Inconel 690 and 693: High-Chromium Nickel Alloys for Nuclear and Extreme High-Temperature Corrosion
Inconel 690 (29–31% Cr, 58% Ni, 9% Fe, C ≤0.05%) was developed specifically to address the PWSCC failures of Alloy 600 in PWR steam generator tubing from the 1970s–1990s. The high Cr dramatically improves resistance to intergranular oxidation in high-temperature water (PWR primary coolant at 300–330°C). The same high Cr also provides superior resistance to oxidising acids and high-temperature oxidation — Inconel 690 is sometimes specified as an alternative to Inconel 600 for chemical process piping in oxidising environments where Alloy 600 is borderline.
Inconel 690 — The Nuclear Grade
Inconel 690 (29–31% Cr, 58% Ni, 9% Fe, C ≤0.05%) was developed specifically to address the PWSCC failures of Alloy 600 in PWR steam generator tubing from the 1970s–1990s. The high Cr dramatically improves resistance to intergranular oxidation in high-temperature water (PWR primary coolant at 300–330°C). The same high Cr also provides superior resistance to oxidising acids and high-temperature oxidation — Inconel 690 is sometimes specified as an alternative to Inconel 600 for chemical process piping in oxidising environments where Alloy 600 is borderline.
Welding filler for Inconel 690 in nuclear service is FM52 (ERNiCrFe-7, 29% Cr) — replacing FM82 (ERNiCrFe-3, 20% Cr) formerly used for Alloy 600. FM52 is susceptible to ductility dip cracking (DDC) in thick-section welds — a known challenge that has driven the development of FM52M and FM52i (modified compositions with improved DDC resistance).
Inconel 693 — Metal Dusting and Carburisation Resistance
Inconel 693 adds 2.5–3.5% Al and 0.5–2.5% Nb to the Inconel 690 base, forming a mixed Al₂O₃/Cr₂O₃ protective scale. This scale is significantly more resistant to carbon ingress (metal dusting, carburisation) than the pure Cr₂O₃ film of standard austenitic alloys. Metal dusting trials at 650°C in CO-H₂ gas have shown Inconel 693 to outperform both Inconel 601 and Alloy 602CA in carbon activity environments — placing it at the top of the commercial alloy hierarchy for reformer piping and syngas cooler internals where metal dusting is a dominant failure mechanism.
Availability and Specification
Inconel 690 buttweld fittings are available to order (ASTM B366 WP690 or equivalent) but are not stocked items — lead times of 16–24 weeks are typical. Inconel 693 fittings are made to order only with limited suppliers globally. Both grades command a significant premium over Inconel 625. For most applications where Inconel 690 or 693 is under consideration, confirm with the design engineer whether Inconel 625 or Alloy 602CA would not serve equally well at lower cost and lead time — the specialty grades are justified only where their specific property advantages (PWSCC resistance for 690; metal-dusting for 693) are the design driver.