Chemical Equipment

Application Scenarios

Acids are not one problem — they are several. Reducing acids (HCl, dilute H₂SO₄) demand high nickel + molybdenum; oxidizing acids and mixed acids demand chromium + molybdenum; hydrofluoric acid is its own special world ruled by nickel-copper. Match the alloy to the acid, its concentration and its temperature.

Hydrochloric Acid (HCl) — the toughest common acid

Hydrochloric acid is a strong reducing acid and attacks almost every stainless steel at useful rates. It is the classic domain of the nickel-molybdenum Hastelloy B family and of nickel metal.

• Hastelloy B2 / B3 (UNS N10665 / N10675): the standard choice for HCl across a wide concentration and temperature range, including boiling dilute-to-medium acid. B3 adds better thermal stability and fabrication tolerance over B2.
• Nickel 200 / 201 (UNS N02200 / N02201): good in HCl at low-to-moderate concentration and temperature; Nickel 201 is preferred above ~315 °C for lower carbon.
• Avoid: 300-series stainless, 904L, 254SMO, Inconel and the C-family Hastelloys — they are rapidly attacked by HCl. Use them only for trace HCl in otherwise mild streams.

Sulfuric Acid (H₂SO₄) — concentration and temperature dependent

Sulfuric acid behavior flips with concentration. Dilute/medium H₂SO₄ is reducing (favors high-Ni/Mo); very concentrated hot H₂SO₄ becomes oxidizing (favors high-Cr). Selection is a moving target.

• ≤ ~50%, up to ~60 °C: Alloy 20 (N08020), 904L (N08904), 254SMO, even 316L/317L for the mildest cases.
• Higher concentration / higher temperature / contaminated: Hastelloy C276 (N10276), C22 (N06022), Alloy 31 (N08031) — the versatile workhorses for aggressive H₂SO₄.
• Hastelloy B2/B3 also performs well in H₂SO₄ but is less versatile than the C-family when chlorides or oxidizers are present.

Phosphoric Acid (H₃PO₄) — moderate, but watch the impurities

Phosphoric acid is milder than HCl. Commercial (wet-process) phosphoric acid, however, carries fluoride and chloride impurities that sharply raise aggressiveness.

• Pure / mild H₃PO₄: Alloy 20, 904L, 254SMO, 316L/317L are widely used.
• Hot, concentrated, or impurity-laden (F⁻/Cl⁻): Hastelloy C276 / C22, Alloy 31 give the margin you need.
• Alloy 20 was essentially designed around phosphoric and sulfuric acid service and remains a cost-effective favorite here.

Hydrofluoric Acid (HF) — the nickel-copper rule

Hydrofluoric acid is chemically unique. The classic, field-proven material is Monel 400 / K500 (Ni-Cu), with Nickel 200/201 also performing well. A non-negotiable rule: avoid silicon-bearing alloys (including many high-silicon stainless and cast irons not specifically rated for HF) — silicon dramatically accelerates attack.

The same acid can demand different materials depending on where it lives in the plant. Thin-wall tubing, agitated vessels and tall columns each create their own corrosion and mechanical challenges.

Heat Exchangers

Heat-exchanger tubes run thin walls (often 0.5–2.0 mm) with high surface area and thermal cycling, so a small corrosion rate means a short life. Tube sheets are thicker and often a different, heavier grade or explosively clad. U-tubes need excellent ductility (annealed condition) to bend without cracking.

• General corrosive service: 904L, 254SMO, Alloy 20 seamless tubes.
• Aggressive acid / chloride: Hastelloy C276, B2/B3, or Titanium/Monel for specific streams.
• Tube sheets: often solid Hastelloy or a carbon-steel base explosively clad with the tube alloy to control cost.

Selecting the alloy is only half the job. A corrosion-resistant grade fabricated with the wrong procedure can corrode faster than a cheaper grade done correctly. Welding, heat treatment and forming all change the microstructure at the joint — precisely where failures start.

The weld zone is the weak point

Welding heats and cools the metal rapidly. In austenitic stainless steels this can sensitize the heat-affected zone (chromium carbide precipitation at grain boundaries), creating intergranular corrosion routes. In the nickel alloys, improper filler or excess heat input can dilute chemistry and lose corrosion resistance. The controls are well established:

• Use matched or over-alloyed filler. Hastelloy C276 is typically welded with ERNiCrMo-4 (C276) filler; Alloy 625 with ERNiCrMo-3; Monel with ERNiCu-7. Under-matching filler is the most common cause of weld-line attack.
• Control heat input and interpass temperature. Lower heat input and a cool interpass reduce sensitization and distortion, especially in thin exchanger tubing.
• Solution anneal when required. After welding thin-wall or critical components in 300-series stainless and some nickel alloys, a full solution anneal restores corrosion resistance. For field-fabricated large vessels, low-carbon (L) grades and stabilized (321/347) grades reduce the need for post-weld heat treatment.
• Avoid contamination. Copper, zinc, lead and sulfur contamination from tools, paints or shop dust causes rapid selective attack. Dedicate tools and keep the alloy separate from carbon steel.

Forming, cladding and inspection

Cold forming (e.g., U-tube bending, dished heads) work-hardens the metal and can raise residual stress — a driver of stress corrosion cracking in chloride service. Stress-relief or solution anneal after heavy forming is standard for critical parts. For clad construction, the bond between the carbon-steel base and the corrosion-resistant layer must be verified by ultrasonic inspection, and the overlay thickness confirmed to cover the design corrosion allowance. Final acceptance relies on NDE (PT/MT/UT/RT as specified) plus a mill test certificate to EN 10204 3.1 or 3.2.

UNS numbers, family and the acids/equipment each grade is best known for. Use it to shortlist, then confirm with corrosion data.

*Indicative continuous-service ceiling for corrosion resistance; mechanical design limits under pressure are set by ASME Section VIII and may be lower.

All grades are available as pipe, tube, plate, sheet, bar, forging, flange and fitting, with mill test certificates and third-party inspection on request.

• Stainless steel 347                

• Stainless steel 321                

• Stainless steel 904L               

• Stainless steel 254SMO/F44 

• Duplex S31803/F51               

• Duplex S32205/F60              

• Super duplex S32750/F53