The performance of Alloy K-500 has been revisited considering its extensive oilfield experience across short and long-term applications and a recently completed test program. Specifically, three heats of Alloy K-500 were sulfide-stress cracking tested between 70°F (21°C) and 275°F (135°C) in two sour and coexisting phases, a 120,000ppm chloride brine and a dense phase with 400psi (2.76 MPa) H2S and 1600psi (11.03 MPa) CO2; complementarily, the same heats were evaluated for hydrogen embrittlement in NACE TM0177 Solution A with and without −1050mV vs. Ag/AgCl potential. In all corrosion tests under tensile loads, Alloy K-500 was only stressed 30-to-80% of its minimum specified yield strength (SMYS) in an effort to validate historical tool practices, with all new results confirming that Alloy K-500 is environmentally-assisted cracking resistant when used per recommended guideline. Additionally, the alloy was characterized for weight-loss and pitting corrosion, including in three heavy brines with pH between 2.8 and 12.2. This testing reveals that Alloy K-500 does not suffer from elevated weight-loss, pitting, or crevice corrosion in the absence of sour gas, and thus may be safely operated up to the investigated temperatures. However, when exposed to a sour environment, Alloy K-500 can experience a rapid sulfide scale conversion with a characteristic tarnishing, overall limiting the alloy useful time in oilfield applications. Despite often being out of favor, Alloy K-500 still has its place as an oilfield alloy, particularly among niche and specialized applications in drilling, wireline, artificial lift, among others.
Alloy K-500 (UNS N05500) is concomitantly a centurial material and the very first precipitation-strengthened nickel-based alloy, then developed in the 1920s by the newly-formed International Nickel Company, or Inco. 1 Derived from Monel 400 (UNS N04400) that was invented in 1901, Alloy K-500 shares many of the same corrosion and tribological characteristics. 1 Being a pioneer alloy with so-called "stain-less" characteristics, Alloy K-500 also established itself as the first high-strength oilfield nickel alloy, having survived sour service conditions exceeding the capabilities of the low-alloy steels of the time. From early naval propeller shaft applications to general cross-industrial uses, Alloy K-500 has always been considered a corrosion-resistant alloy, or CRA. For instance, it has been included in the NACE MR1075 document right from the first 1975 edition. 2-3 With the emergence in the 1980s of NiCrMo alloys such as Alloy 925 (UNS N09925), and the adoption of Alloy 718 (UNS N07718) for deep and sour fields, some shortcomings of Alloy K-500 both downhole and subsea made Alloy K-500 lose some of its original appeal. 4,19 Today, Alloy K-500 is either banned or heavily restricted for permanent completions, wellheads, Christmas trees, and general subsea bolting. 5-8 When not exposed to in-service tensile loads, including burst pressures, Alloy K-500 is still permissible in completions, including in gas-lift valves and pressure regulating accessories. 3 Since beginning, Alloy K-500 has remained popular in electric submersible pumps (ESP), particularly for pump shafts, pump head and base fasteners, as well as for wireline tool threaded adapters where Alloy K-500 has been proven fit-for-service in ultra-sour environments despite otherwise recommendations. 3 In the current NACE MR0175/ISO 15156 standard and guideline, the operational limit (partial pressure) of Alloy K-500 for general use is only set to 0.5 psi (3.45 kPa) H2S, but the alloy remains unrestricted in otherwise disclosed applications, including permitted exclusions such as wireline logging equipment. 3 For the latter applications, the alloy is generally fit-for-service because associated to components and/or applications with generous safety factors or design margins. Figure 1 shows an example of a simple mechanical component that had been successfully deployed in over 1600 psi H2S (11.03 MPa) with both carefully bound tensile loads and short environmental exposures, resulting in the alloy tarnishing but not cracking. 9 The literature referring to the field failures of Alloy K-500 often invokes hydrogen embrittlement, implicitly hydrogen-induced cracking and galvanically-induced cracking under cathodic protection, thus types of failures predominantly active at ambient and low temperatures. 4 Sulfide-stress cracking (SSC), exacerbated by dubious alloy microstructures, has also been a common root-cause of failures, as exemplified by Figure 2. 9 Figure 2 depicts a fractured bolt, conclusively explained by a substandard-quality control in the procurement of Alloy K-500 fasteners (i.e., poor microstructures and non-compliances to the NACE MR0175/ISO15156 document). Grain-boundary precipitates along with a hardness consistently in excess of 35 HRC were often characteristics of the encountered Monel K-500 failures. 4,9
