Hastelloy C276 vs Stainless Steel: the Measured Gap

Hastelloy C276 is more corrosion resistant than stainless steel, and a page that says so without naming the stainless steel and without naming the test has not said anything a fabricator can use. Stainless steel is a family, and the distance between its weak end and its strong end is larger than the distance between its strong end and this alloy.

The comparison is worth making properly, because the alloy’s own developer has published it. The measurements below come from three standard tests, run with 316L and 254SMO as the representative austenitic stainless steels and with alloy 625 as a second nickel comparator. Every one of them puts a number on a gap that is otherwise described with the word superior.

C276 is UNS N10276, W.Nr. 2.4819, a nickel-chromium-molybdenum alloy with a deliberate tungsten addition. The published description of its purpose is worth quoting because it explains the composition: it was the first wrought nickel-chromium-molybdenum material to reduce the concern over welding, by virtue of extremely low carbon and silicon contents. That is the design idea of the grade, and it is the reason the carbon limit is 0.01 % and the silicon limit is 0.08 %.

Two alloys that behave completely differently both get called stainless steel, so the first useful step is to separate them. The usual screening number is the pitting resistance equivalent, built from the chromium, molybdenum and nitrogen contents. The common form of the formula counts those three elements, and variants add half the tungsten content as well, which matters here because this alloy has several per cent of tungsten and the simple form therefore understates it.

The two comparators used below are therefore chosen from opposite ends of the family. 316L is the general purpose austenitic grade and the baseline against which the other numbers are read, with a screening value near the low twenties; it resists pitting in fresh water and fails in chlorides once the temperature or the chloride level rises. 254SMO is a super austenitic stainless and the strong end of the family, with a screening value in the forties, and published guidance treats a value around 32 as the minimum likely to protect against pitting in seawater. Alloy 625 is carried through as a second nickel comparator, because it is the grade a buyer most often considers alongside this one.

Against those three, this alloy is not the strongest by a small margin. The screening number is the least useful figure on this page, and the measured critical temperatures in the next section are the ones to read.

The standard way to rank resistance to chloride pitting and crevice attack is to measure the temperature at which each begins in an acidified ferric chloride solution, held for 72 hours, to ASTM G48. The result is a critical temperature for each alloy, and it can be read as the highest temperature the material tolerates in that solution.

Read the two columns as different kinds of statement. On pitting, the gap between 316L at 15 °C and this alloy at 150 °C is a change of regime and not an improvement. It is why a 316L heat exchanger that pits in warm chloride service does not pit when the same duty moves to this alloy. On crevice attack the gap is narrower but still decisive: 316L has a critical crevice temperature of 0 °C, so it is outside its own limit for any real seawater or process duty, while this alloy tolerates 55 °C.

Two harsher test solutions are used to rank the same property, and the published results are worth recording because they describe conditions a process plant actually produces. In the solution known as Green Death, which is 11.5 % sulfuric acid with 1.2 % hydrochloric acid and additions of ferric and cupric chloride, the lowest temperature at which pitting has been observed in this alloy is its boiling point. In Yellow Death, which is 4 % sodium chloride with ferric sulfate and hydrochloric acid, the alloy showed no pitting at the maximum test temperature of 150 °C, and its critical crevice temperature in that solution is 60 °C.

Chloride stress corrosion cracking is the failure mode that decides against austenitic stainless in hot chloride service, and it is the other half of the case for a nickel alloy. The standard test is boiling 45 % magnesium chloride with stressed U-bend samples, to ASTM G36.

The tests were stopped at 1,008 hours, which is six weeks, so the nickel alloys are recorded as having survived a test that the two stainless grades failed inside a day. Both nickel alloys reach the same result on this test, which is worth knowing when the choice is being made on price: on chloride cracking alone, alloy 625 is as good, and the difference between the two sits in the acid and oxidising duties and not here.

Seawater is the most common chloride environment, and crevice corrosion in it is the trap for a material that passes a coupon test in a beaker. A United States Navy study tested two samples of each alloy in still and in flowing seawater at 29 °C for 180 days, each sample carrying two potential crevice sites.

That table is the one to keep when a design has a dead leg, a gasket face, or a support that traps water. Every other alloy in the row above attacks in flowing seawater as well as in still water; this one attacks only in the still condition, and its worst measured depth in 180 days is about a tenth of a millimetre. The result is a design permission as much as a material property: where the water moves, the alloy is not pitting.

