- Published on Tuesday, 02 October 2012 08:32
- Written by Don Bush
Q: Are the nickel-copper alloys (such as UNS N04400) acceptable for use in accordance with the NACE sour service standards?
A: This question has been asked frequently, usually in relation to a valve for an upstream hydrocarbon application involving sour water that also contains chlorides. Nickel-copper alloys can be an economical option for such applications. However, changes in the NACE standards within the last decade have complicated the issue.
The original version of NACE MR0175 released in 1975 allowed N04400 material with a 35 HRC maximum hardness requirement. In 1982, N04405, the higher-sulfur, free-machining version of N04400, was balloted, approved and added by addendum. M35-1 and M35-2, two cast “equivalents” of N04400, were balloted and added to a 1991 revision.
In addition to weak grades, the precipitation-hardenable N05500 was also included in the original 1975 version, with a maximum hardness of 35 HRC provided the material was hot-worked and age-hardened, solution-annealed, or solution-annealed and age-hardened.
When the MR0175 rewrite was under production as a precursor to ISO 15156, the weak grades (N04400, N04405, M35-1 and M35-2) were omitted for general usage and relegated to specific oil and gas field applications (i.e., they were no longer allowed for valve components). Many end users felt this alloy family was not needed for general sour upstream petroleum applications.
In addition, because of some field failures in N05500 fasteners, an effort was made to eliminate this material as well. In the end, N05500 was retained in the new standard, but was relegated to use in:
- Wellhead and Christmas tree components other than bodies or bonnets, with specific pH and hydrogen sulfide partial pressure restrictions
- Non-pressure-containing internal components in valves, pressure regulators or level controllers, and
- The same specific oil and gas field applications as soft alloys.
Because NACE defines valve shafts and stems as pressure-containing components, this material could no longer be used for shafts and stems when compliance with MR0175 was required.
Shortly after NACE MR0175-2003 was released, it was converted to ISO 15156, which NACE then adopted back as NACE MR0175/ISO 15156. No changes were made regarding the nickel-copper materials.
In the meantime, NACE MR0103, the downstream (oil refining) sour service standard, was released (in 2003), and included N04400, N04405, M35-1, M35-2 and M30C (another N04400 cast equivalent), as well as N05500 with the same metallurgical requirements that were specified in the pre-2003 versions of NACE MR0175. No restrictions regarding the types of components that could be made from the N05500 grade were made.
In 2005, ISO 15156-3:2003 Technical Corrigendum 2 (essentially an addendum to NACE MR0175/ISO 15156) was released, allowing use of N04400 and N04405 for general purposes. However, the cast grades were not included in this change.
When NACE MR0175/ISO 15156 Second Edition was published in 2009, N04400 and N04405 were inserted into the main document along with the other solid solution nickel alloys.
A summary of the acceptability of these various nickel-copper materials is shown in Table 1.
The ramification of this situation is that it is not possible to produce many valve designs using an all nickel-copper construction in accordance with the current version of NACE MR0175. If the body style requires a casting (such as a globe valve), the cast alloys are not permitted. In addition, N04400 and N04405 are rarely strong enough to use for shafts or stems, and N05500 is not allowed for those components.
Therefore, unless the valve body can be produced from a wrought material form such as a plate or forging, and the shaft can be made from the lower-strength N04400 or N04405, some other, non-nickel-copper material will need to be used for the body and the stem or shaft.