Fog & Weathering Testing: Salt Fog, Acid Fog & ASTM B117 Methods

What Is Fog Testing?

Fog testing — most commonly known as salt spray (fog) testing — is one of the oldest, most widely used accelerated corrosion test methods in the world. It creates a controlled, reproducible, artificially corrosive environment by generating a continuous fine mist of salt water solution within an enclosed test chamber and exposing material specimens, coated panels, or finished components to this environment for defined periods.

The objective is to assess the corrosion resistance of metals, coatings, platings, anodized layers, and surface treatments in a fraction of the time required for natural outdoor exposure—enabling manufacturers to screen materials, validate protective coatings, qualify supplier finishes, and conduct quality-control inspections quickly and efficiently.

The Science Behind Salt Fog Corrosion

Corrosion is the electrochemical degradation of metals through oxidation in the presence of moisture and electrolytes. In natural environments, chloride ions from marine salts, road de-icing salts, and industrial atmospheres are among the most aggressive corrosion promoters, disrupting protective oxide layers and accelerating the anodic dissolution of metal surfaces.

Salt fog testing concentrates this chloride-rich, humid environment — continuously maintaining the conditions that most aggressively attack metal surfaces and protective coatings. The controlled, reproducible nature of the test allows comparison between materials, coatings, and treatment processes on a consistent, standardized basis.

ASTM B117 — The Global Standard for Salt Spray Testing

ASTM B117 is the primary international standard for salt spray (fog) testing — originally published in 1939 and the most widely referenced corrosion test standard worldwide alongside ISO 9227.

Test Conditions (ASTM B117)

• Test solution: 5% sodium chloride (NaCl) in distilled water, pH 6.5–7.2
• Chamber temperature: 35°C ± 2°C
• Fog collection rate: 1–2 mL/hour per 80 cm² horizontal collection area
• Exposure: Continuous — specimens remain in constant fog throughout the test duration

Specimen Preparation

Specimens must be clean, free of oils, grease, and surface contamination. Scribed panels (with a controlled scratch through the coating to the substrate) are commonly used to evaluate corrosion protection at coating defects — measuring undercutting (creep) from the scribe as a key performance indicator.

Evaluation Criteria

After defined exposure periods (ranging from 24 hours for plated parts to thousands of hours for high-performance coatings), specimens are evaluated for:

• Time to first rust or corrosion product appearance
• Area of corrosion as a percentage of exposed surface
• Creep from scribe (in mm) for coated panels
• Blistering, adhesion loss, and other coating degradation signs
• Rating per ASTM D1654 (scribe) or ASTM D714 (blistering)

Types of Salt Fog Tests

Neutral Salt Spray (NSS) — ASTM B117, ISO 9227 NSS

The standard 5% NaCl test is the baseline corrosion test for most industrial applications. Most widely applicable to metals, alloys, metallic coatings, conversion coatings, and organic coatings.

Acetic Acid Salt Spray (AASS) — ISO 9227 AASS

The salt solution is acidified with acetic acid to pH 3.1–3.3. Provides more aggressive conditions — particularly useful for evaluating decorative nickel-chromium, copper-nickel-chromium, and anodized aluminum coatings where the neutral NSS test may be insufficiently discriminating.

Copper-Accelerated Acetic Acid Salt Spray (CASS) — ASTM B368, ISO 9227 CASS

Acetic acid plus copper(II) chloride in the solution. The most aggressive fog test — used for decorative chromium, copper, and nickel plating, anodized aluminum, and zinc-aluminum alloy coatings. CASS testing produces results faster than NSS for very corrosion-resistant surfaces.

Modified Salt Spray Tests — ASTM G85

ASTM G85 provides several annexes covering modified fog protocol,s including:

• Annex A1 (AASS): Acetic acid salt fog
• Annex A2 (CASS)
• Annex A3 (SWAAT): Seawater acidified test — for aluminum alloys and marine coatings
• Annex A4: Salt/SO₂ spray test — simulates industrial acid rain environments
• Annex A5 (PROHESION): Dilute electrolyte salt fog/dry cyclic test — better correlation to real atmospheric corrosion for painted steel

Cyclic Corrosion Testing — A More Realistic Alternative

A key limitation of continuous salt fog testing (ASTM B117) is its poor correlation to real-world atmospheric corrosion for many coating systems — particularly automotive and architectural coatings. Continuous wetness in a salt fog chamber does not simulate the wet-dry cycling, UV exposure, and temperature fluctuations of real outdoor environments.

Cyclic corrosion tests (CCT) alternate between salt fog, humidity, and drying phases — better simulating real atmospheric corrosion. Key standards include:

• ASTM D5894: Cyclic salt fog/UV exposure for organic coatings
• SAE J2334: Automotive cyclic corrosion (used by GM, Ford, Chrysler)
• VDA 233-102: German automotive cyclic corrosion standard
• Volvo STD 423-0014 / BMW GS 90002: OEM-specific CCT protocols

These cyclic tests have a significantly better correlation with outdoor automotive and infrastructure corrosion performance, and are increasingly preferred over the static ASTM B11 test 7 for service life prediction.

Industry Applications

Automotive: Coated body panels, fasteners, brake components, chassis members, and under-hood parts are tested per ASTM B117 for production QC and per SAE J2334 or VDA 233-102 for corrosion life prediction.

Aerospace: Structural aluminum alloys, cadmium-plated fasteners, zinc-chromate primer systems, and hydraulic fittings are qualified by salt fog testing per MIL-STD-889 and aerospace prime specifications.

Electronics: PCB surface finishes (HASL, ENIG, OSP), connector plating (gold, nickel, tin), and metal enclosures are tested per ASTM B117 for corrosion resistance in humid, salt-laden environments.

Marine: Hardware, fittings, structural fasteners, and deck equipment are salt fog tested to simulate the highly corrosive marine atmosphere — typically to 1,000+ hours for high-grade marine hardware.

Infrastructure: Bridge fasteners, rebar coatings, and structural steel protective coatings are tested per ASTM B117 for specification compliance and comparative evaluation of coating systems.

Conclusion

Fog and salt spray testing — spanning neutral salt spray, acetic acid, CASS, and cyclic corrosion protocols per ASTM B117, G85, ISO 9227, and automotive OEM standards — provides the accelerated corrosion resistance data essential for qualifying metals, coatings, platings, and surface treatments across automotive, aerospace, electronics, marine, and infrastructure applications. Selecting the right fog test protocol and exposure duration for the specific coating system, substrate, and service environment determines whether corrosion test results accurately predict real-world protection performance and service life — making test method selection as critical to meaningful corrosion qualification as any decision about coating formulation or surface treatment process.

Why Choose Infinita Lab for Salt Spray and Fog Testing?

Infinita Lab is a trusted USA-based testing laboratory offering comprehensive salt spray (fog) testing services — including ASTM B117, B368, G85, and ISO 9227 — as well as cyclic corrosion testing per SAE J2334 and customer-specific OEM protocols — across an extensive network of accredited facilities. Our advanced equipment and expert professionals deliver highly accurate, prompt corrosion test results, helping businesses achieve quality compliance and validate coating systems.

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. Request a Quote.

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