High temperature alloy chloridation at 850°C

Abstract

The resistance of eight alloys against chloridation was tested at 850 C in Ar/Cl 2 (2.5% Cl 2 ) for 15 min. Pre-oxidation treatments were performed for 1 h and 8 h at 850 C in order to produce a thin, adherent and protective oxide scale able to improve the chloridation behaviour of the tested materials. The chloridised sample morphologies were compared to the morphologies observed on the non pre-oxidised samples. The alloys containing a large amount of iron did not exhibit any chloridation resistance, even after preoxidation, and were severely damaged. The nickel based alloys gave interesting results but were also attacked by chloride, probably by the "active oxidation" mechanism. The duration of the pre-oxidation treatment plays an important role, since the 8 h pre-oxidation appears more beneficial than the 1 h pre-oxidation, to delay the chloridation, probably because of the best quality of the oxide layer grown during 8 h. For the nickel based materials, the effects of chloride appear less severe than for the iron-based alloys, but are not stopped. The "active oxidation" mechanism is proposed to be responsible for the degradation of the tested materials.

HCl-Induced High Temperature Corrosion of Stainless Steels in Thermal Cycling Conditions and the Effect of Preoxidation

“…This type of behaviour is frequently observed [1][2][3][4][5][6]8] and is in accordance to the wellknown chlorine cycle [5]. However, the metal chloride evaporation is often associated with a subsequent transformation into metal oxides at higher oxygen partial pressures.…”

“…(i) The chlorine cycle, involving inward diffusion of molecular chlorine, formation of volatile metal chlorides at the metal/oxide interface and transformation into metal oxides at the scale/gas interface [4][5][6]8]. (ii) Ionic transport of Fe 2?…”

“…Regardless of which chlorine species interacts with the metal, the formation of metal chlorides is considered central in the corrosion degradation process [1][2][3][4][5][6][7][8]. The formation and volatilisation of metal chlorides seems to be of particular importance for the iron-based ferritic and austenitic steels which are widely used in the applications of interest.…”

“…Combustion of waste, and to some extent also biomass, releases high levels of gaseous HCl which is considered a primary reason for the fast corrosion of critical components such as superheater tubes. Despite numerous studies on the oxidation-chlorination of both pure metals [1][2][3][4][5] and alloys [4][5][6][7][8], the mechanisms behind the breakdown are still under debate. The term ''active oxidation'' is frequently used to describe corrosion in the presence of poorly protective oxide scales [2][3][4][5][6]8].…”

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

“…This severe materials degradation induced by the formation of volatile metal chlorides at alloy/oxide interface, which evaporate, diffuse outwards and are converted into oxides in regions of higher oxygen partial pressure, has been termed as an 'active oxidation'. [15][16][17][18] It is interesting to note that iron rich oxide former alloys, which are usually more prone to 'active oxidation' by HCl or Cl 2 than chromia forming alloys, [18][19][20][21] exhibit less reactivity with direct attack by KCl. For instance, due to lower Cr concentration, the pre-oxidation of Fe-9Cr (P91) alloy produces an iron rich oxide top layer followed by an (Fe,Cr) spinel underneath (Fig.…”