Alloys for Acid and Alkali Service: Which Grade for Which Chemical
Every grade below is placed from its producer’s own published data, with the concentration and temperature limits the producer gives.
Table of Contents:
- Oxidising or Reducing Matters More Than Acid or Alkali
- Quick Selection by Chemical
- Alkali Service: Caustic Soda, Caustic Potash and Ammonia
- Sulphuric Acid
- Hydrochloric Acid
- Hydrofluoric Acid
- Phosphoric Acid
- Nitric Acid
- Acetic and Formic Acid
- Chlor-Alkali: Caustic and Chlorine in One Plant
- What Changes the Answer
- What to Send Us When You Enquire
- Where These Figures Come From
- Frequently Asked Questions
Nickel resists alkalis. Chromium resists oxidising acids. Molybdenum and copper resist reducing ones, and which of those an alloy carries decides where it survives. So the question usually arrives as “acid or alkali?”, and that is not enough to choose a grade. Monel® 400 resists hydrofluoric acid better than almost any engineering alloy and is nearly as good as pure nickel in caustic soda – yet nitric acid stronger than 0.5% attacks it rapidly. Hastelloy® C-276 is the usual answer for hot, strong sulphuric acid – yet in dilute sulphuric acid Special Metals ranks Incoloy® 825 above it. What decides the choice is whether the chemical oxidises or reduces, how strong and how hot it is, and what it carries with it.
Oxidising or Reducing Matters More Than Acid or Alkali
Acids are either oxidising, like nitric acid, or reducing, like hydrochloric and dilute sulphuric acid. Special Metals groups the alkalis with the reducing chemicals. An alloy resists one kind or the other according to what it is made of, and Special Metals sets out what each element contributes:
| Element | What it adds | Grades that rely on it |
|---|---|---|
| Nickel | Resistance to reducing acids and to caustics, and to stress-corrosion cracking, particularly in chlorides and caustics | Nickel 200 and 201, Monel 400, Inconel 600 |
| Chromium | Resistance to oxidising corrosives, and to pitting and crevice corrosion | Inconel 690, Hastelloy C-22, Hastelloy G-30, Alloy 28 |
| Molybdenum | Resistance to reducing acids, and to pitting and crevice corrosion in chloride-bearing water | Hastelloy B-2 and B-3, C-276, Inconel 625 and 686 |
| Copper | Resistance to reducing acids, particularly non-aerated sulphuric and hydrofluoric; in Ni-Cr-Mo-Fe alloys, improved resistance to hydrochloric, phosphoric and sulphuric acid | Monel 400, Alloy 20, Incoloy 825, 904L, Hastelloy C-2000 |
| Tungsten | Resistance to reducing acids and to localised corrosion | Hastelloy C-276 and C-22, Inconel 686 |
Two consequences run through every section below. First, an alloy with little or no chromium – Nickel 200, Monel 400, Hastelloy B-2 and B-3 – depends on the acid staying reducing. Special Metals is blunt about the B grades: with an absence of chromium, Ni-Mo alloys have no resistance to environments containing oxidising species. Ferric or cupric ions, dissolved oxygen or nitric acid in the stream can drastically change how a reducing acid behaves.
Second, sulphuric acid changes character by itself. It is reducing up to about 25%, takes on oxidising characteristics above that, and is oxidising at the 87%-plus strength it is usually sold at – so dilute and concentrated sulphuric acid call for different grades.
Titanium is the mirror image of the nickel alloys. TIMET rates it highly resistant to oxidising acids, only moderately resistant to reducing ones, and rapidly attacked by hydrofluoric acid of even very dilute concentration.
Quick Selection by Chemical
Where each producer’s data places the grades – a starting point, not a guarantee for a particular process. The sections that follow give the limits behind each entry.
