Cast Iron with its ancient history, traced back to 6th century BCE1, has been used for centuries to anything from manhole covers & fire hydrants to bridges. However, the development of Spheroidal Graphite Cast Iron (SGCI) or Nodular Cast Iron, in the 1940’s, with resulting improvement in mechanical properties such as ductility and fracture toughness, paved the way for further growth in industrial usage of cast iron.² The material has been adopted by several industries such as automotive-, nuclear-, and wind turbine industry. During the last decade, SCGI has gained increased attention as construction material for subsea equipment in offshore oil & gas production, mainly competing with welded and bolted steel assemblies.

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.

The purpose of this research was to investigate the influence of microstructure on hydrogen trapping in Armco iron by analyzing the trapping ability of grain boundaries and dislocations. Hydrogen traps were introduced into the material by systematically subjecting it to various grades of heat treatment and mechanical deformation. By combining different treatment steps (annealing at different temperatures cold rolling at various deformation degrees severe plastic deformation) a wide range of different grain sizes and dislocation densities was created.SEM EBSD TEM and XRD imaging were carried out to obtain a detailed characterization of the microstructure and an estimation of dislocation densities.Electrochemical permeation experiments and thermal desorption spectroscopy (TDS) were performed to render a classification and characterization of hydrogen traps. Electrochemical permeation yields information on the diffusivity of hydrogen in the material and the influence of traps on the diffusivity. An experimental setup according to Devanathan and Stachurski was used.TDS allows the estimation of the amount of hydrogen stored in the different traps and the determination of the trap’s binding energy for hydrogen.By combining the information on the microstructure obtained from the material characterization with the results of the permeation experiments and TDS as well as a model based data interpretation the trapping efficiency of grain boundaries and dislocations in iron can be precisely determined.