What Is the Difference Between Hastelloy C-22 and C276?

The two alloys are the same family, the same base chemistry and the same intended service, and the published data settles the choice on two questions and not on a general preference. The first is whether the medium is oxidising or reducing, and the second is whether the part will be welded. Everything else about the pair is either a small difference or a misunderstanding that circulates widely.

Both are nickel-chromium-molybdenum alloys with a tungsten addition. C-22 is UNS N06022 and W.Nr. 2.4602. C-276 is UNS N10276 and W.Nr. 2.4819. The composition difference is a trade between two elements, and it is the whole of the technical difference: one alloy carries more chromium and less molybdenum, the other the reverse.

The Composition Difference, Stated the Right Way Round

The published nominal figures are set side by side below, and the direction of the trade is the point. Chromium provides resistance in oxidising conditions, molybdenum provides resistance in reducing conditions and against pitting. The alloy with the higher chromium is the one for oxidising service, which is why the comparison has a direction at all.

Two sentences in circulation get this backwards and both are worth correcting because they invert the selection. A description that gives C-22 the higher molybdenum and then quotes 12.5 to 14.5 % for C-22 against 15 to 17 % for C-276 is contradicting itself in one sentence, because 12.5 to 14.5 is the lower figure. And the tungsten difference is usually presented as though it were decisive, when the specification ranges overlap and the chromium and molybdenum are what separate the grades.

The Critical Temperatures, in the Same Solution

The most useful measurement published for this pair is a critical pitting and critical crevice temperature test run on both alloys in one solution, so the comparison is like for like. The solution is the oxidising one: 4 % sodium chloride with 0.1 % ferric sulfate and hydrochloric acid at pH 2, giving about 24,300 ppm of chloride. Pitting was assessed after 24 hours and crevice corrosion after 100 hours, with the temperature raised in 5 degree steps.

Read the crevice column as the deciding figure, because it is the one with the narrower margin and the one that governs a real vessel with gasket faces and dead legs. C-22 tolerates 102 °C where C-276 stops at 80 °C, which is a gap of 22 degrees. The pitting figures are closer, since C-22 reaches the ceiling of the test at above 150 °C against 150 °C for C-276, and the honest reading is that both are at the top of the scale and neither is far ahead.

The stainless grades are carried in the table because they show the scale. The three grades usually considered for chloride service, AL-6XN, 254SMO and 904L, all sit between 40 and 45 °C on crevice, which is below the temperature of most process streams and below the temperature of most cooling water returns.

The Wet Chlorine Test, Which Is the Sharpest Single Result

Saturated wet chlorine is the environment that separates these two most clearly, and the comparison was run as a dedicated study and not as one row of a general table. The environment was 3,000 ppm chloride solution adjusted to pH 1.5 with hydrochloric acid and chlorine gas bubbled through it for the whole 30 day period, with the exit gas led into water to hold a slight chlorine back pressure in the vapour phase. The pH at the end was 0.9.

The published conclusion is that C-22 is significantly more resistant to wet chlorine than C-276. The detail inside the result is more useful than the conclusion, and it is where C-276 failed. At every temperature where it was tested, the attack on C-276 was concentrated in the weld metal and the heat affected zone and not in the parent plate. The parent metal of a welded fabrication was not the weak point. The weld was.

The Thermal Stability Comparison, Which Is the Published Reason the Grades Differ

The reason C-22 exists as a separate grade and not as a variation on C-276 is thermal stability in the heat affected zone of a weld, and the difference is published as a welded coupon test and not as an assertion.

That table is the technical reason for the chromium difference. The higher chromium in C-22 is what keeps the heat affected zone from forming the intermetallic phases that leave it vulnerable, and the published description of the grade states that it resists the formation of grain boundary precipitates in the weld heat affected zone and is therefore suitable for most chemical process applications in the as-welded condition.

One terminology correction belongs here, because the published version of this comparison uses the wrong word. The mechanism is described in circulation as sensitisation, which is the term for chromium carbide precipitation in an austenitic stainless steel. The published description of this alloy states the mechanism as the formation of grain boundary precipitates in the heat affected zone, which for a nickel-chromium-molybdenum alloy means the intermetallic phases and not a chromium carbide. The practical consequence is the same and the mechanism is different, and a specification that asks for a sensitisation test is asking for the wrong test.

Where Each One Wins, Medium by Medium

The two grades are not ordered against each other overall. The published aqueous corrosion data, run on both alloys in the same media at the same temperatures, shows a pattern and not a ranking, and there are media in which C-276 is the better choice.

