A Fracture Mechanics Approach to Characterizing Hydrogen Embrittlement of Fasteners

This paper elaborates on a method previously reported that characterized a materials susceptibility to hydrogen embrittlement in a seawater environment under CP by monitoring the crack growth rate of ASTM ⁽¹⁾ E1820¹ pre-cracked CT specimens at various constant stress intensity (K-levels) and at varying cathodic protection potentials. For the fastener grade ASTM A320 L7 & L43² grade carbon alloy material the initial reported results showed that the material susceptibility to hydrogen embrittlement followed a pattern influenced by the yield strength level of the material, specifically that material susceptibility is distinguished by the response to varying CP potential & applied K levels. Intermediate yield strength (< 930 MPa) for example, showed multiple K_1EAC threshold responses; while known susceptible material with very high strength levels showed reduced threshold response and insensitivity to more positive CP potentials.

The paper presents additional results from those previously reported covering the yield strength levels between 725 MPa (105ksi) and 964 MPa (140ksi) and highlighting the transition between non-susceptibility to susceptibility right about 939 MPa (135ksi) and close to the hardness limit of HRC 34 identified in API² 20e³. These results can be represented graphically and can serve to establish design operating limits for the materials in a seawater environment under CP.

Materials properties that are used in specific oil and gas environments are de-rated due to the risks associated with hydrogen embrittlement cracking. In oil production environments the concern is for the onset of stress corrosion cracking (SCC), while in seawater environments the concern is for Hydrogen Induced Stress Cracking (HISC). Both are hydrogen embrittlement phenomena with the distinction being the source of hydrogen for each.

In SSC the source of hydrogen is from the presence of H₂S in the oil production fluids, and in HISC the source of hydrogen is from the dissociation of water from the cathodic protection system. This paper is focused on the latter phenomena and aims to characterize the susceptibility of carbon alloy steels as applied in fastener applications, in a seawater environment under cathodic protection.