Corrosion Inhibition of Welded of X – 52 Steel Pipelines by Sodium Molybdate in 3.5 NaCl Solution

The demand for improved reliability and prolong the lifetime of piping and pipelines has led to the development of various methods to protect piping and pipelines from corrosion attack, one of which is the use of inhibitors such as molybdate. Molybdate is a corrosion inhibitor that is non-toxic and environmentally friendly. The present paper aims to study effect of molybdate on corrosion behaviors of various regions in submerged arc API 5L X-52 steel welded joints including weld metal (WM), heat affected zone (HAZ) and base metal (BM) in a 3.5 NaCl solution. In this study, corrosion characteristics of welded API 5L X-52 steel pipelines were studied using Tafel polarization method by immersing the specimens in 3.5 NaCl solution with different percentages of Na2MoO4 inhibitors, namely 0.1, 0.2, 0.3, and 0.4 combined with SEM microanalysis of corrosion products. Results show that HAZ seems to be the most susceptible region for corrosion attack, and this undesirable effect is associated with bainitic microstructure present in the region. The additions of molybdate have two effects. First, the corrosion rate decreases with increasing inhibitor concentration and secondly; the corrosion potential is shifted to the positive direction in the polarization curve. The molybdate inhibition of corrosion could be related to its ability to form passive film, and it is discussed in the present study. © 2020 Egyptian Petroleum Research Institute