The healing mechanism of hydrogen-attacked cracks in low carbon steel and Cr-Mo steel and its influencing factors during the healing process were studied by recovering heat treatment of split specimens in vacuum. The result showed that crack pacing turns much smaller under the condition of pure heating, especially for crack tips. The healing effect is well related to the length of cracks with the shorter in priority. By the primary mechanism of thermal diffusion, iron and carbon atoms must diffuse at the high speed in steel to realize that plasticity deformation energy exceeds and overcomes surface tensile force energy. In addition, phase transformation and stress-stain relationship also have positiwe effects on the process.

The effects of SO2 on the initial atmospheric corrosion of AZ91D magnesium alloy were investigated in laboratory. Metallographic observation, SEM (Scanning Electron Microscopy), XRD (X-ray Diffraction) and XPS (X-ray Proton Spectrograph) were used to analyze and discuss the initial surface morphology of corrosion layers and corrosion products. The corrosion rate of the alloy increases with increasing the content of SO2. The initial attack has the characteristics of localized corrosion and preferentially concentrates on a phase. MgO and Mg(OH)2 form at first, which provide a protective layer, then the existence of SO2 decreases the pH of the thin solution on the alloy, accelerates dissolution process, and promotes the formation of MgSO3.6H20 and MgSO4.6H20, meanwhile cracks were found on the corrosion products with corrosion continuation. These soluble corrosion products and the cracks provide the paths for filtering oxygen and corrosion pollutants into the matrix, which results in severe localized corrosion and the loss of protective function of film.

The experiments were aimed at obtaining crack healing of hydrogen attack (HA) in low carbon steel by heat treatment. It was observed by SEM that HA cracks experienced different healing effects. The results indicated that the HA cracks≤10μm length can be healed completely by cyclic heat treatment between room temperature and 1000℃. Five cycles totalling 10 hours were used. The studies performed showed that crack healing is controlled by diffusion and that the recovery kinetics depend on the plastic deforming energy Es arising from the growth of HA bubbles or cracks. These findings led to the conclusion that the critical condition for healing of bubbles or cracks is Es≥2γ/r (where γis the surface tension, r is the radius of bubbles or half length of cracks) provided the atoms of H, C, Fe can diffuse rapidly.