ASTM B117 Salt Spray Test — Procedure, Standards, and Applications
What Is the ASTM B117 Salt Spray Test?
ASTM B117 is the Standard Practice for Operating Salt Spray (Fog) Apparatus — the most widely referenced and executed accelerated corrosion test in the world. It defines the equipment, salt solution concentration, temperature, spray rate, and exposure protocol for creating a standardized corrosive fog environment in which metallic materials, coatings, and finished products are evaluated for corrosion resistance. The test has been in continuous use since its first publication in 1939 and is referenced in thousands of product specifications, customer standards, and regulatory requirements across the automotive, aerospace, marine, and metal coatings industries.
Test Procedure
Salt Solution Preparation
A 5% (mass/volume) sodium chloride (NaCl) solution is prepared in deionized or distilled water, adjusted to pH 6.5–7.2 at 35°C. Impure water or salt containing iodide or excessive impurities must be avoided — trace contaminants alter corrosion chemistry and reproducibility. Solution conductivity is typically monitored as a quality indicator.
Chamber Conditions
The salt spray chamber maintains:
- Temperature: 35°C ± 2°C throughout the exposure zone
- Spray rate: 1.0–2.0 mL/hour per 80 cm² of horizontal collection area (measured by funnel collectors placed in the exposure zone)
- pH of collected solution: 6.5–7.2
- Fog distribution: Uniform throughout the exposure zone — no direct impingement on specimens
Specimen Positioning
Flat panels are exposed at 15–30° from vertical. Cut edges must be sealed with wax, tape, or other materials to prevent preferential edge corrosion unless edge performance is being specifically evaluated. Test panels must not touch each other or the chamber walls.
Test Duration and Evaluation
Exposure durations range from 24 hours (rapid screening of thin coatings) to 2,000+ hours (heavy-duty industrial coating qualification). Evaluation at defined intervals records: time to first rust spot, percentage of rusted area, blister rating (per ASTM D714), adhesion retention (per ASTM D3359), and overall corrosion rating (per ISO 10289).
Correlation to Real-World Corrosion
ASTM B117 is a comparative test — it ranks materials and coatings relative to each other under reproducible conditions. Direct year-equivalent predictions are system-specific and environment-dependent. Commonly cited approximations for automotive systems:
- 240 hours NSS ≈ 1–2 years mild outdoor exposure
- 500 hours NSS ≈ qualification threshold for many automotive OEM primer systems
- 1,000 hours NSS ≈ requirement for automotive topcoat systems on treated steel
Cyclic corrosion tests (GMW14872, Volvo STD1027,3, SAE J2334) show significantly better real-world correlation than constant NSS for automotive applications.
Industry Specifications Referencing ASTM B117
- Automotive: GM GMW14872 (uses cyclic methods referencing NSS), Ford FLTM BI 103-01, Chrysler LP-461P-B-01
- Military: MIL-DTL-5541 (chromate conversion coatings), MIL-PRF-23377 (epoxy primer)
- Fasteners: ISO 4042, ASTM F1941 (electroplated fasteners)
- Marine hardware: ASTM B456 (electrodeposited coatings)
- Electronics: IEC 60068-2-11 (salt mist test for components)
Conclusion
ASTM B117 remains the global baseline for accelerated corrosion screening — valued for its reproducibility and universal industry recognition. While it does not directly predict real-world service life, it provides a reliable comparative framework for ranking coatings and materials. For automotive and marine applications requiring stronger field correlation, cyclic corrosion protocols complement B117 qualification data effectively.
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Infinita Lab is a leading provider of ASTM B117, ISO 9227, and cyclic corrosion testing services, with 2,000+ accredited labs across the USA, offering NSS, AASS, and CASS testing, along with comprehensive project management, confidentiality, and fast turnaround.
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Frequently Asked Questions
What is the difference between ASTM B117 and ISO 9227? Both standards specify 5% NaCl at 35°C with identical chamber conditions. ISO 9227 additionally covers Acetic Acid Salt Spray (AASS) and Copper-Accelerated Acetic Acid Salt Spray (CASS) methods. European specifications reference ISO 9227 while US specifications reference ASTM B117.
How is ASTM B117 used for coating qualification? Coated specimens are scribed to bare metal before exposure, creating a controlled defect. After exposure, corrosion creep from the scribe and blister formation on intact coating are measured. Creep within specification limits — typically 3–4 mm at defined test duration — determines pass or fail for automotive coating systems.
What does a salt spray test NOT tell you about coating performance? ASTM B117 does not replicate wet-dry cycling, UV exposure, freeze-thaw effects, mechanical abrasion, or stone chip damage driving real-world corrosion. It cannot predict absolute service life. Its value is strictly comparative — ranking coating systems under reproducible conditions rather than predicting actual field durability.
What surface preparation is required before salt spray testing? Specimens must be free of fingerprints, oils, and manufacturing residues. Mild solvent wiping is acceptable for bare metal panels. Coated panels must be fully cured before exposure. Edges must be sealed, and scribe lines must penetrate cleanly to bare metal without burrs that artificially concentrate corrosion initiation.
What are the most common reasons for variability in ASTM B117 results? Chamber temperature uniformity, spray nozzle condition, fog droplet size distribution, salt solution pH drift, specimen positioning, inadequate edge sealing, and panel cleanliness are primary variability sources. Regular chamber calibration per ASTM B117 Section 5 and participation in proficiency testing programs effectively minimize inter-laboratory and intra-laboratory result variation.
