Atmospheric corrosion FeOOH

When steel or iron is exposed to an atmospheric environment, a thin layer of magnetite, Fe304, is formed, covered by a layer of FeOOH. Atmospheric oxygen then penetrates though the almost water-free, porous outer layer of FeOOH and oxidizes the magnetite to hydrated ferric oxide, Fe203, or FeOOH. The presence of Fe " in the electrolyte initiates the precipitation of various corrosion products. The electrochemical mechanism of atmospheric corrosion of iron suggested by Evans is briefly summarized in this chapter [8]. 

The rust layer usually consists of dense inner regions of amorphous FeOOFI and crystalline magnetite. The outer layer consists of crystalline a-FeOOH (goethite), y-FeOOFI (lepidocrocite), and y-Fe203 (maghemite) [9]. In summary, atmospheric corrosion of iron occurs in several steps ... 

The model for atmospheric corrosion tmder high chloride concentration su ested by Kamimura et al. [32] is based on the separation of cathode and anode sites under the rust and the thin electrolyte. The pH at the anode compartment is affected by the chloride ion concentration and decreased to 1.5 by the hydrolysis of ferric ions and the formation of P-FeOOH. Chloride ions accumrrlate at the anode site and initiate the oxidation of ferrous ions to ferric ions. Accumulated chloride ions increase ferric ion solubility in the electrolyte and accelerate the hydrolysis of ferric ions, causing the pH at the anode to decrease. Low pH at the metal-electrolyte interface accelerated the formation of P-FeOOH. The atmospheric corrosion process is summarized as follows ... 

Horton et al. [55] observed that when steels containing Cu and Ni are exposed in industrial and marine atmospheres, the Cu and Ni appear in the mst layers both in the loose outer and adherent inner mst on skyward and ground ward surfaces. Also it was shown by chemical analysis that Ni, Cu, Cr and Mn from weathering steel appear in the mst layer and provides protection. Presence of chlorides in the atmosphere accelerates corrosion of steels leading to the formation of basic Fe ", Fe chlorides and jS FeOOH. Townsend et al. [56] conducted 8-year atmospheric corrosion tests on weathering steel in mral, industrial and marine environments with different heated conditions and indicated that heat treatments have no effect on the corrosion resistance/performance of weathering steels. 

Dunnwald and Otto [137] found phase transformation of iron corrosion product to Fe(OH)3 in the atmosphere containing SO2 with humidity by Raman spectroscopy. Subsequently, Fe(OH)3 gets transformed to crystalline FeOOH with amorphous FeOOH. It has been shown that the amorphous rust is the primary product of atmospheric corrosion, which later transforms to crystalline forms in the absence of copper. Yamashita et al. [144] smdied the long-term growth of the protective rust layer formed on weathering steel under atmospheric corrosion in an industrial region involving an exposme for 26 years. The outer layer of rust was... 

The less dense exterior part of the rust layer contains mostly the phases y-FeOOH and a-FeOOH. The latter has the higher thermodynamic stability of the two but forms more slowly. For this reason, accelerated atmospheric corrosion tests yield lepidocrocite (y-FeOOH) primarily. 

The various phases of rust formed under atmospheric corrosion conditions often contain sulfate or chloride ions. In a marine atmosphere, for example, the )3-FeOOH phase contains a small amount of chloride. 

The term green rust designates a variety of intermediate reaction products, generally amorphous, that are found during the atmospheric corrosion of steel in the presence of chlorides and/or sulfates. More precisely, these products form during the transformation of Fe(OH)2 into y-FeOOH. Green rust consists of mixed ferric and... 

There are other types of pores in a single solid particle. a-FeOOH is a precursor material for magnetic tapes, a main component of surface deposits and atmospheric corrosion products of iron-based alloys, and a mineral. The a-FeOOH microcrystal is of thin elongated plate [83],... 

In the previously described work, low levels of lead were found in the rust layer near the paint-rust interface, within 30 tm of the rust-paint interface. Thomas suggests that because lead salts do not appear to reach the metal substrate to inhibit the anodic reaction, it is possible that lead acts within the rust layer to slow down atmospheric corrosion by interfering with the cathodic reaction (i.e., by inhibiting the cathodic reduction of existing rust [principally FeOOH to magnetite]) [33], This presumably would suppress the anodic dissolution of iron because that reaction ought to be balanced by the cathodic reaction. No conclusive proof for or against this theory has been offered. 

Corrosion of iron or steel (anodic reaction Fe —> Fe + 2e and cathodic reaction 4Fe + O2 + tOH20 4Fe(OH)3 + 8H+) leads to formation of surface hydroxides. The main constituents of oxides formed during atmospheric corrosion of steels are y-FeOOH (lepidocrocite), a-FeOOH (goethite), y5-FeOOH (akaganite) and -FeOOH (feroxyhite). These hydroxyl groups in rast have a tendency to react with hydroxyl groups of hydrolyzed metal alkoxide from sol-gel coatings. 

In the specific case of iron or steel exposed to dry or humid air, a very thin oxide film composed of an inner layer of magnetite (FejO ) forms, covered by an outer layer of FeOOH (rust). " Atmospheric corrosion rates for iron are relatively high and exceed those of other structural metals. They range (in pm/ year) from 4 to 65 in rural, 26 to 104 in marine, 23 to 71 in urban and 26 to 175 in industrial areas. ... 

Yamashita et al. [32] observed that atmospheric msts on weathering steels are composed of Cr substituted a FeOOH, y FeOOH and a small amount of y Fe20s and/or Fe304. The dark Cr substituted a FeOOH area was located in the inner layer while the bright y FeOOH area was in the outer layer. Thus, the innermost Cr substituted a FeOOH layer may be the final form of the protective rust layer which suppresses and prevents the transport of corrosive species through the rust layer to retard further corrosion. 

Figure 5.8 displays the Fe L-edge absorption spectra of the iron film deposited on a quartz crystal wafer before and after several hours of corrosion at relative hnmidity above 90% in a flow of atmospheric pressure of air. The quartz aystal wafer was connected to an electronic oscillator circuit monitor the small mass changes in the deposited thin film. Clearly, the Fe L absorption edge was shifted to higher energy after corrosion, consistent with the oxidation of iron due to corrosion. The quartz crystal microbalance (QCM) also revealed a mass change of approximately -h 15 ag/cm, likely due to the corrosion of the iron film into FeOOH. 

When steel is exposed in a clean, dry atmosphere, the surface is covered with a 20- to 50-A-thick oxide film consisting of an inner layer of Fe20v This film practically prevents further corrosion. If small amounts of water vapor are present FeOOH may also form. 

In atmospheres with chlorides present the corrosion of carbon steel proceeds in local cells that resemble the sulfate nests. They may form around chloride particles deposited on the surface, where the concentrated local chloride solution destroys the passivating film of FeOOH. The chlorides are concentrated in the anodic areas formed by migration, while the surrounding area covered by rust acts as a cathode. 

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