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This paper will discuss some of the reasons why some Owner/Operators are choosing to invoke NACE MR0175-20021 instead of NACE MR0175/ISO 15156. Examples of exceptions taken will be examined. And cost issues involved in meeting MR0175/ISO 15156 will be presented.
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Recently, there have been several cracking failures of type HL carbon steel sucker rods with evidence of fatigue striation. In order to find out the reason for the fracture failure, the fracture morphology, material properties, loading and corrosion product of the failed type HL 35CrMoA sucker rod were analyzed, combined with the corrosion environment. The results show that the material properties of the failed 35CrMoA sucker rod meet the GB/T 26075-2010 standard requirements. Also the result of loading analysis shows that the loading is far from the fatigue limit stress. The type HL sucker rod’s yield strength is almost 1049 MPa and hardness is 35.2 HRC. In addition, the low in-situ pH and high H2S-CO2 partial pressure places the sucker rods in the Region 3 of sulfide stress corrosion cracking, which means the quenched and tempered sucker rod’s yield strength should not exceed 863 MPa and hardness should not exceed 30 HRC, according to the ISO 15156-2:2015 standard. It is concluded that the high hardness level of the type HL sucker rod led to sulfide stress corrosion cracking in the high H2S-CO2 partial pressure and low in-situ pH condition.
The use of Duplex Stainless Steels (DSS) in refinery sour environments is governed by ANSI/NACE MR0103/ISO 17945NACE “Metallic materials resistant to sulfide stress cracking in corrosive petroleum refining environments” which limits DSS base materials to be used in Hydrogen Sulfide (H2S) services to a maximum hardness of 28 HRC for materials with a PREN ≤ 40 and to a maximum hardness of 32 HRC for those materials with PREN > 40.1 These hardness values are in line with the hardness requirements of solution annealed as produced straight tubes, but when the heat exchanger design requires the use of integral finning or u-bend tubes, these are subject to significant work hardening that results in as bent and as finned heat exchanger tubes with hardness measurement as high as 418 HV0.5 or 35.6 HRC which clearly exceeds the allowable limits stated above.
Materials qualification testing of corrosion resistant alloys (CRAs) typically involves the use of simple pass/fail tests. Modification of existing standards is recommended for environments in which pit initiation is statistically improbable but pit propagation is rapid, e.g. low chloride/high H2S.