Corrosion Resistance by Media × Alloy: Instrumentation Material Selection Matrix
The corrosion-by-media × alloy matrix is a two-axis dataset. It cross-tabulates the process media an instrumentation system carries against the corrosion-resistant alloys it can be built from, and gives each media-alloy pairing a single resistance rating you can read, compare, and cite. It answers one question in one place: given the fluid your tube fittings, valves, and tubing actually see, which alloy will survive, and which is over- or under-specified. A generic corrosion-resistant alloys list won't tell you that. This matrix names the best corrosion-resistant alloys and corrosion-resistant materials for each specific medium, because the right material changes with the fluid.
These ratings apply to instrumentation fluid-system components: the wetted alloy in compression tube fittings, instrumentation valves, and tubing that carries process media. They don't cover structural, plumbing, or electrical service. And every alloy listed in the columns below? Crestflo manufactures it as a fitting, valve, or tube.
Also Known As: Corrosion Resistance Chart, CRA Selection Guide & Chemical Compatibility Matrix
Engineers and specifiers ask for this same dataset under a lot of different names. Some call it a corrosion resistance chart, others a corrosion resistance table, an alloy corrosion chart, or a metal corrosion resistance chart. Procurement and design documents might reference it as a chemical compatibility chart, a material compatibility chart, a chemical resistance chart for stainless steel and alloys, or a corrosion compatibility matrix. In material-selection workflows, it goes by corrosion-resistant alloy (CRA) selection chart, alloy selection guide, or isocorrosion guide. Every one of these names points to the same media-vs-metal decision table presented here for instrumentation service.
How Corrosion-Resistant Alloys Resist Process Media
A corrosion-resistant alloy doesn't corrode uniformly the way carbon steel does. It survives aggressive media instead by forming a stable, self-repairing passive film on its surface. That film's chemistry, and whatever breaks it down, decides every rating in the matrix.
Chromium builds the base passive layer, a chromium-oxide film. More chromium generally means broader resistance to oxidizing acids and to the atmosphere.
Molybdenum fights pitting and crevice attack in chlorides. It's the single most important element behind the Pitting Resistance Equivalent Number (PREN).
Nickel carries resistance to reducing acids and to chloride stress-corrosion cracking (SCC). High-nickel alloys dominate the acid columns that austenitic stainless simply can't hold.
- Copper (in Monel 400 and Alloy 20) unlocks hydrofluoric and certain sulfuric-acid service.
Titanium holds up well in oxidizing chlorides and seawater, thanks to a tough titanium-oxide film that forms on its surface. Hydrofluoric acid is a different story: it attacks the metal outright, so titanium doesn't belong there.
Those elements sort into four alloy families, and together they form the matrix columns. First, the austenitic stainless steels: 304/304L, 316/316L, 904L. Next, duplex and super-austenitic grades, Duplex 2205, Super Duplex 2507/Zeron 100, 254 SMO, AL-6XN, Alloy 20/Carpenter 20. Then the nickel alloys, Inconel 625, Incoloy 825, Monel 400, Hastelloy C-276, Hastelloy B, Nickel 200/201, and finally titanium (Grade 2). None of them rust, not in the iron-oxide sense, because none depends on bare iron sitting exposed. What changes from one to the next, and what the ratings actually measure, is how well each one's passive film holds up under attack from a particular medium.
The Master Corrosion Resistance Chart: 14 Process Media × 14 Instrumentation Alloys
The table below is the asset. It stacks 14 process media, rows with formula, against 14 instrumentation alloys, columns with UNS number, and each cell carries a resistance rating. It also works as a corrosion resistance comparison, an alloy corrosion comparison across every column, since each one lets you set the alloy grades side by side for a single medium. The ratings come from established, published isocorrosion and chemical-compatibility references for general corrosion at ambient-to-moderate temperature. Read the legend and caveats in the next section before you apply any cell.
