Low-carbon structural steels 195Steels for sour gas service
In recent years, the demand has increased for linepipe with resistance to envi-
ronmental fracture due to the exploitation of sour gas wells, i.e. those containing
significant levels of H2S and CO2. The National Association of Corrosion Engi-
neers (NACE) has determined that a fluid is designated sour when it contains
greater than 0.0035 arm. partial pressure of H2S. Both H2S and CO2 become
corrosive in the presence of moisture and the corrosivity of natural gas is deter-
mined solely by the levels of these compounds. Although gas pipelines do not
normally operate under corrosive conditions, a temperature drop in the gas to
below its dewpoint or failure in dehydration plant can lead to the introduction of
moisture. Similarly, other operational measures such as desulphurization and the
use of inhibitors are not considered to be totally adequate and therefore there is
a need for steels with inherent resistance to sour environments.
Two types of fracture can be introduced by H2S, namely hydrogen-induced
cracking (HIC) and sulphide stress corrosion cracking (SSCC). The latter
form of attack is generally confined to steels with yield strengths greater than
about 550 N/mm 2 and therefore does not feature prominently in linepipe steels.
However, the fact that high hardness levels can be generated in the heat-affected
zones of welds should not be overlooked since SSCC can result in catastrophic
brittle fracture. On the other hand, HIC results in a form of blistering or
delamination and can take place in the absence of stress. Atomic hydrogen is
generated at cathodic sites under wet sour conditions and diffuses into the steel,
forming molecular hydrogen at the interface between non-metallic inclusions
and the matrix. When the internal pressure due to the build-up of molecular
hydrogen exceeds a critical level, HIC is initiated. Long elongated inclusions
such as Type II MnS are particularly favourable sites for crack initiation but
planar arrays of globular oxides are also effective. Cracking can proceed along
segregated bands containing lower temperature transformation products such as
bainite and martensite. Cracking tends to be parallel to the surface but can be
straight or stepwise.
The susceptibility to HIC is assessed by immersion of unstressed coupons in a
synthetic solution of seawater, saturated in H2S with a pH of 5.1-5.3 (BP test)
or in the more aggressive solution of 0.5% CH3COOH + 5% NaC1 + H2S sat. at
a pH of 3.5-3.8 (NACE test). In either case, the test duration is 96 h and the
test parameters include crack length, crack width or blister formation.
From the remarks made earlier, it could be anticipated that the control of
non-metallic inclusions would feature prominently in the development of HIC-
resistant steels. The sulphur content is therefore generally reduced to below
0.01% and additions of calcium or rare earth metals are made to produce a
globular sulphide morphology. In some Japanese practices, the sulphur contents
are reduced to 0.001-0.003% with calcium additions of 0.0015-0.0035%.
Segregation effects can be minimized by restricting the levels of elements such
as carbon, manganese and phosphorus and fortunately the use of controlled rolling
enables high strengths to be obtained at low carbon levels. On the other hand,
manganese is beneficial in improving toughness by refining the ferrite grain size