Author: RAS Thermal Applications Team · Updated 2026-10-10
TL;DR PREN ranks pitting resistance; SCC and galvanic pairings decide whether the joint survives. Both tables are here.
Corrosion is the most common cause of bolted joint failure in high-temperature service, not mechanical overload. A bolt that retains 90% of its tensile strength at 600 °C can still fail within weeks if the wrong material is specified for a wet chloride or sour gas environment. The PREN table on the corrosion data page ranks the six grades on pitting resistance; the two sections below cover the mechanisms PREN does not capture.
Stress corrosion cracking (SCC) susceptibility
Chloride SCC is the Achilles’ heel of austenitic stainless steels such as 304 and 316, but it becomes less relevant as nickel content increases. Above approximately 45% Ni, chloride SCC resistance approaches immunity. All six of the superalloy grades we stock are considered resistant or immune, with A-286 the borderline case at its 25% nickel content.
| Grade | Boiling MgCl2 | NaCl / seawater | H2S / sour gas | Notes |
|---|---|---|---|---|
| A-286 | Susceptible | Resistant (moderate) | Susceptible at high hardness | Iron-based; the 25% Ni is borderline for chloride SCC resistance. Passes NACE MR0175 at 35 HRC or below. |
| Inconel 718 | Resistant | Resistant | Resistant at 40 HRC or below | Nickel content above 50% provides chloride SCC immunity. NACE compliant at 40 HRC or below with testing. |
| Nimonic 80A | Resistant | Resistant | Resistant | High nickel, controlled hardness. Good SCC resistance but limited by the absence of molybdenum. |
| Hastelloy C-276 | Immune | Immune | Immune | Solid-solution alloy, low hardness, highest Mo content. The reference material for SCC resistance in chloride service. |
| Monel K-500 | Resistant | Immune | Conditional (35 HRC or below) | The nickel-copper matrix is inherently resistant to chloride SCC. Hydrogen embrittlement risk at high hardness in sour service. |
| Inconel 625 | Immune | Immune | Immune | Solid-solution grade with 9% Mo. Essentially immune to chloride SCC in all practical conditions. NACE compliant. |
Galvanic compatibility
When dissimilar metals are joined, the less noble material corrodes preferentially. In bolted joints the bolt is typically the smaller component and therefore at higher risk of galvanic attack.
Compatible pairings (galvanic potential difference below 0.15 V)
- Nickel alloy bolt with an austenitic stainless steel (304 / 316) flange
- Monel K-500 bolt with copper-nickel piping
- A-286 bolt with carbon steel, with a protective coating in wet environments
- Inconel 625 bolt with duplex stainless steel
Risky pairings (require insulation or protective measures)
- Nickel alloy bolt with an aluminium flange: severe galvanic corrosion of the aluminium is expected
- Monel K-500 bolt with carbon steel in seawater: the steel corrodes sacrificially
- Nickel alloy bolt with a titanium flange: titanium is cathodic to nickel, and the bolt may be attacked in reducing acids
- Any nickel alloy bolt with a graphite gasket in an electrolyte: a severe galvanic couple is possible
For a site-specific corrosion assessment, send the process fluid composition, the pH, the temperature range and the chloride or sulphide concentrations. Our metallurgy team evaluates them against our internal corrosion database and provides a written material recommendation within 48 hours.
How to work through this topic
- 01 Rank the candidate grades on pitting resistance. Compute or look up PREN = %Cr + 3.3 x %Mo + 16 x %N for each candidate. Above 32 is considered seawater-resistant for stainless steels; nickel alloys generally want above 40 for the same confidence. Treat it as a ranking tool, not a pass/fail test.
- 02 Check stress corrosion cracking in the actual environment. Chloride SCC stops being a threat as nickel content rises above roughly 45%. Read the SCC table against your specific environment — boiling MgCl2, NaCl or seawater, and H2S or sour gas behave differently, and hardness limits apply in sour service.
- 03 Check the galvanic couple at the joint. The less noble metal corrodes preferentially, and in a bolted joint the bolt is usually the smaller component and therefore at higher risk. Compare the bolt and flange materials against the pairing rules, and insulate or coat the interfaces that are flagged as risky.
- 04 Confirm the certificate and the acceptance criteria. Name the standard, the heat-treat condition and the certificate grade on the line item, and state the hardness limit where NACE applies. Order trial quantities early, because first-article lots run from 50 pieces.
Frequently Asked Questions
Which superalloy bolt grades resist chloride stress corrosion cracking?
Above roughly 45% nickel, chloride SCC resistance approaches immunity. Inconel 718, Nimonic 80A, Hastelloy C-276, Monel K-500 and Inconel 625 all sit in that band; A-286 is the borderline case at 25% nickel and passes NACE MR0175 only at 35 HRC or below.
Why does PREN not decide the material on its own?
PREN = %Cr + 3.3 x %Mo + 16 x %N ranks pitting resistance in chlorides only. It does not account for crevice geometry, temperature or pH, and it does not apply at all to a chromium-free grade such as Monel K-500, which resists corrosion through its nickel-copper matrix instead.
What happens when a nickel alloy bolt is fitted to an aluminium flange?
Severe galvanic corrosion of the aluminium is expected: the aluminium is the less noble metal and the bolt is the smaller component, so the joint is at risk of attack at the flange face. Insulate the pair or coat the interface.
Is a Monel K-500 bolt safe against a carbon steel flange in seawater?
No, not uncoated. The steel corrodes sacrificially, because Monel sits higher in the galvanic series. Use insulation or a protective coating, and check the pairing rules before the joint is assembled.
Bottom line: Corrosion is the most common cause of bolted joint failure in high-temperature service, not mechanical overload. Rank the grades on PREN, then read SCC and the galvanic couple against the real environment before the material is fixed.