How to Check a Material Against the Fluid It Will Touch

Why this matters

Compatibility charts get used as if a letter grade were a yes or a no. It is neither. Every rating on every chart was produced at one concentration, one temperature, one duration, and with the sample sitting still and unstressed, and your part almost never matches all four. Reading a rating without its conditions is how a material rated "excellent" splits open in service, and the failure looks like a defective part rather than a misread chart.

Before you open a container

Chemical exposure is the hazard here, and it comes before any of the lookup work.

• Read Section 8 of the safety data sheet and wear what it names, not what is on the truck. Glove material is itself a compatibility decision: thin disposable nitrile is permeated by ketone solvents within minutes, and a glove that has taken up the chemical is worse than no glove because it holds it against your skin. Butyl or a laminate glove is the usual answer for ketones, and the sheet's Section 8 will say.
• Eye protection is not optional for decanting. Splash goggles, not safety glasses, for anything you are pouring.
• Ventilate, and know where the eyewash and shower are before you start, not after.
• Never identify or test a fluid by touch or smell, and never mix two products to see what happens. Chlorine-based and acid-based cleaners are both routine in the trades and together they generate chlorine gas.

The gate: four conditions, all required

Here is the rule the rest of this procedure serves. Evaluate it per material-and-fluid pair, at the worst condition that pair ever sees, not the average condition:

A published rating may be accepted as-is only when all four of these hold: the service concentration is at or below the chart's stated concentration, AND the service temperature is at or below the chart's stated temperature, AND the contact is intermittent or short relative to the part's replacement interval, AND the part carries no sustained tensile stress. If any one of the four fails, the rating is provisional, and the pair needs either a derated rating from the material manufacturer or a coupon test before it goes in.

The Boolean is AND across all four, and the failure of a single condition is enough to move the pair to provisional. That is stricter than most people read a chart, and the two cases at the end show why.

Step 1: write the fluid list, which is longer than you think

The process fluid is one entry. The list also includes:

• the cleaning or sanitizing solution, at its use concentration
• the descaling or flush chemical, at its concentration, which is usually the most aggressive thing the part ever sees
• the assembly lubricant or grease
• any leak-detection spray, thread sealant, primer, or solvent cement used at the joint
• residue from whatever was in the system before, if this is a retrofit

Skip this and you specify against the process fluid and the part dies during a maintenance flush that lasts twenty minutes twice a year. That is the single most common way a correct-looking material selection fails.

Step 2: read the two SDS sections that name materials

Under the hazard communication standard at 29 CFR 1910.1200, the sheet supplied with a hazardous chemical carries a fixed sixteen-section format, and two of those sections do work here:

• Section 10, stability and reactivity, lists incompatible materials and conditions to avoid. This is where you find out that a product attacks a specific polymer or metal.
• Section 7, handling and storage, names container and storage material requirements, which is frequently the clearest statement of what the chemical can sit against.

Read them for the product you are actually using, at the dilution you are actually using, because a concentrate and its use dilution can have different sheets and different answers.

Skip this and you rely entirely on a generic chart entry for a chemical family, when the product in front of you is a formulation with surfactants, inhibitors, and solvents that the family name does not cover.

Step 3: pin the four numbers

For each pair on your list, write down the service concentration, the service temperature, whether contact is continuous or intermittent with a duration, and whether the part is under sustained tensile stress. Sustained stress includes hoop stress from internal pressure, a torqued fastener, a bent or sprung part, and molded-in stress in a plastic fitting that was over-tightened.

Skip this and you have nothing to compare the chart's conditions against, which means you cannot run the gate at all.

Step 4: read the rating with its conditions attached

Copy the chart's stated conditions down next to the rating. If the chart does not state its conditions, it is not usable as evidence and you need a better source, usually the material manufacturer's own document rather than a general-purpose wall chart.

Most published charts are room temperature, one stated concentration, static immersion, and unstressed. Assume that unless it says otherwise.

Case A: the gate holds

A polypropylene drain fitting handles equipment condensate at ambient temperature. Twice a year it sees a dilute acid descaling flush, roughly 20 minutes of contact, then a water rinse. The fitting is not pressurized and sits in a gravity drain.

