Premium
Oxidation of Nickel and Cobalt‐Based Alloys in the Presence of Condensed Sodium Sulphate
Author(s) -
Johnson D. M.,
Whittle D. P.,
Stringer J.
Publication year - 1975
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.19750260803
Subject(s) - chromium , alloy , cobalt , metallurgy , materials science , nickel , corrosion , oxidizing agent , superalloy , sodium , inorganic chemistry , chemistry , organic chemistry
The hot corrosion behaviour of a number of nickel and cobalt‐based superalloys has been examined by exposing samples to a high temperature oxidizing environment supersaturated with sodium sulphate vapour. This test seems more able to reproduce typical service behaviour than other laboratory tests. Pure cobalt is unaffected by the presence of the condensed sulphate, whereas CoW and low‐chromium, CoCrW alloys undergo acidic fluxing. However, the major change produced by the continuous supply of Na 2 SO 4 , as opposed to the limited amount of salt available in the coating test is in the behaviour of the high chromium alloys, when the protective Cr 2 O 3 layers are removed due to the formation of a Na 2 CrO 4 species. Thus, the normally resistant Co25Cr7.5W alloy suffers acidic fluxing. Similarly, the Cr 2 O 3 layer on the binary Co25Cr alloys is rendered ineffective; considerable ingress of sulphur into the alloy occurs. Aluminium and manganese additions seem to reduce this effect slightly by stabilising the protective oxide layer. Both these alloying additions have a higher affinity for sulphur than chromium, and this could be important. Binary Ni20Cr seems less susceptible to accelerated attack than the Co25Cr alloys, presumably due to its greater ability to maintain a protective Cr 2 O 3 layer. However, addition of 3 vol.‐% Y 2 O 3 virtually prevents any attack by the Na 2 SO 4 , preventing sulphur penetration into the alloy and promoting the formation of a protective Cr 2 O 3 layer. Under non‐condensing conditions, all of the alloys tested oxidize in an unaccelerated manner, supporting the view that condensation of sodium sulphate is necessary for hot corrosion.