Key takeaways
Neutral salt spray (NSS) simulates a neutral corrosive environment; acetic acid salt spray (AASS) accelerates an acidic environment by adding acetic acid; cyclic salt spray alternates hot/cold cycles with dry and wet phases, coming closer to the wet-dry cycling seen in real use. Automotive electronic parts usually take a combination of NSS and cyclic salt spray — the first for long-term salt exposure, the second for alternating wet and dry conditions.
The three types of salt spray test are often confused. They simulate different environments, use different acceleration factors and suit different cases. Choose the wrong type and the result maps to nothing.
1. Neutral salt spray (NSS)
The method defined in ISO 9227 uses a sodium chloride solution at pH 5.0–6.5, with no acid or alkali adjustment.
Environment simulated: neutral atmospheric corrosion, such as salt spray in coastal areas or salt-bearing industrial atmospheres.
Suits: verifying the corrosion resistance of zinc, cadmium and steel substrate protective coatings, and assessing salt spray resistance of some plastic coatings.
Limitation: it departs considerably from real service conditions; the corrosion morphology in the laboratory often does not match what is seen outdoors, and the result cannot be converted directly into a service life.
2. Acetic acid salt spray (AASS)
Acetic acid is added to the NSS solution, bringing the pH to around 3.2–3.5.
Environment simulated: acidic corrosion, such as industrial exhaust or acid rain areas.
Acceleration: acidification markedly speeds up corrosion, which suits rapid screening of protective coating performance.
Limitation: the corrosion mechanism differs from a neutral environment, so AASS results cannot substitute for NSS.
3. Cyclic salt spray
Heating, cooling, drying and immersion phases are layered on top of the salt spray to form a cycle. A typical programme: salt spray exposure → drying → damp heat → heating → cooling → repeat.
Environment simulated: outdoor service with alternating wet and dry, closer to reality than constant salt spray.
Advantage: the cycling lets corrosive liquid accumulate at coating defects, which exposes protective coating defects and sealing problems far more readily.
Limitation: long duration and complex conditions, so cost is higher than conventional salt spray.
4. Choosing for automotive electronic parts
- Metal parts and coatings: neutral salt spray for baseline corrosion resistance
- Connectors and terminals: cyclic salt spray, to verify contact resistance stability under wet-dry cycling
- Seals and structural parts: cyclic salt spray plus a water ingress check afterwards
- Export products: per the target market standard — the European market commonly cites the relevant clauses of ISO 9227
5. What to inspect after the test
Salt spray testing on its own only reports a degree of corrosion. A real assessment needs the post-test checks: corrosion rating to the ISO 9227 scale, change in contact resistance, X-ray and cross-section to trace the corrosion path, and a check of sealing condition.
A report that runs salt spray but reports no post-test data has limited practical value.
