C276 or G3: picking a Hastelloy for wet chlorine versus mixed acid

Both handle aggressive chemistry, but they fail in different places. C276 is the chlorine and hypochlorite alloy; G3 is the sulphuric-phosphoric workhorse with better thermal stability and a lower alloy cost.

The two Hastelloys most often quoted against each other are C276 and G3, and the confusion is understandable: both are nickel-chromium-molybdenum alloys with low carbon, both are used in chemical plant, and both appear on the same approval lists. The difference is where the molybdenum and the iron sit.

Composition drives the duty

	C276 (N10276)	G3 (N06985)
Nickel	balance	36.9 – 45.4%
Chromium	14.5 – 16.5%	21.0 – 23.5%
Molybdenum	15.0 – 17.0%	6.0 – 8.0%
Tungsten	3.0 – 4.5%	≤ 1.50%
Iron	4.0 – 7.0%	18.0 – 21.0%
Copper	—	1.50 – 2.50%
Carbon	≤ 0.010%	≤ 0.015%

C276 is built around molybdenum and tungsten: roughly 20% of the alloy is there to resist chloride pitting and to survive oxidising and reducing media in the same vessel. G3 trades that molybdenum for chromium, iron and a little copper — which is exactly the combination that keeps sulphuric and phosphoric acid service clean, and incidentally makes the alloy considerably cheaper per kilogram.

Choose C276 when the medium contains chloride

• Wet chlorine, hypochlorite, chlorine dioxide bleach lines.
• Seawater-cooled heat exchangers where 316L and duplex have already pitted.
• Contaminated sulphuric acid — the mixed-acid situations where the oxidiser content varies day to day.
• Flue gas desulphurisation ducting, stack liners and dampers.

One fabrication note we repeat on every C276 order: after heavy cold work, solution annealing is not optional. Cold forming leaves the alloy susceptible to stress-corrosion cracking in exactly the environments it was bought for.

Choose G3 when the acid is the process

• Phosphoric acid production, including the wet-process route with chloride and fluoride impurities.
• Sulphuric acid pickling lines, where the alloy also has to survive the oxidising stage.
• Equipment that cycles between hot acid and ambient cleaning — G3's thermal stability is better in that duty.
• Projects where the C276 price premium cannot be justified by the actual chemistry.

The question to ask before ordering

Ask for the operating window, not just the medium name. "30% sulphuric acid at 80 °C, with 200 ppm chloride and occasional nitric addition" points at G3. "Sodium hypochlorite at 60 °C with salt carry-over" points at C276. A quote built on the medium name alone is a guess, and we would rather ask twice than ship the wrong plate.

Values quoted here follow the ASTM / EN edition named in each paragraph. For a specific heat, the governing numbers are those printed on the EN 10204 certificate that ships with the material. Written by the technical desk at Shanghai Hangbo Alloy Group.