A New High Strength, Corrosion Resistant Alloy for Marine Corrosion and Fastener Applications

The C-22HS alloy was introduced recently for applications requiring both corrosion resistance and high strength.  Being a C-type alloy (Ni-Cr-Mo), the new material has excellent corrosion resistance in both reducing and oxidizing environments.  The alloy is capable of being age-hardened to effectively double the yield strength over other C-type alloys.  The C-22HS alloy has generated interest in the oil & gas industry due to its resistance to sour gas environments. C-22HS is being balloted for inclusion in NACE MR0175/ISO 15156 at NACE Level VII (highest severity) with data showing resistance to elemental sulfur in the severe sour brine environment. In this paper, properties of the new alloy relevant to marine corrosion and Navy fastener applications are presented. This material has also been shown to be resistant to hydrogen embrittlement in marine and other severe environments, exhibiting potential for corrosion immunity.  Efforts in developing alternate processing routes for C-22HS alloy for very high strength applications are detailed.  The high strength of C-22HS alloy is imparted through the formation of Long Range Ordering (LRO) domains of Ni₂(Cr, Mo) which develop during a two-step age-hardening treatment given to the alloy in the fully annealed condition.  In standard C-22HS alloy, the formation of the LRO domains results in a near doubling of the room temperature yield strength up to values around 100 ksi.  The two-step age-hardening treatment is 1300°F/16h/fce cool to 1125°F/32h/air cool. The yield strengths of the annealed and age-hardened bars ranged from 114 to 126 ksi, the as-cold worked bars ranged from 191 to 205 ksi, and the cold-worked + age-hardened bars ranged from 199 to 206 ksi. In these material conditions, C-22HS also exhibits excellent ductility and Charpy-V-Notch fracture toughness of ≥150 ft-lbs from -300ºF to +550ºF. The C-22HS alloy is presently being tested for Navy Subsafe high strength fastener approval.