Materials of Construction for Process Equipment: A Practical Selection Guide

Material of construction (MOC) selection for process equipment determines equipment life, maintenance cost, safety, and capital cost. The selection must account for the corrosive nature of the process fluid at operating temperature, mechanical requirements (strength at temperature), fabricability, weldability, ASME material code compliance, and cost. This guide covers the most commonly specified materials in chemical, pharmaceutical, oil & gas, and food processing industries.

Carbon Steel (CS): ASTM A516 Grade 60/70, A285

Carbon steel is the default, lowest-cost material for pressure vessels and heat exchanger shells. It is appropriate when the process fluid is not significantly corrosive and operating temperature is within range.

Stainless Steel 304 (SS 304): ASTM A240/A312 TP304

SS 304 (18% chromium, 8% nickel) is the most widely produced austenitic stainless steel. It provides excellent resistance to oxidation and corrosion by weak acids, organic compounds, and most neutral chemicals.

• Temperature range: Cryogenic (−196°C) to +870°C for non-pressure applications. For pressure applications, allowable stress decreases above ~450°C.
• Pitting corrosion in chloride-containing media: SS 304 is NOT suitable for chloride service above 60°C. Chloride stress corrosion cracking (SCC) can cause catastrophic failure even at chloride concentrations of 200 ppm at elevated temperatures. For any service with chlorides above 50 ppm at temperatures above 50°C, consider SS 316 at minimum.
• Excellent for: Food-grade applications, dilute acid service (excluding HCl), nitric acid, caustic (up to ~70°C), most organic chemicals.
• ASME Material Spec: SA-240 TP304 (plate), SA-312 TP304 (seamless pipe/tube).

Stainless Steel 316 (SS 316): ASTM A240/A312 TP316

SS 316 adds 2–3% molybdenum to the SS 304 composition, significantly improving resistance to pitting and crevice corrosion in chloride-containing environments.

• Chloride tolerance: SS 316 can tolerate chloride concentrations up to approximately 1,000 ppm at 60°C, compared to ~200 ppm for SS 304 at the same temperature. Above 80°C, even SS 316 is at risk from chloride SCC in seawater or high-chloride brines.
• Suitable for: Seawater at low temperature (< 25°C), pharmaceutical clean steam and WFI systems, dilute HCl (< 1%), acetic acid, many salt solutions.
• Not suitable for: Concentrated chloride solutions at elevated temperature; reducing acids (HCl, dilute H₂SO₄ above 10%).
• SS 316L vs SS 316: The 'L' grade has lower carbon content (max 0.03% vs 0.08%), preventing sensitisation during welding. Always specify 316L for welded fabrication.
• Cost premium over SS 304: approximately 15–25%.

Duplex Stainless Steels: 2205, 2507

Duplex stainless steels have a mixed austenite-ferrite microstructure that gives them approximately twice the yield strength of austenitic grades (SS 304/316) and excellent resistance to chloride stress corrosion cracking and pitting corrosion.

Duplex grades are the preferred material for seawater heat exchangers, desalination plants, offshore equipment, and any high-chloride service that is beyond SS 316 capability but does not require the full corrosion resistance of Hastelloy or Titanium. The high strength allows thinner wall sections, partially offsetting the higher material cost.

Hastelloy C-276 and C-22: ASTM B575

Hastelloy alloys (nickel-molybdenum-chromium base) offer exceptional resistance to a wide range of aggressive chemical environments, including oxidising and reducing acids, chlorides at high temperature, and mixed acid streams.

• Hastelloy C-276: Ni-Mo-Cr alloy (Ni ~57%, Mo 15–17%, Cr 14–16%). Outstanding resistance to HCl, H₂SO₄, HF (dilute), wet chlorine, ferric chloride, seawater at elevated temperature, phosphoric acid. Used in chemical reactors, FGD (flue gas desulfurisation) systems, pharmaceutical reactors.
• Hastelloy C-22: Improved resistance to oxidising acids (HNO₃, HNO₃/HCl mixtures) compared to C-276. Better for mixed acid environments.
• Temperature limit: Up to 1,000°C for oxidation resistance; for pressure applications, allowable stress governs.
• Cost: Hastelloy tubes are approximately 8–12× the cost of SS 316 tubes. Solid Hastelloy construction is typically reserved for extreme services. For many applications, SS 316 shell with Hastelloy tubes (or Hastelloy tube-side lining) provides the necessary protection at lower cost.
• Weldability: Hastelloy requires skilled welders and qualified procedures. Ensure the fabricator has certified Hastelloy welding procedures and welder qualifications before ordering.

Titanium (Grade 2 and Grade 12): ASTM B265

Titanium offers outstanding corrosion resistance in oxidising and chloride-containing environments, including seawater at elevated temperatures, wet chlorine, chlorine dioxide, hypochlorite, and many organic acids.

• Grade 2 (commercially pure titanium): Standard grade for heat exchanger tubes. Excellent seawater resistance, even at 150–180°C. Used in seawater-cooled condensers, coastal power plants, desalination exchangers.
• Grade 12 (Ti-0.3Mo-0.8Ni): Better resistance to reducing conditions and crevice corrosion compared to Grade 2. Used where Grade 2 is marginal.
• Limitations: Titanium is not resistant to dry chlorine gas (only wet), hydrofluoric acid, concentrated H₂SO₄, or concentrated HCl. Embrittlement can occur at temperatures above 300°C in the presence of hydrogen.
• Cost: Titanium tubes are approximately 10–15× the cost of CS tubes, comparable to Hastelloy. However, the thin wall sections possible with titanium (owing to its strength-to-weight ratio and zero corrosion allowance needed) can partially offset material cost.
• Galvanic corrosion: Never use titanium tubes with carbon steel tubesheets without proper isolation — galvanic corrosion will damage the CS tubesheet. Use titanium or duplex tubesheets, or apply protective coatings.

Quick Reference MOC Selection Matrix

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