Determining Inherent Properties of Rust Layers by Mossbauer Spectrometry

3 DETERMINING INHERENT PROPERTIES OF RUST LAYERS BY MOSSBAUER SPECTROMETRY 

We mentioned earlier that a careful quantitative characterization of the iron oxides and oxyhydroxides present in the rust layers allows (i) determining the protective ability of the rust layers, (ii) calculating the atomic fraction (a) of iron species coming from the steels that after the corrosion processes form part of the adherent rust, and (iii) understanding the mechanisms associated with the deterioration processes. 

Let us talk about each aspect a little bit more. T o determine the protective characteristics of the rust layers adhered to the steel surfaces, we can use the ternary rust diagram proposed by Hara et al. [4]. This diagram compares the following parameters (i) the protective ability index PAI = G/(A + L + S) and (ii) the ratio (A + S)/(A + L + S), where G, A, L, and S are the relative phase abundances of a-FeOOH, (3-FeOOH, 7-FeOOH, and spinel phases (magnetite/maghemite), respectively. Hara et al. [4] proposed a criterion for the classification of the rust layers as (i) protective if PAI I, (ii) nonprotective but active if PAI 1 and (A + S)/(A + L + S) 0.5, and (iii) nonprotective but inactive if PAI I and (A + S)/(A + L + S) 0.5. Although this criterion was defined for field experiments, it can also be applied in accelerated tests. 

in order to calculate a, we need to measure the mass of the steel before the corrosion test, m (S), the mass of the steel after the corrosion test taking into consideration the mass of the adherent rust, mf(S +AR), and the mass of the steel after the corrosion test but without the rust, that is, the rust completely removed, mj(S). a can be calculated using the following formula [18]  

Finally, a deep quantitative characterization of all corrosion products, which includes detailed identification and determination of relative phase abundances and physical properties, allows a better comprehension of possible mechanisms of formation of the iron phases. 

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