Page 54 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

32 INTRODUCTION AND FORMS OF CORROSION

hydrogen peroxide solution. ASTM A262 test consists of six practices for detect-
ing susceptibility to IGC in austenitic stainless steels using different oxidizing agents
at different temperatures, and kinetics is examined by microscopic investigation of
the etched structure for Cr C sensitization and weight loss measurement. Similarly,
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testing for susceptibility to IGC has been described in ASTM 763 for ferritic stainless
steels and ASTM G28 for wrought Ni-rich, Cr-bearing alloys.
A laboratory procedure for carrying out nondestructive electrochemical reacti-
vation (EPR) test on types 304 and 304L stainless steels to quantify the degree of
sensitization is given. The metallographically mounted and highly polished sample
is potentiodynamically polarized from the normal passive condition in 0.5 M sulfu-
∘
ric acid and 0.01 M potassium thiocyanate solution at 30 C to active potentials – a
process known as reactivation. The amount of charge passed is related to the degree
of IGC associated with Cr C precipitation, which predominantly occurs at grain
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boundaries. After EPR test, the microstructure is examined. Certain media have been
commonly used for the evaluation of the susceptibility of Mg, Cu, Pb, and zinc alloys
to IGC.

1.5.1.15 Weldment Corrosion The factors that can initiate or propagate differ-
ent forms of corrosion of welded regions are numerous, interrelated, and difficult
to define. However, some pertinent factors to take into account are: weldment design,
fabrication technique, welding practice, welding sequence, moisture contamination,
organic or inorganic chemical species, oxide film and scale, weld slag and spat-
ter, incomplete weld penetration or fusion, porosity, cracks (crevices), high residual
stresses, improper choices of filler metal, and final surface finish. Consequently, the
corrosion resistance of welds may be inferior to that of the properly annealed base
metal because of microsegregation, precipitation of secondary phases, formation of
unmixed zones, recrystallization and grain growth in the weld heat-affected zone
(HAZ), volatilization of alloying elements from the molten weld pool, and contami-
nation of the solidifying weld pool (4).
Welded microstructures can be extremely complex and often change drastically
over a very short distance. The fusion zone or weld metal is a dendritic structure that
solidified from a molten state. Bordering the fusion zone are transition, unmixed and
partially melted zones, and the HAZ. These zones can be reheated and altered by
subsequent weld passes in multipass welding. For alloys with structures dependent
on thermal history such as steels, the final microstructure can be very complex. As
welded structures are often quite susceptible to corrosion, overalloyed filler metals
are often used to increase the weld corrosion resistance. In the case of stainless steels
with high levels of carbon content, sensitization in the HAZ is another problem (4).

1.5.1.16 Weld Corrosion of Carbon Steels The corrosion behavior of carbon steel
weldments obtained by fusion welding can be because of preferential corrosion of the
HAZ or weld metal or associated with geometric aspects such as stress concentration
at the weld toe, or creation of crevices because of joint design.