Which Alloys for Chemical Processing Equipment?

Choosing the wrong alloy for chemical processing equipment can lead to catastrophic failure, costly downtime, and serious safety hazards. This guide answers a critical engineering question: which alloys are best suited for chemical processing equipment?

We evaluate ten widely used alloys - spanning stainless steel grades, nickel-based alloys, and titanium - across corrosion resistance, mechanical performance, application environments, and cost. Objective data tables are provided throughout to support informed decision-making.

Which Alloys for Chemical Processing Equipment

Key Takeaway

No single alloy is universally best. Selection depends on the specific chemical environment, temperature, pressure, contamination risk, and lifecycle cost. This guide maps those variables to the right material.

Chemical processing plants handle some of the most aggressive substances on earth - concentrated acids, caustic alkalis, oxidizing gases, and halide-rich brines. Equipment such as reactors, heat exchangers, storage tanks, piping, valves, and columns must resist corrosion for 15–30 years of continuous service.

Poor alloy selection causes:

Uniform corrosion - general thinning of vessel walls

Pitting corrosion - localized penetration in chloride-rich media

Stress corrosion cracking (SCC) - sudden brittle failure under tensile stress + corrosive environment

Crevice corrosion - attack in confined spaces such as flange faces and under gaskets

Galvanic corrosion - when dissimilar metals are in electrical contact in an electrolyte

According to NACE International (now AMPP), corrosion costs the global chemical and petrochemical industry an estimated USD 170 billion per year. Materials selection is the single most effective way to reduce that cost.

When specifying an alloy for chemical service, engineers evaluate the following factors:

Corrosion Resistance

The alloy must withstand the specific corrosive media without unacceptable material loss over the equipment's design life. Relevant metrics include:

Corrosion rate (mm/year or mils per year, mpy)

Pitting Resistance Equivalent Number (PREN = %Cr + 3.3×%Mo + 16×%N) - higher PREN indicates better chloride pitting resistance

Critical Pitting Temperature (CPT) and Critical Crevice Temperature (CCT)

Mechanical Properties

Equipment must meet pressure-containing code requirements (ASME, EN 13480, PED). Critical mechanical data include: ultimate tensile strength (UTS), yield strength, elongation, and hardness.

Temperature Range

Elevated temperatures accelerate corrosion and reduce allowable stresses. Many alloys have a maximum service temperature in chemical media below their metallurgical limits.

Fabricability

Alloys must be weldable, machinable, and formable into pressure vessel geometries. Nickel alloys in particular require specialized welding procedures (ASME Section IX).

Total Cost of Ownership (TCO)

Material cost is only one element. TCO includes installation, inspection, maintenance, and avoided downtime costs. A Hastelloy C-276 reactor vessel may cost 4× more than 316L SS but last 5× longer in hydrochloric acid service, delivering a superior TCO.

Table 1 provides a fast-scan overview of ten alloys commonly specified for chemical processing equipment. For detailed data, see Tables 2–4 below.

Alloy	Family	Typical Application	Resistant To	Avoid	Corrosion Resistance	Relative Cost
304 / 304L SS	Stainless	General aqueous, food, pharma	Nitric acid, mild organics	Hydrochloric, sulfuric (conc.)	Moderate	Low
316 / 316L SS	Stainless	Brine, marine, pharmaceuticals	Dilute H₂SO₄, phosphoric, acetic	Hot chlorides, HCl, strong oxidants	Good	Low
317L SS	Stainless	Higher Mo for better Cl⁻ resistance	Dilute acids, bleach plants	Reducing acids at high temp.	Good	Low
904L SS	Stainless	Sulfuric acid service	H₂SO₄ (<50%), phosphoric, acetic	HCl, hot chloride salts	Very Good	Low-Mod
Alloy 20 (N08020)	Ni-Fe-Cr	Sulfuric acid process equipment	H₂SO₄ (all conc.), phosphoric	Hydrofluoric acid	Excellent	Moderate
Hastelloy C-276	Nickel	Broad chemical duty, FGD systems	HCl, H₂SO₄, HF, wet chlorine	Strong oxidizing acids (HNO₃ hot)	Excellent	High
Hastelloy C-22	Nickel	Mixed acid / oxidizing environments	HNO₃ + HCl mixtures, chromic acid	Reducing HCl (>20%)	Excellent	High
Inconel 625	Nickel	High-temp, fatigue, seawater	Phosphoric, organic acids, seawater	Hot HCl, H₂SO₄	Excellent	High
Monel 400 (N04400)	Ni-Cu	Hydrofluoric acid, seawater	HF (all conc.), brine, alkalis	Oxidizing acids, HNO₃, aerated HF	Very Good	Moderate
Ti Gr. 2 / Gr. 12	Titanium	Oxidizing acid & chloride service	HNO₃, chlorine water, wet Cl₂	HF, anhydrous reducing acids	Excellent	High

