On Novel Aspects of Hydrogen Effects on Applied Stress - Coupled Micromagnetic Activity in a Mild Steel After Exposure to NaCl – Water Solution: A Combined Approach

Abstract

Hydrogen produced after exposure of a low – carbon steel to corrosive NaCl – Water solution may affect various its tensile mechanical and magnetic microstructural behaviour in a complex manner. This was investigated by introducing a relevant micromagnetic specific emission (ME) - response of this ferromagnetic material, where related processes and parameters of micromagnetic activity and mechanical response were implemented. In this manner, it was demonstrated that an increase in the hydrogen accumulation with corrosion time leads to an associated increase in the embrittling effect expressed by a substantial loss in the ductility of material. The competive and opposing effects of cumulative hydrogen, applied stress and plastic strain – induced microstructural damage were related to the specific ME- response parameter by which an increased magnetic hardening tendency of material with corrosion time was possible to establish. In this fashion and by using a stress as well as strain mode of presentation- aided combined approach, the complex interplay between micromagnetic activity, hydrogen accumulation and applied stress-strain was better revieled and analysed. It was also shown that the embrittlement is a product of hydrogen accumulation introduced by two highly localized processes. As such, accumulation occurs in two characteristic parallel ways: one of a common lattice diffusion and one of hydrogen transport and redistribution by moving dislocation towards the affected sites. Concerning the highly localized effects the dominating role of hydrogen – induced damage in form void initiation and growth over the hydrogen – assisted stress relief was reasonably demonstrated by using a simple modelling approach. Based on a mechanism of moving dislocation – assisted interaction between commulative hydrogen and magnetic domain walls, a Portervin – Le Chatelier – type micromagnetic process of a cooperative-corelated domain wall transport was proposed to explain certain subtle, quasiperiodic behaviour of ME- response. In the frame of the above findings the superior sensivity of ME – response compared to the mechanical one in early detecting cumulative hydrogen – assisted microstructural damage changes can be d educed.