In chemical transfer applications, the majority of pump failures are not mechanical but material-related. Even if the correct hydraulic components are selected and the capacity is accurately calculated, an incorrect seal or an incompatible housing can render the pump inoperable within weeks.
This guide covers how to select wetted parts, the limitations of materials, and the most common mistakes made in the field.
The “weakest link” rule
In a pump, the parts that come into contact with the fluid are not limited to the housing:
- Housing and manifolds
- Impeller, rotor, or diaphragm
- O-rings and gaskets
- Valve balls and seating surfaces
- Shaft, bushings, and bearings
- Mechanical seal faces
If any one of these parts is incompatible, the entire pump is incompatible. Installing an EPDM O-ring on a PVDF-bodied pump renders the body’s resistance meaningless. The material of each item must be confirmed separately during the quotation and ordering stages.
Thermoplastics
| Material | Maximum Temperature (approx.) | Strengths | Limit |
|---|---|---|---|
| PVC | ≈ 60 °C | Economical; dilute acids/bases, water treatment | Low-temperature limit; soluble in solvents |
| PP (polypropylene) | ≈ 100 °C | Dilute acids and bases, salt solutions; economical | Aromatic/chlorinated solvents; oxidizing agents (including hypochlorite) limited |
| PVDF (Kynar®) | ≈ 135 °C | Very broad resistance: strong acids, halogens, solvents, hypochlorite | Not suitable for use in hot concentrated caustic soda |
| PTFE (Teflon®) | ≈ 260 °C | Nearly universal chemical resistance; widest temperature range | Mechanically soft, creeps under load; expensive |
The greatest advantage of thermoplastics over metal is that their corrosion resistance is inherent: they do not wear away like a coating to expose the underlying material. The thermoplastic pumps we offer meet this need; if leak-tightness is also required, models with thermoplastic housings and magnetic couplings can be considered.
Elastomers (gaskets, O-rings, diaphragms)
| Material | Temperature | Suitable | Not suitable |
|---|---|---|---|
| EPDM | Up to ≈ 150 °C | Hot water, steam, bases, ketones, dilute acids | Oils and hydrocarbons |
| FKM / Viton® | Up to ≈ 200 °C | Aromatic/chlorinated hydrocarbons, oils, aggressive acids, high temperatures | Hot concentrated bases, ketones, amines |
| NBR (Buna-N) | Up to ≈ 100 °C | Petroleum products, oils, fuels | Strong acids, ozone, ketones |
| PTFE | Up to ≈ 260 °C | Almost everything | Not flexible; requires special design for sealing |
EPDM and FKM behave exactly opposite to each other: EPDM performs well in bases but poorly in oil; FKM performs well in oil and acid but poorly in hot bases. The most common mistake in the field is the habit of “using Viton everywhere”—in a caustic line, this choice causes the gasket to swell and break apart.
Metals
- 304 stainless steel: Food and general water services. Limited resistance to chlorides and acids.
- 316 / 316L stainless steel: The most common process material. The molybdenum addition increases chloride resistance. However, it should not be used in hydrochloric acid and carries a risk of pitting and stress corrosion cracking in chloride environments.
- Alloy 20: Developed for sulfuric acid applications.
- Hastelloy C: Harsh environments containing oxidizing agents and chlorides; high cost.
- Duplex stainless steel: High strength + chloride resistance; seawater applications.
For metal options, please see our stainless steel pumps and alloy pumps categories.
Quick Compatibility Chart
A = suitable · B = conditional/limited · C–D = not suitable. Values are for room temperature.
| Chemical | PP | PVDF | PTFE | EPDM | FKM | 316 |
|---|---|---|---|---|---|---|
| 10% sulfuric acid | A | A | A | A | A | B |
| 20% hydrochloric acid | A | A | A | A | A | D |
| Phosphoric acid 20% | A | A | A | A | A | B |
| Sodium hydroxide 20% | A | B | A | B | D | B |
| Sodium hypochlorite | C | A | A | C | B | C |
| Acetone | A | D | A | C | D | A |
| Toluene | C | A | A | D | B | A |
This table is only a summary. You can search for 1,460 chemicals and 26 materials using our Chemical Compatibility Table tool to directly look up the liquid you will be transporting.
Three key points to note in the table:
- 316 stainless steel cannot be used with hydrochloric acid. The assumption that “stainless steel can withstand anything” is one of the most costly misconceptions.
