On the effect of microstructure in CO2 corrosion of carbon steel welds

Oil companies have experienced considerable corrosion in weld zones of hydrocarbon piping systems made of carbon steels. In CO2-containing environments corrosion failures are often explained by the varying tenacity of protective corrosion scales on different carbon steel microstructures. Preferential corrosion of weld metal and heat affected zone (HAZ) has mostly been explained by galvanic effects caused by differences in alloy content and microstructure. This paper reviews opinions on how the microstructure affects corrosion on carbon steel materials for toy side production piping, but because only a few sources deal specifically with these materials carbon steel in general and pipe line materials have also been reviewed. Possible hypotheses for the effects of microstructure are discussed. In the experimental part metallographic studies were carried out by nital etching and colour etching on corroded welds from top side production piping. Coupons of uncorroded welds were used as references. Certain theories on the tenacious nature of the corrosion scale are challenged and discussed against the background of the metallographic studies. Initiation seems to be important for weld corrosion. Propagation seems to have the nature of localised corrosion that is not significantly affected by the microstructure. The result of the metallographic study lends support to the hypothesis that CO2 corrosion may propagate beneath a corrosive scale.