Page 57 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

METALLURGICALLY INFLUENCED CORROSION 35

because of the formation of chromium carbides. Knife-line corrosion is a form of IGC
encountered in stabilized austenitic stainless steels. During welding, the base metal
adjacent to the fusion line is heated to high enough temperatures to dissolve the stabi-
lizing carbides followed by sufficiently rapid cooling to prevent carbide precipitation.
When weldments in stabilized grades are then heated in the sensitizing temperature
∘
range of 425–815 C during stress-relieving operations, high-temperature service,
or subsequent weld passes chromium carbide may precipitate. The precipitation of
chromium carbide leaves the narrow band adjacent to the fusion line susceptible to
IGC. Knife-line attack can be avoided by the proper choice of welding variables and
by the use of stabilizing heat treatments (4).

1.5.1.20 Localized Biological Corrosion of Stainless Steels There are three sets
of conditions under which localized biological corrosion of austenitic stainless steel
occurs. These conditions should be examined for metals that show active–passive
behavior. Microbiological corrosion in austenitic steel weldments has been studied
and documented (4, 48).

1.5.1.21 Prevention of IGC In North America, susceptibility to IGC is achieved
by the use of low carbon steels such as 316 L (0.03% carbon maximum) in place of
type 316 (0.08% C maximum). In Europe, it is common to use steels with 0.05% C,
which are fairly resistant to sensitization, in particular, when they contain molyb-
denum and nitrogen, which appear to raise the tolerable level of carbon and/or heat
input (4, 48). This method is not effective for eliminating sensitization for long-term
∘
exposure at 425–815 C.
∘
Above 815 C titanium and niobium form more stable carbides than chromium
and are added to stainless steels to form stable carbides and prevent formation of
chromium carbide.
Duplex alloys with a composition around Fe–26Cr–6.5 Ni–3Mo with low carbon
and nitrogen have been found to be resistant to chloride SCC, pitting corrosion, and
IGC in the as-welded condition (48).
Choosing the proper welding parameters, balancing alloy compositions to inhibit
precipitation reactions, shielding molten and hot surfaces from reactive gases in the
weld environment, removing chromium enriched oxides, and chromium depleted
base metal from thermally discolored surfaces are the factors to note in the welding
process (48).

1.5.1.22 Corrosion Resistance of Aluminum Alloys Both welded and unwelded
∘
aluminum alloys are corrosion resistant in uninhibited nitric acid up to 50 C. Above
this temperature, most of the aluminum alloys exhibit knife-line attack adjacent to
the welds. No knife-line attack was observed for any commercial aluminum alloy or
∘
weld at 70 C in inhibited fuming nitric acid containing 0.1% hydrofluoric acid.
Weldments of nonheat-treatable alloys are resistant to corrosion. In the case of
heat-treatable alloys, corrosion is selective in the weld or in the HAZ. Welding can
crack because of mercury-zinc amalgam.