The Advantages of Nickel Alloys for Seawater Service

Hibner, Edward L. (Special Metals Corporation) | Shoemaker, Lewis E. (Special Metals Corporation)


The excellent corrosion resistance of nickel-alloys has been put to good use in marine engineering for many years. Some applications, such as bolting, require high levels of strength as well as corrosion resistance. New high strength nickel- alloys and their weldments exhibit excellent resistance to hydrogen embrittlement and seawater corrosion. Solid solution nickel-based alloys such as alloy 686 (UNS N06686) obtain their strength through cold work. Other highly corrosion resistant nickel-alloys such as alloy 925 (UNS N09925) and alloy 725 (UNS N07725) are precipitation hardened. Both the cold worked and the precipitation hardened alloys exhibit exceptional strength, ductility and toughness.

The U.S. Navy often uses corrosion resistant fasteners with corrosion sensitive materials such as steel, which requires cathodic protection. For example, MONEL alloy K-500 (UNS N05500) fasteners are used with alloy steel in a seawater environment. The steel receives cathodic protection flom sacrificial anodes. The protection is extended to the alloy K-500 fasteners. Failures of the alloy K-500 have occurred due to hydrogen embrittlement problems and also due to corrosion resulting from galvanic interaction with more noble materials. The U.S. Navy currently has a need to replace alloy K-500 fasteners, which can suffer hydrogen embrittlement, with a high strength corrosion resistant alloy. INCOLOY alloy 925, and INCONEL alloys 686 and 725 are highly corrosion resistant nickel-based alloys, which exhibit high strength, toughness and superior corrosion resistance and therefore are excellent candidate materials to replace alloy K-500 as a fastener material for the Navy in various applications.

Alloy 686 is a solid solution nickel-base alloy capable of being cold worked to high yield strengths, such as 90 to 100 ksi (690 MPa). Alloy 686 was originally developed for Flue Gas Desulfurization (FGD) and chemical process applications. Alloys 925 and 725 are age-hardenable nickel-base alloy capable of being aged to the minimum yield strengths of 110 ksi and 120 ksi (758 MPa and 827 MPa), respectively. Alloy 925 and alloy 725 are strengthened by precipitation of gamma prime [Ni3 (Ti, Al)] and gamma double-prime [Ni3 (Nb, Ti, A1)], respectively. Alloys 925 and 725 were developed for Oilfield applications such as tubing hangers, subsurface safety valves, Christmas trees, valve trim, packers, and other down hole equipment for severe sour service.

Alloys 686, 925 and 725 are resistant to hydrogen embrittlement in the NACE International TM0177 t sulfide stress cracking test and are listed in the NACE MR01752 document "Standard Material Requirements- Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment" and to chloride stress corrosion cracking in severe sour brine environments. Sulfide stress cracking is considered in the Oilfield to be the most severe for hydrogen embrittlement. Depending on the alloy, applications include use in chemical and food processing, marine and offshore platform equipment, oilfield wellhead and subsurface equipment and tubular goods, salt plant evaporators, air pollution control systems, condenser tubing, service water piping and feedwater heaters in the power industry.

Unless otherwise specified, duplicate corrosion specimens were tested, and alloys 925 and 725 were tested in the standard solution annealed plus age-hardened conditions listed in Table 3.

Composition and Mechanical Properties
The complete limiting chemical composition is given in Table 1.

Table 2 exhibits the Room Temperature Tensile properties for cold worked alloy 686. The material displays excellent stren