| Chemical | How it behaves | Start with | Avoid |
|---|---|---|---|
| Caustic soda, caustic potash | Alkali; attack falls as nickel content rises | Nickel 200 (Nickel 201 above 315 °C); Monel 400; Inconel 600 where hot caustic carries sulphur | Austenitic stainless steels in hot, concentrated caustic; titanium above 77 °C at pH 12 or more |
| Sulphuric acid, up to 50% | Reducing to about 25%, then increasingly oxidising | Incoloy 825, Alloy 20, 904L, Alloy 926; Monel 400 if air-free; Hastelloy B-3 or C-2000 when hot | Monel 400 with ferric, cupric or other oxidising salts; unalloyed titanium beyond about 5% at room temperature |
| Sulphuric acid, over 50% or hot | Oxidising above about 87% | Inconel 686, Hastelloy C-276, C-22; Alloy 28 at 40–70% deaerated and above 85% | Hastelloy B-3 in boiling acid above 70%; Monel 400 above 85% |
| Hydrochloric acid | Reducing at every concentration | Hastelloy B-3 in pure acid; C-276, Inconel 686; C-2000 up to 10%; Nickel 200 and Monel 400 when dilute and air-free | Hastelloy B-2 and B-3 if ferric or cupric ions or air are present; unalloyed titanium beyond about 7% at room temperature |
| Hydrofluoric acid | Few compounds are more corrosive; aeration makes it worse | Monel 400; Nickel 200 below 82 °C and in anhydrous hydrogen fluoride; Inconel 600 when dilute, to 70 °C | Titanium at any concentration; the Incoloy alloys |
| Phosphoric acid, pure | Non-oxidising | Incoloy 825 up to boiling 85%; Monel 400 up to 80 °C | Nickel 200 and Inconel 600 in hot acid |
| Phosphoric acid, wet-process | Carries chlorides and fluorides from the phosphate rock | Hastelloy G-30, Alloy 28, Inconel 625, Hastelloy C-276, Inconel 686 | Incoloy 825 where chlorides or fluorides pit it; Monel 400 where ferric ions are present |
| Nitric acid | Oxidising | Incoloy 825 up to boiling 65%; Inconel 690 and 625; Hastelloy C-22 and G-30; Alloy 28; titanium below boiling | Nickel 200 and Monel 400 above 0.5%; Hastelloy B-2 and B-3; titanium in red fuming nitric acid |
| Acetic and formic acid | Weak and non-oxidising | Incoloy 825, Alloy 20, Inconel 625; Hastelloy C-276 for evaporators and reboilers; Alloy 28; titanium for acetic | Nickel 200 and Monel 400 in aerated acid |
| Wet chlorine, hypochlorite | Oxidising | Titanium; Hastelloy C-276 | Nickel 200 above 500 ppm available chlorine in continuous service; titanium in dry chlorine |
Alkali Service: Caustic Soda, Caustic Potash and Ammonia
Resistance to alkalis generally improves with increasing nickel content, Special Metals finds, and the nickel alloys it names as most used in alkali processes are Nickel 200, Monel 400 and Inconel 600 and 625. Austenitic stainless steels and other low-nickel materials, by contrast, may suffer either stress-corrosion cracking or general corrosion in hot, concentrated caustic.
Nickel’s protection in strong caustic is a black nickel oxide film, and its effect grows with exposure. In a Special Metals test in flake caustic at 412–443 °C, Nickel 201 corroded at 0.53 mm/a over the first 24 hours, at 0.07 mm/a averaged over the first week, and at the same 0.07 mm/a over a second week.
| Grade | UNS | What the producer publishes | Watch for |
|---|---|---|---|
| Nickel 200 | N02200 | All concentrations up to and including the molten state. Below 50%, usually under 0.003 mm/a even boiling | Not recommended at 315–650 °C, because prolonged exposure at 425–650 °C precipitates graphite; chlorates and oxidisable sulphur compounds raise the rate |
| Nickel 201 | N02201 | The low-carbon grade, preferred wherever nickel is used above 315 °C | Chlorates and oxidisable sulphur compounds, as for Nickel 200 |
| Monel 400 | N04400 | Nearly as resistant as Nickel 200 through most of the concentration range | Stress-corrosion cracking in strong alkalis at elevated temperature; faster attack in highly concentrated, hot caustic soda and potash |
| Inconel 600 | N06600 | Resists boiling sodium hydroxide through 80%; preferred to Nickel 201 in high-temperature caustic that contains sulphur | Can crack in some alkali environments – stress relieve before service and keep operating stresses low; chlorates raise the rate |
| Inconel 625, 686, Hastelloy C-276, C-22 | N06625, N06686, N10276, N06022 | Not normally required in uncontaminated caustic | Can offer an advantage where the caustic carries halides |
| Incoloy 825, Alloy 20, Alloy 926 | N08825, N08020, N08926 | 0.01 mm/a in boiling 50% sodium hydroxide, but less resistant than Nickel 200; seldom used in alkali unless other corrosives are present | Stress-corrosion cracking in hot, concentrated alkalis |
| Alloy 28 | N08028 | 0.074 mm/y in 43% sodium hydroxide at 135 °C (904L 0.301). No cracking in boiling 43% NaOH + 6.7% NaCl at 142 °C for 500 h. At moderate temperature and concentration, a suitable alternative to pure nickel, which may be attacked by erosion corrosion | General corrosion rises at high temperature, and chlorides raise the cracking risk |
| 904L | N08904 | In hot, concentrated caustic, resistance is set mainly by nickel content; a good alternative to conventional stainless steels | Cracked in Alleima’s boiling 43% NaOH + 6.7% NaCl test, where Alloy 28 did not |
| Titanium Grade 2, 7, 12 | R50400, R52400, R53400 | Generally 0.127 mm/y or less in sodium, potassium, calcium and ammonium hydroxide, whatever the concentration | Hydrogen pickup and possible embrittlement above 77 °C at pH 12 or more; ATI lists very strong caustic as incompatible |
Three things raise the rate on nickel in caustic. Chlorates, which should be removed as thoroughly as possible before high-temperature evaporation. Oxidisable sulphur compounds – sulphides and mercaptans most of all – whose effect Special Metals avoids by adding enough sodium peroxide to oxidise them to sulphate. And temperature above 315 °C on Nickel 200, which is why the low-carbon Nickel 201 exists.
