Surface layer evolutions of pre-oxidised Fe–30Cr alloy induced by gaseous KCl

Abstract

This study reports on the evolutions of surface structure and composition of Fe-30Cr alloys that were first oxidised in oxygen atmosphere then followed by an exposure in gaseous KCl environment. After the pre-oxidation treatment at 800uC for 10 h, a continuous, dense and protective chromia scale was produced. During the following exposure in KCl contaminated atmospheres at 650uC, the surface chromia layer was gradually transformed into worm-like potassium chromate textures, which quickly coalesced on the substrate surface along with simultaneous volatility. The protective oxide layer was then severely deteriorated by enhancing cracking, spallation, volatility and porosity. Voluminous iron migrated outward through these defective sites and iron oxide became the dominating reaction products. Both chromate and residual chromia were finally incorporated into the thick and porous iron oxide scale. This process accounts for the major catastrophic destruction of chromia forming materials involving reactions with alkali chloride salts, even at minor amounts.

Influence of preoxidation on high temperature corrosion of a Ni-based alloy under conditions relevant to biomass firing

“…By comparison of Figures 5, 8 and 13, it is evident that preoxidation has a positive effect with respect to reducing the corrosion attack on Nimonic 80A, under laboratory conditions mimicking biomass firing. The fact that no significant attack occurred on the preoxidation layer containing a Cr/Ti-oxide is intriguing because previous work has suggested that KCl can destroy Cr-rich oxides according to reaction (1) [6,27,30]. With appreciable amount of KCl vapour (> 10 -6 atm) [42] being generated at 560 o C, this reaction is highly favourable with a Gibbs free energy change (ΔG 0 ) of -60.…”

“…Furthermore, an investigation with chromia forming alloys (EN 1.4982, EN 1.4301 and EN 1.4845) revealed a positive effect of preoxidation against attack by HCl at low temperatures (400 o C) [28]. On the other hand, no positive effect was reported for preoxidation of Fe-30Cr alloys in oxygen, when they were subsequently exposed to a gaseous KCl environment [27]. This implies that although Cr-oxide rich preoxidation layers may be resistant to attack by HCl [28,29], they can exhibit poor resistance to KCl-induced attack [27], possibly due to the reaction between KCl and Cr2O3 [30].…”

“…preoxidation) provides an avenue for rendering protection to materials under aggressive conditions. This approach has been widely investigated under sulphidation conditions [20][21][22][23][24], and in a few cases, under conditions related to biomass firing [25][26][27][28][29]. With respect to investigations related to biomass firing, the effect of preoxidation for protection against corrosion depends on the composition and crystallography of the preoxidized layer, preoxidation conditions, as well as the corrosive environment [29].…”

Pre-oxidation as a Means to Increase Corrosion Resistance of Commercial Superheater Steels

“…One approach for material improvement could be temperature-and time-based controlled pre-oxidation of the heat-transfer materials, resulting in oxide scales thick or dense enough to withstand material degradation at higher material temperatures and for longer times than materials, which have not been pre-oxidized. The possibilities of pre-oxidation have previously been addressed in the case of alumina formers [13], Ni-based superalloys [14], and in studies addressing corrosion resistance to gaseous chlorine species [15][16][17]. Generally speaking, pre-oxidation may improve the high-temperature corrosion resistance of an alloy compared with a non-oxidized material.…”