ASTM B117 Salt Spray Test Procedure: Step-by-Step Overview Guide
What Is ASTM B117?
ASTM B117 — Standard Practice for Operating Salt Spray (Fog) Apparatus — is the most widely referenced accelerated corrosion test standard globally. It defines the operating conditions for a salt spray (salt fog) chamber that exposes metallic materials, protective coatings, and finished products to a continuous, controlled salt fog environment to evaluate their corrosion resistance. ASTM B117 does not define acceptance criteria — it is a practice for operating the test apparatus — the pass/fail criteria are specified by the applicable product or material specification.
Since its first publication in 1939, ASTM B117 has become the foundational accelerated corrosion test referenced in thousands of product specifications across the automotive, aerospace, electronics, fastener, coatings, and industrial equipment industries worldwide.
ASTM B117 Test Conditions
Salt Solution
A 5% sodium chloride (NaCl) solution in distilled or deionised water, adjusted to pH 6.5–7.2. Neutral salt spray (NSS) at 5% NaCl is the standard condition — distinct from CASS (copper-accelerated acetic acid salt spray, ASTM G85 Annex A3) or AASS (acetic acid salt spray, ASTM G85 Annex A1) which use acidified solutions for more aggressive testing of decorative chromium plating.
Chamber Temperature
The test chamber interior is maintained at 35°C ± 2°C. The fog collecting receivers inside the chamber should accumulate 1.0–2.0 mL/80 cm²/hour of salt solution.
Fog Distribution
The salt fog must be distributed uniformly throughout the test zone — specimens positioned at defined angles (15–30° from vertical for flat specimens) must not drip collected solution onto other specimens.
Test Duration
ASTM B117 does not specify a fixed duration — test duration is defined by the applicable product standard. Common durations range from 24 hours (thin decorative platings) to 500–2000 hours (heavy-duty corrosion protection coatings for marine and offshore applications).
What ASTM B117 Testing Evaluates
Coatings and Platings
- Hot-dip galvanised, zinc electroplated, and zinc-alloy coated fasteners and components: Evaluated for white rust (zinc corrosion) and red rust (base steel) appearance per ASTM B117 combined with ASTM D1654 (corrosion of painted specimens) or ASTM B537 (rating of electroplated panels)
- Organic coatings (paint, powder coat, epoxy): Evaluated for blistering (ASTM D714), loss of adhesion at scribe, and field corrosion after defined exposure hours
- Conversion coatings (chromate, trivalent chromium): Evaluated for base metal corrosion appearance
Metallic Materials
Bare metals are tested for corrosion resistance — stainless steel grades, aluminium alloys, copper alloys, and cast irons are ranked by time to first corrosion appearance.
Product and Assembly Testing
Assembled hardware, connectors, fastener systems, and finished components are tested to verify that all plating, coating, and material combinations provide the required service life in corrosive environments.
Limitations of ASTM B117
ASTM B117 is widely criticised for poor correlation with real-world corrosion performance — the constant wet salt fog environment does not replicate the cyclic wet-dry conditions of most outdoor environments. Cyclic corrosion tests (SAE J2334, ISO 11997, GM9540P) provide much better correlation with automotive field corrosion. Nevertheless, ASTM B117 remains the most specified test because of its global standardisation, reproducibility, and decades of data.
Key Evaluation Methods After ASTM B117 Exposure
ASTM D1654: Evaluation of painted or coated specimens subjected to corrosive environments — measuring rust creep from scribe and field blistering. ASTM D714: Rating blisters in paint films. ASTM B537: Rating electrodeposited coatings for corrosion.
Why Choose Infinita Lab for ASTM B117 Testing?
Infinita Lab provides ASTM B117 salt spray testing alongside post-exposure evaluation services through our nationwide accredited corrosion testing laboratory network.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
What is the difference between NSS (ASTM B117) and CASS or AASS testing? NSS (Neutral Salt Spray, ASTM B117) uses a pH-neutral 5% NaCl solution at 35°C. AASS (Acetic Acid Salt Spray) adds glacial acetic acid to reduce pH to 3.1–3.3 — more aggressive for decorative chromium plating evaluation. CASS (Copper-Accelerated Acetic Acid Salt Spray, ASTM G85 A3) adds CuCl₂ to AASS — the most aggressive variant, primarily for anodised aluminium and decorative plating evaluation.
How should scribing be performed before ASTM B117 testing of painted panels? Scribes are cut through the full coating to bare metal in a defined pattern (typically an X or single line) using a scribing tool of defined width and tip angle (ASTM D1654). The scribe provides a controlled corrosion initiation site — measuring rust creep and delamination from the scribe characterises the coating's wet adhesion and cathodic protection capability.
Does ASTM B117 predict outdoor service life accurately? ASTM B117 provides comparative corrosion resistance ranking and specification compliance verification — but correlation with specific outdoor service life years is unreliable because continuous salt fog does not replicate real outdoor wet/dry cycling, temperature variation, UV exposure, and varying chloride concentrations. Cyclic tests (SAE J2334, Prohesion) provide better outdoor correlation.
What is the required fog collection rate in ASTM B117? ASTM B117 requires 1.0–2.0 mL of salt solution collected per 80 cm² of horizontal collecting area per hour when measured at two or more positions in the test chamber. This verifies that the fog distribution is uniform and within the specified intensity range — a critical chamber performance parameter verified during setup and periodically during operation.
What pass/fail criteria are used for ASTM B117 testing of zinc-plated fasteners? Common specifications (ASTM F1941, ISO 4042) specify that zinc-plated fasteners must show no red rust (base metal corrosion) for defined hours: typically 24–72 hours for standard zinc plating, 120–240 hours for zinc-nickel alloy plating, and 480–1000 hours for hot-dip galvanised fasteners.