Ammonia is the exception for nickel. Nickel 200 is not attacked by anhydrous ammonia or 1% ammonium hydroxide, but stronger solutions attack it rapidly when dissolved oxygen is present. Monel 400 resists anhydrous ammonia and ammonium hydroxide up to 3% without dissolved oxygen. Inconel 600, because of its chromium, is almost entirely resistant to ammonia solutions across the complete range of concentrations and temperatures.
Sources: Special Metals aqueous corrosion handbook and Nickel 200 & 201 bulletin (nickel alloys, Incoloy 825, Alloy 20 as INCOLOY 020, Alloy 926 as INCOLOY 25-6MO); Alleima Sanicro 28 datasheet (Alloy 28, and the 904L comparison); Outokumpu Ultra range datasheet (904L); TIMET and ATI (titanium).
Sulphuric Acid
For dilute sulphuric acid, the nickel alloys Special Metals says are most commonly used are Incoloy 825, Alloy 20 and Alloy 926; for aggressive, hot acid, Inconel 625, C-22, C-276 and 686. Its ranking reverses with strength. Below 50%, Incoloy 825 and Inconel 686 lead, followed by C-22, Inconel 625, C-276 and Alloy 926. Above 50%, the order is Inconel 686, C-276, C-22, Incoloy 825, Inconel 625 and Alloy 926.
So C-276’s reputation for sulphuric acid is earned at the hot, strong end, and it is not the automatic answer in dilute acid. Haynes’ answer for this acid is C-2000, which carries a deliberate copper addition that Haynes says greatly enhances its resistance to sulphuric acid, and which Haynes shows outperforming C-22, C-276 and 625 at concentrations up to 80%.
| Grade | What the producer publishes | Watch for |
|---|---|---|
| Monel 400 | In air-free 5–6% acid, very low rates at all temperatures. Suitable in boiling acid up to about 15%; stores 80% acid at room temperature; at elevated temperature, up to about 65% | Ferric sulphate, chromates, nitrates, cupric salts and other oxidising salts make the acid very corrosive to it; above 85% the rate rises sharply |
| Inconel 600 | Cold, non-aerated acid up to about 70% | Not for hot acid except at low concentration; aeration raises the rate |
| Incoloy 825 | Below 0.13 mm/a at 40–80% and 50 °C. Oxidising salts other than chlorides actually help | Continuous immersion in chloride-contaminated acid needs a higher-molybdenum grade |
| Alloy 20 | Performs like Incoloy 825, its composition being similar | As for Incoloy 825 |
| Alloy 926 | Useful resistance at boiling below 10%, up to 50% at 60 °C, and at all concentrations up to 50 °C | A contender for intermediate-strength acid contaminated with chlorides |
| 904L | Originally developed for sulphuric acid at ambient temperature. With Alloy 825, the best across the whole concentration range of the grades Outokumpu compares | Chlorides are detrimental; at high concentration, 316L and duplex grades can outperform it |
| Alloy 28 | Suited to 40–70% deaerated acid and to acid above 85%; about the same resistance as Alloy C in 98% acid. Better than 904L in chloride-contaminated acid | Naturally aerated acid is more corrosive than deaerated in the intermediate range |
| Hastelloy C-276 | Useful resistance up to 10% at boiling, and at all concentrations up to about 80 °C | Chloride contamination lowers its resistance, as it does for all nickel alloys |
| Inconel 625, C-22, 686 | Resistance similar to C-276, with higher chromium giving them good resistance in more concentrated acid. Ranked 686, then C-22, then 625 | As for C-276 |
| Hastelloy C-2000 | A copper addition that greatly enhances resistance to sulphuric acid, with high chromium for acid carrying ferric ions or dissolved oxygen | See the Haynes figures below |
| Hastelloy B-3, B-2 | B-3: outstanding resistance to non-oxidising sulphuric acid, 0.05 mm/y or less at boiling up to 60% | No resistance once oxidising species are present; 4.76 mm/y in boiling 80% acid |
| Titanium Grade 2, 12, 7 | Unalloyed, such as Grade 2: about 20% at 0 °C and 5% at room temperature. Grade 7: about 45% at room temperature and 7% boiling. Grade 12: about 1% boiling | Unalloyed titanium corrodes fast in boiling acid of only 0.5%; cupric and ferric ions inhibit the attack |
Hastelloy grades side by side
| Acid, temperature | B-3 | C-2000 | C-276 | C-22 | C-4 | G-30 |
|---|---|---|---|---|---|---|
| 10%, boiling | 0.01 | 0.09 | 0.18 | 0.29 | 0.43 | 0.78 |
| 30%, boiling | 0.02 | 0.42 | 0.83 | – | 1.85 | 1.53 |
| 50%, 93 °C | 0.04 | 0.16 | 0.62 | 0.77 | 0.99 | 0.56 |
| 70%, 93 °C | 0.01 | 0.42 | 0.50 | 0.94 | 0.94 | 0.98 |
| 80%, 93 °C | 0.01 | 0.99 | 0.60 | 2.16 | 2.47 | 4.52 |
| 96%, 93 °C | 0.02 | 0.19 | 0.18 | 1.10 | – | 1.86 |
B-3’s figures hold only while the acid stays free of oxidising species. Reagent-grade test acid is; process acid may not be, and Special Metals warns that contaminants are notorious in intermediate-strength sulphuric acid. The same alloy corrodes at 4.76 mm/y in boiling 80% acid.
