34 Fundamentals of Corrosion
For this form of attack to occur, there must be a specic environment.
Many environments do not cause intergranular corrosion in stainless
steels. Acids containing oxidizing agents, such as phosphoric acid con-
taining ferric or cupric ions and nitric acid, as well as hot organic acids
such as acetic and formic acids, are highly specic for this type of attack.
Seawater and other high chloride waters cause severe pitting in sensitized
areas, but low chloride waters (e.g., potable water) do not, except in spe-
cic situations such as might occur under the inuence of microbiological
corrosion.
If the carbon content is held to less than 0.030%, chromium carbide pre-
cipitation can still occur upon sensitization, but in such small amounts
that no signicant chromium depletion occurs. Such low carbon grades
are practically immune to weld decay. However, sensitization can occur
under prolonged heating in the critical temperature range, such as during
service at elevated temperatures, or during very prolonged thermal stress
relief. Refer to Figure 3.1. For all practicality, the low carbon grades can be
welded, hot formed, and even thermally stress relieved without sensitiz-
ing occurring.
Sensitization can also be prevented using stabilized stainless steels. These
are stainless steels to which titanium, columbium (niobium), or niobium–
titanium mixtures have been added.
Titanium and niobium additions equal to ve or ten times the carbon con-
tent, respectively, permit the carbon to precipitate as titanium or niobium
carbides during a sensitizing heat treatment. The carbon precipitation does
not reduce the chromium content at the grain boundaries.
Three problems are presented by this approach. First, titanium-stabi-
lized grades, such as type 321, require a stabilizing anneal to tie up the
carbon in the form of titanium carbide before welding. Second, titanium
does not transfer well across a welding arc and thus loses much of its
effectiveness in multipass or cross-welding. Third, although niobium does
not have this drawback, the niobium carbides (as can the titanium car-
bides) can be redissolved by the heat of welding. Consequently, multipass
or cross-welding can rst redissolve titanium or niobium carbide and then
permit carbide precipitation in the fusion zone (not the HAZ). This can
cause a highly localized form of intergranular corrosion known as knife-
line attack, seen particularly in alloys such as type 347, alloy 20Cb3, and
alloy 825.
Titanium and niobium carbides precipitate at higher temperatures than
chromium carbides. For example, niobium carbide precipitates in the tem-
perature range of 1498 to 2246°F (815 to 1230°C) where chromium carbide
dissolves. During stabilization (cooling down from the melt), niobium car-
bide will form in this temperature range, leaving no carbon to form chro-
mium carbide at temperatures below 1498°F (815°C). However, above 2246°F
(1230°C), niobium carbide dissolves. When a stabilized steel is heated above
2246°F (1230°C), all carbides are dissolved, and a rapid cooling to room