In chemical transfer applications, the majority of pump failures are caused by material issues rather than mechanical ones. Even if the correct hydraulic specifications are selected and the capacity is accurately calculated, an incorrect gasket or an incompatible casing can render the pump inoperable within weeks.
This guide covers how to select wetted parts, the limitations of materials, where compatibility charts come from, and the most common mistakes made in the field. If you have a damaged part, you can start by referring to the damage diagnosis chart.
The "weakest link" rule
The parts of a pump that come into contact with the fluid are not limited to the casing:
If any one of these parts is incompatible, the entire pump is incompatible. Installing an EPDM O-ring on a pump with a PVDF casing renders the casing’s resistance meaningless. The material of each item must be confirmed separately during the quotation and ordering stages.
There are two additional items that are often omitted from the list, and both can cause leaks: bolt/nut material (exposed to external chemical vapors and spills) and auxiliary equipment on the line—strainer basket, capillary manometer, level float. Selecting the right pump but installing the line with the wrong materials does nothing more than shift the problem a few meters down the line.
Thermoplastics
| Ingredients | 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 their inherent corrosion resistance: they do not wear away like a coating to expose the underlying material. The thermoplastic pumps we offer meet this need; if sealing is also required, models with thermoplastic casings and magnetic couplings can be considered.
In contrast, thermoplastics have three weaknesses that metals do not have, and these are often overlooked when making a selection:
Elastomers (gaskets, O-rings, diaphragms)
| Ingredients | 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 design for sealing |
EPDM and FKM behave exactly opposite to each other: EPDM performs well in alkaline solutions but poorly in oil; FKM performs well in oil and acid but poorly in hot alkaline solutions. 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.
Elastomers also pose a risk of explosive decompression: gas that enters the elastomer while it is under pressure expands when the pressure drops rapidly, causing the material to bulge from the inside and crack. When selecting O-rings for fluids containing dissolved gases and for lines that are frequently commissioned and shut down, this behavior must be taken into account.
With PTFE gaskets, however, sealing is achieved more by design than by the material itself: Since PTFE is not flexible, spring-energized or envelope-type gasket configurations are used. Simply substituting a flat PTFE sheet for an O-ring will not provide sealing.
Metals
For metal options, please see our stainless steel and alloy pump categories.
A numerical criterion for selecting stainless steel: PREN
There is a measurable answer to the question, "Which stainless steel is better?" PREN (Pitting Resistance Equivalent Number) quantifies an alloy’s resistance to pitting corrosion in a chloride environment as a single number and is calculated from the alloy’s composition:
PREN = %Cr + 3.3 × %Mo + 16 × %N
(Percentages are by mass; for tungsten-containing alloys, the molybdenum content is calculated as %Mo + 0.5 × %W. Some sources use a nitrogen coefficient of 30—the formula used must be specified.)
In practical terms, this means that as the PREN value increases, resistance to chloride corrosion increases. Alloys with a PREN value above 32 can be used in seawater under suitable conditions; for seawater service, a PREN value greater than 40 is typically required—because crevice corrosion is more severe than corrosion on exposed surfaces. This explains, in numerical terms, why 316 is not safe for use in seawater.
PREN is a ranking tool, not an absolute guarantee: the type of acid, temperature, flow rate, and surface quality all affect the result. Still, it helps clarify the "316 or duplex?" debate when comparing two offers.
Material certificate: What are you verifying?
For critical applications, the designation "316 stainless steel" alone is not sufficient; an inspection report verifying that the material actually has that composition is required. EN 10204 defines the types of these documents:
The difference lies in traceability. On a line that carries hazardous liquids or falls under the category of pressurized equipment, the difference between 2.2 and 3.1 is decisive in a post-failure investigation.
Quick Compatibility Chart
A = suitable · B = conditional/limited · C–D = unsuitable. 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 points to note in the table:
Where do these tables come from?
The letters "A/B/C" in the compatibility tables do not represent an opinion but are the result of a standard laboratory procedure. The method commonly used for plastics is ASTM D543: material samples are immersed in specified chemical solvents (or exposed to the solvent under stress), and changes in weight, dimensions, appearance, color, and mechanical properties are measured and reported. The standard also defines separate procedures for exposure at elevated temperatures and under stress.
Knowing this has two practical implications:
For this reason, a table value is not independent of the question, “What does the manufacturer’s data sheet say?” For critical applications, an immersion test using your own fluid and at your own temperature is required instead of relying on the table.
Corrosion does not occur in just one form
The question "Is the material durable?" is not one-dimensional; damage to metals occurs through several different mechanisms, and some of them do not appear in the compatibility chart:
In thermoplastic materials, the equivalent phenomena are stress cracking (ESC) and swelling/softening: the material does not dissolve but changes in size and strength occur as it absorbs chemicals. Just because a part does not "break" does not mean it is compatible—its dimensions and strength must be maintained.
In plastic, pressure decreases with temperature
In metal pumps, the pressure limit remains virtually constant over a wide temperature range. With thermoplastic materials, however, the situation is different: as the temperature rises, the permissible operating pressure drops significantly. This is why manufacturer data sheets include a pressure–temperature (derating) curve.