Sources: Special Metals aqueous corrosion handbook (Monel 400, Inconel 600, 625, 686, C-22 as INCONEL 622, C-276, Incoloy 825, Alloy 20, Alloy 926); Haynes International alloy pages (Hastelloy grades and table); Outokumpu Ultra range datasheet (904L); Alleima Sanicro 28 datasheet (Alloy 28); TIMET (titanium). Compare grades within one table, not across producers: each laboratory tests differently.
Hydrochloric Acid
Hydrochloric acid is reducing across its entire concentration range, and its strong acidity combined with the chloride ion makes it a very severe corrosive. The high-nickel alloys are among the few metallic materials with useful resistance to it. Oxidising contaminants such as ferric or cupric ions drastically change its behaviour, and in opposite directions for the two nickel alloy families: they can actually decrease the corrosion rate of chromium-bearing Ni-Cr-Mo alloys, and they increase that of the Ni-Mo B grades.
Special Metals ranks, in descending order: Inconel 686 with C-276, then C-22, Inconel 625, Incoloy 825 and Alloy 926.
| Grade | What the producer publishes | Watch for |
|---|---|---|
| Nickel 200, Monel 400 | Similar to each other. Up to 10%, below 0.25 mm/a in air-free acid at 30 °C. Nickel 200 to 30% at room temperature, aerated or not; Monel 400 at room temperature under 10% aerated and 20% air-free. Both withstand the below-0.5% acid formed by hydrolysis of chlorides up to 149–204 °C | Aeration: in air-saturated acid above room temperature, usually limited to under 3–4% |
| Inconel 600 | Fair: good in cold aerated acid below 2%, satisfactory at room temperature up to 20% | Not as good as Monel 400 or Nickel 200 |
| Incoloy 825, Alloy 20, Alloy 926 | Good resistance, greatly improved in 825 and Alloy 20 by their copper, molybdenum and higher nickel | For aggressive, hot acid, Special Metals moves to the Ni-Cr-Mo grades |
| Hastelloy C-276 | Useful resistance up to 3% at boiling, 5% at 80 °C and all concentrations up to about 66 °C | See the Haynes figures below, measured differently |
| Hastelloy C-22, Inconel 686 | Very good, particularly in strong acid or oxidising conditions; more resistant than Nickel 200 or Monel 400 when the acid carries cupric, ferric or oxygen | – |
| Inconel 625 | Good in dilute acid, from its 9% molybdenum | – |
| Hastelloy B-3, B-2 | B-3: extremely high resistance to pure hydrochloric acid; 0.29 mm/y in boiling 20% acid | No resistance to oxidising species: ferric or cupric ions, or air, raise the rate |
| Hastelloy C-2000 | Superior to C-22, C-276 and 625 at concentrations up to 10% | – |
| Alloy 28 | More resistant than stainless steels lower in chromium and molybdenum; useful where process solutions are contaminated with hydrochloric acid | Alleima positions it for process solutions contaminated with hydrochloric acid |
| 904L | A significant advantage over 316L | All stainless steels are sensitive to halide acids |
| Titanium Grade 2, 12, 7 | At room temperature, about 7% (Grade 2), 9% (Grade 12) and 27% (Grade 7) | Much lower near boiling. Ferric ions inhibit the attack, but pure acid used to clean titanium has caused severe damage |
Hastelloy grades side by side
| Acid, temperature | B-3 | C-2000 | C-276 | C-22 | C-4 | G-30 |
|---|---|---|---|---|---|---|
| 1%, boiling | 0.01 | 0.01 | 0.33 | 0.06 | 0.48 | 0.01 |
| 2%, boiling | 0.04 | 0.09 | – | – | 1.99 | 9.47 |
| 5%, 66 °C | 0.25 | <0.01 | 0.31 | 0.44 | 0.42 | 1.33 |
| 10%, 66 °C | 0.24 | 0.65 | 0.46 | 0.98 | 0.57 | 1.48 |
| 20%, 66 °C | 0.21 | 0.69 | 0.55 | 0.90 | 0.55 | 1.24 |
| 20%, boiling | 0.29 | – | – | – | – | – |
G-30 shows how abruptly a grade can run out: 0.01 mm/y in boiling 1% acid and 9.47 mm/y in boiling 2%. Haynes positions it for phosphoric acid and strong oxidising solutions, not hydrochloric acid.
