🧂 Salt spray corrosion test
Accelerated corrosion test in salt spray chamber. Three methods: NSS (neutral), AASS (acetic acid), and CASS (with copper chloride). Assessment of corrosion resistance of metals and protective coatings.
Overview
The salt spray test is one of the oldest and most widely used accelerated methods for assessing the corrosion resistance of metallic materials and protective coatings. ISO 9227 defines three test variants: NSS (Neutral Salt Spray), AASS (Acetic Acid Salt Spray), and CASS (Copper-Accelerated Acetic Acid Salt Spray).
In the automotive industry, the salt spray test is a standard qualification test for galvanic coatings (zinc, nickel, chromium), paint coatings, anodic oxide layers, and conversion protective coatings. OEMs (BMW, VW, Toyota, Stellantis) require suppliers to provide confirmed corrosion resistance of chassis, body, connector, and fastener elements.
The NSS method (5% NaCl, pH 6.5–7.2, 35°C) is most common and used for most metals and coatings. AASS (pH 3.1–3.3 with acetic acid) is more aggressive and intended for testing decorative Cu/Ni/Cr coatings. CASS (with CuCl₂ addition) is the fastest method — exposure usually does not exceed 48 hours, making it an economical alternative to long-term NSS tests.
Important: ISO 9227 defines only the apparatus, reagents, and procedure — it does not specify acceptance criteria, exposure time, or result evaluation method. These parameters are defined in product standards and OEM specifications (e.g., VW TL 211, BMW GS 90011).
Method principle
In a closed climate chamber, a controlled fog (aerosol) is generated from a sodium chloride (NaCl) solution. The solution is sprayed through pneumatic nozzles fed by compressed air, creating a uniform fog that settles on samples placed at an angle of 15–25° from vertical. Chamber temperature is maintained at 35±2°C (NSS and AASS) or 50±2°C (CASS). Condensing brine creates a thin electrolytic film on sample surfaces, which initiates and sustains electrochemical corrosion processes (formation of galvanic micro-cells). After a specified exposure time, samples are removed, rinsed, dried, and visually or instrumentally assessed for degree of corrosion, blistering, flaking, or coating adhesion loss.
Applications
- Qualification of galvanic coatings (zinc plating, nickel plating, chromium plating) in automotive
- Testing corrosion resistance of chassis and suspension components
- Assessment of paint coatings (KTL, powder paint) on body components
- Testing of connectors, bolts, and fasteners with protective coatings
- Quality control of aluminum anodizing (architecture, aviation)
- Assessment of corrosion inhibitor and protective oil effectiveness
- Comparative testing of different coating systems
- Complaints and analysis of corrosion causes in service
Key parameters
| Parameter | Value |
|---|---|
| NaCl concentration (NSS/AASS) | 50 ± 5 g/L (5%) |
| NSS solution pH | 6.5–7.2 (at 25°C) |
| AASS solution pH | 3.1–3.3 (glacial acetic acid) |
| NSS/AASS chamber temperature | 35 ± 2°C |
| CASS chamber temperature | 50 ± 2°C |
| Fog settling rate | 1.0–2.0 mL/h per 80 cm² (horizontal collector) |
Standard
- Standard number
- PN-EN ISO 9227:2017
- Title (PL)
- Badania korozyjne w sztucznych atmosferach — Badania w rozpylonej solance
- Title (EN)
- Corrosion tests in artificial atmospheres — Salt spray tests
Step-by-step procedure
1. Salt solution preparation
Dissolve 50±5 g NaCl in 950 mL deionized water. For AASS — add acetic acid to pH 3.1–3.3. For CASS — additionally 0.26 g/L CuCl₂·2H₂O.
2. Solution pH check
Measure solution pH at temperature 25±2°C. NSS: 6.5–7.2, AASS: 3.1–3.3, CASS: 3.1–3.3. Correct with NaOH or HCl if necessary.
3. Sample preparation
Clean samples of dirt and grease (do not remove coating!). Protect cut edges and areas not subject to testing with protective tape or wax.
4. Placing samples in chamber
Set samples at angle 15–25° from vertical. Do not allow mutual dripping. Place fog collectors.
5. Test start
Set chamber temperature (35°C NSS/AASS, 50°C CASS). Start spraying. Check fog settling rate (1–2 mL/h/80 cm²) after first 24 h.
6. Monitoring during test
Daily check solution level, temperature, and fog settling rate. Replenish solution. Do not open chamber without necessity.
7. End of exposure
After required time (24 h, 48 h, 96 h, 240 h, 480 h, 720 h, 1000 h — according to specification) remove samples from chamber.
8. Sample rinsing and drying
Gently rinse samples with running water (temperature <40°C) to remove salt deposits. Dry at room temperature.
9. Result assessment
Assess degree of corrosion according to reference standards (ISO 10289, ISO 4628, ASTM D1654). Document: blistering, flaking, white/red rust, percentage of corroded surface.
10. Photographic documentation and report
Take photos of samples before and after test. Prepare report with test parameters, exposure time, visual assessment, and compliance result with requirements.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Salt spray chamber | Ascott S450, Weiss Technik SC/KWT, Erichsen 606/608, Atlas CCX | 40 000–200 000 PLN |
| Air compressor with filtration | Atlas Copco, FIAC, pressure 0.7–1.4 bar, oil-free | 5 000–15 000 PLN |
| Laboratory pH meter | Mettler Toledo SevenCompact, WTW inoLab pH 7310 | 3 000–10 000 PLN |
| Analytical balance | Mettler Toledo ME204, Sartorius Quintix 224 | 5 000–15 000 PLN |
| Fog collectors | Glass funnels Ø80 mm with graduated cylinder, 2 pcs according to standard | 200–500 PLN/pc |
| Sample racks / holders | Glass or plastic (PP, PTFE), angle 15–25° | 500–2 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Sodium chloride NaCl analytical grade | 7647-14-5 | Purity ≥99.5%, without iodine addition, Cu content <0.3 ppm, NaI <0.1%, 25 kg |
| Deionized water | — | Conductivity <20 µS/cm at 25°C, for salt solution preparation |
| Glacial acetic acid (AASS) | 64-19-7 | Purity ≥99.8%, for solution acidification to pH 3.1–3.3 |
| Copper(II) chloride dihydrate (CASS) | 10125-13-0 | CuCl₂·2H₂O, concentration 0.26 ± 0.02 g/L in CASS solution |
| Sodium hydroxide 1 mol/L | 1310-73-2 | For pH correction (if too low) |
| Hydrochloric acid 1 mol/L | 7647-01-0 | For pH correction (if too high) |
Health and safety (OHS)
- Salt fog — highly corrosive, chamber must be sealed; do not open during operation without necessity
- Glacial acetic acid — corrosive, irritating, work under fume hood, gloves and goggles
- Copper(II) chloride — toxic to aquatic organisms, irritating, avoid skin contact
- Chamber effluent — neutralize and dispose of according to environmental regulations
- Safety glasses, acid-resistant gloves, and laboratory coat mandatory