Table 1 | Alloy Quick-Reference Guide - Chemical Processing Equipment. Corrosion suitability ratings are general guidance; always verify with corrosion coupon testing or published isocorrosion charts for specific media, concentrations, and temperatures.

Stainless steels are iron-chromium alloys containing a minimum of 10.5% chromium. The chromium reacts with oxygen to form a passive chromium oxide (Cr₂O₃) film that self-heals when damaged. Molybdenum, nickel, and nitrogen are added to enhance corrosion resistance.

Stainless Steel Alloys for Chemical Processing

■ General-Purpose Workhorse

Type 304 is the most widely produced stainless steel globally. With 18% Cr and 8% Ni, it performs well in mildly corrosive aqueous environments, food-grade service, pharmaceutical manufacturing, and atmospheric exposure. The L variant (304L) has restricted carbon (≤ 0.03%) to minimize sensitization during welding, preventing intergranular corrosion.

However, 304 is not suitable for chloride-containing media above trace concentrations, nor for hydrochloric or sulfuric acids at meaningful concentrations. This is a common specification error in plants that later experience rapid pitting or SCC.

■ Most Widely Used Grade in Chemical Industry

The addition of 2–3% molybdenum to 316 significantly improves chloride pitting resistance versus 304 and provides better performance in dilute sulfuric, phosphoric, and acetic acid environments. PREN of approximately 24 makes it the industry workhorse for chemical service.

316L SS is the standard of construction for pharmaceutical reactors, acetic acid process piping, seawater-cooled heat exchangers, and dilute acid storage. It is not appropriate for hydrochloric acid service at any meaningful concentration.

■ Higher Molybdenum for Improved Chloride Resistance

317L contains 3–4% molybdenum and higher nickel than 316L, pushing PREN above 30. It is often specified as an upgrade from 316L in bleaching plants, pulp and paper equipment, and dilute chloride service. 317L offers modestly better corrosion resistance at modest cost premium over 316L.

■ High-Performance Austenitic for Sulfuric Acid

904L is a high-alloy austenitic stainless steel with 4–5% molybdenum, 23–28% nickel, and approximately 1.5% copper. It was specifically developed for sulfuric acid service and delivers excellent resistance to H₂SO₄ across a wide concentration range at ambient temperature. Its PREN of ~37 puts it in the same league as 6% Mo super-austenitic grades.

904L is cost-effective for sulfuric acid piping, heat exchangers, and storage tanks where Alloy 20 or Hastelloy grades would be over-specified.

Nickel-based alloys offer superior corrosion resistance compared to stainless steels, particularly in highly aggressive environments such as concentrated acids, reducing media, and high-temperature chemical service. They carry a significant cost premium - typically 4–8× the price of 316L SS - but are often the only technically viable option.

Nickel Alloys for Chemical Processing

■ The Sulfuric Acid Specialist

Alloy 20 was engineered specifically for sulfuric acid service. Its composition - 20% Cr, 29% Ni, 2.5% Mo, 3.5% Cu, balanced in iron - delivers exceptional resistance to H₂SO₄ across virtually all concentrations at ambient to moderate temperatures. The copper addition is critical to its performance in reducing sulfuric acid environments.

Alloy 20 is widely used in sulfuric acid pickling equipment, chemical storage tanks, phosphoric acid process systems, and explosives manufacturing. It represents an economical bridge between stainless steel and the more expensive nickel alloys.

■ The Industry Benchmark for Severe Corrosion Service

Hastelloy C276 is one of the most corrosion-resistant alloys available for chemical plant construction. Its high molybdenum (15–17%) and tungsten (3–4.5%) content provides outstanding resistance to pitting, crevice corrosion, and stress corrosion cracking across a remarkably broad range of chemicals.