Sources: Special Metals aqueous corrosion handbook; Haynes International alloy pages; Alleima Sanicro 28 datasheet; Outokumpu Ultra range datasheet; TIMET. Haynes notes 20% as the azeotrope, above which its hydrochloric acid tests are less reliable.
Hydrofluoric Acid
Chemically a weaker acid than hydrochloric or sulphuric, hydrofluoric acid is nonetheless among the most corrosive compounds there are: only gold and platinum are completely resistant to it in aqueous solution, and the engineering materials that survive it do so by forming fluoride films. As with other non-oxidising acids, aeration or oxidising chemicals increase the attack. The nickel alloy most commonly used in it is Monel 400, and Special Metals’ ranking puts it first: Monel 400; then Inconel 686, C-22 and C-276 with Nickel 200; then Inconel 600 and 690; then Incoloy 825.
| Grade | What the producer publishes | Watch for |
|---|---|---|
| Monel 400 | In non-aerated acid, resists all concentrations up to the boiling point. Comparatively insensitive to velocity, so used for bubble caps and valves in flowing acid; small amounts of sulphuric acid or reducing sulphur compounds do no harm | Stress-corrosion cracking in moist, aerated hydrofluoric or hydrofluosilicic acid vapour; one hour at 538–648 °C and a slow cool is usually sufficient to prevent it |
| Nickel 200 | Excellent in anhydrous hydrogen fluoride, even at elevated temperature; in aqueous acid usually limited to below 82 °C | Severe corrosion at room temperature in commercial 60–65% acid |
| Inconel 600 | Dilute, non-deaerated acid up to 70 °C; oxidising salts usually help | Weld metal carrying significant niobium corrodes selectively |
| C-276, C-22, Inconel 686 | Not as resistant as Nickel 200 or Monel 400 in aqueous hydrofluoric acid | Haynes’ own hydrofluoric figures count external attack only, because the acid also attacks nickel alloys internally |
| Incoloy 825, Alloy 20, Alloy 926 | Generally not used in hydrofluoric acid service; 825 occasionally, for equipment that cannot be stress relieved | – |
| Alloy 28 | Resists hydrofluoric and hydrofluosilicic acid very well where they occur as impurities | Alleima’s claim is for these acids as impurities |
| 904L | A significant advantage over 316L | All stainless steels are sensitive to hydrofluoric acid |
| Titanium, all grades | – | Rapidly attacked by even very dilute acid; not recommended for hydrofluoric acid or fluoride solutions below pH 7 |
Sources: Special Metals aqueous corrosion handbook and MONEL alloy 400 bulletin (SMC-053); Haynes International; Alleima; Outokumpu; TIMET and ATI.
Phosphoric Acid
Pure phosphoric acid is non-oxidising, much like dilute sulphuric. Commercial acid is another matter: acid made by the “wet” process carries chlorides and fluorides from the phosphate rock, which markedly increase its corrosivity, and may carry ferric salts as well. For pure acid Special Metals names Incoloy 825, Alloy 20 and Alloy 926 as the most commonly used grades; for hot acid, especially acid contaminated with halides, Inconel 625, C-22, C-276 and 686. In commercially pure acid it ranks Inconel 686 first, then C-276, Inconel 625, Incoloy 825 and Monel 400.
| Grade | What the producer publishes | Watch for |
|---|---|---|
| Incoloy 825 | Excellent in pure acid at all concentrations and temperatures up to and including boiling 85% | Significant pitting or crevice corrosion in commercial-grade acid |
| Alloy 20, Alloy 926 | Normally less resistant than Incoloy 825, and Alloy 926 less than Alloy 20 | – |
| Monel 400 | Below 0.25 mm/a in pure acid at all concentrations up to 80 °C | Higher temperatures and aeration raise the rate; in crude acid, as little as 0.4% ferric ion can raise it tenfold |
| Nickel 200, Inconel 600 | Both resist pure acid at room temperature | Rates rise too far for either in hot or concentrated acid |
| Inconel 625, C-276, C-22, 686 | No better than Incoloy 825 in pure, concentrated, boiling acid, but superior where chlorides and fluorides are present, as in wet-process evaporators. Inconel 625: 0.05 mm/a in boiling 25% phosphoric acid with 2% hydrofluoric acid | – |
| Hastelloy G-30 | Highly resistant to wet-process phosphoric acid; very successful as phosphoric acid heat exchanger tubing in the fertiliser industry | Its hydrochloric acid rates rise steeply (see above) |
| Alloy 28 | The most widely used metallic material for wet-process evaporator tubes. In contaminated 95% superphosphoric acid at 200 °C over 20 days: 0.03 mm/y, against 0.10 for 904L and 0.23 for Alloy 20 | – |
| 904L | Much lower corrosion than 316L in wet-process acid at 60 °C | – |
| Titanium Grade 2, 7 | Grade 2: about 30% at room temperature, 10% at 60 °C, 2% at 100 °C. Grade 7: about 80%, 15% and 6% respectively. Grade 12 lies between them | Boiling acid accelerates the attack significantly |
Hastelloy grades side by side
| Acid, temperature | B-3 | C-2000 | C-276 | C-22 | G-30 |
|---|---|---|---|---|---|
| 50%, boiling | 0.09 | 0.03 | – | 0.07 | 0.01 |
| 70%, boiling | 0.21 | 0.15 | – | 0.23 | 0.35 |
| 80%, 93 °C | 0.02 | 0.01 | 0.02 | – | – |
| 85%, boiling | 0.10 | – | – | 0.66 | 0.84 |
These are pure-acid figures, and G-30’s case does not rest on pure acid. Haynes positions it for wet-process acid, whose chlorides and fluorides a reagent-grade test leaves out.
