How to Choose a High-Temperature Sealer for Fasteners
"High temperature" on a datasheet can mean three different things: the curing temperature, the temperature the dry film survives without visible change, and the temperature at which the film still protects the substrate. They are not interchangeable. Selection goes wrong when a buyer reads one number and assumes the other two.
Four dimensions, in priority order
1. Temperature resistance — and what it is measured against
Ask for the test description, not just the number. A statement like "no discolouration after 1 h at 250 °C" describes appearance. A statement like "corrosion resistance retained after dehydrogenation at 200 °C for 8 h" describes function. Only the second one answers the question a fastener plant is actually asking.
2. Salt-spray retention after the bake
This is the number that matters most and is almost never printed on a TDS. Ask for paired data: same substrate, same zinc thickness, same passivation, sealed and tested once before baking and once after. A film that gives 240 h before baking and 48 h after has failed the job, even if its "temperature resistance" looks impressive.
3. Film appearance and coverage
Appearance is a customer acceptance criterion, not cosmetics. Typical points to check: uniform silver-white after blue-white or yellow passivation, no runs or drip marks at thread roots, no rainbow mottling on large flat faces, no colour drift after baking. Film build also matters — a film thick enough to hide the passivation colour may fail a torque or fit requirement.
4. Bath operation
The chemistry that is easiest to run is the chemistry that gets run correctly. Compare:
| Item | Why it matters on a real line |
|---|---|
| Working concentration | A 750–1000 ml/L bath is mostly water you pay for, heat and drag out. A 100–400 ml/L bath changes the cost structure and the drag-out load on rinsing. |
| Operating temperature | Baths that need 40–50 °C cost electricity all year and lose temperature overnight. |
| pH control | Frequent pH adjustment means labour and a failure mode: an out-of-range bath silently produces poor films. |
| Agitation and filtration | Continuous low-shear mixing and 80–150 µm filtration are real capital and downtime items, and they are often skipped under production pressure. |
Why side-by-side "spec tables" mislead
Two products rarely solve the same problem. One may be optimised for low friction on engine-fastener lines, another for maximum salt spray on construction parts, a third for surviving a dehydrogenation bake. Lining them up row by row invites a conclusion the data cannot support — a blank cell means "not published", not "worse".
A more useful comparison is a problem map: write down the three failures you actually have (red rust on edges? torque scatter? discolouration after bake?) and check which product's published data addresses each one. Where data is missing, ask for a sample and run the test yourself.
A practical validation protocol
- One production batch, one rack position, one zinc thickness window. Split into control and test.
- Seal at three concentrations (low, middle, high of the recommended range).
- Bake half of each group at production temperature and time.
- Run neutral salt spray to the customer specification, and record first white rust and first red rust separately.
- Measure friction coefficient on plated bolts if the part is torque-controlled (ISO 16047 or the customer's own method).
- Keep photo records at fixed intervals — most disputes are about what "white rust" looked like.