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The corrosion behavior of Fe‐Cr alloys containing Co, Mn, and/or Ni and of a Co‐base alloy in the presence of molten (Li,K)‐carbonate
Author(s) -
Biedenkopf P.,
Spiegel M.,
Grabke H. J.
Publication year - 1997
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.19970481103
Subject(s) - corrosion , alloy , spinel , oxide , materials science , metallurgy , metal , carbonate , base (topology) , diffusion , base metal , inorganic chemistry , chemical engineering , chemistry , mathematical analysis , physics , mathematics , welding , thermodynamics , engineering
The corrosion behavior or commercial Fe ana Co base alloys and Fe‐Cr model alloys with different contents of Co and/or Mn was investigated by continuous exposure tests in the presence of a thin carbonate film. All alloys studied form multi‐layered corrosion scales consisting of outer Li containing oxides and inner Cr rich oxides, i.e. spinels or LiCrO 2 . The LiCrO 2 is formed on alloys with high Cr contents (≤ 20 wt.%), whereas mixed (Fe,M) 3‐ x Cr x O 4 spinels (M = Co, Mn, Ni) were found on alloys with lower Cr content (15–20 wt.%). Insoluble Cr containing oxides occur only in the inner layers of the corrosion scale, whereas on the surface of corroded specimens soluble chromates were detected. Alloys with Mn contents greater than 15 wt.% form Mn 2 O 3 in the initial stages of the experiments, this oxide reacts with the melt and formation of Li 2 MnO 3 takes place. In exposure tests up to 500 h Fe‐Cr alloys with low contents of Mn and Co (10 wt.% Co or Mn) form iron rich oxides (LiFeO 2 and LiFe 5 O 8 ) with varying amounts of dissolved Mn or Co. In the later corrosion stages outward diffusion of Mn and/or Co takes place and LiCoO, and Li 2 MnO 3 are formed on top of LiFeO 2 , whereby the concentration of Mn and/or Co in the inner layers (LiFeO 2 and spinel) decreases. The outer Li containing oxides LiFeO 2 , LiCoO 2 and Li 2 MnO 3 are nearly insoluble in the melt and when present at the surface protect the metallic material from further corrosive attack. Fe‐Cr model alloys containing Co and Mn form multi‐layered corrosion layers after 2000 h of exposure. These layers consist of four oxides in the following sequence from the metal‐scale to the scale‐melt interface: (Fe,Cr,Co,Mn) 3 O 4 spinel, LiFeO 2 , Li 2 MnO 3 and LiCoO 2 .