Nacéra Sabrina Meck, Paul Crook and Lee Pike
Haynes International, Inc.
1020 West Park Avenue, Kokomo, IN 46904, USA

 ABSTRACT

 For many chemical processes, the moderate strength exhibited by the family of Ni-Cr-Mo (C-type) alloys is more than sufficient.  Nevertheless, some applications require a material with even greater strength.  For this reason, a new nickel alloy, essentially Ni-21Cr-17Mo (wt. %), has been recently developed and introduced into the market.  This new alloy not only exhibits corrosion resistance comparable to other C-type alloys, but it can also be strengthened through a two stage treatment to obtain approximately double the yield strength.  The aim of this study was to ascertain the corrosion characteristics of this age-hardenable Ni-Cr-Mo composition, especially the influence of the strengthening treatment.  The alloy was tested in both the annealed and age-hardened conditions, in several aggressive inorganic acids, namely hydrochloric, sulfuric, hydrobromic, phosphoric, nitric, and hydrofluoric, at different concentrations and temperatures.  To assess the effects of the strengthening treatment upon resistance to chloride-induced pitting, crevice attack and stress corrosion cracking (SCC), both forms were tested in acidified 6% ferric chloride, and under stress in boiling 45% magnesium chloride.  For comparison, standard alloy C-22 was tested simultaneously under identical conditions.  Hence, the paper will discuss the results obtained in both forms of the alloy along with those concerning the passivation behavior of the materials in hydrochloric acid containing ferric ions as contaminants, at different temperatures.

 Key words: C-22HS alloy, age-hardenable, strength, nickel-chromium-molybdenum alloys, iso-corrosion diagrams