Effects of Cathodic Protection on Cracking of High-Strength Pipeline Steels

A comparison was made between four strength levels of pipeline steels (X-70, X80, X-100 and the X-120) from the point of view of their susceptibility to hydrogen embrittlement under cathodic protection. The main aim was to determine whether the development of higher strength materials led to greater susceptibility to hydrogen embrittlement. This was achieved by straining at 2×10⁻⁶ s⁻¹ after cathodic charging in a simulated dilute groundwater solution (NS4) containing 5% CO₂/95% N₂ (pH approximately 6.7). The results showed quantitatively the loss of ductility after charging, and the loss of ductility increases with strength level of the steel. All four steels exhibited a loss of ductility at overprotected charging potential and an increasing amount of brittleness on the fracture surface. Ductility in solution was measured under four different levels of cathodic protection, ranging from no cathodic protection to 500 mV of overprotection with respect to the usually accepted criterion of −850 mV vs. Cu/CuSO₄ reference electrode. Experiments were carried out by straining during cathodic polarization in a simulated dilute ground water solution (NS-4 solution). Strain rates used were 2×10⁻⁶ s⁻¹. After failure, the fracture surfaces were characterized by examination using scanning electron microscopy (SEM). Under cathodic protection, all four steels showed loss of ductility and features of brittle fracture. The loss of ductility under cathodic polarization was larger the greater the strength of the steel and the more active (i.e., more negative) the applied potential. The Ductility Reduction Index (DRI) was defined to quantify the reduction in ductility.