- PVDF dissolves in acetone but performs very well in toluene. PP behaves exactly the opposite. There is no single “good plastic” when selecting a solvent.
- Sodium hypochlorite is aggressive toward PP, EPDM, and 316; the correct choice is PVDF or PTFE. For details on the dosing side, see our dosing pump guide.
Corrosion doesn’t occur in just one way
The question “Is the material resistant?” is not one-dimensional; damage to metals occurs through several different mechanisms, and some are not listed in the compatibility chart:
- Uniform corrosion: Balanced thinning across the entire surface. It is predictable; service life can be calculated using the “mm/year” values in the tables.
- Pitting corrosion: Localized pitting while the surface appears intact. Chlorides trigger this in stainless steel. Since the total mass loss is minimal, it goes undetected by measurement—until the pump eventually fails.
- Crevice corrosion: Concentrates in stagnant areas such as under gaskets or between flanges. It is as much a material issue as it is a design flaw.
- Stress-Corrosion Cracking (SCC): Sudden cracking caused by the combination of stress, chlorides, and temperature. This is a classic weakness of 304/316 stainless steel in hot, chloride-containing environments.
- Galvanic corrosion: When two different metals come into contact, the more active one corrodes more rapidly. This is why compatibility between the pump body and the bolt/flange material is crucial.
In thermoplastics, the equivalent phenomena are stress cracking (ESC) and swelling/softening: the material does not dissolve but absorbs chemicals, altering its dimensions and strength. Just because a part does not “rupture” does not mean it is compatible—its dimensions and strength must be maintained.
Pressure in plastics decreases with temperature
In metal pumps, the pressure limit remains nearly constant over a wide temperature range. In thermoplastics, however, the situation is different: as temperature increases, the allowable operating pressure drops significantly. This is why pressure–temperature (derating) curves are included in manufacturer’s data sheets.
Practical implication: A PP body rated for 6 bar at 20 °C can withstand only a fraction of that pressure at 80 °C. Base your selection on the point where the highest temperature and highest pressure occur simultaneously—checking them separately can be misleading.
The same logic applies to piping systems; the derating curve must not be ignored when selecting plastic pipes for hot lines.
Bearing, shaft, and packing materials
This is the group that comes into contact with the fluid and is often overlooked:
- Silicon carbide (SiC): Very high wear resistance and broad chemical compatibility. It is brittle and sensitive to thermal shock and dry running. It is the standard bearing material for seal-less pumps—see the magnetic coupling pump guide.
- Carbon-graphite: More tolerant of dry running, self-lubricating; wear resistance is lower than that of SiC. Not suitable for strong oxidizing agents.
- Alumina ceramic: Economical and chemically resistant; more brittle than SiC.
- PTFE-coated shaft / coated surfaces: Because it is a coating, the underlying metal is exposed in the event of scratching or wear; exercise caution with abrasive fluids.
Three pitfalls of the tables
1. Temperature
Compatibility tables are generally for room temperature. As a general rule, the corrosion rate roughly doubles for every 10 °C increase in temperature. A combination rated “A” at 20 °C may be rated “C” at 60 °C. Request data at the operating temperature.
2. Concentration
Different concentrations of the same chemical behave completely differently. Sulfuric acid is a classic example of this: its dilute form acts as a reducing agent, while its concentrated form acts as a strong oxidizing agent, and the suitable materials vary accordingly. Simply stating “sulfuric acid” is not enough; specify the percentage concentration.
3. Mixtures and Impurities
The tables are for pure chemicals. Actual process fluids are mixtures; even trace amounts of a chloride or oxidizing agent can compromise the material selected for the primary chemical. In cases of uncertainty, the immersion test (coupon test) is the most reliable method: material samples are immersed in the actual fluid at the actual temperature, and changes in weight and appearance are measured.
Selection Process
- Fully characterize the fluid: chemical name, concentration, operating and peak temperatures, solid content, impurities. The Safety Data Sheet (SDS) is the starting point.
- Determine the body material (thermoplastic / stainless steel / alloy).
- Select the elastomers separately—do not assume they are the same as the body material.
- Verify the bearing/shaft/seal materials (SiC, carbon, ceramic).
- Check the temperature and pressure limits for all selected materials; the lowest limit is the system’s limit.
- Conduct a coupon test for critical applications or obtain written approval from the manufacturer.
If you share the name, concentration, and temperature of your fluid, we can work together to determine the appropriate material combination—contact us for technical support or review our thermoplastic, stainless steel, and alloy pump series.