| Media \ Alloy | 316/L (S31600) | 904L (N08904) | 2205 (S32205) | 2507 (S32750) | 254 SMO (S31254) | AL-6XN (N08367) | Alloy 20 (N08020) | Inconel 625 (N06625) | Incoloy 825 (N08825) | Monel 400 (N04400) | Hast C-276 (N10276) | Hast B (N10665) | Nickel 200 (N02200) | Ti Gr2 (R50400) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sulfuric acid H₂SO₄ | Xc | Bc | Cc | Cc | Bc | Bc | Ac | Bc | Bc | Bc | Ac | Ac | Cc | Xc |
| Hydrochloric acid HCl | X | Cc | X | Cc | Cc | Cc | Cc | Bc | Cc | Bc | Ac | Ac | Cc | Xc |
| Hydrofluoric acid HF | X | X | X | X | X | X | Cc | Bc | Bc | Ac | Bc | Bc | Bc | X |
| Nitric acid HNO₃ | A | A | A | A | A | A | A | Bc | Bc | Xc | Cc | Xc | Xc | A |
| Phosphoric acid H₃PO₄ | Bc | Ac | Bc | Ac | Ac | Ac | Ac | Ac | Ac | Bc | Ac | Bc | Cc | Bc |
| Chlorides / seawater | Cp | Bp | Bp | Ap | A | A | Bp | A | A | A | A | Bp | Bp | A |
| Sour / wet H₂S¹ | B | B | B | A | A | A | B | A | A | B | A | B | X | B |
| Caustic soda NaOH | Bs | Bs | Bs | Bs | Bs | Bs | B | B | B | A | A | B | A | Bc |
| CO₂ / carbonic (wet) | Bp | A | A | A | A | A | A | A | A | B | A | B | C | A |
| Oxygen (GOX)² | A | A | A | A | A | A | A | A | A | A | A | B | A | A |
| Steam / condensate | A | A | A | A | A | A | A | A | A | A | A | B | B | A |
| Ammonia / amines | A | A | A | A | A | A | A | A | A | Xs | A | B | B | A |
| Organic / acetic acids | Ac | A | A | A | A | A | A | A | A | Bc | A | Bc | Cc | A |
| General atmospheric | A | A | A | A | A | A | A | A | A | A | A | A | A | A |
Caveat superscripts: c = concentration-dependent · T = temperature-limited · p = pitting/crevice risk · s = SCC risk. ¹Sour service falls under NACE MR0175 / ISO 15156 hardness and environmental limits; the rating here only tells you the alloy family. ²Oxygen is a different matter: the alloy itself is compatible, but the real gatekeeper is cleanliness, per ASTM G93, not corrosion resistance. Take Monel 400 in ammonia: it's rated X, since copper alloys are prone to ammonia stress-corrosion cracking.
European specs label these same columns with Werkstoff (EN material) numbers, which line up one-to-one with the UNS designations in the header wherever the grade sees common use in EU service: 316L is Werkstoff 1.4404, 904L is 1.4539, Duplex 2205 is 1.4462, Super Duplex 2507 is 1.4410 (Zeron 100 is 1.4501), and 254 SMO is 1.4547. When matching a European material spec, treat the UNS number in the column and the Werkstoff number here as the same alloy.
Read the ratings as guidance, not a warranty. They're pulled together from published isocorrosion and compatibility references covering general corrosion at ambient-to-moderate conditions. Temperature, concentration, aeration and velocity all push these numbers around, so every application needs to be checked against its exact conditions. Final material selection rests with the specifying engineer. Values follow applicable published references and can be adjusted to fit your requirements.
How to Read This Alloy Corrosion Chart: Rating Legend & the Three Caveats
Each cell gets an A/B/C/X rating, based on the standard engineering bands for corrosion rate.
| Symbol | Meaning | Typical corrosion-rate band* |
|---|---|---|
| A | Fully resistant / recommended | < 0.05 mm/yr (< 2 mpy) |
| B | Useful resistance with limits, watch temperature, concentration, or localized attack | 0.05–0.5 mm/yr (2–20 mpy) |
| C | Limited / marginal, case-by-case only | 0.5–1.25 mm/yr (20–50 mpy) |
| X | Not recommended, general attack, pitting, or SCC likely | > 1.25 mm/yr (> 50 mpy) or SCC risk |
Bands follow the conventional NACE/engineering corrosion-rate classification. The exact rate, though, always depends on temperature, concentration, aeration, and velocity.
