Water Vapour Effects on the Oxidation of Chromia-Forming Alloys

Abstract

The electronic properties of chromia scales grown between 800°C and 900°C on chromium metal and chromia-forming ferritic stainless steels were determined using room temperature PhotoElectroChemistry (PEC) experiments and the relative importance of the n- and p-character of the scales could be assessed. According to the thermodynamic previsions of defects structures, the external part of all the scales grown in oxygen exhibits band gap energy around 3.5 eV, with a marked p-type character on chromium and a possibly n-type behaviour on stainless steels. On the contrary, the internal part of the scales is always n-type, with predominant interstitial chromium defects. A major change appears when chromium or stainless steels are oxidised in water vapour-argon mixtures, where the absence of a p‑type semiconductor in the scales could be evidenced. Hydrogen defects are thought to be responsible of this particular behaviour which leads to a strong reduction of residual stresses due to increased high temperature relaxation. Moreover, the inversion of the growth direction resulting from high mobility of the OH defects makes the chromia scales grown in water vapour more adherent than when grown in oxygen.

Intergranular Oxidation of Additively Manufactured Ni-Base Alloy 625: The Role of Si

“…1). Being more uniform and homogeneous, the H 2 -H 2 O grown chromia scales additionally demonstrate a better adherence to the metal [40,44] and show no signs of delamination over GBs. However, smooth oxide bulges are visible over the GBs accompanied by abundant porosity in the alloy subsurface.…”

“…To check the influence of the chromia scaling mechanism, i.e., oxide growth direction, on intergranular corrosion, the exposures in a flow of Ar-5 H 2 %− 3%H 2 O were performed in a closed, gas-tight quartz tube. It is well known that water vapor promotes inward growth of Cr 2 O 3 and this inwardly grown chromia usually grows faster and adheres better to metal [35,39,40]. The Ar-5%H 2 gas mixture supplied by Linde gas (AGA AB, Lidingö, Sweden) was bubbled through a humidification setup held at 24.4 • C to obtain 3% absolute humidity (30 mbar of H 2 O).…”

CHAPTER 5 Effect of Water Vapour on the High Temperature Oxidation of Stainless Steels

“…For thermally grown chromia on chromium and stainless steels, the photoelectrochemistry measurement showed that the semiconducting type of the oxide turned from amphoteric p+n when oxidised in oxygen into n when oxidised in water vapour [37,39]. This effect is less for stainless steel since the iron doping into the oxide might weaken its p-type property [37,39].…”

“…For thermally grown chromia on chromium and stainless steels, the photoelectrochemistry measurement showed that the semiconducting type of the oxide turned from amphoteric p+n when oxidised in oxygen into n when oxidised in water vapour [37,39]. This effect is less for stainless steel since the iron doping into the oxide might weaken its p-type property [37,39]. From these results, we may consider that, for the oxidation of chromium or the chromia-forming stainless steel in water vapour, the hydroxyl ion might be incorporated into the oxide in the substitutional site and diffuses via the oxygen vacancy, or it might be incorporated into the oxide grain boundary and further diffuses to the metal/scale interface.…”

“…So, the cracks and spallation occur faster than that exposed to dry condition. Since the cracks occur, Fe2O3 could be thermodynamically reproduced and this oxide is indeed observed to grow [39]. Ehlers et al [47] pointed out that the transformation of protective chromia into non-protective iron-rich oxide is governed by the ratio of H2O/O2 rather than the absolute level of H2O.…”