Secondary ion mass spectrometry and surface profilometry characterization of oxide scales developed over various regions of weldments of 2.25Cr1Mo steel

Microstructurally different regions (namely the weld metal, the heat-affected zone (HAZ) and the base metal) were characterized in weldments of 2.25Cr1Mo steel and composite specimens comprising the three regions were oxidized in air at 773 K for durations from 2 to 100 h. For all exposures a thicker oxide scale developed over the HAZ than on the weld metal and base metal. As measured by a surface profilometer, the thickness difference was found to be a fraction of a micrometre higher over the HAZ after 2 h of exposure but had increased to more than 5 μm after 100 h. Detailed secondary ion mass spectrometry (SIMS) depth profiles of Fe and Cr in the scales developed over the various regions of the weldments suggest a considerably higher Cr content of the inner scales on the weld metal and base metal than that of the scale over the HAZ. SIMS profiles have also corroborated the formation of a thicker scale over the HAZ as assessed earlier by surface profilometry. Secondary ion images of Cr acquired from an area comprising the boundary between the scales grown over the weld metal and HAZ have confirmed the formation of a Cr-rich scale over the weld metal. The higher Cr content of the weld metal scale has been further confirmed by combined scanning electron microscopy and energy-dispersive X-ray examinations.