The chart also flags the failure mode behind each rating: general corrosion, pitting, crevice corrosion, stress-corrosion cracking (SCC), intergranular attack, or galvanic corrosion. A single letter never tells the whole story. Three variables move almost every cell.
- Temperature: a B rating at 20 °C can turn into an X at 80 °C. That's not an anomaly; it's the whole reason isocorrosion curves exist. Corrosion resistance has a temperature limit, full stop.
- Concentration: acids don't behave consistently across their concentration range. Many swing from reducing to oxidizing as concentration climbs, and that's exactly why so many acid cells carry the "c" flag.
- Aeration and velocity: dissolved oxygen and flow don't have one fixed effect. Depending on the alloy and the media involved, they can just as easily protect the surface as tear it apart.
One more caution: mix dissimilar alloys in the same wetted system and you can drive galvanic corrosion at the couple. Check the galvanic relationship first when combining grades or brands, see the galvanic risk of mixing tube-fitting brands.
PREN: The Pitting Resistance Equivalent Number Shortcut for Chlorides
For chloride and seawater service, no single test cell predicts pitting and crevice resistance as well as one number: the Pitting Resistance Equivalent Number. PREN = %Cr + 3.3 × %Mo + 16 × %N. Push that figure up, and the critical pitting temperature climbs with it, giving more margin against chloride attack.
| Alloy | UNS | PREN (approx.) | Chloride verdict |
|---|---|---|---|
| 316L | S31603 | ~24 | Pits in warm chlorides |
| 904L | N08904 | ~34 | Better, still limited in seawater |
| Duplex 2205 | S32205 | ~35 | Good, with strength |
| Super Duplex 2507 | S32750 | ~42 | Strong seawater performer |
| 254 SMO | S31254 | ~43 | 6-moly seawater grade |
| AL-6XN | N08367 | ~45 | Highest of the group |
These PREN figures are approximate, standard published values. Exact per-heat PREN isn't set by them; it's set by the certified composition. For the full ranking, see the PREN comparison table.
Media-by-Media Alloy Selection Notes
Each block below flags the alloy or alloys rated A, lists the grades to steer clear of, spells out the caveat behind the choice, and points to where the fuller selection detail lives. The first five blocks deal with the acid-resistant alloys, the metal choices suited to sulfuric, hydrochloric, hydrofluoric, nitric, and phosphoric acid, and here it's an alloy's reducing versus oxidizing behavior that drives the pick.
Best Alloy for Sulfuric Acid (H₂SO₄)
Alloy 20 (Carpenter 20) has long been the workhorse for sulfuric acid across a wide range of concentrations. Hastelloy B and C-276 cover narrower windows, while 904L and 254 SMO handle the dilute end well. 316 and 316L won't hold up in sulfuric service. Titanium fares no better once the acid turns reducing. Check concentration and temperature before specifying anything: see Alloy 20 for sulfuric acid.
Best Alloy for Hydrochloric Acid (HCl)
Hydrochloric acid behaves as a reducing acid, and that's where nickel-molybdenum alloys shine. Hastelloy B (B-2/B-3) and Hastelloy C-276 top the list, with C-22 belonging to the same family. Stainless grades don't hold up here, they're rated X, since they suffer rapid general attack and pitting. For more detail, see Hastelloy C-276 for HCl.
Best Alloy for Hydrofluoric Acid (HF)
Monel 400 is the answer for hydrofluoric acid and remains the reference choice for anhydrous and aqueous HF. Titanium must be avoided, HF strips its passive film and attacks it aggressively. See Monel 400 for HF service.
Best Alloy for Nitric Acid (HNO₃)
Nitric acid oxidizes aggressively, and that favors high-chromium steels. 304L, 316L (the low-carbon versions of 304 and 316), 904L, and Alloy 20 all earn an A rating. Skip the reducing-service nickel-molybdenum alloys, Hastelloy B among them; they corrode fast when exposed to oxidizing acids. Titanium does well here too, holding up under oxidizing nitric conditions.
