Hydrated passive film on iron

Schematic representation of the hydrated passive film on iron (From Pou et at., 1984)...

Corrosion and Passivity. The inhibition of dissolution is important m the corrosion of metals and building materials. Passivity is imparted to many metals by overlying oxides, the so-called passive films the inhibition of the resolution of these passive layers protects the underlying material. Figure 13 eves a schematic model of the hydrated passive film on iron. 

These results point to the importance of hydration effects on the structure of passive films on iron. However, these results were obtained ex situ and therefore are subject to some uncertainty. 

Fig. 12.65. A polymeric film of hydrate iron oxide consistent with Mossbauer spectra of passive film on iron.

Froment and co-workers " have employed REFLEXAFS (vide supra) for studying passive films on iron and nickel. Their early studies were concerned with demonstrating the applicability of the REFLEXAFS technique to electrochemical systems. Most recently, they have used this technique to study the structure of passive films on Ni and on Ni-Mo alloy electrodes. For the Ni electrodes, they performed studies after reduction at — 700 mV (vs. saturated mercurous sulphate electrode) as well as in the passive (-l-3(X)mV) and transpassive (-1-800 mV) regions. The Fourier transforms for the films in the passive region have a Ni—O peak at a distance that corresponds closely to that in bulk nickel oxide. However, no Ni-Ni interactions were observed. These investigators interpreted these results as consistent with a model that postulates an amorphous hydrated polymeric oxide. ... 

A layer of Fc203 (mol. wt. = 159.7 d = 5.12 g/cm ) is formed equal to a minimum of (0.01 X 159.7)7(6 x 96,500 x 5.12) = 5.4nm thick (based on apparent area). The hydrated oxide would be thicker. This value compares in magnitude with measured values for the thickness of the decomposed passive film (2.5-10 nm). It is the decomposed passive film that is presumably isolated in experiments designed to remove the passive film from iron. 

The passive film formed on austenitic stainless steel is duplex in nature, consisting of an inner barrier oxide film and an outer deposit of hydroxide or salt film. Passivation takes place by the rapid formation of surface-absorbed hydrated complexes of metals that are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that rapidly deprotonates to form an insoluble surface oxide film. The three most commonly used austenite stabilizers—nickel, manganese, and nitrogen—all contribute to the passivity. Chromium, a major alloying ingredient, is in itself very corrosion resistant and is foimd in greater abundance in the passive film than iron, which is the major element in the alloy. 

The effect of metalloids on the corrosion resistance of alloys also varies with the stability of polyoxyanions contained in their films. Phosphorus and carbon contained in iron-chromium-melalloid alloys do not produce passive films of phosphate and carbonate in strong acids, and so do not interfere with the formation of the passive hydrated chromium oxyhydroxide... 

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... 

During hydration of cement a highly alkaline pore solution (pH between 13 and 13.8), principally of sodium and potassium hydroxides, is obtained (Section 2.1.1). In this environment the thermodynamically stable compounds of iron are iron oxides and oxyhydroxides. Thus, on ordinary reinforcing steel embedded in alkaline concrete a thin protective oxide film (the passive film) is formed spontaneously [1-3]. This passive film is only a few nanometres thick and is composed of more or less hydrated iron oxides with varying degree of Fe and Fe [4j. The protective action of the passive film is immune to mechanical damage of the steel surface. It can, however, be destroyed by carbonation of concrete or by the presence of chloride ions, the reinforcing steel is then depassivated [5j. 

The presence of a hydrated film (FeOOH) was found to depend on the concentration of iron cations at the passive layer-solution interface [64]. In situ surface X-ray diffraction studies indicated that the film consists of a spinel crystal structure [65]. 

The corrosion resistance of high-silicon cast iron is attributed to the development of a thin passive barrier film of hydrated oxides of silicon on the metal surface. This film develops with time due to the dissolution of iron from the metal matrix, which leaves behind silicon that hydrates due to the presence of moisture. Any flaws in the barrier film will reduce its effectiveness. 

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