Sources: Special Metals aqueous corrosion handbook; Haynes International; Alleima Sanicro 28 datasheet; Outokumpu Ultra range datasheet; TIMET.
Nitric Acid
Nitric acid is strongly oxidising, so the alloys that resist it are those that form passive oxide films – the chromium-bearing ones. Nickel alloys offer good resistance to pure nitric acid and are particularly effective in mixed acids, nitric with sulphuric or phosphoric; where small amounts of chlorides or fluorides are present, the Ni-Cr-Mo alloys are superior. This is also where titanium is at its best.
| Grade | What the producer publishes | Watch for |
|---|---|---|
| Nickel 200, Monel 400 | – | Acid above 0.5% attacks both rapidly |
| Incoloy 825 | Excellent at all concentrations and temperatures up to and including boiling 65%. Titanium-stabilised, so welding and stress relieving do no harm and no post-weld heat treatment is normally needed | – |
| Inconel 600 | Resistant above 20% at room temperature, and excellent in boiling 10% | Can be sensitised by exposure to intermediate temperatures, and then shows high intergranular corrosion in hot acid |
| Inconel 690 | Significantly better than 600, with 30% chromium against 16%; lower rates than Inconel 625 in mixed nitric and hydrofluoric acid | – |
| Inconel 625 | Good: about 0.76 mm/a in boiling 65% acid. In boiling 15% nitric with 3% hydrofluoric acid, 0.86 mm/a against 5.99 for AISI 316 | Stabilised; no post-weld heat treatment needed before oxidising acid service |
| Hastelloy C-276 | Resistant above 20% at room temperature | Sensitisation, as for Inconel 600 |
| Hastelloy G-30 | High chromium; with G-35, Haynes’ premier choice for strong oxidising solutions and mixed-acid pickling | – |
| Hastelloy B-2, B-3 | – | Little or no chromium: no resistance to oxidising species |
| Alloy 28 | Below 0.15 mm/y in the ASTM A262 Practice C (Huey) test, five 48-hour periods in boiling 65% acid | – |
| 904L | 0.47 mm/y in 20% nitric plus 4% hydrofluoric pickling acid at 25 °C, against more than 6 for 316L | – |
| Titanium | Excellent across the full concentration range below boiling; titanium’s own corrosion product inhibits hot acid in recirculating streams | Significant general corrosion in hot, very pure 20–70% acid. Never in red fuming nitric acid, where a pyrophoric reaction can follow |
Hastelloy grades side by side
| Acid, boiling | C-2000 | C-276 | C-22 | G-30 |
|---|---|---|---|---|
| 10% | – | 0.26 | 0.01 | – |
| 20% | 0.02 | 0.66 | 0.06 | – |
| 40% | 0.24 | – | 0.26 | – |
| 50% | 0.51 | – | 0.59 | 0.08 |
| 65% | 1.00 | – | – | 0.16 |
Sources: Special Metals aqueous corrosion handbook; Haynes International; Alleima Sanicro 28 datasheet; Outokumpu Ultra range datasheet; TIMET.
Acetic and Formic Acid
Most organic acids are weak and non-oxidising. Formic is the most corrosive of the common ones, then acetic, and as with the other non-oxidising acids, aeration and temperature usually increase the attack.
| Grade | What the producer publishes | Watch for |
|---|---|---|
| Nickel 200 | Excellent at all concentrations when little dissolved oxygen is present | Aerated acid: 10.2 mm/a in air-saturated 85% acetic acid at ambient temperature |
| Monel 400 | Below 0.10 mm/a in air-free acetic acid at room temperature, all concentrations; used for fatty acid distillation up to 260 °C | Generally less resistant than the other nickel alloys; cupric ions from its own corrosion accelerate the attack |
| Incoloy 825, Alloy 20, Alloy 926 | Highly resistant, even to boiling concentrated acetic acid, acetic-formic mixtures, and maleic and phthalic acids | – |
| Inconel 625 | Below 0.03 mm/a in boiling glacial acetic acid; 0.08 mm/a in boiling 5% formic | – |
| Hastelloy C-276 | Preferred by Special Metals for the more severe services, such as acid evaporators and reboilers. Haynes: below 0.01 mm/y in boiling 99% acetic, 0.04 in boiling 88% formic | – |
| C-22, B-3, G-30 | Haynes, boiling 99% acetic: C-22 0.00, B-3 0.02, G-30 0.03 mm/y. Boiling formic (88–89%): C-22 below 0.01, B-3 0.01, G-30 0.05 | – |
| Alloy 28 | Completely resistant to pure acetic acid at all temperatures and concentrations at atmospheric pressure, and far more resistant to formic acid than 316L | Formic contamination makes acetic acid more corrosive |
| 904L | 0.06 mm/y in a tall-oil fatty acid distillation column at 235 °C, against 0.88 for 316L | – |
| Titanium | Resistant to acetic acid over a wide range of concentrations and temperatures, well beyond the boiling point | Hot non-aerated formic, hot oxalic, concentrated trichloroacetic and sulphamic acids attack it |
Sources: Special Metals aqueous corrosion handbook; Haynes International; Alleima Sanicro 28 datasheet; Outokumpu Ultra range datasheet; TIMET.