Best Alloy for Phosphoric Acid (H₃PO₄)
904L, Alloy 20, 254 SMO, and the nickel alloys all handle clean phosphoric acid without trouble. Bring contaminants into the mix, chlorides or fluorides, and you're pushed toward the higher grades. 316/316L holds up too, but only within limited concentrations.
Best Alloy for Chlorides, Seawater & Brackish Water
6-molybdenum grades like 254 SMO and AL-6XN earn an A rating for seawater and chloride exposure, and so do Super Duplex 2507 and Titanium Grade 2. 316 pits, though, and austenitic grades run a real risk of chloride stress corrosion cracking once temperatures climb past roughly 60 °C. PREN is what predicts where each grade lands in that ranking: the higher the PREN, the more pitting margin you get. For more, see offshore chloride selection and Super Duplex 2507 for chlorides.
Best Alloy for Sour Service / Wet H₂S
NACE MR0175 / ISO 15156 hardness and environmental limits drive sour (wet H₂S) selection, not general-corrosion rating alone. 316, Duplex and Super Duplex, Inconel 625/825, and Hastelloy C-276 all qualify, each within its own defined limits. What decides each case: hardness, PREN, and partial-pressure envelopes. See sour-service (NACE) selection, NACE MR0175, and ISO 15156.
Best Alloy for Caustic Soda (NaOH)
For hot, strong caustic soda, Nickel 200/201 sets the benchmark, though Monel 400 and Hastelloy C-276 hold up well too. Austenitic stainless grades don't get the same pass: at higher temperatures and concentrations, caustic stress corrosion cracking becomes a genuine risk. See Nickel 200 for hot caustic for more detail.
Best Alloy for CO₂ / Carbonic Acid (Wet CO₂ & Carbon Capture)
Wet CO₂ forms carbonic acid, and that's what changes the calculus. 316L and Duplex 2205 hold up fine at low partial pressure. But push CO₂ partial pressure higher, and chloride content along with it, and you need to move up in alloy selection. This is a common question for carbon-capture and amine-plant engineers; see carbon capture / CCS.
Best Alloy for Oxygen Service (GOX)
For gaseous oxygen (GOX), the alloy holds up well. Cleanliness is what matters here, not corrosion. Components need oxygen cleaning to strip out hydrocarbons and particulates, per ASTM G93. See oxygen-service cleaning (ASTM G93).
Best Alloy for Steam, Condensate, Ammonia, Amines & Organic Acids
Steam and condensate service isn't a problem for practically any of the stainless or nickel grades. Ammonia and amine service works fine across the board too, with one exception: copper-bearing Monel 400, which falls victim to ammonia stress-corrosion cracking (rated X). Austenitic and nickel grades hold up well against organic and acetic acids, though the leaner alloys need some care around concentration. As a rule of thumb, nickel-molybdenum alloys are the better bet for reducing acids, while high-chromium grades win out against oxidizing acids. That distinction is really the axis the whole matrix turns on. For the decision workflow behind these calls, see material selection by service.
Standards & Proof: NACE MR0175/ISO 15156, EN 10204 MTC & PMI Verification
A rating means nothing if the metal on your dock isn't the metal you actually chose. That's the gap Crestflo closes, from chart to part:
Instrumentation products come available NACE MR0175 / ISO 15156 compliant wherever the alloy and hardness qualify for sour service.
Material traceability follows the EN 10204 3.1 material test certificate standard as a baseline, with 3.2 available on request. Every part carries its heat or lot number.
Positive Material Identification (PMI) confirms that the fitting, valve, or tube actually delivered matches the alloy specified. It's the bridge between "the chart says alloy X" and having a part in hand that's verified as alloy X.
Testing covers NDT done in-house, destructive testing at ISO/IEC 17025-accredited laboratories, and third-party inspection on request.
- Quality system: ISO 9001 / 14001 / 45001 & PED certified.
