In situ synchrotron X-ray diffraction study of the effect of chromium additions to the steel and solution on CO2 corrosion of pipeline steels

•We studied the effect of chromium on CO2 corrosion processes.•Chromium addition accelerates the onset of siderite and chukanovite precipitation.•One of the key effects is to decrease the critical supersaturation for siderite nucleation.

We demonstrate the important effects of chromium in the steel composition and of Cr3+ ions in solution on the nucleation and growth of corrosion layers in a CO2 environment. We propose that high-valent metal cations in solution (within the boundary layer) catalyse the nucleation of siderite, which otherwise has a high critical supersaturation for precipitation. One of the key effects of small alloy additions to the steel is to put into the local solution species that decrease the critical supersaturation for siderite and modify the growth rate of the scale, thereby promoting the formation of an adherent and protective scale.