Salt Spray Test (ASTM B117) Explained: Procedure, Ratings, and Limits
ASTM B117 is the most widely referenced accelerated corrosion test in industry, used to compare the corrosion resistance of metals and coatings inside a controlled neutral salt fog chamber. This guide develops the standard’s chamber conditions, specimen preparation and scribing practice, the rating standards used to evaluate results, related more aggressive variants, and the well-documented limitations engineers should understand before treating salt spray hours as a proxy for real-world service life.
Key Takeaways
- ASTM B117 neutral salt spray (NSS) uses a 5% NaCl solution at pH 6.5-7.2, atomized continuously in a chamber held at 35°C ±2°C, with a controlled fog fall-out rate of 1.0-2.0 ml per 80 cm² per hour.
- The standard itself specifies no fixed test duration; exposure time is set by the referencing product specification and commonly ranges from 24 hours for thin decorative platings to 500-2000+ hours for heavy-duty protective coatings.
- Coated specimens are typically scribed to bare metal before exposure, and results are evaluated using ASTM D1654 (rust creep and corrosion rating), ASTM D714 (blister rating), or ASTM B537 (electrodeposited coating rating).
- More aggressive related variants include acetic acid salt spray (AASS, pH ~3.1-3.3) and copper-accelerated acetic acid salt spray (CASS, ASTM B368, elevated temperature), both primarily used for decorative plated finishes.
- Continuous salt fog exposure correlates poorly with real-world atmospheric corrosion because it lacks the wet-dry cycling, temperature variation, and UV exposure that dominate outdoor service conditions; salt spray hours cannot be reliably converted into equivalent years of service life.
- Cyclic corrosion tests such as SAE J2334 and the Prohesion test generally show substantially better correlation with real field exposure and are increasingly specified where realistic service life prediction is required.
What Is the ASTM B117 Salt Spray Test?
ASTM B117 is a standardized accelerated corrosion test method that exposes metallic and coated metallic specimens to a continuous neutral salt fog inside a controlled chamber. First published in 1939 and in continuous use since, it is referenced in thousands of product specifications across the automotive, aerospace, marine, and metal finishing industries as a comparative, repeatable screening tool for corrosion resistance and specification compliance. Its role complements the electrochemical corrosion mechanisms covered in our corrosion mechanisms overview and the selective degradation discussed in our dezincification and dealloying corrosion guide, providing a practical, standardized way to compare candidate materials and coatings rather than to model any single corrosion mechanism directly.
Standard Chamber Conditions
ASTM B117’s neutral salt spray (NSS) procedure specifies tightly controlled chamber conditions to ensure repeatable, comparable results between laboratories and test runs:
| Parameter | Specified Condition |
|---|---|
| Salt solution | 5% NaCl by mass, in purified (distilled or deionized) water |
| Solution pH | 6.5-7.2 (neutral), adjusted with reagent-grade HCl or NaOH as needed |
| Chamber temperature | 35°C ± 2°C |
| Fog fall-out rate | 1.0-2.0 ml per 80 cm² per hour, verified at multiple chamber locations |
| Relative humidity | Approximately 95% ± 5% (near-saturated fog condition) |
| Specimen angle | Typically 15-30° from vertical, avoiding drip-through onto adjacent specimens |
Chamber pre-conditioning to the operating temperature and verified fog collection rate is required before specimens are introduced, and fog fall-out rate is checked using calibrated collection funnels positioned near and away from the atomizing nozzle to confirm uniform distribution throughout the chamber’s working volume.
No Fixed Test Duration
ASTM B117 itself specifies no fixed exposure time; the required duration is set entirely by the applicable product or material specification that references the standard. Common durations range from as little as 24 hours for thin decorative platings up to 500-2000 hours or more for heavy-duty protective coatings intended for marine, offshore, or other severe-service applications, so the appropriate duration must always be confirmed against the governing specification rather than assumed from the standard alone.
Specimen Preparation and Scribing
Test specimens must be thoroughly cleaned of fingerprints, oils, and manufacturing residues before exposure, and are handled by their edges wherever possible to avoid contaminating the test surface. Coated panels must be fully cured before testing, and panel edges are commonly sealed or protected if the edge condition is not part of the evaluation, since edge effects can otherwise dominate and mask the coating performance under evaluation.
For coated specimens, a controlled scribe line, cut cleanly through the coating to bare metal without burrs that would artificially concentrate corrosion initiation, is standard practice before exposure. This deliberately introduced defect allows evaluation of both the coating’s general field performance and, separately, its ability to resist corrosion creep migrating outward from the scribe line, a distinction directly relevant to coating systems discussed alongside the surface protection content in our corrosion protection coatings guide.
