Investigation of causes of failures of nickel alloy pipelines at high temperatures

By microstructure studies and electrode potential measurements, it has been found that prolonged exposure of Nickel 201 nickel samples to high temperatures results in graphite film grain boundaries resulting in corrosion cracking of the intergranular type. Galvanic vapor formation is observed on different sections of inner surface of pipes due to non- uniformity of their surface. At the same time, areas with a more electronegative potential (welds, deformed surfaces of branches) act as an anode, which leads to their dissolution. In some cases, through corrosion is observed by the pitting mechanism. Hardness measurements confirmed the inhomogeneity of the properties of the weld joint: the hardness values of the base metal and weld differ from the new joint by more than 4 units on the Rockwell scale. During pipeline operation, weld hardness decreases, which increases the probability of metal erosion at a high flow rate, in which nickel oxide crystals act on the material as abrasive particles and, as a result, leads to thinning of the pipeline wall. Heat treatment of the weld compound followed by cooling in air ensures that the hardness over the entire surface of the sample is equalized, while when the samples are cooled together with the furnace in laboratory conditions, higher hardness of the weld was obtained. After heat treatment, the potential difference of the "weld - base metal" decreases, i.e. the corrosion rate of the weld decreases