Deterioration of the corrosion resistance of welded joints in API5L X52 steel isothermally aged

Corrosion behavior of welded joints in API5L-X52 pipe steel aged at 250 °C at different times was investigated under electrochemical technique like tafel polarization, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies. The electrochemical results which were performed in a solution of brine containing hydrogen sulfide at 25 °C, revealed an increase of the general corrosion rate in the weld bead, the heat affected zone and the base metal as the aging time was elapsed. The corrosion rate higher was linked to the transgranular precipitation of fine nanoparticles of cementite and niobium carbide ocurred during accelerated aging process until 500 hours and their coarsening process after this time. The SEM study of the corrosion products at the three different zones showed the presence of oxide and sulphur of iron very brittle and porous on metallic surface. The highest corrosion rate was reached by the weld bead owing to the largest increase in the amount of nanocarbides, lesser coarsening and the presence of corroding products with larger brittleness and porosity.