Guides / Grades

What PEEK Resists, and the Few Things That Attack It

September 20, 2026

Sources: manufacturer datasheets and published standards. See editorial standards.

The short version: PEEK resists nearly everything you are likely to put it in. The list of chemicals that attack it is short enough to memorise, and the failures people do see in the field are usually about stress, temperature or pressure rather than chemistry.

What it resists

At room temperature and at elevated temperature, PEEK is stable in:

• Most acids and bases at working concentrations
• Aliphatic and aromatic hydrocarbons, fuels and oils
• Alcohols, ketones and esters in normal service
• Steam and hot water, continuously
• Hydraulic fluids, drilling fluids and hydrogen sulfide
• Cleaning chemicals used in clean in place systems
• Sterilising agents: steam, ethylene oxide, hydrogen peroxide vapour, gamma and electron beam

This is why PEEK ends up in fluid paths, pump and valve parts, analytical hardware, downhole tools and anything that gets cleaned aggressively.

What attacks it

Concentrated sulfuric acid. The one real answer. It attacks PEEK and at high concentration dissolves it. Dilute solutions are manageable but the margin narrows quickly with concentration and temperature.

Strong oxidising acids. Concentrated nitric acid and similar oxidisers attack the polymer. Mixed acid systems are worth testing rather than assuming.

Halogenated solvents, under stress. PEEK resists these chemically, but a part carrying high residual or applied stress can craze or crack in contact with some of them. This is environmental stress cracking, and it is a stress problem as much as a chemical one.

Sustained UV, unfilled and unshielded. Natural PEEK ages slowly outdoors. Black grades and shielded installations are the normal fix.

The property that wins medical work

Hydrolysis resistance. Plenty of engineering plastics survive a chemical splash. Far fewer survive water at 134 °C, several hundred times.

PEI loses strength in repeated steam. Polysulfone and polycarbonate are worse. PEEK holds up, which is the reason instrument handles, trays, clamps and guides that live in an autoclave end up machined from unfilled PEEK rather than something cheaper. Note the grade: unfilled is the tough one, and on a filled part the fibre ends at a machined surface give steam a path into the part. More on the grade choice in which PEEK grade should be on your drawing.

Three things that change the answer

Temperature. A chemical that is harmless at 20 °C may not be at 150 °C. Every resistance table is a temperature specific statement, and PEEK spends its life near its limits, so read the table at your service temperature.

Stress. A part at 60% of its yield stress behaves differently in a solvent than an unloaded coupon. Residual stress from machining counts, which is one more reason for the annealing step described in annealing PEEK parts.

Dissolved gas and pressure. In downhole and process service, gas dissolves into the polymer under pressure. If the pressure drops quickly, that gas expands inside the part. The material is chemically fine and the part still fails. This is why oil and gas work qualifies the part, not just the polymer, as covered in PEEK parts for oil and gas.

How to specify it properly

Put the media on the RFQ, with concentration and temperature. Four lines is enough:

• Media and concentration, including cleaning chemicals
• Continuous and peak temperature
• Pressure, and whether it cycles quickly
• Contact type: immersion, splash, or occasional cleaning

That tells the shop which grade to quote and whether documentation is needed. It also tells you whether you should test. For anything close to the limits, a soak test on a machined coupon at service temperature is cheap next to a field failure.

When PEEK is not the answer

• Concentrated sulfuric or oxidising acid service. PTFE or PFA, accepting that they creep under load.
• Pure friction and no load. PTFE is cheaper and slicker.
• Nothing hot, nothing aggressive. Acetal costs a fraction and machines faster.

The comparison across PEEK, PTFE, acetal and PEI is worth reading before you commit a design.

Running a soak test that means something

If the service is close to the limits, test. A useful soak test costs a few hundred dollars and takes a few weeks, which is nothing against a field failure.

What makes the difference between a test and a gesture:

• Machined coupons, not moulded bars. The stress state of a machined part is what you are testing.
• Service temperature, not room temperature. Chemistry that is harmless cold is the whole question hot.
• Under load if the part is loaded. A stressed coupon in a solvent behaves differently from a loose one.
• Long enough to see a trend. Measure mass, dimensions and strength at intervals rather than only at the end.
• The real media. Including the cleaning chemicals, the inhibitor and whatever else is in the line.

Ask the stock supplier what data already exists for the grade before you start. For standard grades in common media, the work has often been done.

Chemical limits are one of the constraints in our guide to PEEK grade selection.

Have a part that lives in something aggressive? Send the drawing with the media and temperature and we match you with shops that already run that kind of work.

Frequently asked questions

Keep reading

• X-Ray Opaque PEEK: What Barium Sulfate Changes in a Medical Part
• When PEEK Runs Out of Heat and Polyimide Takes Over