Corrosion behaviour of welded joints within chloride and chloride/CO[2] environments.

Electrochemical measurements and metallographic studies were performed on welded carbon steel pipeline materials. Corrosion tests were performed within naturally chloride solution with and without CO[2] additions at ambient temperature. The pH within different environments, chloride solution alone and buffered chloride solution with CO[2] additions was measured at 6.3 +/- 0.3 and 6.2 +/-0.1 respectively. The objective of this study was to evaluate the corrosion behaviour (rates) of carbon steel welds using both traditional and novel electrochemical scanning techniques within two different chloride environments. Corrosion tests were conducted using DC steady state and scanning electrochemical techniques. These tests consisted of linear polarisation resistance (LPR), corrosion potential (Ecorr)} zero resistance ammetry (ZRA), cathodic polarisation (CP) and the scanning vibrating electrode technique (SVET), which were used to semi-quantitatively assess the corrosion activity of the different microstructures, i.e., weld metal (WM), heat-affected zone (HAZ) and parent plate (PP) respectively. Samples were freshly ground before exposure to the different aqueous chloride solutions. SVET was used during these investigations to evaluate preferential corrosion susceptibilities of weldments. SVET results were compared with results from (long term-immersion) DC-based electrochemical corrosion tests. SVET was found to be a sensitive technique with good resolution allowing differences in corrosion response to be determined within hours as compared to other corrosion tests that require several hours to days.