Potentialabhängigkeit der H-induzierten Korrosion (HIC) und H-induzierten Spannungsrißkorrosion (HSCC) bei Röhrenstählen in schwach sauren und neutralen Medien

Influence of Potential on Hydrogen-Induced Cracking (HIC) and Hydrogen-Induced Stress Corrosion Cracking (HSCC) of Line-Pipe Steels in Weak Acidic and Neutral Environments In weak acidic (pH 5.5), sulfide-containing environments, high-strength line-pipe steels (0.2% yield strength 430 to 700 Nmm^?2) show a critical mechanical stress level σ_cr ≈ 60% YS above which HSCC occurs. The given value is independent of the mechanical properties. σ_cr decreases with decreasing pH (pH 3: σ_cr ≈? 30% YS), increases with increasing pH and becomes identical with the YS at sufficiently high pH. In buffer solution pH 5.5, σ_cr corresponds to a critical limiting potential for the occurence of HSCC, which depends on the mechanical stress level, ranging between U_H = ?0.42 and ?0.55 V for σ = 60% YS. At potentials more negative than a cathodic limiting potential, which ranges from U_H = ?0.8 to ?1.0 V, no HSCC occurs. This is probably due to the formation of protective layers or of an alkaline environment on the surface. This limiting potential is shifted into the positive direction with decreasing pH and does no more exist in acidic environments. With anodic polarization to potentials more positive than the free corrosion potential, uniform attack occurs resulting in failures of the specimens by reduction of area. Susceptibility to HSCC has a maximum at ambient temperature. There exists also a critical potential U_HIC for the occurrence of longitudinal cracks due to HIC which is more positive than U_HSCC. There is no connection found between U_HIC and U_HSCC nor between σ_cr and U_HIC. From the occurrence of HIC one can not derive any conclusions for the susceptibility or resistance of the given material to HSCC. As a summary, high strength line-pipe steels display no higher susceptibility to HSCC in sulfide-containing media than steels with minor mechanical properties. The HSCC-behaviour of the steels under investigation in buffer solution pH 5.5 is completely equivalent to the behaviour in stronger acidic environments (pH 3).