The published comparison of corrosion rates against concentration and temperature is where the difference stops being a matter of degree. In hydrochloric acid the two materials are not on the same curve at all above a low concentration.

Two figures put the acid behaviour in its proper place. In 10 % hydrochloric acid at 100 °F the published corrosion rate for the wrought alloy is 0.17 mm per year. In 30 % sulfuric acid at the same temperature it is 0.01 mm per year. Those are rates a plant can design around.

One further number belongs on this page because it is the one most often missed when a material is chosen on base metal data. The same published programme tested all-weld-metal samples taken from a welded assembly. In 30 % sulfuric acid at 150 °F the wrought base metal corroded at 0.03 mm per year while the weld metal corroded at 1.2 mm per year, and in 10 % hydrochloric acid at 100 °F the figures were 0.17 and 0.22 mm per year. A welded vessel is therefore governed by its welds, and the allowance should be taken from the weld figures instead of from the plate figures. The published assessment attributes the difference to the inhomogeneous nature of a weld microstructure, which applies to any welded alloy and is quantified here.

The same published acid data defines the boundary, and the boundary is real. Nitric acid is the clearest case, because the failure is concentration and temperature dependent instead of gradual.

The nitric acid row is the one that separates this alloy from a general reputation for resisting everything. It is not a universal material. It is a material whose published acid data has a shape: broad in reducing acids and in chlorides, and narrow once the medium turns strongly oxidising and hot.

Each of the four numbers that decide the behaviour comes from a deliberate choice in the chemistry, and the two smallest limits on the specification are the ones that made the grade weldable.

The carbon and silicon limits are the pair worth checking on a mill certificate, because a heat that meets the corrosion elements and misses those two will pass a composition review and then fail in a weld. The published history of the grade is that this pair of limits is the whole reason it displaced its predecessors in chemical process service.

Yes, and the gap is measurable and not marginal. In acidified ferric chloride to ASTM G48 the critical pitting temperature is 150 °C against 15 °C for 316L and 60 °C for 254SMO, and the critical crevice temperature is 55 °C against 0 °C and 30 °C. In boiling 45 % magnesium chloride the stainless grades cracked in 2 and 24 hours while this alloy and alloy 625 survived the full 1,008 hour test. In flowing seawater over 180 days it recorded no crevice attack where every other alloy tested attacked at both sites.

The one that has failed, or the strongest one that might still work. Compare it against 316L if you are considering a general purpose grade, and against 254SMO or a super duplex if you are at the top of the stainless family. Comparing it against 316L alone overstates the case, since 254SMO is a genuinely strong material and is cheaper than this alloy.

In strongly oxidising and hot media, and the published nitric acid data shows the shape of it. At 10 % concentration and 100 °F the corrosion rate is 0.01 mm per year. At 40 % and 200 °F it is 0.88 mm per year. At 65 to 70 % and boiling it runs between 18 and 22 mm per year, which is a destroyed vessel. Concentrated boiling sulfuric acid has a similar boundary. A strongly oxidising duty is where a chromium bearing or high chromium grade belongs.

The published figure is resistance to oxidising atmospheres up to 1900 °F, which is 1038 °C. That is a real limit and it applies to furnace work, to flue gas equipment and to any component sitting in hot air. It is also a different limit from the strength limit, which is higher, and the two are often confused when a high temperature duty is being specified.

Because carbon at the grain boundaries is what degrades corrosion resistance after welding. At 0.01 % maximum, together with a silicon limit of 0.08 % maximum, the alloy resists the formation of grain boundary precipitates in the heat affected zone, and the published history of the grade is that this is what allowed it to be welded and then used in chemical process service without losing the corrosion resistance around the weld. Those two limits are the reason the grade was adopted.

Yes, and the published comparison quantifies it. In 30 % sulfuric acid at 150 °F the base metal corroded at 0.03 mm per year while the all-weld-metal sample corroded at 1.2 mm per year. In 10 % hydrochloric acid at 100 °F the figures were 0.17 and 0.22 mm per year. The reason given is the inhomogeneous nature of a weld microstructure, which is true of any welded alloy. The practical consequence is that a corrosion allowance on a welded vessel is calculated from the weld figures.

