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Revisiting Alloy K-500 with Improved Performance Limits for Short and Long-Term Subterranean Applications

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. 
Derived from Monel 400 (UNS N04400) that was invented in 1901, Alloy K-500 shares many of the same corrosion and tribological characteristics.  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. 

Product Number: 51323-18787-SG
Author: Manuel Marya
Publication Date: 2023
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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. 

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. 

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