Rust layers steels

In oxygenated water of near neutral pH and at or slightly above room temperature, hydrous ferric oxide [FelOHla] forms on steel and cast irons. Corrosion products are orange, red, or brown and are the major constituent of rust. This layer shields the underl3dng metal surface from oxygenated water, so oxygen concentration decreases beneath the rust layer. 

Figur 3.2 Incipient rust layer on steel in oxygenated water. (Courtesy of National Association of Corrosion Engineers, Corrosion 91 Paper No. 84 by H. M. Herro.)...

When a clean steel coupon is placed in oxygenated water, a rust layer will form quickly. Corrosion rates are initially high and decrease rapidly while the rust layer is forming. Once the oxide forms, rusting slows and the accumulated oxide retards diffusion. Thus, Reaction 5.2 slows. Eventually, nearly steady-state corrosion is achieved (Fig. 5.2). Hence, a minimum exposure period, empirically determined by the following equation, must be satisfied to obtain consistent corrosion-rate data for coupons exposed in cooling water systems (Figs. 5.2 and 5.3) ... 

Figure 5.6 Rust layer on carbon steel mill water supply pipe. Note the partially exfoliated region at the bottom of the photo.

The distinguishing feature of the behaviour of the slow-rusting low-alloy steels is the formation of this protective rust layer. Corrosion in conditions where it cannot form is little different from that of unalloyed steel, although the particular alloying elements present will have some influence on the actual rate at which corrosion occurs. 

The adsorption isotherms for metallic surfaces are reported in the literature however, an important part of the atmospheric corrosion process takes place under rust layers, which play a decisive role in the long-term course of corrosion because of its sorption capacity for water. The influence of the chloride and sulfate anions has a real effect only when the corrosion products layer is already formed. Thus, the adsorption isotherms of the steel corrosion products formed in different atmospheres were determined. 

In the presence of chloride ions, a local breakdown of rust layers makes an anode channel for localized corrosion of underlying steel, and the chloride ions tend to accumulate in the channel as the anodic metal dissolution progresses. Assuming the ferrous chloride concentration at 1 mol dm 3 in the anode channel, we obtain the proton level at pH 4.75, where no ferrous hydroxide precipitation is expected to occur because of its solubility greater than 1 mol dm 3. The hydrated ferrous chloride produced by corrosion is then oxidized by air-oxygen in the anode channel ... 

Once in dry dock, the outside of the ship was pressure-washed with mains water to remove marine growths attached to the steel plates as well as the loosely adherent corrosion products. This was repeated several times to assist in the removal of chloride ions from the rust layers. Approximately 11 tons of debris were removed from the external structure of the ship by this process. Several sections of the steel plates were found to have very thin areas less than 1 mm... 

The addition of Cu to steel increases the corrosion resistance against atmospheric corrosion due to the reduced oxygen reduction inside the rust layers that are formed on these alloys [20]. 

The way these metals form a protective oxide, or rust layer, is the unique property of stainless steel that makes it so useful as a material for cooking tools and eating utensils. When the oxide layer is scratched away and the metal is exposed, it causes the underlying steel to oxidize again. This oxide layer forms quickly and is quite strong. The metal inside is protected by the outer oxide layers. 

Misawa et al. [32] proposed that an inner cohesive protective rust film is formed on low-alloy steels after long atmospheric exposure (industrial or urban). It consists of amorphous 5-FeOOH, the formation of which is catalyzed by copper and phosphorus on the steel surface alternate drying and wetting favors its protective qualities. Keiser et al. [33] confirmed that the typical inner adherent rust layer consists mostly of 5-FeOOH. 

Ferrous hydroxide [Fe(OH)2] and hydrated ferrous oxide (Fe0.nH20) is first diffusion barrier layer formed on the surface. As the pH of saturated Fe(OH)2 is about 9.5, the surface of steel corroding in aerated pure water is always alkaline. Due to incipient oxidation green coloured Fe(OH)2 is formed. Ferrous oxide is converted to hydrous ferric oxide or ferric hydroxide at the outer rust layer as dissolved oxygen is available by the following reaction. 

After extensive studies it has been found that Cr compounds are effective for obtaining the protective rust layer in a short period of time [24]. Cr substituted a FeOOH forms rapidly in presence of Cr2(S04)3 solution. This accelerates the dissolution of steel and promotes the formation of goethite. On the other hand, Cr " forms fine particles of Cr substituted goethite and improves the protection ability of the rust layer. 

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. 

In another study, corrosion rates of Mn-steel and Cu-Mn weathering steel in a simulated coastal environment were measured by wet-dry cyclic test. The rust layer was observed and analysed by SEM and XRD. The experimental results... 

Thibeau et al. [143]. have used Raman and infrared spectroscopy to investigate the structure of the inner rust layer formed on weathering steels exposed to an industrial environment for 4.5 and 8 years. The inner rust layer on weathering steel was composed primarily of 5 FeOOH with 10-20 %, y FeOOH and some a FeOOH irrespective of the exposme period. 

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... 

Misawa, T., Yamashita, M., Miyuki, H., Nagano, H. Protective rust layer ftmned on weathering steel by atmospheric corrosion for a quarter of a century, J. Iron Steel Inst. Jpn. 

Ohtsuka, Toshiaki Passivation Oxide Films and Rust Layers on Iron Characterization of Corrosion Products on Steel Surfaces. Springer, Berlin Heidelberg (2006)... 

Yamashita, M., Nagano, H., Misawa, T., Townsend, H.E. Structure of protective rust layers formed on weathering steels in industrial atmospheres of Japan and North America. ISU Int 38(3), 285-290 (1998)... 

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