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GH5188 (old GH188, Haynes 188, UNS R30188, W.Nr. 2.4683) is a solid-solution cobalt-base alloy — Co bal (~39%), Cr 21–23%, Ni 20–24%, W 13–15%, and the one additive that actually decides its oxidation behavior: lanthanum 0.03–0.12% (typical 0.04–0.08%). It is not age-hardenable. You don't buy it for creep strength at 760°C; you buy it because its scale stays stuck to the metal when the gas temperature swings between 870°C and 1095°C and back, thousands of times.
Haynes International's own brochure puts it plainly: continuous exposure in air or combustion gas is good up to 2000°F (1093°C), and short-time excursions can go higher. That 1095°C number is an oxidation ceiling, not a stress-bearing temperature. At 1095°C the alloy is still structurally alive (Rm ~200–260 MPa short-time), but nobody specs it for 1000 h loaded service there — it's a liner, a flame holder, a transition duct, not a disk.
Why the scale doesn't fall off
Below ~900°C the surface is essentially Cr₂O₃ — dense, slow-growing, parabolic kinetics. The Cr comes from the 22%-ish chromium; the W (14%) just slows the base metal so the scale doesn't crack when the sheet flexes. Ni (~22%) keeps the FCC matrix ductile so the substrate follows thermal expansion instead of tearing the film.
The trick is La. It segregates to the oxide/metal interface, pins Cr₂O₃ grain boundaries, cuts the interfacial voids that normally let a scale lift and spall. Wang et al. (Journal of Chinese Society for Corrosion and Protection, 2024) ran GH5188 with 0 / 0.029 / 0.060 / 0.571% La at 1100°C: parabolic rate constant Kp dropped from 0.0494 mg²·cm⁻⁴·h⁻¹ (no La) to 0.0068 (0.029% La) and 0.0089 (0.060% La), then climbed back to 0.0155 at 0.571% La. Optimum La window: 0.029–0.060%. Too much La and you grow a brittle La-rich phase and the scale adherence gets worse again — which is why the spec caps it at 0.12%, not "the more the better".
Above ~1000°C, Cr₂O₃ starts volatilizing as CrO₃. GH5188 still holds because La delays breakaway and a little MnCr₂O₄ spins up at the scale outer layer to patch it. But push past 1095°C continuous and you're betting on time, not on the alloy.
Hard numbers people actually quote
From AMS 5608 / mill data and cyclic rig reports:
980°C, 1000 h static air: weight gain +0.2–0.5 mg/cm², no spall.
1095°C (2000°F), 100 h continuous air: +0.8–1.5 mg/cm², scale intact, edges sharp.
Room-temp ⇄ 980°C thermal cycle, 100 cycles (30 min hold): only minor edge spall, metal loss typically < 0.1 mm; L-605 (Haynes 25, no La) loses scale in roughly half the cycles under the same swing.
1093°C cyclic (ASTM G54-type): spallation mass an order of magnitude lower than non-La Co-Cr-W alloys.
1100°C isothermal, 100 h (research): Cr₂O₃ + MnCr₂O₄, parabolic; with 0.06% La the Kp is ~7× lower than base alloy.
Those are oxidation numbers. They do not mean "GH5188 is good for 1095°C loaded rotor service" — they mean the skin survives, the sheet doesn't thin out, and you can pull it off the engine and still see metal.
Oxidation limit vs. stress-bearing limit (don't mix them)
Oxidation, continuous: ~1095°C (2000°F) — Haynes ceiling.
Oxidation, short-time / intermittent: 1100–1150°C possible, life drops fast.
Low-stress static sheet (liner, shroud): 870–1095°C is normal work.
Loaded service, meaningful creep: ≤ 870–900°C is the honest redline; 980°C only for low stress and hundreds-of-hours life (982°C/100 MPa rupture ~100–200 h typical, not 10⁴ h).
> 1095°C metal temperature: GH5188 exits; go TBC + single crystal, or accept Co-base is no longer the right answer.
The confusion in most specs is that someone reads "resistant to 2000°F" and writes "GH5188, 2000°F turbine disk". It isn't. The 2000°F line is about the skin not burning away.
What kills the oxidation resistance in practice
La out of range: mill cert shows La <0.03% or >0.12% → not Haynes 188, don't treat it as such.
Over-forging / long anneal above 1200°C: La can segregate, carbides coarsen, but the bigger risk is grain growth making the sheet brittle — scale still forms, but the part cracks underneath it.
Sulfur / salt environment: GH5188 handles mild sulfidation better than Ni-base Hastelloy X, but molten chloride or heavy Na₂SO₄ deposit at 900°C+ still attacks Cr₂O₃. It's good, not magic.
Weld without matching filler (ERNiCoCrMo-1 / AMS 5796): dilution drops Cr/La at the seam, local spall starts there first.
GH5188's temperature story is simple: Cr makes the Cr₂O₃, W holds the metal together, La keeps the Cr₂O₃ stuck on through thermal cycling, and 1095°C is where the oxidation warranty stops. Past that, you're on borrowed time; below 870°C with load, it's a comfortable long-termer; between 870°C and 1095°C it lives its real life — thin-sheet hot-gas parts that heat and cool every flight cycle and are not allowed to drop their skin.
