Stainless as suffix is because this material has good resistance against getting rust/ corroded. It steel is protected from corrosion by a thin, impervious, invisible surface layer – the passive layer-that consists mainly of chromium oxide. The oxygen content of the atmosphere is normally sufficient to create and maintain this passive layer. Unfortunately, surface defects and imperfections introduced during manufacturing operations may drastically disturb this “self-healing” process and reduce resistance to several types of local corrosion.
Welding Heat tint:
Heat tint is the result of the thickening of the naturally occurring transparent oxide layer on the surface of the steel. The colors formed are similar to the ‘temper colors’ seen on other steel surfaces following heat treatment and range from pale straw hues to dark blue.
Heat tints are often seen in heat affected areas of welded stainless steel fabrications, even where good gas shielding practice has been used (other weld parameters such as welding speed can affect the degree of heat tint color formed around weld bead).
As heat tints are formed on the stainless steel chromium is drawn to the surface of the steel, as chromium oxides forms more readily than the iron in the steel. This leaves a layer at and just below the surface with a lower chromium level than in the bulk of the steel, and so a surface with reduced corrosion resistance.
Thus, visible welding heat tint on stainless steel fabrications must be removed. This can be removed by chemical as well as mechanical means. Chemical means can be acidic brush on pastes or gels, spray pickling, immersion tank pickling or electrochemical cleaning methods. Mechanical means can be sand blasting or grinding or abrading.
The table represents the temper colors that are likely to form on stainless steel type (AISI 304) if heated in air.
|Colour Formed||Approx Temperature deg.C|
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