Electrochemical Assessment of X70 Steel With Non-Conventional Heat Treatment

Abstract

Corrosion behavior of an API X70 steel by potentiodynamic polarization curves was carried out. X70 steel was heat treated at a temperature of 1050°C (onset temperature solution of niobium carbonitrides) for 15 and 30 minutes hold followed by quenching in water. Test solutions for electrochemical evaluation were NS4 solution and congenital water (CW) to assess external and internal corrosion pipelines respectively. The polarization curves were performed within a range of -500mV to 1000mV for NS4 solution and the -500mV to 600mV by congenital water respect to open circuit potential (OCP) at a scan rate of 1mV/s. The tests were conducted at room temperature. The surfaces of the samples were observed by scanning electron microscope (SEM). A localized corrosion type was observed. According to polarization curves it can be observed that oxidation reaction in the anodic branch belongs to a charge transfer process. Cathodic branches reveal a process where the charge transfer resistance is influenced by a process of mass transfer. The non-conventional heat treatment improved the corrosion resistance compared to as received material.