Page 56 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

34 INTRODUCTION AND FORMS OF CORROSION

molybdenum. The thermal cycle of heating and cooling during the welding process
affects the microstructure and surface composition of welds and adjacent base metal.
Hence the corrosion resistance of welds made without the filler metals and welds
with filler metals may be inferior to that of properly annealed base metal because
of microsegregation, precipitation of secondary phases, formation of unmixed zones,
recrystallization, and grain growth in the weld HAZ as well as volatilization of alloy-
ing elements from the molten pool and contamination of the solidifying weld pool
(22, 48).
Unmixed zones result when welding stainless steels with a filler metal. An
unmixed zone has the composition of base metal with the microstructure of an
autogenous weld. The microsegregation and precipitation phenomena typical of
autogenous weldments decrease the corrosion resistance of an unmixed zone relative
to the parent metal. Unmixed zones bordering welds made from overalloyed filler
metals can be preferentially attacked on exposure on the weldment surface (4, 48).

1.5.1.18 Sensitization Welding is the common cause of sensitization of stainless
steels to IGC. The main weld metal precipitates in austenitic stainless steels are
-ferrite, -phase, and M C with small amounts of M C carbide. Although the
23 6
6
cooling rates in the weld and the adjacent base metal are high enough to avoid car-
bide precipitation, the weld thermal cycle brings part of the HAZ into the carbide
precipitation temperature range. The carbides can precipitate and a zone somewhat
removed from the weld becomes susceptible to intergranular corrosion. Reheating
∘
the alloy to above 1035 C and cooling it rapidly removes the carbide precipitate, and
this practice is termed solution annealing (4).
The well-known weld-related corrosion problem in stainless steels is weld decay
(sensitization) caused by carbide precipitation in the weld HAZ. This sensitized
microstructure is much less corrosion resistant as the chromium depleted layer and
carbide precipitate are subject to preferential attack.

1.5.1.19 Pitting and SCC Under moderately oxidizing conditions, as in pulp
and paper bleach plants, weld metal austenite may undergo preferential pitting
in alloy-depleted regions. This mode of attack is independent of any weld metal
precipitation and results from microsegregation or coring in weld metal dendrites
(48). In the 18-8 austenitic steels, the ruptures of SCC are intergranular when steel
is subjected to nonsuitable thermal treatment such as in the zone of carbide precip-
∘
itation, between 400 and 800 C. When the steel is subjected to “hyperquenching,”
ruptures are generally transgranular.
Austenitic stainless steels that are susceptible to IGC are also prone to intergran-
ular SCC. The problem of intergranular SCC of sensitized austenitic stainless steels
in boiling high-purity aerated water has been well studied. Cracking of sensitized
stainless steels in boiling water nuclear reactors has been observed. Cracking of sen-
sitized stainless alloys in polythionic acid that is formed during the shutdown of
desulphurization units in petroleum refineries has been observed. IGC of 430, 434 and
446 steels during welding has been observed (48). Weld decay (sensitization) in an
austenitic steel weldment occurs. The grain joints become impoverished in chromium