Oxidation of the root bead and adjacent HAZ during welding of stainless steels is commonly called heat-tint. A chromium-rich scale is formed, which is typically not very protective, and the stainless steel surface becomes chromium-depleted: these effects may impair the corrosion resistance of stainless steel welds.
Tests performed in various corrosive environments have shown that susceptibility to pitting and crevice corrosion is greater when the surface is heat-tinted, and the colour of the heat-tint is an indicator of the degree of susceptibility to corrosion. For example, purple-blue oxides are generally the most susceptible to corrosion attack. The critical pitting temperature, which is an indicator of corrosion resistance in aqueous chloride media, may be reduced from 60 to 40°C for a type 316 stainless steel weld in 0.1% NaCl solution with an applied potential of +300 mV SCE. In order to restore corrosion resistance, the scale and the Cr-depleted layer must be removed by post-weld cleaning.
An efficient back purge will minimise the formation of heat-tint. The oxygen content in the back shield must be controlled prior to the start of welding and should be minimised. Most specifications require that the back purge be maintained until the root and hot pass, and sometimes the first few fill passes, are completed or until the temperature of the root region is below about 250°C. However, it may be beneficial to reduce the gas flow pressure slightly after the root pass to stop an over-pressure build up. This is particularly important where the root bead is thin and is nearly re-melted by the second bead. The use of pure argon as backing gas is recommended. Argon/nitrogen backing gases produce good results when welding highly alloyed stainless steels, such as duplex and super-austenitic stainless steels.
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