Corrosion and Materials Degradation

In the original publication [...]

The present study evaluates the electrochemical behaviour of reinforcing steel embedded in an alkali-activated eco-cellular geopolymer concrete designed for applications in environments with high chloride exposure. The material was formulated using a ternary precursor composed of fluid catalytic cracking residue (FCC), Class F fly ash, and ground granulated blast furnace slag (BFS), activated with an alkaline solution and combined with preformed foam to generate a microstructure characterised by predominantly closed porosity and low capillary connectivity. The electrochemical response of the system was assessed through open circuit potential (OCP) measurements, Tafel polarisation curves, electrochemical impedance spectroscopy (EIS), and potentiodynamic tests under accelerated exposure to NaCl solutions. The results demonstrate a markedly improved electrochemical performance, evidenced by shifts in OCP towards more noble values, reductions of 45–65% in corrosion current density (Icorr), and increases of up to fourfold in charge transfer resistance (Rct), together with the development of broader and more stable passive regions. This behaviour is attributed to the synergistic interaction between the formation of dense N-(C)-A-S-H (sodium/calcium–aluminosilicate hydrate) and C-(A)-S-H (calcium–aluminosilicate hydrate) gels, the eco-cellular architecture with low capillary connectivity, and the stable high alkalinity of the activated matrix, which collectively restrict ionic transport and promote the passive stability of the reinforcing steel—defined here by noble OCP values, low Icorr, high Rct, and sustained passive domains in polarisation curves. Overall, the findings position the developed eco-cellular geopolymer concrete as a sustainable, high-performance alternative for infrastructure exposed to chloride-rich environments.

The growing demand for hydrogen-based energy systems has intensified the need for structural materials with enhanced resistance to hydrogen-induced degradation. This study presents a comparative investigation of hydrogen-induced mechanical behavior and embrittlement susceptibility of laser powder bed fusion (LPBF) manufactured 316L steel and conventionally manufactured (CM) 316H steel. Tensile/Charpy testing, hydrogen charging (up to 115 h), OM, SEM, TEM, and EBSD analysis were employed to assess microstructure, strength, ductility, fracture characteristics, and phase stability. In the uncharged state, LPBF steel exhibited significantly higher strength but lower ductility than CM steel, attributed to its fine cellular sub-grain microstructure. Both steels showed similar hydrogen saturation kinetics, reaching ~9 ppm, with residual hydrogen levels of ~3.3 ppm after 90 days of desorption. Hydrogen exposure led to a more pronounced degradation of the tensile properties of the LPBF steel, with an up to 22% reduction in the ductility-based embrittlement index, while CM steel remained much less affected. Impact toughness in both materials resisted hydrogen embrittlement, retaining over 96% of initial values. Fractographic analysis of tensile specimens revealed subsurface brittle zones consistent with calculated hydrogen diffusion depths. EBSD data indicated that hydrogen-stabilized austenite in LPBF steel was achieved by suppressing deformation-induced martensitic transformation, despite increased dislocation activity. These findings suggest that, while LPBF steel is more vulnerable to hydrogen embrittlement under tensile loading via the HELP mechanism, its microstructure mitigates impact toughness degradation through hydrogen-induced austenite stabilization.