C-276 is the primary alloy of choice for flue gas desulfurization (FGD) scrubbers, hydrochloric acid process equipment, wet chlorination systems, waste acid handling, and pharmaceutical synthesis vessels. Its PREN of approximately 70 is among the highest of any commercial alloy.

■ Superior in Mixed Oxidizing/Reducing Environments

C-22 has higher chromium (22%) and slightly lower molybdenum than C-276. This composition gives C-22 an advantage over C-276 in oxidizing environments and mixed-acid service (e.g., HNO₃ + HCl mixtures, chromic acid). C-22 is often preferred in pharmaceutical API synthesis and environmental engineering applications where variable, mixed-chemistry conditions are encountered.

■ High-Temperature and Fatigue Resistance

Inconel 625 derives its strength from the combined solid-solution strengthening of niobium and molybdenum in a nickel-chromium matrix. It resists cyclic thermal fatigue, has excellent cryogenic toughness, and maintains strength at temperatures up to approximately 815°C.

In chemical processing, 625 is specified for heat exchanger tubing in aggressive environments, offshore and marine process equipment, chemical injection tubing, and seawater-cooled condensers. Its PREN of ~51 makes it highly resistant to pitting in seawater and halide-containing process streams.

■ The Choice for Hydrofluoric Acid Service

Monel 400 is a nickel-copper alloy (67% Ni, 23–28% Cu) with a unique combination of properties: excellent resistance to hydrofluoric acid at virtually all concentrations, resistance to seawater and brine, and good mechanical properties across a wide temperature range.

Monel 400 is the preferred alloy for hydrofluoric acid alkylation units in petroleum refining, HF acid storage and handling equipment, marine hardware, and potable water service. It must never be used in the presence of oxidizing acids such as nitric acid.

Commercially pure titanium (Grades 1 and 2) and titanium alloys form an extremely stable, self-healing TiO₂ passive film that provides outstanding corrosion resistance in oxidizing environments. Grade 12 (Ti-0.3Mo-0.8Ni) extends this resistance into mildly reducing conditions.

Titanium's key advantages in chemical service are its excellent resistance to wet chlorine, hypochlorite, chlorinated brines, and nitric acid. Its low density (4.5 g/cm³) is approximately half that of nickel alloys, offering weight savings in large pressure vessels. However, titanium is attacked by hydrofluoric acid, strongly reducing acids, and anhydrous conditions, and requires special handling during welding.

All chemical processing equipment must be designed to applicable pressure vessel codes. Table 2 provides minimum mechanical properties per ASTM standards at room temperature. Design allowable stresses must be sourced from ASME BPVC Section II-D or equivalent.

Alloy	UTS (MPa)	Yield (MPa)	Elongation (%)	Hardness (HB max)	Density (g/cm³)	Key Alloying Elements
304 / 304L SS	515 / 485	205 / 170	40	200	8.0	18–20 / 8–12 Ni
316 / 316L SS	515 / 485	205 / 170	40	217	8.0	16–18 / 10–14 Ni / 2–3 Mo
317L SS	515	205	40	217	8.0	18–20 / 11–15 Ni / 3–4 Mo
904L SS	490	215	35	220	8.0	19–23 / 23–28 Ni / 4–5 Mo
Alloy 20	621	262	30	241	8.0	19–21 / 32–38 Ni / 2–3 Mo
Hastelloy C-276	790	355	40	217	8.9	15–17 Cr / 15–17 Mo / Bal. Ni
Hastelloy C-22	690	310	45	220	8.7	20–22.5 Cr / 12.5–14.5 Mo / Bal. Ni
Inconel 625	827	414	30	220	8.4	20–23 Cr / 8–10 Mo / Bal. Ni
Monel 400	550	240	35	180	8.8	28–34 Cu / Bal. Ni
Ti Gr. 2	345	275	20	160	4.5	Cp Titanium

Table 2 | Typical Minimum Mechanical Properties at Room Temperature (per relevant ASTM standards: A240, A276, B574, B575, B443, B127, B265). Actual values may vary by product form and heat treatment. Confirm with material test reports (MTR).

Table 3 maps common process chemicals to the most appropriate alloy choices and flags critical cautions for each environment. This table is intended as a screening tool; detailed corrosion evaluation using isocorrosion charts and supplier data is essential before final specification.

