INVESTIGATING THE CORROSION SUSCEPTIBILITY OF WELDED STEEL PIPES AT ELEVATED TEMPERATURE

The drive behind this research work cannot be far-fetched from the quest to precisely predict how effectively welded low carbon steel pipes will function in industrial service applications requiring subjecting them to service use at elevated temperature as a function of inferences drawn from a critical study of the corrosion behavior of these welded steel pipes in both acidic and salty environment. To this end, a particular low carbon steel pipe of known chemical composition was sectioned into the required number of samples needed to carry out the corrosion experiment and this finally amounted to twenty-four samples, out of which eight were un-welded; the other eight samples were welded with tiny weld pool and the last eight were welded with large weld pool. For adequate comparison and effective corrosion evaluation, the samples were appropriately immersed into two different corrosive media of 0.3M H2SO4 and 0.3M NaCl respectively with special subjection of twelve samples to corrosion at the prevailing room temperature and the other twelve samples were subjected to corrosion at a controlled elevated temperature of 950C with the aid of the blast-air oven. The corrosion behaviours of these welded steel pipes were evaluated using the weight loss analysis method and the overall corrosion exercise took 61 days. Subsequently, necessary readings including potential difference; pH value and resulting weight losses were taken at four days intervals. After the corrosion exercise, series of calculations were done with the available data in order to compute the appropriate corrosion result through the plot of different graphs and provide effective evaluation of the corrosion behaviors of the aforementioned steel sample. Microstructural analyses of pre-corrosion control samples X, Y and Z were undertaken as well as macrostructural analyses of the as-corroded steel samples with the aid of the optical metallurgical microscope. The results obtained revealed that corrosion rates were much higher in 0.3M H2SO4 than in 0.3M NaCl at both temperatures. Another notable point is the action of weldment against the propagation of corrosion in H2SO4 and the anomaly in the corrosion behaviours of welded samples in NaCl particularly at elevated temperature. However, the corrosion rates of all samples reduced over time as a result of the hygroscopic nature of the corrosive media used which resulted into the loss in potency of the solutions as a function of reduction in the concentration of the environments which also led to the decrease in potential differences of the corrosive media with time and corresponding increase in their pH values with time. Nevertheless, all samples immersed in H2SO4 experienced active corrosion between 22 mg/mm2/yr to 165mg/mm2/yr. while samples immersed in NaCl experienced passive corrosion between 0.3mg/mm2/yr to 1.97mg/mm2/yr. Therefore, the research work has been able to show that welded low carbon steel will perform better in NaCl environment than in H2SO4 environment even at elevated temperature provided that there are no inherent welding defects in the steel sample.