Corrosion, by various chemical pathways, is perhaps the highest threat to a huge array of chemical processing equipment. From acidic solutions to aggressive solvents, from reactive liquid-gas mixtures to hot catalytic reactions, materials in these environments must endure the harshest of conditions while maintaining strength, integrity, performance and longevity. And in many cases, that equipment must be cleanable and easily repairable as well.

Corrosion is the enemy of industrial process equipment, from pharmaceuticals to food and beverage to energy production. Advances in methods and understanding have transformed reliability and functional effectiveness across all sectors, although that effect is perhaps most profound in materials science. And there remain several materials that are well-known, but advancements in their manufacturability has increased the scope of suitable applications.

Here are 10 material types that laugh in the face of corrosion due to materials processing. And some initial criteria to evaluate their performance differentials.

1. Titanium (Grade 2/Grade 7)

Titanium offers excellent resistance to a wide range of corrosive media, including chloride ionic solutions, seawater and many acids. Grade 2 is widely used due to its weldability and moderate strength, while Grade 7 includes palladium for enhanced corrosion resistance in reducing or acid environments. Common uses include heat exchangers, pressure vessels and reactor linings.

2. Hastelloy (C-22, C-276)

This nickel-molybdenum-chromium super-alloy is highly resistant to aggressive chemicals, including hydrochloric, sulfuric and nitric acids. Hastelloy is frequently used in scrubbers, reactors and flue gas desulfurization systems. Its exceptional resistance to pitting, crevice corrosion and stress corrosion cracking renders it ideal for the most demanding environments. C-22 is ideal for oxidizing environments due to its higher chromium content, where C-276 is more resilient in reducing environments because of its higher molybdenum levels.

3. Inconel (600, 625)

Inconel super-alloys provide excellent corrosion and oxidation resistance, even at elevated temperatures. Inconel 625 (Ni-Ch-Ni-Mo) is preferred for its performance versus marine environments, strong oxidizers and acidic solutions due to the low/zero iron content. Inconel 600 (Ni-Cr-Fe) is a more general grade, but still offers very high corrosion and chloride pitting and cracking resistance. Typical applications include distillation columns, valves and high-temperature vessels.

4. Tantalum

Tantalum possesses remarkable corrosion resistance, especially against strong acids (sulfuric and nitric acids). Though very costly, it is used in niche, high-purity chemical applications where relevant exposures are at the extreme end of scale and temperature. Common uses include heat exchangers, thermowells and linings for reaction vessels.

5. Duplex stainless steel (2205, 2507)

Duplex steels combine the strength of ferritic and corrosion resistance of austenitic stainless steels in a twin-structure alloy. They both offer superior resistance to stress corrosion cracking, pitting and general corrosion, where 2205 is lower cost/performance and 2507 is significantly better in extra harsh and high stress environments. They are commonly used in chemical tanks, piping systems and offshore platforms.

6. Alloy 20

Alloy 20 is a unique group of materials of nickel-iron-chrome formulation with copper and molybdenum, designed specifically for performance in sulfuric acid applications. However, it also performs well in environments with chlorides and phosphoric acid. It provides a cost-effective alternative to more expensive alloys and is used in storage tanks, pumps and valves.

7. PTFE (Teflon) and PFA-lined components

Fluoropolymer surface coatings and linings are chemically inert and resistant to virtually all corrosive chemicals. PTFE and PFA are used to line pipes, tanks, valves and gaskets, particularly in high-purity or highly corrosive and high temperature environments where metals would degrade quickly. They allow for the use of lower cost/performance alloys in more demanding applications.

8. Glass-lined steel

Steel vessels lined with enamel glass provide a corrosion-resistant, non-reactive surface. Glass-lined equipment is widely used in pharmaceuticals, fine chemicals and food processing, especially where hygiene and chemical inertness are critical. It is highly susceptible to mechanical damage and, when cracked, can expose the metal substrate to extreme pitting corrosion. This solution requires careful handling and close monitoring in use, but it is a low-cost solution allowing high grade performance from inexpensive materials.

9. Nickel 200/201

Commercially pure nickel offers excellent resistance in presence of caustic materials, like sodium or potassium hydroxide. Nickel 200 has a slightly higher carbon content, where Nickel 201 offers lower carbon, resulting in better properties at high temperatures. These materials are ideal for evaporators, piping systems and heat exchangers in alkaline chemical processes.

10. Stainless steel 316L

Stainless steel 316L is a moderate performance material that is sufficient to a wide spectrum of fairly aggressive operational conditions. While not suitable for the most aggressive environments, 316L offers good corrosion resistance for many chemical processes, including phosphoric and acetic acid applications. It's widely used due to its sweet spot in price, ease of fabrication and broad chemical compatibility.

First steps of selection

Selecting the most appropriate corrosion-resistant material for a more aggressive environment involves balancing safety, performance and cost, usually in that order. What follows are some initial questions or specifications to consider, although each application may have different challenges or industrial standards with which to comply.

Step 1: Assess the chemical environment

Start with a detailed analysis of the chemical media, temperature, pressure and physical state (e.g., liquid, gas, solid). Effective selection starts with understanding of material concentration, alkalinity or acidity, the presence of chlorides or oxidizers. the expected operating and peak temperatures. and flow velocity and turbulence.

Step 2: Understand the corrosion mechanism

It is important to select materials that offer high resistance to the dominant form of corrosion that is to be experienced. These forms are generally categorized as:

• Uniform corrosion: even material loss over a surface
• Pitting/crevice corrosion: highly localized attacks, particular to chloride-rich environments
• Stress corrosion cracking (SCC): cracks formed and propagated by combined tensile stress and corrosive exposure
• Galvanic corrosion: electrochemical driven corrosion between charge-dissimilar metals in conductive media.

Step 3: Evaluate mechanical and structural needs

In addition to corrosion, equipment must handle mechanical stresses. Some chemically resilient materials are less adept in handling various forms of stress and this is a critical factor in material selection and system design.

These stress sources can be combinations of pressure containment, fatigue from oscillating loads, material and structural changes resulting from fabrication processes, or the combination of high/cyclic heat induced expansion combined with other forces.

Some high-resistance materials may lack strength or be difficult to fabricate without chemical changes that affect corrosion resistance, requiring trade-offs or mechanical reinforcements.

Step 4: Cost versus performance

Exotic alloys like Hastelloy or tantalum provide top-tier resistance but at a high price. A balance is required between initial and lifecycle cost, maintenance or replacement frequency required and factor of safety/risk analysis in mode and consequence of failure.

For many applications, stainless steel or coated steel will offer sufficient performance at a lower total cost and offer manageable risks in terms of lifespan.

Step 5: Coatings and linings

When full construction from expensive alloys isn’t feasible, linings or coatings of a more basic metal, using corrosion-resistant materials such as PTFE, enamel, glass or even synthetic rubbers can offer cost-effective options of sufficient performance.

These can provide adequate protection at considerably lower cost than exotic materials, though these solutions require increased inspection and will increase the need for periodic replacement of vulnerable parts.

Step 6: Always consult industry standards

Search for relevant standards through GlobalSpec's standards search engine.

Summary

Through careful analysis of the environment, corrosion risks, mechanical and endurance needs, and cost factors, a material can be identified that ensures longevity, safety and cost-effectiveness — and keeps operations running smoothly. This is often a component-by-component process, rather than a one size fits all decision, as localized factors can be overwhelming, on a zone-by-zone basis.