Salt spray testing is the most widely cited measure of corrosion resistance for sintered NdFeB magnets — yet the hours on a datasheet are one of the most misread numbers in magnet sourcing. A magnet rated for “72 hours” is not guaranteed to survive a fixed number of years in the field, and two coatings carrying the same rating can perform very differently in service. This guide explains what NdFeB salt spray testing actually measures, how the ISO 9227 and ASTM B117 standards define failure, how many hours common coatings achieve, and how to choose the right requirement for your application.
What Is NdFeB Salt Spray Testing?
NdFeB salt spray testing is an accelerated corrosion test in which coated neodymium magnets are exposed to a continuous salt fog inside a sealed chamber, and the time until corrosion appears is recorded. The two governing standards, ISO 9227 and ASTM B117, describe the most common variant: the neutral salt spray (NSS) test.
Under NSS, samples sit in a chamber held at roughly 35 °C and are continuously exposed to an atomized fog of about 5% sodium chloride solution, mounted at an angle so condensate runs off rather than pools. Exposure runs continuously, and parts are inspected at set intervals. The goal is to compress corrosion that might take months or years in ambient air into a controlled, repeatable window of hours.
Why Neodymium Magnets Need Corrosion Testing
Sintered NdFeB gets its magnetic strength from a neodymium-rich phase along the grain boundaries of its microstructure. That phase is electrochemically active: when moisture and chloride ions reach it, corrosion attacks the boundaries first. Left unprotected, the magnet doesn’t simply discolor — it can lose grain cohesion, pulverize, and shed magnetic performance.
Because the base material is inherently reactive, the coating is the corrosion barrier. A salt spray result on a finished magnet is therefore a test of the coating system — its thickness, adhesion, and how completely it covers difficult edges and corners. This is exactly why the number matters: it is a direct window into the one layer standing between the environment and the magnet.
What the Salt Spray Hours Actually Mean
How failure is defined
A salt spray result is only meaningful alongside its failure criterion. In most specifications, failure is the first appearance of red rust — corrosion of the base magnet showing through the coating — or a defined percentage of surface corrosion, blistering, or coating detachment. The reported figure is the time to reach that criterion. Without knowing the criterion, “72 hours” is incomplete information.
A benchmark, not a lifespan
Here is the distinction that prevents costly mistakes: an accelerated test does not map linearly to real-world life. It is tempting to read “72 hours” as a fixed number of outdoor years, but NSS was never designed to produce a calibrated field-life figure. Real environments vary in humidity cycling, temperature swings, pollutants, mechanical wear, and actual chloride exposure — none of which a steady salt fog reproduces.
What NSS does reliably is provide a comparative, reproducible stress. Use the hours for three purposes:
- A pass/fail gate against a specification.
- A like-for-like comparison between coating options under identical conditions.
- A consistency check confirming that production lots hold their quality over time.
Read this way, the number is dependable and decision-useful. Read as “years of service,” it will mislead you.
Salt Spray Ratings by Coating Type
Coating choice is the single biggest driver of the result. The ranges below are indicative of typical NSS performance; the real figure for any given part depends on layer thickness, sealing quality, and geometry.
| Coating system | Typical NSS range | Best suited for |
|---|---|---|
| Phosphate | A few hours | Temporary transit/storage protection, or as a base layer |
| Zinc (Zn), passivated | 24–72 h | Cost-sensitive, low-to-moderate humidity |
| Nickel / Ni-Cu-Ni | 16–72 h | The default decorative + barrier coating for general use |
| Epoxy | 48–120 h | Humid and chemically aggressive environments |
| Ni-Cu-Ni + epoxy (composite) | 72–120 h+ | Demanding indoor / industrial applications |
| Aluminum (IVD) | 96–500 h+ | Automotive, aerospace, harsh or under-hood conditions |
| Parylene | Conformal barrier | Medical and electronics, thin uniform coverage |
One practical point: edges and corners are where coatings thin out, and where corrosion almost always appears first. A supplier who reports strong salt spray numbers and controls edge coverage delivers a more durable part than the headline hours alone suggest.
How to Choose the Right Salt Spray Requirement for Your Application
The useful move is to match the requirement to your environment tier, rather than chasing the highest number available.
- Sealed or indoor assemblies (encapsulated in a rotor, potted in resin, or housed indoors): a modest NSS rating is often sufficient, because the coating is not the only barrier the environment must cross.
- General industrial or automotive-interior use: a mid-range coating with proven lot-to-lot consistency is usually the sweet spot.
- Outdoor, high-humidity, coastal, or marine exposure: specify both a higher NSS figure and a coating chemistry suited to sustained moisture, such as epoxy or aluminum systems.
The sharper question to ask a supplier is not only “how many hours?” It is: “Which coating, at what thickness, tested to which standard, and against which failure criterion?” Two magnets can both claim “72 hours” and differ substantially once those details are on the table.
Beyond Salt Spray: Other Corrosion Tests
Salt fog probes chloride-driven corrosion well, but some applications are limited by different failure modes that other tests describe more accurately:
- 85 °C / 85% RH humidity testing and accelerated moisture tests such as HAST or pressure-cooker testing (PCT) reveal how well a coating resists moisture ingress and holds adhesion — often more relevant for sealed electronics.
- Thermal cycling exposes adhesion problems caused by expansion mismatch between coating and magnet.
- CASS (copper-accelerated acetic acid salt spray) offers a harsher, faster screen when you need to differentiate high-performing coatings quickly.
For automotive and EV drive applications, a combination of these is commonly specified alongside NSS.
Frequently Asked Questions
How many salt spray hours does an NdFeB magnet need?
It depends entirely on the environment. A sealed indoor assembly may be well served by 24–48 hours, while outdoor, coastal, or automotive applications often call for 96 hours or more with an appropriate coating chemistry.
Does 72 hours of salt spray equal a set number of years?
No. Salt spray is an accelerated, comparative test, not a calibrated field-life prediction. It reliably compares coatings and verifies lot consistency, but real-world durability depends on factors the test does not reproduce.
What is the difference between ISO 9227 and ASTM B117?
Both describe neutral salt spray testing under near-identical conditions (about 5% NaCl, 35 °C, continuous fog). ISO 9227 is the international standard and ASTM B117 the North American equivalent; results are broadly comparable, but the referenced standard should always be stated.
Which coating has the best salt spray resistance?
Among common options, aluminum (IVD) and composite systems such as Ni-Cu-Ni + epoxy typically achieve the highest NSS hours, while phosphate offers only temporary protection. The right choice balances corrosion resistance against cost, thickness, and application.
Can salt spray hours be compared between suppliers?
Only when the coating, thickness, standard, and failure criterion match. Two “72-hour” ratings measured against different failure criteria are not directly comparable.
Match Your Coating to Real Conditions
Salt spray hours are a strong signal when read for what they are: a controlled, comparative measure of coating integrity and lot consistency. Pair the number with the coating system, thickness, standard, and failure criterion, and you have a genuinely reliable basis for a sourcing decision.
If you are specifying sintered NdFeB and want to match a coating and test protocol to your actual operating environment, our engineering team is glad to work through it with you — including sample testing to the standard your application requires. Reach us at tony@xh-magnet.com.
