Hastelloy C276, C22 or 625 Plate: Which Nickel Alloy Plate Do You Actually Need?

Hastelloy C276, C22 or 625 Plate sit on the same warehouse rack. They look identical - same 2000 × 6000 mm footprint, same 2B-ish mill finish, same stencilled heat number. If you hand a buyer the mill certificates and ask which one to put in the vessel, most will pick by molybdenum content and move on.

That shortcut is how projects end up with the wrong plate.

Here is a number that stops the conversation. On ASTM G28 Method A - boiling 50% sulfuric acid with ferric sulfate - a typical C276 plate reports 6 to 9 mm/year. A C22 plate from the same family reports 0.6 to 1.5 mm/year. Read that as a scorecard and C276 looks ten times worse.

It is not. G28A is an oxidizing test. It is designed to catch intergranular attack in an oxidizing solution, and C276 is the one grade in this group that was never built for oxidizing conditions. The test is doing exactly what it should. What matters is whether you are reading a material property or a pass/fail screen. Almost every supplier page treats them as the same thing.

Chemistry and mill properties of the three plates

Three things fall out of that table immediately.

The molybdenum spread is more than two to one. C276 carries 15–17%, C22 12.5–14.5%, 625 only 8–10%. Molybdenum is what gives a nickel alloy its resistance to reducing acids - hydrochloric, dilute sulfuric, phosphoric under reducing conditions. A plate with 16% Mo behaves very differently from one with 9% in an HCl-bearing stream, and no amount of surface treatment changes that.

Chromium runs the opposite direction. C22 has the most (20–22.5%), 625 next (20–23%), C276 the least (14.5–16.5%). Chromium builds the passive film that resists oxidizing attack - dissolved oxygen, ferric ions, nitric acid, chlorine, hypochlorite. A low-Cr plate in a strongly oxidizing chloride stream has nothing to rebuild the film with.

625 is a different animal wearing similar clothes. It is 8–10% Mo and 20–23% Cr, which is respectable, but it also carries 3.15–4.15% niobium and is alloyed for high-temperature strength more than for maximum acid resistance. Its minimum tensile of 827 MPa against 690 MPa for the two Hastelloys is the tell - this plate was designed for structural duty at temperature, with corrosion resistance as a strong secondary. Specify it for a 400 °C structural application and it is the right plate. Specify it as a substitute for C276 in a cold HCl stream because "they are both nickel alloys" and you will be replacing it.

Corrosion rate in simulated mixed-gas condensate, 168 h (mm/year, with mpy in brackets)

Read the two rows against each other. In a clean oxidizing acid mixture, C22 leads by an order of magnitude - 0.05 against 0.51 mm/year for C276 - exactly what the chromium difference predicts. Add chlorides and abrasive fly-ash solids and the two Hastelloys converge to 1.07 and 1.27 mm/year, while 625 falls to 3.20.

That is the whole selection logic in two lines: the cleaner and more oxidizing the stream, the more the C22 premium pays; the dirtier and more chloride-loaded it gets, the more the C276 molybdenum content pays, and the further 625 falls behind.

The pitting side is worth one more number set, because it explains why chloride projects so often go to C22 over C276. Critical crevice temperature per ASTM G48 Method D, in 6% FeCl₃ + 1% HCl, is roughly:

A 25 °C gap between C276 and C22 is not an incremental difference. In a stagnant, chloride-bearing crevice at 60–70 °C, one plate is operating above its threshold and one is comfortably inside. Where a plate is used for gasketed joints, lap welds or under-deposit areas, that margin is the deciding factor.

Supply form and specification by grade

Two practical notes on that table.

Thickness availability is not the constraint - mill route is. All three are rolled to comparable gauge ranges, but the supply base narrows as you move up in thickness and down in order quantity. Above about 60 mm, lead times for all three extend and the number of qualified mills drops sharply. For a vessel shell where the design calls for a thin wetted layer over a structural backing, the roll-bonded plate is the form that actually gets bought, and the grade choice is expressed as "C276 clad, 3 mm on SA-516 Gr.70" rather than as a solid plate callout.

Grade 1 and Grade 2 matter for 625 and rarely for the Hastelloys. 625 is supplied as Grade 1 (annealed, 827 MPa min) and Grade 2 (solution annealed + cold worked, 1034 MPa min). Grade 2 buys strength at the cost of ductility and is a structural form, not a corrosion form. If a specification asks for Grade 2 plate in a wet corrosive service, it is worth questioning the reasoning before ordering.

Fabrication behaviour that decides the plate you order

Two plates that perform identically in a coupon test can behave very differently in a fabrication shop, and this is a real part of product selection rather than an afterthought.

C276 needs low heat input. Interpass temperature should stay below about 90 °C, with stringer beads and controlled heat input. The 375–875 °C range is where sigma and mu phases precipitate and pull molybdenum out of the matrix, and a weld that sits there too long will show it as knife-line attack later. Solution annealing at 1121 °C with a rapid water quench is the recovery route if the material has been over-exposed - not a stress relief at 650 °C, which would make it worse.

C22 is the more forgiving plate to weld. The higher chromium and lower molybdenum give it a wider tolerance to heat input variations, which is why it is often chosen on projects where site welding quality is the risk rather than the corrosion environment. ERNiCrMo-10 is the matching filler.

625 tolerates heat best of the three and needs different filler. ERNiCrMo-3, and the niobium in the weld metal stabilises it against sensitisation, which is why 625 is the standard choice for weld overlay on carbon steel where the overlay will see thermal cycling.

FAQ

If C276 has more molybdenum, why does C22 show lower corrosion rates in some tests?

Because those tests run an oxidizing solution. ASTM G28 Method A uses ferric sulfate, which is an oxidant; chromium, not molybdenum, is what resists that. C22 has 20–22.5% Cr against C276's 14.5–16.5%, so it leads in oxidizing media. Put both plates in a reducing acid with chloride and the ranking reverses - G28 Method B shows C276 at 1.4–1.5 mm/year against C22 at roughly 0.2–0.5, and the mixed-gas condensate data shows C276 leading where chlorides and solids are present.

Can I use 625 plate where the specification says C276?

Only if the stream is oxidizing or the duty is structural at temperature. 625 has 8–10% Mo against C276's 15–17%, and in the mixed acid Method B test it dissolves at a rate that makes the test unusable - above 89 mm/year. It is an excellent plate for high-temperature strength, oxidation resistance and weld overlay, and a poor substitute in a reducing chloride stream.

What is the largest plate thickness I can realistically order in these grades?

All three roll to roughly 100 mm in the Hastelloys and 80 mm in 625, but practical availability drops steeply above 60 mm and the qualified mill list shortens. For thick-section work the usual answer is clad plate - a thin alloy layer on carbon steel backing - which is supplied to ASTM B898 and is the form most large vessels are actually built from.

Do I need a corrosion test report with every plate?

Not as routine practice. What a plate order should carry is the heat's chemical analysis and mechanical certificate to the product standard, plus the bond examination report if it is clad plate. Corrosion tests such as G28 are qualification and screen tests on the mill's production, and they are normally supplied on request rather than per heat.

Sending a plate enquiry? Give us the grade, thickness, standard and quantity - or send the service conditions and we will tell you which of the three plates actually fits, including the ones where the cheaper option is the correct one. C276, C22 and 625 plate, clad and solid, in ASTM B575, B443 and B898.

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