Oxide scales coatings

Metal Treatment. After rolling, the oxide scale on sheet steel is removed by acid treatment (pickling) (see Metal surface treatments). Phosphoric acid, a good pickling agent, leaves the steel coated with a thin film of iron phosphates. This process improves mst resistance but presents a problem if the steel is to be electroplated. 

A near white surface finish corresponds to Swedish standard Sa 2-1/2 or BS4232 second quality. This finish is defined as one from which is removed all oil, grease, dirt, mill scale, rust, corrosion products, oxides remaining coatings, except for a very light shadows of light streak discolouration. At least 95% of the surface shall have the appearance of a white metal surface finish and the remainder shall be limited to light discolouration. 

Critical for the coating system lifetime is the formation of a thermally grown oxide scale (TGO) at the interface BC/TBC during service, which mainly consists of alumina [3]. The possible oxygen diffusion in zirconia itself and the open columnar structure of the ceramic coating allows oxidation of bond coat aluminum. The scale s growth and the difference in... 

Fig.8. Optical micrograph of a cross section of Fe Al, oxidized in H2/H2O at 900°C. (A Ni coating is applied for the preparation of the cross section). Internal oxidation (black intrusions), iron protrusions on top of oxide scale (bright half circles on top of oxide scale)...

A. Strawbridge, H. E. Evans Spallation of oxide scales from NiCrAlY overlay coatings, Materials Science Forum, in press (1996). 

Figure 8.14 Optical micrograph showing transverse section of a nickel specimen coated with 0.5 mg cm 2 32804 and then oxidized 20 h in 1 atm oxygen at 1000 °C. The oxide scale is NiO and is composed of a porous and a dense zone. The dense portion forms beneath the porous zone after the accelerated period of oxidation.

Figure 8.32 Comparison of the microstructure of Co-Cr-Al-Y specimens after isothermal oxidation at 1000 °C in 1 atm of oxygen where (a) the specimens were coated with 5 mg cm of Na2S04, or (b) pre-sulphidized for 20 s in an H2S-H2 gas mixture with H2S/H2 = 0.2 prior to oxidation. Protective oxide scales have been formed on both specimens.

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