Heat-tint oxides

Effects of Heat-Tint Oxides on the Corrosion Resistance Of Austenitic Stainless Steels... 

The chemical composition of the base metal beneath the heat-tint oxide... 

The adherence of the heat-tint oxide to the base metal. 

The defects, internal stresses, and composition of the heat-tint oxide make it a poor barrier to any corrosive media that might initiate localized corrosion in the chromium-depleted layer of base metal. 

The severity of localized corrosion at heat-tinted regions exposed to oxidizing chloride solutions is directly related to the temperature of the hot metal surface during welding. A heat-tint oxide on an austenitic stainless steel exposed to air first becomes obvious at approximately 400 °C (750 °F). As the surface temperature is increased, differently colored oxides develop that appear to be superimposed upon the oxides formed at lower temperatures (Table 12.6). 

Dark blue heat-tint oxides are the most susceptible to localized corrosion. It should be noted that gas-shielded surfaces do not form the same distinctly colored oxides as surfaces exposed to air during welding, but gas-shielded surfaces can also be susceptible to preferential corrosion. 

Grinding or wire brushing may not be snfficient to repair a heat-tinted region. Snch abrading operations may only smear the heat-tint oxide and embed the residual scale into the snrface, expose the chromium-depleted layer beneath the heat-tint oxide, and contaminate the snrface with ferrons particles that were picked up by the grinding wheel or wire brush. 

We may also use the method of tint etching to produce color contrast in microstructure. Tint etchants, usually acidic, are able to deposit a thin (40-500 nm) film such as an oxide or sulfide on specimen surfaces. Tint etching require a very high-quality polished surface for best results. Tint etching can also be done by heat tinting, a process by which a specimen is heated to a relatively low temperature in air. As it warms, the polished surface is oxidized. The oxidation rate varies with the phase and chemical composition of the specimen. Thus, differences in the thickness of oxidation films on surfaces generate variations in color. Interference colors are obtained once the film reaches a certain thickness. Effectiveness of heat tinting depends on the material of specimens it is effective for alloy steels and other non-ferrous metals and carbides, but not for carbon or low alloy steels. 

Heat tinting is another type of deposit etch. Exposing a polished sample (not in a plastic mount ) to elevated temperatures in air causes oxidation to occur. On the microscopic level, oxidation rates of microstructural constituents vary according to their reactivity in a given environment. 

Corrosion resistance can usually be maintained in the welded condition by balancing alloy compositions to inhibit certain precipitation reactions, by shielding molten and hot metal surfaces from reactive gases in the weld environment, by removing chromium-enriched oxides and chromium-depleted base metal from thermally discolored (heat-tinted) surfaces, and by choosing the proper welding parameters. 

This is a particnlar problem in applications in which the base metal has marginal corrosion resistance. The effect may be cansed by inadequate heat-tint removal, by the use of lower-alloy stainless steel brushes such as Type 410 or 304, or hy the redeposition of abraded metal or oxides. 

The rate of oxidation for a stainless steel, and consequently the degree of depletion in the base metal, is independent of the alloy composition and is controlled by diffusion through the oxide. The oxidized, or heat-tinted, surface of a welded stainless steel consists of a heterogeneous oxide composed primarily of iron and chromium above a chromium-depleted layer of base metal. The properties of such a surface depend on ... 

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