H2S corrosion:- The presence of H2S corrosion may give rise to three principal types of cracking in steels. These are sulphide stress cracking (SSC) and stress corrosion cracking (SCC), which result from the joint action of corrosion and tensile stress (residual or applied) in the presence of water and H2S corrosion may occur in carbon and low alloy steels as well as in CRA, and hydrogen induced cracking (HIC), which is limited to carbon and low alloy steels.
Sulphide Stress Cracking (SSC) H2S Corrosion
When corrosion occurs in sour service, the iron sulphide formed acts as a catalyst in the absorption of atomic hydrogen into the steel. Sulphur in the water will poison the combination reaction of hydrogen atoms to hydrogen molecules at the corroding surface and allow uncombined hydrogen atoms to be absorbed into the steel. The adsorbed hydrogen atoms enter the steel lattice and change its mechanical properties, making it more brittle and crack susceptible. With the assistance of tensile stress from weld residual stress or from pressure applied during service; the hydrogen can promote stress corrosion cracking (SCC).
Generally, susceptibility to SSC can be reduced by limiting carbon steel hardness to Rockwell 22 HRC (equivalent to about 250 Vickers, VHN). Upgrading to austenitic or more expensive CRAs alloyed with Nickel is another way to avoid SSC since these materials generally have good resistance to SSC. NACE MR0175/ISO 15156 defines the hardness limits of CRAs.
Step wise cracking is caused when atomic hydrogen from corrosion reactions diffuses into the material, but combines to form molecular hydrogen as a gas at inclusions or other micro-structural defects, where it produces an internal pressure. This results in various forms of internal cracking, or blistering if swelling of the cracked area predominates.
Stepwise cracking is formed in steels by the propagation and linking up of small and moderate sized laminar cracks in a step-like manner. As more hydrogen diffuses into the steel, the areas around these laminar cracks become highly strained, which can cause linking of the adjacent cracks to form SWC in the through thickness direction between the individual planar cracks.
Hydrogen Induced Cracking
Hydrogen Induced Cracking (HIC) occurs when hydrogen penetrates into the metal, and concentrates at iron sulphide inclusions in the steel to form lamellar cracks in the steel which may join together in step-wise fashion. This process takes a long time to occur, sometimes only being detected after years in service. This hydrogen damage only occurs in plate steel (in which sulphides in the metal are rolled flat). Thus, all components made of rolled carbon steel including these items and any vessels should be purchased from HIC resistant steel.
Recommendations for HIC resistant materials, including controls on sulphur content are mentioned in ISO 15156. It is also recommended to minimize carbon, manganese and phosphorus limits in the steel, consistent with obtaining satisfactory mechanical properties, to reduce the risk of centerline segregation which may initiate HIC.
Note that HIC resistant steel is not required if the steel is totally internally cladded because that stops the corrosion reaction and no hydrogen enters the steel.
Stress-Oriented Hydrogen Induced Cracking
In some cases a combination of HIC and SSCC can occur because a stacked array of hydrogen induced cracks becomes linked through the wall thickness of the steel by intermediate sulphide stress corrosion cracks. Often the key stress factor resulting in this type of failure is related to the weld residual stress so SOHIC has been found, as an example, associated with girth welds in pipelines made from rolled steel plate.
The measures taken to purchase steels resistant to HIC are generally considered adequate for overcoming SOHIC as well and these are defined in ISO 15156.