Welding is one of the most important fabrication processes that joins metallic structures. Weldment structures are commonly seen in the oil and gas industry such as production tubing, co-rod, gathering pipeline, etc. Corrosion of weldments may lead to a severe production failure. In this study, the environmental effects on intrinsic and galvanic corrosion of weldments in CO2 and CO2/H2S environments have been investigated using different types of electrochemical techniques. A non-alloyed standard carbon steel weldment was selected as the test material in this study. During this study, intrinsic corrosion rates of each weldment segment (i.e. parent metal, heat effected zone and weld material), as well as galvanic currents between each weldment segment were measured under different test conditions.
Various environmental effects including salt concentration, temperature, acetic acid, H2S and addition of inhibitor (24% active Tall oil fatty acid/ diethylenetriamine Imidazoline) on weldment corrosion were studied. Experimental results show that the intrinsic corrosion rates of the different segments of the non-alloyed standard weldment are not significantly different under the same test conditions. It was also found that the galvanic current measured on the weld metal always appeared to be positive, the galvanic current on the heat affected zone (HAZ) was always neutral and the galvanic current on parent metal was always negative. This suggests that for the non-alloyed standard weldment, the corrosion of weld metal becomes worse and the parent metal is protected due to the galvanic effects between the segments.
The experimental results also show that an increase of salt concentration significantly affected the intrinsic CO2 corrosion rate in a nonlinear fashion. High salt concentration leads to a decrease of the corrosion rate at 25¿¿¿¿C, but an increase of the corrosion rate at 60¿¿¿¿C. The weight loss method confirmed the corrosion rate magnitude. The galvanic currents were unaffected by the different salt concentrations (1~10 wt% NaCl). However, in comparison with the galvanic current at 25¿¿¿¿C, the galvanic currents significantly increased at 60¿¿¿¿C.
It was also found the addition of 50 ppm H2S and/or 20 ppm inhibitor reduced the intrinsic corrosion rate of weld segments (weld metal, metal, and HAZ) and led to lower galvanic corrosion rates between different weld segments, compared with that observed in CO2 environment. The addition of acetic acid under the same conditions increased the intrinsic corrosion rate of all weld segments and lead to a higher magnitude of galvanic corrosion rate.
Surface analysis techniques including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and infinite focus microscopy (IFM) were applied to this study to characterize the surface morphology, identify the chemical composition of corrosion products and quantify the possible localized corrosion.