Neodymium magnets are made from highly reactive rare-earth metals. Left bare, they corrode quickly in hot, humid air — which is why almost every NdFeB magnet gets a protective coating. But how do you prove the metal underneath is really corrosion-resistant? That is what weight-loss tests like HAST and PCT are for.
What the test does
Bare, uncoated samples go into a sealed autoclave and are exposed to high-temperature, high-pressure steam — a harsh, accelerated version of humid weather. The steam attacks the surface, grains flake off and the sample slowly loses material. After a set number of hours, the loose powder is brushed off and the samples are weighed again.
The result is weight loss per unit area, usually in mg/cm². The less a sample loses, the more corrosion-resistant the base material. It is the most direct way to compare one magnet material or coating system against another.
HAST or PCT?
Both tests use steam under pressure, but they work differently:
- PCT (pressure cooker test) — saturated steam at 100% humidity; temperature, humidity and pressure rise together. Simple and repeatable, often run on finished coated parts, with "no visible corrosion in 12, 24 or 72 hours" as the pass line.
- HAST (highly accelerated stress test) — humidity and temperature are set independently, so you can run harsh profiles like high temperature plus high humidity. Used to rate bare substrates and to compare coatings in development.
Two things to remember
- Results only compare within the same protocol — same sample size, surface preparation and chamber mode. Always state the full conditions on the test report.
- Corrosion accelerates once it starts: below a few mg/cm² of loss a substrate looks fine, but past roughly 5 mg/cm² the material erodes fast. Treat that as a design limit, not a pass mark.
Read on: corrosion and protection of NdFeB → · coating systems compared →