Chemical Environment	Recommended Alloys	Key Cautions
Sulfuric Acid (H₂SO₄)	Alloy 20 (all conc.); Hastelloy C-276 / C-22 (hot, conc.); 904L (dilute, ambient); Ti Gr.12 (conc. > 80% cold)	Avoid 304/316 SS in hot or concentrated service
Hydrochloric Acid (HCl)	Hastelloy C-276 (primary choice); Hastelloy C-22; Monel 400 (dilute, cold)	All stainless steels susceptible to pitting & SCC
Nitric Acid (HNO₃)	304L / 316L SS (dilute–moderate); Ti Gr.2 (strong oxidizing); Hastelloy C-22 (mixed acids)	Avoid C-276 in hot fuming HNO₃
Hydrofluoric Acid (HF)	Monel 400 (primary choice, all conc.); Hastelloy C-276 (wet/dilute); PTFE-lined systems	Stainless steels & titanium attacked by HF
Phosphoric Acid (H₃PO₄)	904L SS; Alloy 20; Hastelloy C-276; 316L SS (dilute, cold)	Check for chloride impurities in wet-process acid
Caustics / Alkalis (NaOH)	304 / 316 SS (ambient–moderate); Monel 400; Inconel 625 (high temp)	Avoid high temp > 60% NaOH with standard SS
Seawater / Brine	Ti Gr.2 (best choice); Inconel 625; Hastelloy C-276; Super duplex (SAF 2507)	Avoid 304 / 316 SS - susceptible to pitting
Chlorine Gas (Dry/Wet)	Hastelloy C-276 (wet); Monel 400 (dry); Ti Gr.2 (wet chlorine water)	304 / 316 SS attacked by wet Cl₂
Organic Acids (Acetic)	316L / 317L SS; 904L; Alloy 20; Hastelloy C-22	Impurities (Cl⁻, formic acid) increase aggressiveness
High-Temp Oxidizing	Inconel 625 / 600; Hastelloy C-22; Ti Gr.2 (< 315 °C)	Check creep limits above 540 °C for Ni alloys

Table 3 | Alloy Selection by Chemical Environment. Data based on published corrosion data from NACE International, ASM Handbook Vol. 13, and alloy supplier technical literature. Temperature, concentration, and impurity levels significantly affect alloy performance - always conduct site-specific testing.

PREN (Pitting Resistance Equivalent Number) is a calculated index used to rank an alloy's theoretical resistance to chloride-induced pitting corrosion. The formula is:

PREN = %Cr + 3.3 × %Mo + 16 × %N

A higher PREN indicates greater resistance to pitting. Alloys with PREN > 40 are classified as 'super alloys' for pitting resistance. PREN alone does not predict resistance to all forms of corrosion - crevice corrosion, SCC, and uniform corrosion require separate evaluation.

Alloy	PREN Range	Typical PREN	Practical Meaning
304 / 304L SS	18–20	~19	Moderate - suitable for mildly corrosive aqueous service
316 / 316L SS	23–28	~24	Good - widely used in chloride-containing media
317L SS	30–34	~32	Better chloride resistance than 316L
904L SS	34–40	~37	Comparable to 6% Mo grades; very good pitting resistance
Alloy 20	~35	~35	Designed for sulfuric acid; good pitting resistance
Hastelloy C-276	~70	~70	Outstanding - resists localized corrosion across pH range
Hastelloy C-22	~67	~67	Excellent oxidizing and reducing corrosion resistance
Inconel 625	~51	~51	Superior in seawater; resists crevice corrosion
Monel 400	N/A	N/A	PREN not applicable; Ni-Cu system; excels in HF and brine
Ti Gr. 2	N/A	N/A	Passive TiO₂ film; outstanding in oxidizing chloride media

Table 4 | Pitting Resistance Equivalent Numbers (PREN) for Alloys Commonly Used in Chemical Processing Equipment. PREN values are calculated from nominal compositions; actual values vary by heat chemistry. PREN does not predict SCC, uniform corrosion, or resistance in reducing environments.

Material selection does not end with the alloy specification. How the alloy is fabricated into equipment critically affects in-service corrosion performance.

