Oxide scale formation and subsurface phase transformations during long-term steam exposure of the cobalt base alloy 25

High chromium nickel and cobalt base alloys are presently being considered as construction materials for various components in high efficiency steam turbines with envisaged operating temperatures around 700?degrees C. In the present study, the steam oxidation behavior of the cobalt base alloy 25 in the temperature range 650800?degrees C was investigated whereby exposures up to 10?000 were carried out. Post exposure analyses of the oxidation products and alloy microstructures included optical microscopy, scanning and transmission electron microscopy, X-ray diffraction analysis and secondary neutrals mass spectrometry. The experiments showed in all cases formation of oxide scales mainly consisting of chromia with minor amounts of outer Cr/Mn spinel and internal silica. The oxidation induced chromium depletion resulted in a number of microstructural changes in the subsurface depletion layer. First, the intermetallic phase Co3W became enriched at the scale alloy interface. Additionally, the chromium rich M23C6 and the tungsten rich M6C dissolved in the depletion layer. The mechanisms for occurrence of these effects are discussed on the basis of phase equilibria in the binary CoW and the ternary CoCrW system.