The pattern in that table is the one to carry away. C-22 wins wherever an oxidiser is present, which is why it wins in nitric acid, in ferric chloride and in every mixed acid containing ferric or cupric ions. C-276 wins in several plain hydrochloric and sulfuric acid duties at the higher concentrations, where the higher molybdenum is the deciding element and the medium is attacking the metal directly and not through an oxidising mechanism.

Two practical consequences follow. A plant that has a mixture of duties should test the specific medium instead of choosing the grade that looks stronger on a general comparison, because the two swap places across the table. And a duty that includes an oxidiser, or a duty where the chemistry can turn oxidising during an upset, is the case the published description of C-22 singles out, because it offers resistance where reducing and oxidising conditions are both encountered in the process stream.

The Physical and Fabrication Differences

The melting range row settles a claim that circulates in the other direction. C-22 has the higher melting range of the two, 1,357 to 1,399 °C against 1,323 to 1,371 °C, so a comparison that gives C-276 the higher figure is reading a number from somewhere other than the melting range. And a maximum operating temperature quoted anywhere near 1,250 °C is not an operating temperature at all: the useful service limit of both grades in oxidising atmospheres is far below their solidus, and where the service is hot and oxidising the higher chromium in C-22 is the advantage and not the disadvantage.

The restricted composition point is worth separating from the rest, because it affects what arrives on a certificate. The developer states that the grade falls within the UNS number range but is produced to a more restricted composition for improved performance. A mill certificate that satisfies the UNS range has therefore not necessarily satisfied the tighter composition the grade was developed to, and an order that names only the UNS number does not capture that.

Questions Buyers Ask

Which is more corrosion resistant, C-22 or C-276?

It depends on whether the medium is oxidising, and that is the only useful answer. In the same oxidising chloride solution, C-22 has a critical crevice temperature of 102 °C against 80 °C for C-276, and in wet chlorine C-22 showed no attack at 95 °C where C-276 failed in the weld metal and heat affected zone at 65 °C. In nitric acid at 65 % boiling, C-22 runs at 134 mils per year against 888 for C-276. Against that, C-276 is the better choice in several plain hydrochloric and sulfuric acid duties at higher concentrations, where its higher molybdenum is what counts.

Why does C-22 have better resistance if C-276 has more molybdenum?

Because chromium and molybdenum resist different things. Molybdenum carries the resistance to reducing acids and to pitting, and chromium carries the resistance to oxidising media. C-22 has about 22 % chromium and 13 % molybdenum, and C-276 has about 16 % and 16 %. The published description of C-22 singles out its resistance to oxidising aqueous media including wet chlorine and mixtures containing nitric acid or oxidising acids with chloride ions, which is the chromium at work.

Is C-22 better for pitting and crevice corrosion?

On the published head to head figures, yes. The crevice figure is the clearer of the two. In the oxidising chloride solution both were tested in, C-22 reached the ceiling of the pitting test at above 150 °C against exactly 150 °C for C-276, while the critical crevice temperature was 102 °C against 80 °C. The developer also publishes the grade under the heading that it excels in pitting resistance. Note that both figures are for that test solution and other test solutions give different absolute values.

Which is better for a welded fabrication?

C-22, and this is the published reason the two grades differ and not a marginal preference. In a welded coupon test in an oxidising sulfuric acid solution at 150 °C, C-276 suffered severe attack of the base metal, the weld metal and the heat affected zone, and only one third of the coupon thickness in the heat affected zone survived. C-22 showed minimal attack. C-22 also resists the formation of grain boundary precipitates in the weld heat affected zone, which is what makes it suitable for most chemical process applications in the as-welded condition. Both grades are commonly welded with C-22 filler for the same reason.

Does C-276 have a higher maximum temperature than C-22?

No. The two melting ranges are 1,323 to 1,371 °C for C-276 and 1,357 to 1,399 °C for C-22, so C-22 has the higher one. A comparison that quotes about 1,350 °C for C-276 and about 1,250 °C for C-22 and calls those maximum operating temperatures is comparing melting ranges and mislabelling them, and the conclusion drawn from it does not follow. Where the service is hot and oxidising, the higher chromium in C-22 is an advantage.

Can I substitute one for the other?

Not on the basis of the family name, because they are different alloys with different chromium and molybdenum and they swap places across the medium table above. A substitution should be made against the specific medium and temperature, with the welded condition considered separately from the parent plate. The published welded coupon result shows that a substitution made on the base metal data alone can be wrong by a wide margin, because the failure in that test was in the heat affected zone and not in the plate.