Fabrication and Welding Considerations

Stainless Steel Fabrication

Use low-carbon (L) grades or stabilized grades (321, 347) to minimize sensitization risk during welding

Post-weld heat treatment (PWHT) is rarely required for austenitic SS but solution annealing is recommended for thick sections

Avoid iron contamination - dedicated tooling, brushes, and grinding wheels are required

Passivation per ASTM A380 / A967 should be performed after fabrication

Nickel Alloy Fabrication

Nickel alloys work-harden rapidly - use sharp tooling, lower speeds, and higher feeds than for stainless steel

Welding must use matching or overmatching filler metals (e.g., ERNiCrMo-4 for C-276)

Control heat input carefully to minimize heat-affected zone (HAZ) sensitization

Preheat is generally not required; post-weld solution annealing improves corrosion resistance in critical service

Titanium Fabrication

Weld only in an inert atmosphere (argon or helium purge) - oxygen and nitrogen contamination causes embrittlement

Hot-forming above 480°C requires inert atmosphere to prevent oxidation ('fire scale')

Titanium is compatible with most stainless steel and nickel alloy filler metals in clad applications

Equipment using these alloys must conform to recognized codes and standards. The most commonly referenced include:

Material Standards (ASTM)

A240 / A240M - Stainless steel plate for pressure vessels

A276 / A276M - Stainless steel bar and shapes

B574 / B575 - Hastelloy C-276 and C-22 plate, sheet, strip

B443 / B444 - Inconel 625 plate and tube

B127 / B163 - Monel 400 plate and tube

B265 - Titanium strip, sheet, and plate

Design Codes

ASME BPVC Section VIII (Div. 1 & 2) - Unfired pressure vessels (USA)

EN 13445 - Unfired pressure vessels (Europe)

NACE MR0103 / ISO 17945 - Materials resistant to sulfide stress corrosion cracking in petroleum refining

NACE SP0169 - Control of external corrosion on underground piping

Important Note

Allowable stresses for pressure vessel design must be taken from ASME BPVC Section II-D (or equivalent) at design temperature. Never use room-temperature mechanical properties directly for elevated-temperature design calculations.

Q: What is the best alloy for sulfuric acid equipment?

For sulfuric acid across all concentrations, Alloy 20 is the most cost-effective choice. For hot, concentrated H₂SO₄ (> 60%, > 60°C), Hastelloy C-276 or C-22 provides superior performance. 904L SS is effective for dilute H₂SO₄ at ambient temperature.

Q: Can I use 316L stainless steel for hydrochloric acid?

No. 316L stainless steel is not resistant to hydrochloric acid (HCl) at any meaningful concentration. HCl causes rapid pitting, crevice corrosion, and SCC in austenitic stainless steels. Hastelloy C-276 is the industry standard for HCl service.

Q: What is PREN and why does it matter?

PREN (Pitting Resistance Equivalent Number) is a calculated index that ranks an alloy's chloride pitting resistance based on its chromium, molybdenum, and nitrogen content. Higher PREN = better pitting resistance. It is a screening tool - not a substitute for actual corrosion testing.

Q: Is titanium or Hastelloy C-276 better for chlorine service?

Both alloys are excellent for chlorine service, but they suit different conditions. Titanium Grade 2 is preferred for wet chlorine and chlorinated water up to ~315°C. Hastelloy C-276 is preferred for mixed acid/chlorine environments where reducing conditions may be present.

Q: How do I verify alloy quality when purchasing?

Always require a Material Test Report (MTR / Mill Certificate) per EN 10204 Type 3.1 or 3.2. For critical service, supplement with PMI (Positive Material Identification) using XRF or OES analyzers, and verify ASTM/ASME material certification.

Selecting the right alloy for chemical processing equipment is a critical engineering decision with direct implications for plant safety, reliability, and total cost of ownership. The ten alloys reviewed in this guide - from cost-effective 304L stainless steel to high-performance Hastelloy C-276 - cover the vast majority of chemical service environments encountered in modern process plants.

The key decision factors are clear: understand your chemical environment in detail (concentration, temperature, impurities, pH), apply the data in Tables 1–4 as a screening tool, and then validate with isocorrosion charts, NACE publications, and, where possible, coupon testing in the actual process stream.

When in doubt, consult with a qualified materials engineer or your alloy supplier's technical team. The cost of proper materials specification is a small fraction of the cost of equipment failure.

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