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Oxide scale formation and subsurface phase transformations during long‐term steam exposure of the cobalt base alloy 25
Author(s) -
García Fresnillo L.,
Chyrkin A.,
Hüttel T.,
Böhme C.,
Barnikel J.,
Grüner D.,
Schmitz F.,
Quadakkers W.J.
Publication year - 2012
Publication title -
materials and corrosion
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.487
H-Index - 55
eISSN - 1521-4176
pISSN - 0947-5117
DOI - 10.1002/maco.201206683
Subject(s) - chromia , alloy , materials science , cobalt , spinel , oxide , intermetallic , chromium , metallurgy , internal oxidation , microstructure , transmission electron microscopy , corrosion , tungsten , scanning electron microscope , phase (matter) , chemical engineering , chemistry , composite material , nanotechnology , organic chemistry , engineering
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 °C. In the present study, the steam oxidation behavior of the cobalt base alloy 25 in the temperature range 650–800 °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 Co 3 W became enriched at the scale alloy interface. Additionally, the chromium rich M 23 C 6 and the tungsten rich M 6 C dissolved in the depletion layer. The mechanisms for occurrence of these effects are discussed on the basis of phase equilibria in the binary Co–W and the ternary Co–Cr–W system.