Effect of flow rate on the corrosion products formed on traditional and new generation API 5L X-70 in a sour brine environment

The effect of chemical composition on the nature of the corrosion products formed on pipe steels API 5L X-70 traditional and API 5L X-70 new generation was studied. General corrosion was induced under different flow rates (1~4 m/s) in a rotating disc electrode immersed in a sour brine solution with kerosene at 60 °C. Linear polarization resistance indicated that API 5L X-70 NG displayed a higher corrosion resistance because of the presence of copper, which promoted a greater amount of oxides than sulfides on surface in comparison to those formed in the traditional pipe steel. Moreover, ferrite increased (70-95%) and perlite decreased (30-5%) in NG steel to protect steel from further dissolution. Scanning electron microscopy of traditional steel displayed the presence of a larger amount of corrosion products with evident porosity and bush shape morphology, whereas corrosion products on NG pipe steel looked similar but more coherent and both covered most of the steel surfaces. X-ray diffraction analysis showed that the corrosion products are mainly composed of a mixture of oxides (maghemite, hematite, magnetite), and sulfides (mackinawite, troilite, pyrite, marcasite and smithite) only in the immersion tests. The protective function of corrosion products on the API 5L X-70 NG surface turned out to be better because of the presence of one different phase, triclinic pirite, that seems more stable than cubic pyrite in traditional steel. The presence of copper in new generation steel in approximately twice as much compared to the traditional steel, which apparently favored the cathodic reaction where oxygen is formed. Chromium and nickel delayed anode reaction, ferrite dissolution, and helps to decrease corrosion rate.