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220 Gas Purification

Figure 3-16. Crack extension from a hydrogen blister. (AH, 799a)

ASCC (Alkaline Stress-Corrosion Cracking)

As noted earlier, it is thought that ASCC is caused by a film rupture mechanism. In areas
of high residual stress, such as heat affected zones, slip causes the passive film to break,
exposing bare steel, which corrodes to form cracks. If the corrosion rate is greater than the
rate of passive film formation, the protective film will not reform, and pitting or some other
form of localized corrosion will occur. However, if the passive film forms faster than the
metal corrodes, the protective film will be restored. Repetition of this process results in alka-
line stress corrosion crack growth. ASCC is the most common cracking mechanism in alka-
nolamine gas treating plants. It can occur in plants treating C02, H2S, or mixtures of both
acid gases (Richert et al., 1987; 1989). Figure 3-18 shows ASCC in the vicinity of a weld.
ASCC in amine gas treating plants was first reported in 1951 by a NACE Committee
(Schmidt et al., 1951). In 1953, Garwood reported ASCC in MEA plants treating natural gas.
Cracks were found in heat exchanger heads, amine absorbers, piping, and amine regenerators.
The cracks were intergranular and oxide-filled. Since the cracking occurred only in highly
stressed, heat affected zones, PWHT was recommended to eliminate ASCC. As a result of
this early work, the industry adopted a general policy of PWHT for all piping and equipment
(except storage tanks) in contact with amine above a certain temperature. Depending upon the
company and the amine, the selected temperature varied between 38 and 93°C (100 and
200°F) with 66°C (150°F) being a common choice (Richert et al., 1987; 1989).
In 1982, Hughes reported ASCC in non-PWHT carbon steel equipment in an MEA refin-
ery unit. ASCC had occurred in welds in contact with amine at temperatures ranging from 53
to 93°C (127 to 200°F); although no cracking had occurred in PWHT welds operating at
temperatures as high as 155°C (31 1°F). Hughes (1982) concluded that PWHT for all carbon
steel piping and equipment in amine service would eliminate ASCC.