Page 55 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

METALLURGICALLY INFLUENCED CORROSION 33

In addition, some environmental conditions such as temperature, conductivity of
the corrosive fluid, or thickness of the corrosive liquid film can cause localized corro-
sion. In some cases, both metallurgical and geometric factors will influence behavior,
such as in SCC. Preferential weldment corrosion of carbon steels has been studied
since the 1950s, commencing with the problem on icebreakers, but the problem con-
tinues today in different applications (4).
Weldment corrosion has clear microstructural dependence, and studies on HAZ
show corrosion to be more severe when the material composition and welding param-
eters are such that hardened structures are formed. It is a well-known fact that hard-
ened steel corrodes more rapidly in acid solutions than fully tempered steel, as local
microcathodes on the hardened surface stimulate the cathodic hydrogen evolution
reaction (4).

1.5.1.17 Grooving Corrosion This is a particular case of preferential weldment
corrosion occurring in electric-resistance-welded/high-frequency-induction-welded
pipe where attack of the seam weld HAZ/fusion line is affected in aqueous media.
This grooving corrosion has been attributed to inclusions in the pipe material. This
type of corrosion is probably because of the redistribution of sulfide inclusions along
the weld. It has been suggested that MnS is concentrated by the movement of liquid
metal during welding. At these high temperatures, MnS can dissociate into man-
ganese and sulphur and form iron sulfide. A suitable heat treatment can prevent the
formation of iron sulfide. And selection of cleaner alloyed steel is recommended.
Galvanic corrosion between the HAZ/fusion line and the parent material because
of the unstable MnS inclusions produced during the welding cycle has been observed:
corrosion of the weld metal because of electrochemical potential differences between
the weld metal and base metal with the weld metal as anodic metal in the galvanic
couple. Although the potential difference may only be 30–70 mV, the low surface
area ratio of anode to cathode leads to corrosion rates of the order of 1–10 mm.
SCC failure may occur by both active path and hydrogen embrittlement (HE)
mechanism, and in the latter case, failure is likely at low-input welds because of
the greater susceptibility of the hardened structures formed. Most SCC studies
of welds in carbon and carbon-manganese steels have evaluated resistance to
hydrogen-induced-SCC under sour conditions prevalent in the oil and gas industry,
and commonly referred to as sulfide stress cracking (SSC). The overriding influence
of hardness leads to the conclusion that soft transformed microstructures around
welds are preferable. Failures in oil refineries have shown cracks parallel or normal
to welds, depending on the orientation of principal stresses. Both transgranular and
intergranular cracks have been observed.
Equipment such as tanks, absorbers, carbon treater drums, skimming drums, and
piping suffer from cracking. All welds of deaerator vessels made from carbon steel
should be postweld stress relieved to minimize cracking and pitting.
Backing rings are sometimes used when welding pipe. It is necessary that the
backing ring insert is consumed during the welding process to avoid a crevice.
It is important that the welding filler metal must at least match the contents of the
base metal with respect to specific alloying elements such as chromium, nickel, and