Salt spray testing is one of the oldest and most widely cited accelerated corrosion methods in industrial use, and also one of the most commonly misunderstood — NSS, AASS, and CASS are often treated as if they're simply increasing severity levels of the same test, when they're actually three distinct test chemistries, each suited to evaluating different coating and material systems.
What Salt Spray Testing Actually Does
A salt spray (or salt fog) test exposes samples to a continuously or intermittently atomised saline solution inside a sealed, temperature-controlled chamber, accelerating corrosion processes. ISO 9227 itself notes that results shouldn't be treated as a direct guide to real-world corrosion — what it reliably provides is a comparative measure, ranking coatings, materials, or processes against each other under controlled, repeatable, accelerated stress.
Three Test Chemistries, Three Different Purposes
NSS (Neutral Salt Spray), the baseline test, uses a neutral pH (6.5–7.2) 5% sodium chloride solution, typically run at 35°C — the general-purpose test for evaluating basic corrosion resistance of metallic substrates, inorganic coatings, and anodic oxidation finishes.
AASS (Acetic Acid Salt Spray) acidifies the same base solution with acetic acid, dropping pH to around 3.1–3.3, producing a more aggressive environment — useful for evaluating decorative coating systems like nickel-chromium plating.
CASS (Copper-Accelerated Acetic Acid Salt Spray) adds copper chloride and typically runs at ~50°C, producing the most aggressive and fastest-acting of the three — suited to high-performance decorative chrome and anodized aluminium.
| Test | Solution | pH | Temperature | Typical use |
|---|---|---|---|---|
| NSS | 5% NaCl, neutral | 6.5–7.2 | ~35°C | General coatings, substrates, anodic oxidation |
| AASS | 5% NaCl + acetic acid | ~3.1–3.3 | ~35°C | Decorative Ni-Cr / Cu-Ni-Cr plating |
| CASS | 5% NaCl + acetic acid + copper chloride | ~3.1–3.3 | ~50°C | High-performance decorative chrome, anodized aluminium |
What a Compliant Test Setup Actually Requires
Solution preparation and purity. The sodium chloride used must meet specified purity limits, and the resulting solution's specific gravity and conductivity fall within tightly defined ranges.
Saturated air delivery. The compressed air used to atomise the salt solution needs to be pre-humidified and heated before reaching the atomiser. The LISUN YWX/Q-010 is a representative chamber covering these requirements.
Specimen placement and angle. Samples are positioned at a specified angle (typically 15–30° from vertical) to ensure consistent, repeatable fog deposition.
Fog collection rate verification. A genuinely calibrated chamber requires periodic fog collection studies confirming deposition rate falls within the standard's specified range.
Chamber material compatibility. Chamber interior materials need genuine long-term resistance to saline and acidic conditions at elevated temperature.
Beyond Single-Condition Testing: Cyclic Corrosion
Cyclic corrosion testing — alternating salt spray exposure with humidity and drying phases — is increasingly specified for products genuinely exposed to outdoor weathering, where wetting, drying, and salt exposure together better represent actual field conditions than constant salt fog alone.
Practical Test Duration Guidance
Typical recommended durations per ISO 9227 are around 48 hours for NSS and 24 hours for AASS and CASS — though actual product specification requirements vary considerably. Confirming the specific duration, test chemistry, and pass/fail criteria a product standard actually calls for is the first step in any salt spray test plan.