Both Inconel 693 and Inconel 690 are high-chromium Ni-Cr-Fe alloys with excellent high-temperature oxidation resistance, but they were developed for different enemies. 690 fights chloride stress-corrosion cracking in steam-generator tubing; 693 was developed specifically to resist metal dusting in syngas and reformer service. If your problem is metal dusting, 693 earns its premium.
Metal dusting is a catastrophic form of high-temperature corrosion that breaks down alloys into a powdery mixture of metal particles, oxides and carbon. It happens in carburising atmospheres — typically 400–900 °C — where carbon activity is high and oxygen partial pressure is very low. Steam-reformer outlet piping, syngas coolers, ammonia and methanol loops are the usual suspects.
Once dusting starts, the material loss is not gradual — it can eat through a tube wall in months. Switching grades is almost always cheaper than a forced shutdown.
Both grades are nickel-rich with very high chromium, but the aluminium content is where they diverge most.
| Element | Inconel 690 (N06690) | Inconel 693 (N06693) |
|---|---|---|
| Nickel | ≥ 58 % (balance) | ≥ 58 % (balance) |
| Chromium | 27.0 – 31.0 % | 27.0 – 31.0 % |
| Iron | 7.0 – 11.0 % | 2.5 – 6.0 % |
| Aluminium | ≤ 0.5 % | 2.5 – 4.0 % |
| Titanium | ≤ 0.5 % | ≤ 1.0 % |
| Niobium + Ta | ≤ 0.5 % | 0.5 – 2.5 % |
| Manganese | ≤ 0.5 % | ≤ 1.0 % |
**Why aluminium matters:** 3 % Al promotes a dense, adherent alumina layer that resists carbon ingress. That single addition is what gives 693 its metal-dusting resistance — 690's chrome-only oxide layer is much less effective in carburising atmospheres.
690 was developed for nuclear steam-generator tubing, where its high chromium content resists chloride-induced stress-corrosion cracking in pure water at high temperature. It remains the standard answer for:
Specify 693 when your equipment is in, or downstream of, a carburising atmosphere in the 500–900 °C window. Typical cases:
Independent test data (including work published by INCO / Special Metals and follow-up studies) consistently show 693 outperforms 690 by an order of magnitude in metal-dusting coupon tests at 650 °C in 50 % CO / 50 % H₂.
693 is a mill-order grade (minimum tonnage applies) and is supplied principally as plate, bar and seamless tube. Welding uses ERNiCrFe-7A (Inconel Filler Metal 693-equivalent when available, otherwise ERNiCr-3) with strict interpass temperature control — the aluminium addition means you cannot use the same filler you would for 690.
If your corrosion mechanism is metal dusting, pay the premium for 693. The aluminium addition is doing real work. If your mechanism is chloride SCC in pure water, 690 is still the proven, cost-effective answer.
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