🧂 Salt spray corrosion test

Automotive PN-EN ISO 9227

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

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.

   ⏱ Time: 30 min • 🌡 Temperature: 25°C

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.

   ⏱ Time: 10 min • 🌡 Temperature: 25°C

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.

   ⏱ Time: 15–30 min

4. Placing samples in chamber

   Set samples at angle 15–25° from vertical. Do not allow mutual dripping. Place fog collectors.

   ⏱ Time: 10 min

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.

   ⏱ Time: 15 min • 🌡 Temperature: 35°C or 50°C

6. Monitoring during test

   Daily check solution level, temperature, and fog settling rate. Replenish solution. Do not open chamber without necessity.

   ⏱ Time: continuous

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.

   ⏱ Time: per specification

8. Sample rinsing and drying

   Gently rinse samples with running water (temperature <40°C) to remove salt deposits. Dry at room temperature.

   ⏱ Time: 15 min • 🌡 Temperature: <40°C

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.

   ⏱ Time: 30–60 min

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.

    ⏱ Time: 30 min

Required equipment and apparatus

Reagents, media and consumables

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

Frequently asked questions