Base metals that contain both chromium and carbon (such as austenitic stainless steels) can lose their corrosion resistance if those stainless materials are allowed to dwell between 800-1500°F (425-815°C). This temperature region is known as the "sensitization" temperature range wherein carbon can become very active at breaking up chromium oxides. The carbon then bonds with the chromium to form chromium carbides and then moves into the grain boundaries of the metal for a more stable existence.
Of course, it is the presence of the tightly adherent layer of chromium oxide that helps to make the stainless steels to "stain less" than regular carbon steels (i.e. to resist corrosion of the metal). The protective chromium-oxide layer on stainless is what helps prevent corrosion of that metal surface, and steps should always be taken to preserve that layer.
When dwelling within the "sensitization" temperature range, however, those chromium oxides are broken down by the carbon reaction, and the resulting steel surface no longer has the protection it needs to resist corrosion.
Welders may encounter this phenomenon and see it as a narrow rust band running parallel along each side of a weld bead just a short distance away from the weld. There is no rust closer to the weld bead because the metal in that area went through the "sensitization" range too quickly. And no rust is seen on the far side of that narrow rust band because the stainless never got hot enough there to become sensitized. But within that narrow span of the rust band the stainless was able to dwell long enough in that sensitization temperature range for the carbide reaction to occur.
Similarly, in the world of furnace brazing, it is quite possible for assemblies made from standard 304-stainless (or standard 316-stainless) to dwell long enough in the sensitization temperature range – during furnace heating and cooling – for corrosion protection to be lost.
To avoid this, it is necessary to be sure to always specify the "L" (low-carbon) version of the stainless so that there is physically not enough carbon in the metal to cause such sensitization damage. Whereas standard 304 stainless may have up to almost 0.1% carbon, the "L" version of that alloy usually has no more than about a tenth of that (0.01% or so). Thus, anytime someone is going to need 304 or 316 stainless for brazing, it is wise for them to specify only 304L or 316L grades in order to preserve the stainless qualities of the metals being joined.