Slow strain rate tests in ASTM seawater using specimens prepared from a failed nickel-copper alloy bolt have shown that precipitation hardened UNS N05500 (MONEL K-500® is embrittled by cathodic protection with sacrificial aluminum anodes. Some loss of ductility also occurred when annealed UNS N05500 was coupled to aluminum anodes and when the hardened alloy was coupled to steel. Brittle fractures produced by slow strain rate tests were intergranular and were very similar in appearance to the field fractures. While the slow strain rate tests were conducted on an alloy, from only one source, there is no reason to assume that UNS N05500 alloy from other sources would resist hydrogen embrittlement from standard cathodic protection systems.
Bolts of alloy UNS N05500, a precipitation hardening nickel-copper alloy commonly referred to as MONEL K-500® failed in a brittle manner on North Sea platforms sometime prior to August of 1984. The failures, which originated at thread roots where the hardness was about Rockwell C 39, occurred in bolts that had been hardened after threading. The failures were attributed to hydrogen embrittlement that was a result of cathodic protection. It was concluded that hydrogen embrittlement of MONEL K-500 would not occur if the hardness was below HRC 35 which is the acceptable hardness of UNS N05500 in sour systems. To insure proper hardness of the thread roots, it was recommended that annealing and hardening treatments be done after the threading.
Since that time, subsea clamp bolts of UNS N05500 alloy with the recommended heat treatment and hardness have failed in a brittle manner on two North Sea platforms. These failures, occurred while the bolts were coupled to steel and cathodically protected with aluminum anodes or a combination of aluminum anodes and impressed current. The bolts had been roll threaded, annealed at 980 to 1050 C, water quenched, and precipitation hardened at 500 to 600 C for 16 hours. This produced hardnesses ofabout HRC 25.
The later failures occurred in bolts that were loaded to 344 Mpa (49,890 psi) during installation. This was 59 percent of the specified minimum yield strength of the material. Following the failures, the broken bolts and other cathodically protected alloy N05500 bolts that were in critical subsea applications were replaced with steel bolts.
The remainder of this paper discusses the more recent failures. Results of slow strain rate tests on hydrogen charged samples from one of the failed bolts are presented along with studies of the fracture surfaces.
Since the more recent failures occurred in bolts that had the recommended heat treatment and were below the critical hardness level, the material was traced through the various heat treating and threading steps to the original alloy supplier to see if the failures were restricted to the alloy from a particular vendor or processor. It was found that both sets of failed bolts were heat treated and threaded by the same shops, but the material was supplied by two different companies.