Salt Spray Test (Corrosion Resistance Testing)

The Salt Spray Test (also called Salt Fog Test) is one of the most widely used accelerated corrosion testing methods for evaluating the protective performance of surface coatings and finishes. It provides an efficient way to assess how well paints, platings, or other protective treatments can resist corrosion when exposed to a highly saline environment.

Purpose of the Test

The main objective of the salt spray test is to simulate harsh environmental conditions and predict the corrosion resistance of coatings applied on metals. This helps manufacturers, quality control teams, and R&D departments ensure that their coatings meet the durability requirements for automotive, aerospace, marine, construction, and industrial applications.

Applicable International Standards

The Salt Spray Test is standardized worldwide, ensuring reliable and comparable results across industries. Common standards include:

• ISO 9227 – Corrosion tests in artificial atmospheres – Salt spray tests
• ASTM B117 – Standard Practice for Operating Salt Spray (Fog) Apparatus
• DIN EN ISO 9227 – German equivalent of the ISO standard for salt spray testing
• JIS Z 2371 – Japanese Industrial Standard for salt spray testing

These standards define the testing conditions, solution preparation, equipment design, and evaluation criteria.

Working Method

Test Principle:

The test exposes coated or plated metal specimens to a fine mist (fog) of a 5% sodium chloride (NaCl) solution under controlled conditions, simulating a highly corrosive marine-like atmosphere.

Procedure:

1. Preparation of Test Solution
   • A solution of sodium chloride (commonly 5% NaCl in deionized water) is prepared.
   • The pH of the solution is adjusted to 6.5 – 7.2 (neutral salt spray test). Variants like acetic acid salt spray (AASS) and copper-accelerated acetic acid salt spray (CASS) may use modified solutions.
2. Test Chamber Conditions
   • Temperature inside the chamber is maintained at 35°C ± 2°C.
   • Atomizers create a fine fog of the saline solution that continuously settles on the specimen surfaces.
   • Deposition rate: approx. 1.0 – 2.0 mL/hour per 80 cm² of surface area.
3. Specimen Placement
   • Samples are mounted at a 15° – 30° angle from vertical, avoiding direct dripping from one sample to another.
4. Exposure Duration
   • Duration depends on coating type and customer specification (commonly 24, 96, 240, 500, or 1000 hours).
   • Test duration does not directly correlate to real-life service life but provides comparative performance data.
5. Evaluation of Results
   • After exposure, specimens are inspected for:
     • Corrosion products (red rust, white rust, blistering, or pitting)
     • Degree of coating breakdown
     • Time to first corrosion appearance
   • Results are compared against the acceptance criteria defined in the relevant standard or customer specification.

Types of Salt Spray Tests

• Neutral Salt Spray (NSS) – Standard test with neutral pH solution.
• Acetic Acid Salt Spray (AASS) – More aggressive test used for decorative coatings (ISO 9227, ASTM B287).
• Copper-Accelerated Acetic Acid Salt Spray (CASS) – Highly severe test used for nickel/chromium plated parts (ISO 9227, ASTM B368).

Applications

• Automotive components (fasteners, body panels, wheels)
• Aerospace and marine hardware
• Construction materials (steel structures, galvanized steel)
• Paints, powder coatings, and conversion coatings
• Electroplated, anodized, or metallic coatings