Selecting Corrosion-Resistant Seals for Chemical Processing: A Material Selection Guide

Understanding Chemical Attack: Why Seals Fail in Corrosive Environments

To make informed material selections, it is necessary to understand the mechanisms by which chemicals degrade sealing materials.

Elastomeric seals rely on polymer cross-linking to maintain their elastic properties and sealing force. When exposed to aggressive chemicals, several degradation mechanisms can occur. First, chemical swelling—fluid molecules penetrate the polymer matrix, causing dimensional changes that reduce sealing pressure and lead to leakage. Second, chemical degradation—reactive species break polymer chains, causing loss of mechanical strength, cracking, and permanent deformation. Third, extraction—plasticizers and other additives are leached out, resulting in shrinkage and embrittlement.

NBR, for example, offers excellent oil resistance but performs poorly when exposed to strong acids, ketones, or ozone. EPDM resists water, steam, and mild acids but swells excessively in contact with petroleum oils and hydrocarbons. FKM (fluoroelastomer) provides broad chemical resistance but is attacked by amines, hot acids, and certain solvents. Even PTFE, widely regarded as the most chemically inert sealing material, has limitations—it lacks elasticity and requires spring energization to maintain sealing force under low-pressure conditions.

Understanding these limitations is the first step toward selecting the right sealing solution for each specific chemical application.

Material Options for Corrosive Service: A Comparative Overview

The table below compares the chemical resistance and key properties of common sealing materials used in corrosive environments:

PTFE: The Universal Chemical Barrier

For the broadest chemical resistance across the widest temperature range, PTFE (Polytetrafluoroethylene) remains the material of choice. With its exceptional chemical inertness, PTFE resists attack from virtually all industrial chemicals—including strong acids like sulfuric acid and hydrochloric acid, strong alkalis, organic solvents, and aggressive oxidizing agents.

PTFE's unique combination of properties makes it suitable for demanding chemical service:

• Chemical inertness: PTFE is attacked by only a few substances, such as molten alkali metals and elemental fluorine at high temperatures. For practical industrial applications, it offers near-universal chemical resistance.

• Wide temperature range: PTFE maintains its useful properties from -200°C to +260°C, making it suitable for both cryogenic and high-temperature chemical processes.

• Low friction: PTFE's coefficient of friction is among the lowest of any solid material, reducing wear on mating surfaces and extending seal life in dynamic applications.

However, PTFE's primary limitation in sealing applications is its lack of elasticity and tendency to creep under load. Because PTFE does not possess the elastic recovery of rubber, it cannot maintain sealing force through elastic deformation alone. This is where spring-energized seal designs provide the solution—a metal spring within the PTFE jacket supplies the necessary sealing force, compensating for PTFE's creep and maintaining contact pressure against the sealing surface.

PEEK: High-Strength Alternative for Extreme Conditions

For applications requiring high mechanical strength, wear resistance, and chemical resistance combined with elevated temperatures, PEEK (Polyether Ether Ketone) offers a compelling alternative to PTFE.

PEEK delivers outstanding resistance to a wide range of chemicals, including strong acids, bases, hydrocarbons, and many organic solvents. Its continuous working temperature of 250°C exceeds that of PTFE, and its high rigidity and creep resistance enable it to maintain dimensional stability under high-pressure conditions where PTFE would deform.

In sealing applications, PEEK is commonly used for:

• Backup rings in high-pressure chemical service, preventing elastomeric seals from extruding into metal clearances

• Wear rings and guide rings in chemical pumps and compressors

• Sealing components in high-temperature, high-pressure chemical processes where PTFE's creep would be unacceptable

PEEK's higher cost compared to PTFE is justified in applications where mechanical strength, wear resistance, and dimensional stability are critical.

FFKM: Ultimate Chemical and Thermal Resistance

For the most demanding chemical service applications—where no other material can survive—FFKM (Perfluoroelastomer) provides the ultimate combination of chemical resistance and high-temperature performance.

FFKM combines the chemical inertness of PTFE with the elastic properties of an elastomer. It offers resistance to virtually all chemicals, including those that attack FKM, and maintains its elasticity at continuous operating temperatures up to +300°C. FFKM is the material of choice for semiconductor manufacturing, pharmaceutical processing, and aggressive chemical handling where seal failure is not an option.

However, FFKM's exceptional performance comes at a very high cost—typically ten to fifty times that of conventional elastomers. Its use should be reserved for applications where no alternative material can provide adequate service life.

PTFE Spring-Energized Seals: The Practical Solution for Chemical Service

For the majority of chemical processing applications requiring broad chemical resistance and reliable sealing performance, PTFE spring-energized seals offer the most practical combination of performance and cost.

The working principle is straightforward: a corrosion-resistant metal spring (typically 304 or 316 stainless steel, with ELGILOY available for highly corrosive environments) is enclosed within a PTFE jacket. The spring provides initial sealing force at zero or low pressure, while the PTFE jacket conforms to the sealing surface and resists chemical attack. Under system pressure, fluid force further energizes the seal, creating a pressure-assisted sealing effect that remains reliable across zero to high pressure ranges.

This design overcomes PTFE's inherent limitations—the spring compensates for creep and thermal contraction, while the PTFE jacket provides chemical resistance. The result is a seal that combines the chemical inertness of PTFE with the mechanical reliability of a spring-loaded design.

Typical Industrial Applications

With their excellent chemical resistance and reliable sealing performance, PTFE spring-energized seals and PEEK components are widely used in the following chemical processing applications:

These industries involve aggressive chemical environments, temperature extremes, and demanding operating conditions where seal failure leads to safety hazards, product contamination, environmental damage, and costly downtime. PTFE spring-energized seals and PEEK components effectively address these challenges, ensuring reliable operation and improving the overall safety and efficiency of chemical processing equipment.

Conclusion

Seal material selection for corrosive chemical service requires careful consideration of the specific chemicals involved, operating temperatures, pressures, and mechanical loads. PTFE offers the broadest chemical resistance and widest temperature range, while PEEK provides superior mechanical strength for high-pressure, high-temperature applications. PTFE spring-energized seals combine the chemical inertness of PTFE with mechanical reliability, making them the practical choice for most chemical processing sealing requirements.

For the most demanding applications where no other material can survive, FFKM provides ultimate chemical and thermal resistance. However, for the majority of chemical service applications, PTFE spring-energized seals offer the optimal balance of performance, reliability, and cost-effectiveness.