Welding Inspection & Metallurgy API ICP Self Study Notes

Welding can reduce the corrosion resistance of regions of the HAZ of some
austenitic stainless steels. Areas exposed to temperatures between 800°F –
1650°F (427°C – 900°C) for a long enough time may precipitate chromium
carbides at the grain boundaries. This causes a loss of corrosion resistance
due to chromium depletion. Using low-carbon content stainless steels, such
as Type 304L or 316L, or stabilized grades of stainless steels, such as Type
321 and 347 can prevent this phenomenon. It is also important to select the
proper filler metal to prevent a loss in corrosion resistance. Low carbon
electrodes or stabilized grades of bare filler metal should be used. Oxidation
of the backside of welds made without proper shielding can also be
detrimental to the corrosion resistance of austenitic stainless steels. To
prevent a loss in corrosion resistance the root of the weld should be protected
by using an inert backing gas.
http://www.hse.gov.uk/welding/asphyxiation-hazards.htm