Fittings undergo testing to ASTM F1387, the standard covering compression tube-fitting performance qualification. Instrumentation valves take a different path: they qualify to ASME B16.34 and the applicable API valve standards. Approvals get stated plainly, as conformance to whatever standard fits the product type. Certifications live at the corporate level. They're not numbered individually on content pages.
Turn the Chart Into a Part: Every Alloy as a Crestflo Fitting, Valve or Tube
Most mills and distributors that show up in corrosion chart rankings sell just one product form. Crestflo doesn't work that way. Every column of this matrix gets built out as an actual instrumentation component: you choose the media-alloy cell, and the fitting, valve, or tube gets made in that exact alloy.
| Alloy | Tube fittings | Instrumentation valves | Manifolds | Tubing | Grade page |
|---|---|---|---|---|---|
| 316 / 316L | ✓ | ✓ | ✓ | ✓ | 316/316L |
| 904L | ✓ | ✓ | ✓ | ✓ | 904L |
| Duplex 2205 | ✓ | ✓ | ✓ | ✓ | Duplex 2205 |
| Super Duplex 2507 | ✓ | ✓ | ✓ | ✓ | Super Duplex 2507 |
| 254 SMO | ✓ | ✓ | ✓ | ✓ | 254 SMO |
| AL-6XN | ✓ | ✓ | ✓ | ✓ | AL-6XN |
| Alloy 20 | ✓ | ✓ | ✓ | ✓ | Alloy 20 |
| Inconel 625 | ✓ | ✓ | ✓ | ✓ | Inconel 625 |
| Incoloy 825 | ✓ | ✓ | ✓ | ✓ | Incoloy 825 |
| Monel 400 | ✓ | ✓ | ✓ | ✓ | Monel 400 |
| Hastelloy C-276 | ✓ | ✓ | ✓ | ✓ | Hastelloy C-276 |
| Hastelloy B-2/B-3 | ✓ | ✓ | ✓ | ✓ | Hastelloy B-2 |
| Nickel 200/201 | ✓ | ✓ | ✓ | ✓ | Nickel 200/201 |
| Titanium Grade 2 | ✓ | ✓ | ✓ | ✓ | Titanium Gr2 |
Our product range covers compression and double-ferrule tube fittings up to 2" (50 mm) OD, instrumentation valves from 1/16" through 1", 2/3/5-way manifolds, and instrumentation tubing, offered in both imperial and metric sizes. Standard items ship from ready stock; specials are made to order, with delivery running 6–8 weeks. As for material origin, it's Indian by default, though US and European melt can be arranged on request, and full traceability comes standard.
Request a quote for any alloy on this chart, as a fitting, valve, or tube. Distributors can request the line card and distributor program. If you're an OEM, start a private-label program or RFQ. Equipment makers can send a print and talk to an engineer. Need the full cross-reference to your current brand? Request your Swagelok® / Parker® cross-reference from the engineering team.
Swagelok® is a trademark of Swagelok Company. Parker® belongs to Parker Hannifin Corporation. Crestflo isn't affiliated with either one, and neither company endorses it. The interchange lines up dimensionally, and it's backed by ASTM F1387 performance qualification, not just claims.
Why Crestflo for Corrosion-Resistant Instrumentation
Crestflo is the instrumentation division of a four-decade-old export house recognized by the Government of India, with a delivery track record across the Middle East. That heritage stands behind a brand built specifically for instrumentation. Every alloy in this matrix works as a drop-in instrumentation component, matching the dimensions of the major brands, backed by ASTM F1387 performance qualification on compression tube fittings and ASME B16.34 / API qualification on valves. Small minimum order quantities, private-label options, EN 10204 3.1/3.2 MTC, heat-number traceability, and PMI on the alloy: put together, they mean the part you receive is the part the chart specified. That's what it means to Engineer Confidence.
Frequently Asked Questions
What is the most corrosion-resistant alloy?
No alloy wins outright. It all comes down to the medium you're dealing with. Hastelloy C-276 and C-22 cover the broadest range of acids and chlorides, while Monel 400 is the one to reach for with hydrofluoric acid. Nickel 200/201 handles hot caustic best, and Titanium Grade 2 takes oxidizing chlorides. Skip the search for a universal alloy. Read the matrix against your specific medium instead.
