Iron passive films

H. Deng, H. Nanjo, P. Qian, A. Santosa, I. Ishikawa, Y. Kurata, Potential dependence of surface crystal strucmre of iron passive films in borate buffer solution, Electrochim. Acta 52 (2007) 4272-4277. 

Three main mechanisms for passive film breakdown and pit initiation have been suggested in the literature through penetration, adsorption, or film breaking [20—22]. These mechanisms apply to pure metal systems because they do not consider second-phase particles in the passive film matrix, which very often initiates pitting. For example, as already discussed, dissolution of MnS inclusion at the MnS/matrix is the initial pit formation step in steel [15]. In the absence of chloride ions, the protective hydrated iron passive film slowly converts into dissolved ferric ions ... 

Figure 6.44 Thickness of iron passive film measured by XPS after different exposure times to solutions containing 0.1 mol/1 of CF, Br , or 1 anions. Applied potential is = 0.5 V. The films were initially formed in a phthalate solution of pH 5.0 at 1.2 V [43].

Sihcon irons are very resistant to oxidizing and reducing environments, and resistance depends on the formation of a passive film. These irons are widely used in siilfuric acid service, since they are unaffec ted by siilfuric at all strengths, even up to the boihng point. 

It is known that thin (-20 A) passive films form on iron, nickel, chromium, and other metals. In s ressive environments, these films provide excellent corrosion protection to the underlying metal. The structure and composition of passive films on iron have been investigated through iron K-edge EXAFS obtained under a variety of conditions, yet there is still some controversy about the exact nature of... 

As silica is not attacked by any acid other than hydrofluoric it might be expected to act as an effective barrier to attack by any other acid solutions, but in fact, while the high-silicon iron is resistant to attack by most acids, it is corroded relatively severely by hydrochloric, hydrobromic and sulphurous acids. The aggressive character of the two halogen acids may be ascribed to the readiness with which their relatively small anions can penetrate a passive film. 

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... 

Under the hot deaerated (reducing) conditions normally found on the surfaces of pre-boiler FW heaters, FW lines, and boiler surfaces, a dense, passive, black, iron oxide film of magnetite (Fe304) naturally forms. 

Temperatures reached during phosphate-based cleaning programs are only modest, and under these lower temperature conditions the passivated film consists of gamma iron oxide (yFe203) together with primary ferrous phosphate [Fe2H2(P04)2] and tertiary ferrous phosphate [Fe3(P04)2]. 

The formation of a passive film of iron oxide (magnetite, Fe304), under sulfite or hydrazine reducing conditions, is optimized at pH of 11 to 12. The downside is that the decomposition of carbonates and bicarbonates produces carbon dioxide, the primary cause of condensate system corrosion. 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

Figure 5 shows the relationship between the passive film thickness of an iron electrode and the electrode potential in an anodic phosphate solution and a neutral borate solution.6,9 A passive film on an iron electrode in acidic solution is made up of an oxide barrier layer that increases its thickness approximately linearly with increasing electrode potential, whereas in a neutral solution, there is a precipitated hydroxide layer with a constant thickness outside the oxide barrier layer. 

Figure 5. Thickness of the anodic passivating film formed on iron at various potentials.6 9 Lbl and Lr, are the thicknesses of the barrier layer and the precipitated layer, respectively. Temperature is 25°C. , in a 150 mol m 3 phosphate buffer solution at pH 1.85 O, in a 300 mol m 3 borate buffer solution at pH 8.2. (From N. Sato, K. Kudo, and T. Noda, Z Phys. Chem. N. F. 98,271,1975, Fig. 5, reproduced with permission and N. Sato, K. Kudo, and R. Nishimura, / Elec-trochem. Soc, 123,1420,1976, Fig. 1. Reproduced with permission of the Electrochemical Society, Inc.)...

Jin and Atrens (1987) have elucidated the structure of the passive film formed on stainless steels during immersion in 0.1 M NaCl solution for various immersion times, employing XPS and ion etching techniques. The measured spectra consist of composite peaks produced by electrons of slightly different energy if the element is in several different chemical states. Peak deconvolution (which is a non-trivial problem) has to be conducted, and these authors used a manual procedure based on the actual individual peaks shapes and peak positions as recorded by Wagner et al. (1978). The procedure is illustrated in Figure 2.8 for iron. 

XPS was also used for the determination of chlorine in the passive film grown in chlorine containing electrolytes. While chlorine was found in the passive film on pure iron, it was absent for chromium rich stainless steel samples. Chloride content of the passive film is substantially time dependent, increasing with time until film breakdown occurs, and decreasing subsequently [109]. 

The use of surface EXAFS in the study of passive films represents a natural application of the technique and, in fact, the studies by Kruger and co-workers70 73 on the passive film on iron represent the first reported. 

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