Run the four:

• Concentration: the flush is used at the chart's stated dilution. Holds.
• Temperature: ambient, well under the chart's stated room temperature reference. Holds.
• Duration: 20 minutes, twice a year, against a fitting with a multi-year service life. Intermittent by any reading. Holds.
• Sustained stress: gravity drain, no internal pressure, hand-tight assembly. None. Holds.

All four hold, so the rating is accepted as-is. No coupon test, no manufacturer call, no derate. Buy the fitting and move on. This is the case the gate exists to let through quickly, and it is most of the work you do.

Case B: same material, same chemical, opposite verdict

Now move that same polypropylene into a continuous process line carrying the same acid at the same 20 percent concentration, at 150 degrees F, in a threaded fitting under line pressure.

Run the same four:

• Concentration: unchanged at 20 percent. Holds.
• Temperature: 150 degrees F against a chart stated at roughly 70 degrees F. Fails.
• Duration: continuous, for the life of the part. Fails.
• Sustained stress: hoop stress from line pressure plus thread engagement stress from assembly. Fails.

Three of the four fail, and the gate only needed one. The rating is provisional and the pair does not go into service on the chart alone.

What the temperature failure costs you, in numbers. Chemical attack rate follows the usual rule of thumb of roughly doubling for every 18 degrees F of temperature rise. Going from 70 to 150 degrees F is 80 degrees F, which is 80 divided by 18, or about 4.4 doublings. Two to the power of 4.4 is about 21. So the same chemical at the same concentration is attacking that fitting on the order of twenty times faster than the condition the chart was measured at. That is a rule-of-thumb order of magnitude, not a measurement, and it is enough to turn a service life measured in years into one measured in months.

What the stress failure costs you is a different mechanism entirely. Sustained tensile stress plus a chemical that is only mildly aggressive produces environmental stress cracking, where the part cracks at a chemical exposure and a stress level that neither one alone would have caused. It is a brittle crack, usually at a thread root or a molding gate, with no swelling, no softening, and no discoloration to warn you. A tech who inspects the fitting for the usual signs of chemical attack will find nothing and sign it off the week before it splits.

Same material, same chemical, same concentration, opposite answers. Nothing about the material changed. The three conditions did, and none of them appear on the chart line.

The coupon test, when the gate sends you there

Where the manufacturer cannot give you a derated rating, you can generate your own evidence. It is not laboratory work but it is real data.

Take three samples of the actual material, weigh each on a gram scale, and measure a dimension you can repeat, a thickness or a cross-section, with a caliper. Immerse them fully in the actual fluid at the actual service concentration, held at service temperature, for a set period, and keep an identical unimmersed sample as a control. Handle the immersion vessel with the PPE from the sheet's Section 8, keep it covered, and keep it out of the work area.

At the end of the period, re-weigh and re-measure. What you are looking for:

• Mass and volume gain means absorption and swelling.
• Mass loss with dimensional shrinkage means something is being extracted from the material, usually plasticizer, and shrinkage is the more urgent of the two because a shrunken seal loses its squeeze immediately.
• Hardness change in either direction means the material's structure is being altered.
• Any crazing, tackiness, or surface chalking on the immersed samples against a clean control means stop, regardless of what the numbers say.

Get the acceptance limits from the material manufacturer for that specific compound. The shape of them is that a static part tolerates meaningfully more swell than a moving one, and that shrinkage of even a few percent is a reject in both.

How to verify you got this right

• Every pair on your fluid list has a verdict, not just the process fluid. A list with one line was never run.
• Each accepted rating has the chart's four conditions written next to it. A rating recorded without its conditions cannot be re-checked by anybody, including you.
• Any pair that failed one condition is marked provisional in the record and shows what resolved it: a manufacturer derate, a coupon result, or a material change.
• The part that went in is the material you tested, not a visually identical substitute from a different supplier. Two fittings that look the same can be different compounds, and the whole exercise assumed a specific one.

References

• 29 CFR 1910.1200, Hazard Communication, for the safety data sheet's sixteen-section format, including Section 7 handling and storage, Section 8 exposure controls and personal protection, and Section 10 stability and reactivity
• Material manufacturer chemical resistance documentation, including stated test concentration, temperature, duration, and stress condition for each rating
• Trade-standard practice for immersion coupon testing and acceptance limits on mass, volume, and hardness change
• See related: Chemical Compatibility and the Checks Worth Running; The Elastomers and What Attacks Each One