Address
304 North Cardinal St.
Dorchester Center, MA 02124
Work Hours
Monday to Friday: 7AM - 7PM
Weekend: 10AM - 5PM
Acid handling is where fitting and flange selection stops being a matter of strength and becomes a matter of which alloy family the acid chemistry will tolerate. The three acids that dominate industrial service behave in completely different ways against stainless steel, and the grade that works in one will fail in another. This piece sets out the service conditions for sulphuric, hydrochloric and phosphoric acid, the corrosion challenge each presents, and the selection logic that follows from the ASME specifications.
It is written as a selection framework, not a project record. Concentrations, temperatures and duty cycles have to be established from the process data and confirmed against the material supplier’s own corrosion data before any grade is committed to an order, because ASME Section II is a material specification volume and carries no corrosion rates.
What the material specifications actually give you
Hydrochloric acid is reducing and chloride-bearing at the same time, so it strips the chromium oxide passive film that austenitic grades rely on and keeps it stripped, faster as concentration and temperature rise. Sulphuric acid behaves differently across its concentration range: oxidising and passivating when concentrated, but reducing when dilute, and that transition is where most failures happen. Phosphoric acid is milder but is frequently contaminated with chlorides and fluorides from the process, so the selection depends on the impurity budget as much as on the acid. One grade cannot be quoted as the answer to an acid enquiry; the real question is which alloy resists this acid, at this concentration, at this temperature, in this contamination, and whether it is available in the product form you need.
The composition data below is read from the ASME BPVC Section II material specifications, so the grade designations and limits are exact.
| Grade | UNS | Carbon max % | Nickel % | Chromium % | Molybdenum % | Copper % | Other |
|---|---|---|---|---|---|---|---|
| F 20 | N08020 | 0.07 | 32.0-38.0 | 19.0-21.0 | 2.00-3.00 | 3.0-4.0 | Columbium 8xC min, 1.00 max |
| F 316L | S31603 | 0.030 | 10.0-15.0 | 16.0-18.0 | 2.00-3.00 | Nitrogen 0.10 | |
| F 51 | S31803 | 0.030 | 4.5-6.5 | 21.0-23.0 | 2.5-3.5 | – | Nitrogen 0.08-0.20 |
| F 53 | S32750 | 0.030 | 6.0-8.0 | 24.0-26.0 | 3.0-5.0 | 0.50 | Nitrogen 0.24-0.32 |
| F 62 | N08367 | 0.030 | 23.5-25.5 | 20.0-22.0 | 6.0-7.0 | 0.75 | Nitrogen 0.18-0.25 |
| F 44 | S31254 | 0.020 | 17.5-18.5 | 19.5-20.5 | 6.0-6.5 | 0.50-1.00 | Nitrogen 0.18-0.25 |
Read across the molybdenum column and the logic of acid resistance becomes visible. Molybdenum stabilises the passive film against chloride attack, which is why the 6 per cent molybdenum grades dominate hydrochloric and reducing acid service, and why the 2 to 3 per cent molybdenum austenitic grades struggle as concentration and temperature climb. Copper is the other lever, and it is why the F 20 grade exists in this form: its 3.0 to 4.0 per cent copper content, combined with 32 to 38 per cent nickel, is what gives it resistance to reducing sulphuric acid where the standard austenitic grades do not.
Sulphuric acid: the concentration transition
Sulphuric acid at moderate concentration and elevated temperature is a reducing acid, and that is where the copper-bearing nickel-iron-chromium alloys earn their place. The heat treatment matters as much as the composition: F 20 is solution treated and quenched from 1700-1850 F [925-1010 C] with liquid quenching below 500 F [260 C].
The same alloy family in pipe and tube form carries the same three UNS numbers, and the differences between them are the practical selection lever. N08020 has the highest copper and the lowest molybdenum of the three; N08026 has higher molybdenum and nitrogen with lower nickel; N08024 sits between them with columbium added. Where the service moves toward the reducing and hotter end of the range, N08026 is the version to evaluate first, and the pipe specification states the delivery condition explicitly: N08020 is furnished stabilised-annealed, N08026 solution-annealed and N08024 annealed, with recommended annealing temperatures of 1800-1850 F [982-1010 C], 2050-2200 F [1121-1204 C] and 1925-1975 F [1052-1079 C] respectively.
For castings, the same UNS number appears under a different designation in the cast specification: WP20CB, with the same 0.07 per cent carbon, 32.0 to 38.0 per cent nickel, 19.0 to 21.0 per cent chromium, 2.00 to 3.00 per cent molybdenum and 3.0 to 4.0 per cent copper. Common names in the same table map WP6XN to N08367, WP904L to N08904 and WPXM-19 to XM-19, so the grade name you are given in an enquiry does not always match the designation that goes on the purchase order.