Evaluating Results: Rating Standards
After the specified exposure period, specimens are removed, gently rinsed to remove surface salt deposits, dried, and evaluated against one or more standardized rating methods:
| Standard | What It Rates |
|---|---|
| ASTM D1654 | Rust creep from the scribe line and general field corrosion on painted or coated specimens |
| ASTM D714 | Blister size and density in paint films |
| ASTM B537 | Corrosion rating of electrodeposited (electroplated) coatings |
These standardized numerical or descriptive rating scales allow quantitative, reproducible comparison between specimens, coating formulations, or process variants, and against pass/fail acceptance criteria specified in the governing product standard.
Related Test Variants: AASS and CASS
Where NSS conditions are insufficient to differentiate between candidate coatings within a practical test duration, particularly for decorative electroplated finishes, more aggressive standardized variants are used:
| Test | Governing Standard | Key Difference from NSS | Typical Application |
|---|---|---|---|
| Neutral Salt Spray (NSS) | ASTM B117 | pH-neutral 5% NaCl, 35°C | General-purpose corrosion resistance screening |
| Acetic Acid Salt Spray (AASS) | ASTM B287 / ISO 9227 | Glacial acetic acid added, pH reduced to ~3.1-3.3 | Decorative chromium and other electroplated finishes |
| Copper-Accelerated Acetic Acid Salt Spray (CASS) | ASTM B368 | Copper chloride added, elevated temperature (~50°C) | Most aggressive of the three; decorative plated finishes |
Why Correlation with Real-World Service Life Is Limited
Salt Spray Hours Are Not Outdoor Years
Continuous salt fog exposure does not replicate the wet-dry cycling, temperature variation, ultraviolet exposure, and variable chloride concentration that characterize real outdoor atmospheric corrosion. Different corrosion mechanisms and protective film formation behaviours can dominate under continuous wetting versus the intermittent wetting typical of actual service, and constant high humidity in the chamber does not allow the periodic drying that influences real coating and substrate degradation. As a result, salt spray hours cannot be reliably converted into an equivalent number of years or months of outdoor service, and the test is best used for comparative ranking between specimens or specification pass/fail compliance rather than absolute service life prediction, a limitation that should be communicated clearly whenever B117 results are used to support a durability claim.
Cyclic corrosion tests, including SAE J2334 and the Prohesion test (ASTM G85 Annex 5), alternate periods of salt fog or humid exposure with dry-off and, in some protocols, temperature cycling, more closely approximating the wet-dry cycling that dominates real atmospheric corrosion. These cyclic methods generally show substantially better correlation with actual field and outdoor exposure results than continuous salt spray testing, and are increasingly specified by automotive OEMs and other industries where realistic service life prediction, rather than simple comparative screening, is the goal.
Common Sources of Test Variability
- Chamber temperature non-uniformity across the specimen exposure zone.
- Spray nozzle condition and resulting fog droplet size distribution.
- Salt solution pH drift over an extended exposure period.
- Specimen positioning, angle, and spacing relative to the fog source and to adjacent specimens.
- Inconsistent specimen cleaning, edge protection, or scribe line preparation before test.
Because these factors can meaningfully affect measured corrosion rate and reproducibility, calibrated equipment, fog collection rate verification at multiple chamber locations, and disciplined, standardized specimen preparation are essential for generating comparable, defensible results both within a single laboratory over time and between different testing facilities.
Industrial Significance
Despite its well-documented correlation limitations, ASTM B117 remains the most widely specified accelerated corrosion test globally because of its long track record, standardized and widely available equipment, and its usefulness as a comparative screening and specification-compliance tool across an enormous range of coated and bare metal products. Understanding both its proper application, comparative ranking and pass/fail verification, and its limitations, unreliable direct conversion to real-world service years, allows engineers to use B117 results appropriately alongside complementary cyclic and field exposure data where genuine service life prediction is required.
Frequently Asked Questions
What is the ASTM B117 salt spray test?
What are the standard ASTM B117 chamber conditions?
How long does an ASTM B117 test run?
How are salt spray test results evaluated after exposure?
What is the difference between neutral salt spray, CASS, and acetic acid salt spray?
Why does salt spray testing correlate poorly with real-world atmospheric corrosion?
What cyclic corrosion tests are used instead of continuous salt spray for better real-world correlation?
What sources of variability affect ASTM B117 test results?
Recommended Reference Reading
Corrosion Engineering (Fontana)
Classic reference covering corrosion testing methodology and mechanisms.
View on AmazonASM Handbook Vol. 13A: Corrosion Fundamentals, Testing, and Protection
Authoritative reference on accelerated corrosion test methods and their limitations.
View on AmazonHandbook of Accelerated Corrosion Testing
Applied guide to salt spray, cyclic, and other accelerated corrosion test protocols.
View on AmazonCoatings Technology Handbook
Reference for coating systems and their performance evaluation methodology.
View on AmazonDisclosure: MetallurgyZone participates in the Amazon Associates programme. If you purchase through these links, we may earn a small commission at no extra cost to you. This helps support free technical content on this site.
Further Reading
