Page 98 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

76 INTRODUCTION AND FORMS OF CORROSION

The hydrogen atoms combine to form innocuous molecular hydrogen. But this
recombination of hydrogen atoms is prevented by cyanide and sulfide, and hydro-
gen atoms cause HE followed by a failure of the structure leading to general loss in
ductility. Internal blisters may occur when large amounts of hydrogen are present in
localized areas. Small amounts of dissolved hydrogen may also react with microstruc-
tural features of the alloys leading to failures at applied stress much below the yield
strength.
HIC may propagate in either a straight or stepwise manner. Straight growth occurs
in steels with pearlite structures and high levels of Mn and P segregation or the pres-
ence of martensitic or bainitic transformation structures. The Mn level around lean
cracks may be twice the level in the matrix while the P level may be as high as a factor
of 10.

1.8.10.6 SSC SSC is an important cracking phenomenon in hydrogen sulfide
medium and a special case of HIC. Natural aqueous environments contaminated
with hydrogen sulfide are very corrosive. The hydrogen sulfide present in salt water
in sour oil wells places an upper limit of 620 MPa on the yield strength of steel that
can be tolerated.
Sulfide stress cracking is considered to be a form of HE that occurs in high-strength
steels and in localized hard zones in weldments of susceptible materials. In the HAZs
adjacent to welds, there exist very narrow hard zones combined with regions of high
tensile stress that may become embrittled to such an extent by dissolved atomic hydro-
gen that they crack. Sulfide stress cracking is directly related to the amount of atomic
∘
hydrogen dissolved in the metal lattice and usually occurs below 90 C. Sulfide stress
cracking also depends on the composition, microstructure, strength, and the sum of
residual stress and applied stress levels of the steel. Sulfide stress cracking was first
encountered in oil industry failures of tubular steels and wellhead equipment made
from steels with hardness values greater than HRC 22. This type of failure can be
eliminated by heat treatment of steels to hardness less than 22. The base metal of
steel pipe has hardness well below HRC 22; service failures occurred in regions of
high hardness in the weld HAZ. Thus HRC 22 limit is applied to the weld l-HAZ
areas in pipeline steels. There are a number of test procedures (10, 101) for SSC of
which the most widely used is the NACE test.
Comparative features of HIC and SSC are given in Table 1.3.

1.8.10.7 Hydrogen-Induced Blistering and Precipitation of Internal Hydrogen
Hydrogen-induced blistering is a cracking process because of absorbed hydrogen
atoms. The phenomenon is also referred to as HIC and occurs in lower strength
(unhardened) steels with tensile strengths less than about 550 MPa (80 ksi). Pipeline
steels exposed to sour gas are prone to HIC.
Penetration of hydrogen into the metal can result in the formation of blisters. Blis-
ter formation is because of a deterioration of mechanical property.
Consider a steel reservoir containing an acidic electrolyte and the outside of the
reservoir is exposed to the atmosphere. Hydrogen will be evolved inside the walls of
the vessel because of corrosion. A part of the atomic hydrogen will diffuse through