Practical conclusion: A PP casing 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 the 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. Furthermore, water hammer is more dangerous in plastic lines than in metal ones: the pressure spike caused by a sudden valve closure can exceed the derated limit by a factor of two. In plastic lines with fast-closing automatic valves, the closure time and water hammer calculations are just as important as the valve selection.
Bearing, shaft, and mechanical seal materials
This is the group that comes into contact with the liquid and is often overlooked:
There is one more component in gland-sealed pumps: the mechanical seal surface pair. Two surfaces made of the same material (e.g., SiC/SiC) provide high wear resistance but do not tolerate dry running at all; the carbon/SiC pair is more forgiving, but carbon wears down rapidly in abrasive fluids. The choice of surface pair is based on the balance between the fluid’s abrasiveness and the risk of dry running.
Food, Drinking Water, and Medicines
Chemical compatibility indicates whether a material is suitable; food contact, on the other hand, asks what it leaves behind in the product. These are different questions and are addressed by different documents:
In practice, what is required is not simply the statement “suitable for food contact,” but a declaration of suitability for each individual component: the diaphragm, O-ring, valve seat, and casing, separately. Furthermore, in hygienic applications, the choice of material is often determined not by the product itself but by the CIP cleaning chemicals—an elastomer that cannot withstand cycles of hot caustic and acid will not have a long service life in the system, even if it is suitable for the product.
The Three Pitfalls of Tables
1. Temperature
Compatibility tables are generally based on room temperature. As a general rule of thumb, 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 for the operating temperature.
2. Concentration
Different concentrations of the same chemical behave completely differently. Sulfuric acid is a classic example of this: in its dilute form, it acts as a reducing agent, while in its concentrated form, it acts as a strong oxidizing agent, and the appropriate materials vary. It’s not enough to simply say “sulfuric acid”; you must specify the percentage.
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 degrade a material selected based on 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. ASTM D543 specifies how this test is to be conducted and what results are to be reported.
From Damage Pattern to Cause: Material Analysis
A picture of the removed part often directly reveals which mechanism was at work:
| As seen in the figure | Possible mechanism | Action to be taken |
|---|---|---|
| O-ring has swelling, is softened, and has increased in size | The elastomer has absorbed chemicals (incompatible material) | Change the elastomer grade; check for FKM↔EPDM incompatibility |
| Gasket has hardened, cracked, or become brittle | Excessive temperature or aging/oxidative attack | Check the temperature limit and oxidizing concentration |
| Internal bubbles, pinholes, or cracks in the elastomer | Sudden pressure drop (gas expansion) | Review the commissioning/shutdown rate and O-ring class |
| Stainless steel surface is shiny but has pinpoint holes | Pitting corrosion — chloride | Switch to an alloy with a higher PREN rating or switch to thermoplastic |
| Pitting under the gasket surface/flange interface | Crevice corrosion | Eliminate stagnant voids; upgrade the alloy grade |
| Fine, branching cracks in the stress-concentration zone | SCC in metal / ESC in plastic | Break the chloride + temperature + stress triad; check torque values |
| Bright pitting on the impeller inlet edge and inside the elbow | Erosion-corrosion | Reduce flow velocity, separate solids, switch to a harder material |
| Rapid pitting where two different metals come into contact | Galvanic corrosion | Match the bolt/flange material to the casing or insulate it |
| Bolts have loosened in the plastic casing | Creep | Include torque checks in periodic maintenance |
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Frequently Asked Questions
Is 316 stainless steel resistant to all chemicals?
No. 316 is susceptible to pitting and stress corrosion cracking in chloride environments and cannot be used in hydrochloric acid. For applications involving chlorides and halogens, either an alloy with a higher PREN value or a thermoplastic casing is required.
Should I choose PP or PVDF?
It depends on the chemical; one is not superior to the other. PVDF is superior in halogens, hypochlorite, and many solvents, but it cannot be used in hot, concentrated caustic soda. PP is economical and safe in dilute acid, base, and salt solutions; however, it is weak in aromatic and chlorinated solvents and strong oxidizing agents.
I saw a "B" in the compatibility chart—can I use it?
"B" indicates conditional use: it generally applies to lower temperatures, lower concentrations, or intermittent contact. Before selecting a material rated "B" for continuous service, request data at the operating temperature or conduct a coupon test.
Are food-grade materials and chemical-resistant materials the same thing?
No. Chemical compatibility assesses whether a material degrades, while food contact regulations assess whether the material migrates into the product. A material may be chemically resistant but still unsuitable for food contact; the two are certified separately.
What should I ask for as a material certificate?
In the process industry, the standard requirement is EN 10204 Type 3.1: the results pertain to the shipped batch and are approved by an inspection representative independent of production. The results under Type 2.2 come from routine production control and may not represent your specific batch.
If you provide the name, concentration, and temperature of your fluid, we can work together to determine the appropriate material combination—get in touch with us for technical support or explore our thermoplastic, stainless steel, and alloy pump series.