PREN values, pitting resistance, crevice corrosion thresholds, SCC susceptibility, and environmental compatibility for high-temperature bolting materials.
Corrosion, not mechanical overload, is the most common cause of bolted joint failure in high-temperature service. PREN below ranks the six grades on pitting resistance in chlorides.
PREN = %Cr + 3.3×%Mo + 16×%N is the standard metric for comparing pitting resistance in chloride environments. Values above 32 are considered “seawater-resistant” for stainless steels, and nickel alloys generally require above 40 for the same confidence. Note that PREN does not account for crevice geometry, temperature, or pH effects, it is a ranking tool, not a pass/fail test.
| Grade | Cr% | Mo% | PREN | Rating | Notes |
|---|---|---|---|---|---|
| A-286 (UNS S66286) | 15 | 1.3 | ~17 | Moderate | 15% Cr provides an adequate Cr2O3 passive film in mild environments. Low Mo means PREN below that of 316L, so pitting resistance in chlorides is limited. Acceptable in rural and light industrial atmospheres; not suitable for marine or chemical chloride service without protective coating. |
| Inconel 718 (UNS N07718) | 19 | 3.1 | ~28 | Good | Decent PREN combined with nickel base provides good general corrosion resistance. The 19% Cr gives adequate oxidation resistance to 700degC, but the Ni-Nb matrix is attacked by strong reducing acids. Resistant to chloride SCC due to nickel content above 45%. |
| Nimonic 80A (UNS N07080) | 19.5 | 0 | ~19.5 | Moderate/Low | High chromium but zero molybdenum means the PREN is deceptively low. Nimonic 80A is designed for high-temperature oxidation resistance in air, not aqueous corrosion. It resists scaling well to 815degC but performs poorly in wet chloride or acid environments. |
| Hastelloy C-276 (UNS N10276) | 15.5 | 16 | >63 | Excellent | The highest PREN of any standard nickel alloy. 16% Mo + 4% W makes it effectively immune to pitting and crevice corrosion in chloride environments, including seawater, bleach, chlorine dioxide, and wet chlorine gas. The benchmark for industrial corrosion resistance. |
| Monel K-500 (UNS N05500) | 0 | 0 | N/A | Seawater Specialist | Zero chromium means the PREN formula does not apply. Monel resists corrosion through the nickel-copper matrix itself, which forms a stable protective film in seawater and marine atmospheres. Excellent resistance to flowing seawater, cavitation, and biofouling. Not suitable for oxidising acids or chlorine. |
| Inconel 625 (UNS N06625) | 21.5 | 9 | >48 | Excellent | Very high PREN with the highest chromium of the six grades. Combined with 9% Mo, this provides outstanding resistance to pitting, crevice corrosion, and chloride SCC across a wide temperature range. The best choice when both corrosion resistance and high-temperature strength are required. |
Stress corrosion cracking susceptibility across boiling MgCl2, seawater and sour gas, and the galvanic pairing rules for nickel alloy bolts, are in the SCC and galvanic compatibility data.
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