7/27/2019 Hydrogen Cracking Mechanisms

1/1

Hydrogen Cracking MechanismsThe presence of hydrogen in steels and CRAs can cause various f orms of damage and loss of mechanical properties. Hydrogen can enter fr om many sources, including steelmaking, welding, galvanic coupling to

less noble metals, cathodic protection and corrosion. Only corrosion is considered further here as the other mechanisms are not specific to CCS process conditions.

When corrosion occurs in the presence of sulphides, these promote absorption of atomic hydrogen (from the cathodic part of the corrosion reaction) into the steel. Some other species such as arsenic salts and

cyanides can also promote hydrogen absorption.

6.8.2 Hydrogen Induced Cracking (HIC)Hydrogen Induced Cracking (HIC) occurs in carbon steels when absorbed hydrogen recombines to hydrogen molecules at internal defects. This causes internal cracking due to t he pressure of the hydrogen gas.Typical initiation points for cracking are elongated non-metallic inclusions such as sulphides. Cracking is typically lamellar, along microstructural features like pearlite and segregate banding. Cracking does not

require an external applied stress, and the orientation of cracking is not related to the applied stresses

Rolled products such as plates and also welded pipe made from plate or coil, are at most risk from HIC. Seamless pipe and cast or forged products are generally at lower risk of HIC. CRAs are not susceptible to

HIC.

Requirements for HIC resistant materials are outlined in ISO 15156-2 [2] and EFC Document nr. 16 [17]. For seamless pipe, cast and forged products it is normally sufficient to specify a restricted S content in the

steel. In the case of plates and welded pipe, HIC resistance is affected by many processing variables and there are not general v alues of composition or other parameters applicable to all manufacturing routes.

Some measure of quality control testing i s necessary to ensure HIC resistance.

Cold-working above 5% strain should be avoided. Cold-worked items such as dished vessel ends must be heat-treated after forming.

6.8.3 Sulphide Stress Corrosion (SSC)Hydrogen dissolved in the carbon steel matrix reduces the ductility and toughness of the material. Under tensile stresses, the embrittled material may crack to f orm sulphide stress corrosion cracks. This process

can be very rapid in susceptible materials. As well as carbon steels, ferritic and martensitic stainless steels such as AISI 410 grades are also susceptible to SSC. SSC is a form of hydrogen embrittlement, and is

fundamentally different from other forms of stress-corrosion, for example being most severe around room temperature.

Guidance on materials selection in H2S containing conditions is provided by ISO 15156-2/NACE MR0175 for upstream oil and gas service, and by NACE MR0103-2003 for downstream refining service [18].

Whilst these documents are specific to other i ndustries, their guidance can be used in comparable environments within CCS processes.

The risk of SSC is increased by hard microstructures and, other factors being equal, i s greater in high strength, low toughness materials. Prevention of SSC is achieved primarily by control of hardness in the base

material and also in welds. ISO 15156-2/NACE MR0175 specifies maximum hardness limits for carbon steel products in H2S service to avoid SSC: f or example 22 HRC / 248 HV10 for general carbon steel

products. Welding standards such as BS 4145 and NACE RP0472-2000 also contain limits on weld hardness values for sour service. Cold-working should also be avoided and cold-worked items such as dished

vessel ends must be heat-treated after forming.

SSC is generally a much reduced risk at operating temperatures above 80C. However, it is strongly recommended to follow the ISO 15156-2 / MR0175 requirements for all the facilities exposed to H2S,

irrespective of the operating temperature, because there is a risk of cracking of hydrogen saturated materials during, for example, shut-downs at ambient temperature.

6.8.4 Stress-Orientated Hydrogen Induced Cracking

(SOHIC)Stress-Orientated Hydrogen Induced Cracking (SOHIC) is related to both HIC and SSCC. SOHIC appears as stacks or chains of hydrogen induced cracks, linked through the wall thickness of the steel by

intermediate sulphide stress corrosion cracks. The orientation of SOHIC is related to residual or applied stresses. Typically, SOHIC is associated with welds. Qualification of materials against SOHIC is not well

established, and common oil and gas industry practice considers that the m easures taken to ensure steels are resistant to HIC and SSC are adequate for avoiding SOHIC as well.

6.8.5 Stress Corrosion Cracking (SCC) of CRAsMany CRAs are susceptible to stress-cracking in the presence of sulphides. This is distinct from SSC of ferritic steels: SCC of CRAs occurs with minimal corrosion and is generally more severe at higher

temperatures.

ISO 15156-3/NACE MR0175, EFC 17 [19] and NACE MR0103-2003 provide guidance on selection of CRAs to avoid SSC in sour environments. ISO 15156-3/NACE MR0175 lists environmental limits within

which particular CRAs are considered to be acceptable for use: in other words li sted materials can be regarded as pre-qualified for the conditions in the standard. It is important to note t hat ISO 15156-3/NACE

MR0175 often places requirements on materials for sour service additional to the basic properties for the grade, for example restrictions on maximum hardness, or on processing conditions.

The standard also allows the possibility of qualifying a material for specific conditions outside these limits based either on service experience or test evidence. In some cases, the standard limits may be rather

conservative, and qualifying materials for service outside the standard limits i s a useful option