Hydrochloric acid: where molybdenum earns its place
The selection path here runs to the higher-molybdenum nickel alloys: F 62 at 6.0 to 7.0 per cent molybdenum, and the C-276 and C-22 families in pipe and tube form.
For C-276 in the SB-622 pipe and tube specification, the composition limits are 14.5 to 16.5 per cent chromium, 15.0 to 17.0 per cent molybdenum and 4.0 to 7.0 per cent tungsten, with nickel as the balance and 0.010 per cent maximum carbon. The C-22 grade in the same specification carries 20.0 to 22.5 per cent chromium, 12.5 to 14.5 per cent molybdenum and 2.0 to 6.0 per cent tungsten. For C-276 the minimum mechanical requirement is 100 ksi [690 MPa] tensile, 41 ksi [283 MPa] yield and 40 per cent elongation, and the dedicated forging specification carries the same figures for solution-annealed N10276.
One practical point that catches people out on Site B enquiries: SA-182 contains no F 625 and no F C-276. A forged C-276 component is governed by the dedicated nickel alloy forging specification, not the stainless steel forging specification, so a quotation written against SA-182 for a C-276 forging names a grade that does not exist. Our C-276 butt weld fitting range is supplied against the correct forging specification.
Phosphoric acid: the contamination budget decides
Phosphoric acid is the least aggressive of the three, and 316L is often adequate. The qualification is that phosphoric acid plants process impure acid, and the impurities that matter are chlorides and fluorides. As contamination rises the selection moves up the molybdenum scale, which is why the duplex grades F 51 and F 53 and the 6 per cent molybdenum F 44 are the ones to evaluate when the impurity budget is high or the duty runs hot.
The fitting and flange side of the decision
For fittings and flanges, the same alloy choice has to survive the product form. Austenitic forgings go to SA-182 and castings to SA-403, while nickel alloy forgings go to the dedicated nickel alloy forging specification. Heat treatment and mechanical requirements follow the alloy, and they are what the certificate has to state.
| Grade | Product form | Solution treatment | Tensile min | Yield min | Elong. min | Hardness |
|---|---|---|---|---|---|---|
| F 20 | SA-182 forging | 1700-1850 F [925-1010 C], liquid quench below 500 F [260 C] | 80 ksi [550 MPa] | 35 ksi [240 MPa] | 30% | 50% reduction of area |
| F 51 | SA-182 forging | 1870 F [1020 C], liquid quench | 90 ksi [620 MPa] | 65 ksi [450 MPa] | 25% | 45% reduction of area |
| F 53 | SA-182 forging | 1880 F [1025 C], liquid quench | 116 ksi [800 MPa] | 80 ksi [550 MPa] | 15% | 310 HBW max |
| F 62 | SA-182 forging | 2025 F [1105 C], liquid quench | 95 ksi [655 MPa] | 45 ksi [310 MPa] | 30% | 50% reduction of area |
| F 44 | SA-182 forging | 2100 F [1150 C], liquid quench | 94 ksi [650 MPa] | 44 ksi [300 MPa] | 35% | 50% reduction of area |
Surface condition is a separate acceptance decision in acid service. The general requirements for forgings permit surface imperfections not deeper than 5 per cent of the actual wall thickness at the point of interest or 1/16 in. [1.6 mm], whichever is less, and mechanical marks, abrasions and pits are acceptable without grinding provided the depth does not exceed 1/16 in. [1.6 mm]. But the wall thickness at the point of grinding must be determined by deducting the amount removed from the nominal finished wall thickness, and may never be less than the specified minimum. For a thin-wall corrosion-critical fitting, that single line is the difference between a repairable part and a rejected one. Stainless steel and nickel alloy fittings need not be coated, and unmachined surfaces of stainless steel fittings must be passivated or electropolished.
Our Alloy 20 Carpenter 20 fittings and flanges and the Inconel 625 butt weld fitting listing are supplied against these requirements, with the heat treatment condition on the certification.
How to structure the enquiry
Five pieces of information turn an acid service enquiry from a grade guess into a defensible selection: the acid and its concentration, the working temperature, whether the acid is aerated or deaerated, the impurity content, particularly chlorides and fluorides, and the product form needed. With those, the shortlist can be built against the composition limits above and the heat treatment confirmed against the specification.
Send us the service conditions and the product form you need, and we will return the shortlist with the governing specification clause for each grade. enquire about acid service fittings on WhatsApp and telephone +86 186 2137 7760, or through our contact page, and we will confirm the specification, the delivery condition and the documentation package.