Summary

C-22 and C-276 are both nickel-chromium-molybdenum alloys with tungsten, and the difference between them is a trade between chromium and molybdenum. C-22 is UNS N06022 with about 22 % chromium and 13 % molybdenum, and it wins wherever the medium is oxidising: a critical crevice temperature of 102 °C against 80 °C, no attack in wet chlorine to 95 °C where C-276 failed at 65 °C, and 134 mils per year against 888 in 65 % boiling nitric acid. C-276 is UNS N10276 with about 16 % chromium and 16 % molybdenum, and it wins in several plain hydrochloric and sulfuric acid duties at the higher concentrations. The reason the two grades exist separately is the weld: in an oxidising sulfuric acid solution at 150 °C, C-276 lost two thirds of its heat affected zone thickness while C-22 was barely attacked, and C-22 resists grain boundary precipitation in that zone, which is why it is used as the filler metal for both.

Both are supplied from Changzhou, Jiangsu in plate, sheet, strip, bar, wire, pipe, tube, covered electrodes and weld overlay form, from a plant that holds ISO 9001 certification and is audited by SGS. Send the medium with its concentration and temperature, the welded condition of the part, and whether an oxidiser can be present during an upset, and the choice between the two grades follows from those three facts.

References

1. Haynes International brochure for HASTELLOY C-22 alloy. Source for the nominal chemical composition, the description of the alloy as a versatile nickel-chromium-molybdenum-tungsten grade with better overall corrosion resistance than other nickel-chromium-molybdenum alloys including C-276 and C-4 and alloy 625, the resistance to oxidising aqueous media including wet chlorine and mixtures containing nitric acid or oxidising acids with chloride ions, the ability to resist formation of grain boundary precipitates in the weld heat affected zone so that it is suitable for most chemical process applications in the as-welded condition, the statement that the grade falls within the UNS N06022 range but is produced to a more restricted composition, the physical properties table including the density of 8.69 g/cm³, the melting range of 1,357 to 1,399 °C and the coefficient of thermal expansion, the specification list, and the use of the alloy as an overalloy filler wire and weld overlay consumable.
2. The same source, comparative critical pitting and critical crevice corrosion temperatures. Source for the test solution of 4 % sodium chloride with 0.1 % ferric sulfate and hydrochloric acid at pH 2 giving about 24,300 ppm chloride, the 24 hour pitting and 100 hour crevice exposures with 5 degree temperature increments, and the values for C-22, C-276, C-4, alloy 625, G-30, AL-6XN, 254SMO, 904L and type 316 quoted in the body.
3. The same source, thermal stability section. Source for the welded coupon test in 11 % sulfuric acid with 3.9 % ferric sulfate and other chemicals at 302 °F, or 150 °C, overpressurised with oxygen, the statement that C-276 suffered unusually severe base metal, weld metal and heat affected zone attack with only one third of the coupon thickness surviving in the heat affected zone, the note that C-276 is seldom attacked to this degree in other media, and the minimal attack on the C-22 sample.
4. The same source, corrosion resistant weld filler metal section, reproducing tests carried out at Los Alamos National Laboratory in a simulated incinerator off-gas scrub solution over 39 days. Source for the comparison of alloy 625 base metal welded with 625 filler against the same base metal welded with C-22 filler, and for the C-22 base metal and C-22 weld result, in the solution of 3 M sodium chloride with 0.1 M ferric chloride and 0.1 M sodium fluoride at 167 °F, or 75 °C, at pH 1.
5. The same source, comparative aqueous corrosion data tables. Source for the paired C-22 and C-276 corrosion rates in mils per year quoted in the body, including the nitric acid, ferric chloride, phosphorus pentoxide, sulfuric acid, hydrochloric acid and mixed acid and chloride rows, with the note that the figures are the average of four to ten tests and that mils per year converts to millimetres per year by dividing by 40.
6. The same source, field evaluation tables. Source for the reactor vessel result in 10 to 15 % sulfuric acid with solids, the pulp and paper pitting and crevice depths, and the chemical waste incineration quench duct rates used in the body.
7. Haynes International technical article on HASTELLOY C-22 and C-276 alloys in saturated wet chlorine. Source for the test environment of 3,000 ppm chloride adjusted to pH 1.5 with hydrochloric acid and chlorine gas over 30 days with an exit pH of 0.9, the coupon positions and temperatures, the attack found on C-276 in the weld metal and heat affected zone, the absence of attack on C-22 including at 95 °C in the liquid phase and 90 °C in the vapour phase, and the published conclusion that C-22 is significantly more resistant to wet chlorine environments.
8. Published data for HASTELLOY C-276 alloy. Source for the melting range of 1,323 to 1,371 °C, the density of 8.89 g/cm³, the coefficient of thermal expansion, the electrical resistivity, and the critical pitting and crevice temperatures in acidified ferric chloride used elsewhere on the