What alloys do not rust?
"Rust," strictly speaking, is iron oxide. Every alloy on this chart fights it off the same way: by forming a passive film. That covers austenitic stainless (304/316), duplex and super-austenitic grades, nickel alloys, and titanium. None of them are truly immune, though. Put them in aggressive chlorides or acids and they can still pit, corrode at crevices, or crack outright. That's exactly what the A/B/C/X ratings are meant to capture.
Is Inconel a corrosion-resistant alloy?
Yes. Inconel 625 and Incoloy 825 both belong to the nickel-chromium family of corrosion-resistant alloys, and they hold up well against chlorides, sour service, and a wide range of acids. You'll find exact composition figures in their matrix columns and on their individual grade pages.
What is the toughest alloy on this chart?
Mechanical toughness and corrosion resistance aren't the same thing. Not even close. When the corrosion range needs to be as wide as possible, Hastelloy C-276 wins out. Need high mechanical strength paired with resistance to chlorides? Super Duplex 2507 is the better pick. Let the failure mode your service actually produces make the decision.
Which alloy is best for seawater and chlorides?
The 6-molybdenum grades, 254 SMO and AL-6XN, earn an A rating, as do Super Duplex 2507 and Titanium Grade 2. 316 doesn't fare as well; it pits in warm chlorides. The reason comes down to PREN: a higher Pitting Resistance Equivalent Number gives more margin against pitting and crevice attack. For more detail, see offshore chloride selection.
Which alloy is best for sulfuric acid?
Alloy 20 remains the go-to material across a wide concentration range. For specific concentration windows, 904L and the Hastelloy B/C alloys are better suited, while 316 simply isn't recommended. Confirm concentration and temperature before you finalize anything. See Alloy 20.
Which alloy is best for hydrofluoric acid?
Monel 400. Titanium reacts badly with hydrofluoric acid, so keep it out of HF service. See Monel 400.
What alloy meets sour-service (wet H₂S) requirements?
NACE MR0175 / ISO 15156 sets the hardness and environmental limits that govern selection. Within those limits, 316, Duplex and Super Duplex, Inconel 625/825, and Hastelloy C-276 all qualify. For more, see sour-service selection and NACE MR0175.
What is PREN and why does it matter?
PREN, short for Pitting Resistance Equivalent Number, is calculated as %Cr + 3.3 × %Mo + 16 × %N. It ranks how well an alloy stands up to chloride pitting and crevice attack. The higher the number, the better the chloride performance. For a side-by-side look, check the PREN comparison table.
Do these ratings account for temperature and concentration?
The base ratings assume ambient-to-moderate conditions. Temperature, concentration, aeration, and velocity can all shift them. Check the caveat flags and your exact process conditions before you select anything. These ratings are engineering guidance. They aren't a warranty.
Can Crestflo supply fittings in these alloys?
Yes. Crestflo makes instrumentation tube fittings up to 2 inches (50 mm OD), valves from 1/16" to 1", manifolds, and tubing. Every alloy on the chart is available. Each one ships with EN 10204 3.1/3.2 material test certificates, heat-number traceability, and PMI verification.
How fast can I get an exotic alloy, and is there a minimum order?
Standard items ship from ready stock. Specials are different: they're made to order, with 6–8 week delivery and small minimum order quantities. Private-label options are available, and ASTM F1387 backs the compression-fitting interchange.
Related Guides, Alloy Pages & Standards
- Material selection by service, the decision workflow behind this lookup table
- PREN comparison table · 904L vs 316 · Duplex vs Super Duplex · 2205 vs 2507 · 254 SMO vs Super Duplex vs AL-6XN
- Sour-service (NACE) selection · Offshore chloride selection
- NACE MR0175 · ISO 15156 · ASTM G93 oxygen cleaning
- Industries: oil & gas · offshore · subsea · petrochemical & chemical · carbon capture / CCS
- Instrumentation fitting materials hub · Resources hub · Request a quote