The corrosion resistance of stainless steel depends on the chromium.
Austenitic stainless is a super-saturated solution of chromium and nickel in
iron. It is actually a very high temperature phase that has been quenched to
preserve the distribution of elements. Austenitic stainless does not like
middling-high heat. It performs well up to 600F (315C), but higher
temperatures in the range of 800-1600F (425-870C) cause atom diffusion which
causes the properties to change. Temperatures in this range allow the chromium
to diffuse away from the grain boundaries to form chromium carbides, the
preferred crystalline structure in that temperature range. Exposure to the
temperatures that cause diffusion is referred to as being "Sensitized". The
diffusion of chromium away from the grain boundaries results in un-stainless
grain boundaries surrounded by stainless steel. This situation soon leads to
localized corrosion and rapid cracking of the grain boundaries. To correct
this, the metal must be heated to at least 1900F (1040C) for a period of time
in an inert gas atmosphere and then quenched to retain the austenite crystal
structure. Unfortunately, doing this heat treatment to a welded keg would
result in a lot of warping and distortion. It is better to get another keg
and start over.
Welding is a local melting/freezing process that creates high temperature
gradients in the metal around the weld. This Heat Affected Zone (HAZ) is the
region where unwanted atom diffusion can take place if it is hot enough, long
enough. There are time/temperature curves that describe this, and the curve
for alloy 304 is shown in Figure 2. This chart shows that for type 304
stainless (nominal carbon content of 0.08%), five minutes at 600C (1110F) or
above will cause chromium diffusion that will later cause cracking in service.
Type 304L stainless, "L" denoting less carbon (nominal 0.03%), is more
weldable and can spend about 6 hours at 600C before becoming Sensitized. Most
kegs (in North America) are made from 304L to facilitate the welded
construction.