Hastelloy C276 is a nickel-chromium-molybdenum alloy with a tungsten addition, UNS N10276 and W.Nr. 2.4819, and its advantage over stainless steel is a measured set of limits and not a general superiority. The critical pitting temperature in acidified ferric chloride is 150 °C against 15 °C for 316L and 60 °C for 254SMO. The critical crevice temperature is 55 °C against 0 °C and 30 °C. The stainless grades crack in boiling magnesium chloride within a day while this alloy survives six weeks. In flowing seawater it is the only alloy of the four compared that showed no crevice attack at all. Against that, the alloy has a real boundary: concentrated boiling nitric acid corrodes it at rates between 18 and 22 mm per year, the useful limit in oxidising atmospheres is 1900 °F, its weld metal corrodes faster than its plate, and it costs more than any of the stainless grades it is being compared with.

The alloy is supplied from Changzhou, Jiangsu in plate, sheet, strip, bar, wire, pipe, tube and covered electrodes, to the standards covering each form, from a plant that holds ISO 9001 certification and is audited by SGS. Send the medium, its concentration and temperature, and whether the vessel will be welded, and whether the answer is this grade or a super austenitic stainless follows from those facts.

1. Haynes International data for HASTELLOY C-276 alloy, UNS N10276. Source for the nominal composition, the description of the grade as the first wrought nickel-chromium-molybdenum material to alleviate concerns over welding by virtue of extremely low carbon and silicon contents, the statement that it is ductile and easy to form and weld, the resistance to both oxidising and non-oxidising acids, the outstanding resistance to pitting and crevice attack in chlorides and other halides, and the resistance to sulfide stress cracking and stress corrosion cracking in sour oilfield environments.
2. The same source, critical pitting and crevice temperatures. Source for the ASTM G48 acidified 6 % ferric chloride values for 316L, 254SMO, alloy 625 and C-276 at 72 hours, and for the Green Death and Yellow Death results including the boiling point figure and the 150 °C and 60 °C values.
3. The same source, stress corrosion cracking. Source for the ASTM G36 boiling 45 % magnesium chloride times to cracking for 316L and 254SMO, and the no-cracking-in-1,008-hours results for alloy 625 and C-276, with the note that the tests were stopped after six weeks.
4. The same source, seawater crevice corrosion, reproducing a United States Navy study carried out at the LaQue Laboratories and published by D. M. Aylor and others as Paper No. 329 at CORROSION 99, NACE International, 1999. Source for the still and flowing seawater results at 29 °C over 180 days with two crevice sites per sample.
5. The same source, iso-corrosion diagrams and selected corrosion data. Source for the hydrochloric acid comparison against 316L, 254SMO and alloy 625 with the statement about the quantum improvement above about 5 %, the 20 % azeotrope note, the sulfuric acid comparison, and the corrosion rates in hydrochloric and sulfuric acid quoted in the body and the nitric acid table.
6. The same source, corrosion resistance of welds. Source for the all-weld-metal against wrought base metal rates in sulfuric and hydrochloric acid, and the note on the inhomogeneous nature of weld microstructures.
7. The same source, physical and mechanical properties. Source for the density of 8.89 g/cm³, the melting range of 1323 to 1371 °C, the electrical resistivity, the thermal conductivity, the mean coefficient of thermal expansion, the magnetic permeability of 1.0002, the specific heat, the dynamic modulus, the Poisson ratio of 0.31, and the Charpy V-notch impact strengths at room temperature and at −320 °F.
8. Published technical data for alloy C-276 as circulated by reproducers of the producer data. Source for the statement that the alloy has excellent resistance to pitting, stress corrosion cracking and oxidising atmospheres up to 1900 °F, which is 1038 °C.
9. Published guidance on the pitting resistance equivalent number, including the form of the formula counting chromium, molybdenum and nitrogen and the value of about 32 given as the minimum likely to protect against pitting in seawater. The screening number is used in the body only as a screening device, with its limitation stated.
10. Delivery records held for DLX Alloy covering Hastelloy C276 in plate, sheet, strip, bar, wire, pipe, tube and covered electrodes. Source for the forms supplied.

Every value above is a laboratory measurement on the alloy and condition named, and a laboratory coupon is not a plant. The published source notes that its acid tests were run in reagent grade chemicals and that a field trial is recommended before a duty is committed to, which is the right standard of caution: a service with trace impurities, a stagnant pocket or a temperature excursion can move a material out of the range the tables describe. The corrosion figures here are a basis for narrowing a shortlist, and not a replacement for an assessment of the specific medium by a corrosion engineer.