Chemical compatibility charts give you a starting point. They do not tell you whether your specific system will fail. The chart says EPDM is compatible with 30% sulphuric acid — and it may be, under the test conditions used to generate that data. Those conditions may not match your operating temperature, your concentration, your exposure duration, or the other chemicals present in your process. Systems fail on the gap between what the chart tested and what the installation actually sees.
What Compatibility Charts Actually Test
Published chemical compatibility data is generated by immersing material coupons in the test chemical at a standard temperature — typically 20–25°C — for a fixed period, usually 7 or 30 days. The coupon is weighed and measured before and after. A rating of “A” or “Excellent” means weight change and dimensional change were below defined thresholds after the test period. A rating of “C” or “Not Recommended” means the material showed unacceptable degradation.
This test protocol captures long-term equilibrium exposure at a single concentration and temperature. It does not capture dynamic mechanical stress — the flex cycling of a diaphragm, the sliding friction of an O-ring, the fatigue loading of a hose. Material that is chemically stable under static immersion may fail in service under cyclic mechanical loading in the same fluid, because mechanical stress opens micro-cracks that expose fresh material to chemical attack at a rate faster than the surface corrosion captured in the coupon test.
Elevated temperature is the most common source of chart-to-reality mismatch. An EPDM O-ring rated compatible with a given acid at 25°C may degrade rapidly at 60°C. Some manufacturers publish temperature correction factors; most do not. If your process runs hot, the published rating at ambient temperature understates the degradation rate, and the service life in your installation will be shorter than the chart suggests.
Concentration and Mixture Effects
Many chemical processes do not handle single chemicals at a single concentration. They handle mixtures, or chemicals whose concentration varies with process conditions. Compatibility data for mixtures is sparse — most charts address individual chemicals, not blends. The interaction of two chemicals on a polymer can be non-additive: a material that resists Chemical A and resists Chemical B may be severely attacked by A+B together, because one chemical opens the polymer matrix and the second penetrates it.
Concentration also matters in non-obvious ways. Some materials show better resistance to concentrated acids than to dilute ones. Concentrated nitric acid passivates some metals; dilute nitric acid attacks them aggressively. PTFE is generally compatible with both concentrated and dilute forms of most acids, but PVDF has concentration-dependent limits with certain strong bases and polar solvents. A compatibility chart entry for a generic chemical category — “sulphuric acid” — without a concentration qualifier may apply to a concentration range that does not include yours.
System-Level Interactions Not Captured by Component Testing
A dosing system has many wetted components: the pump body, diaphragm, ball seats, O-rings, hose or tubing, fittings, check valves, injection quill, and all pipe joints. Each component is made from a different material. A chart check confirms that each individual material is compatible with the chemical. It does not confirm that all these materials are compatible with each other when the chemical is present.
Galvanic effects occur when dissimilar metals are present in the same conductive fluid — an SS316 fitting and a brass body valve in an electrolyte solution will experience accelerated corrosion of the less noble metal. Extractables from one material can contaminate another: plasticisers or stabilisers leaching from a PVC fitting into a solvent-based fluid can attack a downstream EPDM diaphragm even if EPDM is rated compatible with the process chemical itself.
Thermal cycling causes different materials to expand and contract at different rates. A PTFE-seated check valve in an aluminium body will cycle through periods of tight seal and loose seal as temperature changes, because PTFE’s thermal expansion coefficient is significantly higher than aluminium’s. This is a system-level effect invisible to single-material compatibility testing.
What System-Level Validation Looks Like in Practice
System-level validation does not require a laboratory. It requires a structured review of the actual process conditions — temperature range, concentration range, flow velocity, pressure, duty cycle — against the materials in the wetted flow path, and then a physical trial under operating conditions before full deployment.
For a new chemical application, the trial approach is: install the proposed system on one duty point, run it under operating conditions for 4–8 weeks, then disassemble and inspect all wetted components for dimensional change, surface degradation, discolouration, or loss of elasticity. Compare to baseline measurements taken at installation. If the components are within acceptable limits, the system is validated for that application. If any component shows unexpected degradation, identify which material is failing and which aspect of the process condition is responsible before selecting a replacement material.
This trial period is not a delay — it is risk management. A system that fails in week six during trials costs the price of a replacement diaphragm set. The same failure at a customer site after three months of production service costs multiples of that in downtime, clean-up, and relationship damage.
Where Charts Are Reliable and Where They Are Not
Compatibility charts are most reliable for: single chemicals at ambient temperature, materials with long published service histories in the application (PTFE and PVDF in HF service, for example), and situations where concentration is stable and well-defined. They are least reliable for: mixtures, elevated temperatures, dynamic mechanical applications (diaphragms, hoses), and chemicals at the boundary conditions of a rating — a material rated “B” or “Conditional” should not be selected without further investigation.
PTFE is the safest material selection from a pure compatibility standpoint — it has the broadest chemical resistance of any common elastomer or thermoplastic — but it is not compatible with fluorine gas, molten alkali metals, or certain oxygen-rich fluorinated compounds, and its low hardness and cold flow characteristics make it unsuitable for high-pressure dynamic sealing without careful engineering. Even the safest material choice requires system-level thinking rather than chart-reading alone.
For support on material selection and system-level chemical validation for your dosing application, contact Autoflo at info@autoflotechnology.com.