ions are introduced into the solution indeed a preexisting passive film may be destroyed. Many other ions are detrimental to passivity, such as Br, I, SO, and CIO, but chloride is the worst offender, because of its... 

Passivation of iron under critical conditions is predicted. Hematite (Fe203) may still be the main corrosion product in the neutral water pH (pH 7.2) region, but the passivation potential range is narrower and shifts to negative potentials, compared with regions on the diagram for ambient conditions. For chromium, no solid chromium oxide stable species is predicted within the stable region of neutral water. This indicates chromium oxidation without any passivation oxide film formation. 

Chromium. So-called stainless steels contain in their alloys elements soeh as Cr, Ni, Co, and Mo) that eventually become incorporated into the passive oxide films. Cr(III), as shown by EXAFS studies with hydrous ferric acxie and goethite (18), forms inner-sphere surface complexes in the form of... 

Equation (4.7) corresponds to the potential variation of a metal electrode of the second kind as a function of pH. The Flade potential is used to evaluate the conditions for passive film formation and to determine the stabihty of the passive film. The reversible Flade potential of three important engineering materials is approximately +0.63 V for iron, +0.2 V for nickel, and —0.2 V for chromium [7,8]. The negative value of the Flade potential for chromium (—0.2 V) indicates that chromium has favorable Gibbs free-energy for the formation of passive oxide film on its surface. The oxide film is formed at much lower potentials than in other engineering materials. 

While the same basic mechanisms for passivity of pure metals also applies to alloys, the processes involved in the passivation of alloys have an added complexity. In many cases only one component of the alloy has the property of being passive in a particular environment. Alloys such as steiinless steels, which contain highly passive components (chromium in this case), owe their corrosion resistance to the surface enrichment of the passivating component Thus stainless steels resist corrosion in many acidic systems (where iron or carbon steel would be poorly passive or not passive at all) by a passivating oxide film containing Cr predominantly as Cr(III). Surface analytical techniques such as Auger electron and X-ray photoelectron spectroscopies reveal substantial enrichment of chromium in the passivating oxide film on these alloys " . There are only two ways by which this enrichment can... 

The passivity of metals like iron, chromium, nickel, and their alloys is a typical example. Their dissolution rate in the passive state in acidic solutions like 0.5 M sulfuric acid may be seriously reduced by almost six orders of magnitude due to a poreless passivating oxide film continuously covering the metal surface. Any metal dissolution has to pass this layer. The transfer rate for metal cations from this oxide surface to the electrolyte is extremely slow. Therefore, this film is stabilized by its extremely slow dissolution kinetics and not by its thermodynamics. Under these conditions, it is far from its dissolution equilibrium. Apparently, it is the interaction of both the thermodynamic and kinetic factors that decides whether a metal is subject to corrosion or protected against it. Therefore, corrosion is based on thermodynamics and electrode kinetics. A short introduction to both disciplines is given in the following sections. Their application to corrosion reactions is part of the aim of this chapter. For more detailed information, textbooks on physical chemistry are recommended (Atkins, 1999 Wedler, 1997). 

The role of alloying elements in the passivation process has been briefly discussed. Alloying additions such as chromium and molybdenum can substantially influence the structure and composition of the passive oxide film and thereby the process of passivation. The alloys discussed have been of the fairly simple binary type, where it is easier to analyze the surface oxide films by surface analytical techniques and to understand the results. This treatise provides a basis for the following discussion of stainless steels, where the number of alloy additions is increased as is the complexity of the passivation process. 

The local dissolution rate, passivation rate, film thickness and mechanical properties of the oxide are obviously important factors when crack initiation is generated by localised plastic deformation. Film-induced cleavage may or may not be an important contributor to the growth of the crack but the nature of the passive film is certain to be of some importance. The increased corrosion resistance of the passive films formed on ferritic stainless steels caused by increasing the chromium content in the alloy arises because there is an increased enhancement of chromium in the film and the... 

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

Active metals such as aluminum, titanium, and high-chromium steels become corrosion resistant under oxidizing conditions because of a very adherent and impervious surface oxide film that, although one molecule thick, develops on the surface of the metal. This film is stable in a neutral medium, but it dissolves in an acid or alkaline environment. In a few cases, such as certain acid concentrations, metals can be kept passive by applying a carefully controlled potential that favors the formation of the passive surface film. The ability to keep the desired potential over the entire structure is very critical in anodic control. If a higher or lower potential is applied, the metal will corrode at a higher rate, possibly higher than if it is not protected at all. 

Chromates and molybdates protect the steel by passivation and form a chromium oxide-iron oxide in the case of chromate. While the oxide film of chromium may be several monolayers, molybdenum oxide film is of the order of a monolayer.73... 

If oxide films are responsible for passivity, it is to be expected that an anode will become passive most readily in an electrolyte from which the oxide will separate most easily this expectation is realized in practice. The oxides of iron, cobalt, nickel and chromium are less soluble in alkaline than in acid solution, and passivity sets in more rapidly in the former. The oxides of molybdenum and tungsten, however, are more soluble in alkali than in acid, and so these metals are rendered passive most easily in acid electrolytes. 

It was mentioned above that a chromium anode continues to dissolve in the passive state forming chromate ions in this case the invisible oxide film may be suflSciently porous to allow ions to penetrate it alternatively, the oxide film may become oxidized to C1O3 which dissolves to form chromate, but is immediately regenerated by oxidation of the anode. 

The first ionisation potentials arc not abnormally high for transition metals, but the metals are relatively inert since they easily become passive. Judged potentiometrically, chromium is a strong reducing agent, and molybdenum a moderate one but again, as for the Gp VA metals, reactivity is inhibited by the formation of an adherent oxide film. 

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