Chlor-Alkali: Caustic and Chlorine in One Plant
A chlor-alkali plant produces both halves of this page at once: caustic soda on one side of the cell and chlorine on the other.
- Caustic side. Nickel 200, or Nickel 201 above 315 °C, for evaporators and caustic handling; Inconel 600 where hot caustic carries sulphur. Chlorates in the caustic raise nickel’s corrosion rate, so remove them before high-temperature evaporation.
- Dry chlorine. Every nickel alloy Special Metals discusses resists it, and Monel 400 is a standard material for the trim of chlorine cylinder and tank-car valves and for orifice plates in chlorine lines.
- Wet chlorine below the dew point, and solutions carrying considerable free chlorine, are very corrosive to all of those nickel alloys except C-276 – which is why C-276 is used for the valve stems in Monel 400-seated chlorine valves, against moisture getting in.
- Titanium is fully resistant to wet chlorine and to hypochlorites, chlorates and chlorine dioxide, and titanium anodes are used to produce chlorine, chlorate and hypochlorite. It needs moisture to stay passive, though: ATI lists anhydrous chlorine as incompatible, and TIMET advises caution where the chlorine is low in moisture.
- Hypochlorite on nickel. Keep continuous exposure within 500 ppm available chlorine. At 25 °C Nickel 200 corroded at 0.003 mm/a at 35 ppm, 0.008 at 100 ppm and 0.02 at 500 ppm; intermittent exposure, as in textile bleaching followed by rinsing, can go to 3 g/l.
- Hot, dry chlorine and hydrogen chloride, as in hydrogen chloride synthesis: Nickel 201 and Inconel 600 are the most practical alloys at elevated temperature, rated above the Ni-Mo and Ni-Cr-Mo alloys at 454–538 °C. Keep nickel surfaces about 30 °C above the dew point, because nickel chloride draws in moisture and hydrochloric acid can condense on it.
Sources: Special Metals aqueous corrosion handbook; TIMET; ATI.
What Changes the Answer
Every producer cited here gives its data for pure chemicals and warns that the real process can move the result a long way. These are the variables that move it most:
- Dissolved air. Raises the rate on Nickel 200 and Monel 400 in sulphuric, hydrochloric and organic acids; oxygen is one of the oxidising species the Hastelloy B grades cannot tolerate.
- Ferric and cupric ions. Make sulphuric acid very corrosive to Monel 400 and raise the Hastelloy B rate in hydrochloric acid – while lowering the rate of the Ni-Cr-Mo grades and inhibiting titanium. In crude phosphoric acid, 0.4% ferric ion can raise a rate tenfold.
- Chlorides. Lower the resistance of every nickel alloy in sulphuric acid, pit Incoloy 825 in wet-process phosphoric acid, and raise the cracking risk for stainless steels in hot caustic.
- Fluorides. Attack titanium below pH 7, and make phosphoric acid markedly more corrosive.
- Chlorates and sulphides in caustic. Both raise the rate on nickel.
- Mercury salts. Can cause stress-corrosion cracking of Monel 400 in sulphuric acid, prevented by a stress relief before service.
- Crevices. Passivity is hard to hold under bolting, in threaded connections and at flange faces, where the oxidising ions that maintain it are not replenished.
- Heat treatment and welding. Inconel 600 and C-276 can be sensitised and then attacked intergranularly in hot nitric acid; Incoloy 825 and Inconel 625 are stabilised against it. Hastelloy B-3 needs re-annealing after more than about 7% cold work, or it may crack in the weld region.
What to Send Us When You Enquire
An enquiry that names only the chemical usually needs a second round. Send these and we can confirm the grade in one:
- The chemical or mixture, and its concentration – and whether that changes through the process.
- The maximum temperature, and whether the liquid boils.
- Whether it is aerated or deaerated, and any oxidising contaminants: ferric or cupric ions, chlorates, nitric acid.
- Chloride, fluoride and sulphide content.
- Flow velocity and any suspended solids.
- Whether the equipment is welded, and whether it can be stress relieved.
