Forty-two chemicals used in water and wastewater treatment, listed at fifty-seven concentrations, against thirty-four materials of construction — pipe, lining, gasket, seal and vessel. Concentration is on the page because it is usually what decides the answer: sulfuric acid is rated across four strengths and carbon steel is unusable in the dilute acid and serviceable in the concentrated. Set the service temperature and the ratings move with it too, and every cell that moves tells you which mechanism moved it. Use it to shortlist, then confirm against the manufacturer’s data for the exact grade, concentration and temperature you are actually running.
Read this before you specify anything. Chemical compatibility depends on concentration, temperature, pressure, trace contaminants, mechanical stress and duration — a table cannot capture all of that. Base ratings assume continuous immersion at 20 °C in the concentration stated. The 40, 60 and 80 °C columns are derived from those base ratings by the published rules set out below — they are not four separate sourced tables, and they are deliberately conservative. This is a screening tool for narrowing options, not a substitute for the manufacturer’s chemical resistance data, a materials engineer, or a coupon test in your actual service.
Most compatibility mistakes in water treatment are not obscure. They are a handful of pairings that look reasonable and are not — usually because stainless steel is assumed to be universally corrosion resistant, which it is not.
The single most common failure. Hypochlorite pits and stress-corrosion-cracks 316 rapidly, and the failure is often a pinhole leak in a dosing line rather than obvious general corrosion. Stainless dosing lances and injection quills fail this way constantly.
Use instead: uPVC or CPVC pipe, PVDF or PTFE wetted parts, titanium where metal is unavoidable.
Ferric chloride attacks every standard stainless grade including 316 and 2205 — the chloride and the oxidising ferric ion together are about the worst combination there is. A stainless coagulant dosing skid will not last.
Use instead: uPVC, HDPE, PP, PVDF, FRP, or rubber-lined steel. Titanium works but is rarely justified.
Titanium is outstanding in wet chlorine and hypochlorite — and it ignites in dry chlorine gas. Below roughly 1 % water content the protective oxide cannot re-form and the reaction is violent. This one is a safety issue, not a durability issue.
Use instead: dry chlorine gas lines are carbon steel or Monel. Never titanium.
Carbon steel handles 93–98 % sulfuric acid because a protective iron sulfate film forms. Dilute the same acid and that film cannot form, and the steel is destroyed. Dilution points, drain lines and spill areas are where this bites.
Use instead: for dilute sulfuric, uPVC, PP, PVDF or Alloy 20. Watch velocity limits even at high concentration.
FKM is reached for as the “premium” elastomer, but it is poor in strong sodium hydroxide, poor in ammonia, and degrades in hot water and steam. A caustic dosing pump fitted with Viton seals will fail early.
Use instead: EPDM for caustic, hot water, ozone and most water treatment duty. FKM belongs on acids, hydrocarbons and oils.
The mirror image. EPDM is the right default for water, caustic and ozone — and it swells badly in any petroleum oil or grease. Lubricating an EPDM O-ring with mineral grease during assembly ruins it before it is even in service.
Use instead: NBR or FKM for oil. Assemble EPDM with silicone grease or water only.
PVDF is superb across almost the whole acid and oxidant range, which makes it easy to assume it is universal. Strong sodium hydroxide dehydrofluorinates it, and the attack accelerates with temperature and concentration.
Use instead: PP, HDPE or PTFE for 50 % caustic. PVDF is fine in dilute caustic at ambient.
316 pits under stagnant chloride, and RO concentrate at 4× seawater chloride is worse than seawater. Tail-end pipework, concentrate valves and energy recovery devices are where this shows up.
Use instead: duplex 2205 as a minimum, super duplex or titanium for seawater RO concentrate.
Publishing four thousand individually sourced cells would mean inventing most of them, so this page does not pretend to. The 20 °C ratings are authored per pair from manufacturer resistance data and standard practice. The higher temperatures are then derived from those ratings by six rules, each of which is a real and well established mechanism, and every cell that moves says which rule moved it.
Named exceptions are handled on top of the rules where a material has a specific weakness the rules would miss — FKM in hot water is the one that catches people, because its 200 °C rating is a dry-service rating.
| Material | Continuous max | Notes |
|---|