Discontinuities in Oxidation Kinetics: A New Model and its Application to Cr–Si-Base Alloys

Abstract

Some alloys such as many Cr-based systems show mass gain discontinuities during thermogravimetric measurements which strongly affect the oxidation kinetics. The behaviour cannot be described by the current models available in the literature. Thus, a novel $$k_\mathrm{para}$$ k para –$$k_\mathrm{lin}$$ k lin -P-model was developed to describe oxidation kinetics during the isothermal exposure of materials which show such behaviour. Beside the parabolic rate constant $$k_\mathrm{para}$$ k para and the linear mass loss constant $$k_\mathrm{lin}$$ k lin , the P-value and $$f_P$$ f P are introduced to take into account spontaneous rapid mass gains due to local oxide scale failure. The parameter P serves as a measure for the mass gain due to discontinuous events and $$f_P$$ f P is the frequency of such events. The both parameters can be related to oxide scale detachment and growth stresses. The application of the model is demonstrated for the oxidation of Cr–Si-based alloys in synthetic air at $$1200^{\circ }\hbox {C}$$ 1200 ∘ C for 100 h. For these alloys, the origin of the mass gain discontinuities is discussed and the meaning of P and $$f_P$$ f P is explained in more detail. Using this newly developed model, an insight into growth and nitridation resistance of oxide scales as well as scale adhesion is gained.

Study of the effect of magnetic fields on static degradation of Fe and Fe-12Mn-1.2C in balanced salts modified Hanks’ solution

“…15 ) would result in the square pits detected in the SEM image ( Fig. 6 ) [ [89] , [90] , [92] , [93] ]. Nevertheless, these square pits were only clearly detected in FeDC-C samples, therefore the effect of magnetic domains, or another magnetic feature, cannot be discarded.…”

Oxidation at 1250°C of {Cr, Ta}‐rich carbides‐strengthened {Ni, Co}‐based alloys

“…For characterizing the whole mass gain kinetic during the 70 h of oxidation at 1250°C, many model equations are available in the literature. [22][23][24] Some of them represent the mass gain kinetic by taking into account the mass loss by chromia volatilization. [22,23] Others do not consider the mass loss by volatilization of chromia, [23,24] for instance, when oxidation follows the Wagner's regime thanks to a covering oxide, which does not know a problem of reoxidation in gaseous species (e.g., NiO, Al 2 O 3 , etc.…”