The corrosion environment drives every material call
In oil and gas, corrosion is not one mechanism — it is at least five: sweet corrosion from dissolved CO2, sour corrosion from H2S (sulfide stress cracking), chloride-induced pitting and stress corrosion cracking, galvanic corrosion between dissimilar metals, and erosion-corrosion from produced sand or high-velocity flow. The material for a given part depends entirely on which of these environments it sees, at what temperature and partial pressure, and for what design life.
The most common material mistake is specifying 316 stainless for any corrosive application, then being surprised when it pits in chloride service above 60 C. Type 316 starts pitting in seawater at roughly 200-300 ppm chlorides once temperature climbs past ambient. For subsea or hot brine service you need duplex, super duplex, or nickel alloys.
Material selection by service condition
| Service | Common materials | Notes |
|---|---|---|
| Sweet (CO2, no H2S) | 4130/4140 alloy steel (HT), 410 stainless | Standard wellhead/tree components, inhibit as needed |
| Sour (H2S per NACE MR0175) | Inconel 718, Alloy 825, duplex 2205 | Must meet NACE MR0175/ISO 15156 hardness limits |
| Subsea / seawater (chloride) | Duplex 2205, super duplex 2507, 6Mo, 625 | Chloride pitting resistance is the limiting factor |
| High temperature (>200 C) | Inconel 718/625, Waspaloy, Ti Gr 5 | Retain strength and corrosion resistance at temperature |
| Topside non-wetted | 316 stainless, painted/galvanized carbon steel | Lower cost, not for direct seawater or sour exposure |
Machining duplex and super duplex stainless
Duplex 2205 is the workhorse of modern subsea and sour-service equipment. It is not difficult to machine if you follow the rules. It work-hardens faster than 316 but not as aggressively as Inconel. Use sharp inserts with positive rake geometry, keep feed rates above 0.1 mm/rev to avoid rubbing (which work-hardens the surface and destroys tool life on the next pass), and direct high-pressure coolant at the cutting edge. Do not dwell in the cut.
The catch for sour service is the NACE hardness limit. Duplex stainless cannot exceed 28 HRC (approximately 290 HV) after machining and any heat treatment. An aggressive cut with a dull tool can work-harden the surface above the limit. Control cutting parameters, use a sharp insert for the finish pass at 0.2 mm depth, and verify surface hardness on critical parts. A sharp tool and consistent feed produce a controlled surface without a hardened layer.
- Duplex 2205: 120-160 m/min with carbide inserts, positive rake geometry
- Super duplex 2507: reduce to 80-120 m/min, more rigid setup required
- Inconel 625/718: 30-50 m/min with high-temperature carbide or ceramic for roughing
- Always: sharp inserts, high-pressure coolant, no dwell, controlled finish-pass depth
- Post-machining: verify surface hardness on critical surfaces for NACE compliance
Sealing surfaces and pressure boundaries
Ring-type joint (RTJ) grooves on oil and gas flanges require Ra 1.6 or better with no tool marks crossing the groove profile. In corrosion-resistant alloys, a scratched or chatter-marked groove is not just a leak risk at hydrotest — it is a potential crevice-corrosion initiation point. Produced fluids trapped in surface defects corrode under the ring gasket invisibly until failure.
I machine RTJ grooves in duplex and nickel alloys with form tools ground to the exact groove angle (23 degrees for BX rings per API 6BX). After machining, all sealing surfaces get a solvent clean, 10x visual inspection, and liquid penetrant inspection (LPI/PT) as required by the specification.
NACE MR0175 / ISO 15156 compliance
If a part sees sour service (H2S partial pressure above 0.3 kPa), it must comply with NACE MR0175 / ISO 15156. This is not just a material certificate issue — it limits hardness, welding procedures, cold work, and post-weld heat treatment. Material must be on the NACE list for the intended service; hardness must not exceed 22 HRC for carbon steels, 28 HRC for duplex, or 35-40 HRC for precipitation-hardened nickels depending on grade. Cold work from machining must not push surface hardness over the limit.
Pressure testing and documentation
Oil and gas components get documented thoroughly: EN 10204 3.1 material certificate minimum (3.2 if required by the customer), dimensional inspection report, hardness test results, NDE reports (PT/MT/UT), and hydrostatic pressure test certificates. We hold +/-0.005 mm on critical dimensions with Zeiss CMM inspection, provide FAI reports, and pressure test at 1.5x design pressure per the applicable code.
Frequently Asked Questions
Can I use 316 stainless for subsea components?
316 stainless is generally not recommended for permanent subsea service in seawater. Chloride levels in seawater (approximately 20,000 ppm) cause pitting and crevice corrosion in 316 above 15-20 C. Duplex 2205, super duplex 2507, or 6Mo are standard for subsea wetted components.
What is NACE MR0175?
NACE MR0175 / ISO 15156 is the standard for metallic materials in oil and gas production environments containing H2S (sour service). It specifies acceptable materials, hardness limits, and heat treatment conditions to prevent sulfide stress cracking.
What is the maximum hardness for duplex in sour service?
Duplex 2205 finished surfaces must not exceed 28 HRC per NACE MR0175. This requires sharp tools, controlled feeds, and avoiding aggressive cuts or dwell that work-harden the surface.
What tolerance do RTJ grooves need?
RTJ grooves per API 6BX / ASME B16.5 require Ra 1.6 surface finish or better, groove angle within +/-0.5 degrees, and groove diameter within +/-0.1 mm depending on size and pressure class.
What documentation comes with an oil and gas machined part?
Typically an EN 10204 3.1 material certificate, CMM dimensional report, hardness test results, NDE reports (PT/MT as specified), hydrotest certificate if pressure tested, and FAI report for first articles.