- The product form, size and quantity, and the certification you need – EN 10204 3.1 as standard, 3.2 and third-party inspection on request.
For hydrogen sulphide service the standards are different; see alloys for sour service. Otherwise, send us the enquiry.
Where These Figures Come From
Every limit, rate and ranking on this page is read from the producer of the alloy it describes:
- Special Metals Corporation, High-Performance Alloys for Resistance to Aqueous Corrosion, with the MONEL alloy 400 and Nickel 200 & 201 bulletins – Nickel 200 and 201, Monel 400, Inconel 600, 625, 686 and 690, C-276, and Incoloy 825. Special Metals sells C-22 as INCONEL 622, Alloy 20 as INCOLOY 020 and Alloy 926 as INCOLOY 25-6MO; the UNS numbers are the same, so its data for those names is data for these grades.
- Haynes International, corrosion-resistant alloys and the B-3, C-4, C-22, C-276, C-2000 and G-30 alloy pages – the Hastelloy descriptions and every figure in the side-by-side tables, taken from each page’s reagent-grade solutions table.
- Outokumpu, Ultra range datasheet – 904L.
- Alleima, Sanicro® 28 datasheet – Alloy 28, including its comparisons with 904L and Alloy 20.
- TIMET, Corrosion Resistance of Titanium, and ATI, Corrosion-Resistant Titanium – titanium grades. TIMET states that Grade 16, with 0.05% palladium, behaves almost identically to Grade 7, so its Grade 7 figures stand for Grade 16 as well.
Laboratory figures are for reagent-grade chemicals under controlled conditions. Haynes encourages field tests before industrial use, and Special Metals cautions that the presence or absence of any single condition can significantly alter an alloy’s performance. Treat these as the basis for a shortlist, then confirm against the process.
Frequently Asked Questions
Which alloy is best for caustic soda?
Nickel 200, which Special Metals describes as unexcelled in resistance to caustic alkalies. It resists caustic soda at all concentrations up to and including the molten state, and below 50% its corrosion is negligible even in boiling solutions. Above 315 °C use the low-carbon Nickel 201, because Nickel 200 can precipitate graphite at those temperatures. Monel 400 is nearly as resistant through most of the concentration range but can suffer stress-corrosion cracking in strong alkalis at elevated temperatures, and Inconel 600 is preferred to Nickel 201 where hot caustic contains sulphur.
Is Monel 400 suitable for alkali service?
Yes, within one limit. Special Metals rates it nearly as resistant as Nickel 200 to caustic soda through most of the concentration range, but notes that it is subject to stress-corrosion cracking in strong alkalis at elevated temperatures and corrodes faster in highly concentrated caustic soda and caustic potash when hot. It resists anhydrous ammonia, and ammonium hydroxide up to 3%, when no dissolved oxygen is present.
Is Hastelloy C-276 the best alloy for sulphuric acid?
Not in every case. Special Metals gives C-276 useful resistance up to 10% sulphuric acid at boiling and at all concentrations up to about 80 °C, and above 50% concentration ranks it second only to Inconel 686. Below 50% it ranks Incoloy 825, Inconel 686, C-22 and Inconel 625 all ahead of it. Haynes International gives C-2000 a deliberate copper addition for sulphuric acid, and in Haynes’ own tests boiling 10% acid corroded C-2000 at 0.09 mm/y against 0.18 mm/y for C-276.
Why is Monel 400 used for hydrofluoric acid?
Because its producer calls it perhaps the most resistant of all commonly used engineering alloys to that acid. In non-aerated acid it resists all concentrations up to the boiling point, it is comparatively insensitive to velocity, and small amounts of sulphuric acid or reducing sulphur compounds do it no harm. Aeration and oxidising salts raise the rate, and in moist, aerated hydrofluoric acid vapour it can crack under stress; a stress relief of one hour at 538 to 648 °C followed by slow cooling is usually sufficient to prevent that.
Can titanium be used for acids?
For oxidising acids, yes. TIMET rates titanium highly resistant to nitric, chromic and hypochlorous acid, and fully resistant to hypochlorites and chlorine dioxide. In reducing acids its resistance is moderate: unalloyed Grade 2 handles about 7% hydrochloric and about 5% sulphuric acid at room temperature, while the palladium-bearing Grade 7 reaches about 27% and 45%. It must not be used with hydrofluoric acid, or fluoride solutions below pH 7, which attack it rapidly, or with red fuming nitric acid.
Why must Hastelloy B-2 and B-3 be kept away from ferric and cupric ions?
They are nickel-molybdenum alloys with little or no chromium, and chromium is what resists oxidising conditions. Special Metals states that Ni-Mo alloys have no resistance to environments containing oxidising species, and that oxidising contaminants such as ferric or cupric ions increase their corrosion rate in hydrochloric acid while actually decreasing it for chromium-bearing Ni-Cr-Mo alloys. Where the acid may carry iron, copper or dissolved air, C-22 or Inconel 686 is the grade Special Metals points to.