ASTM G85 Modified Salt Spray Test: A Guide
What Is ASTM G85?
ASTM G85 is the standard practice for modified salt spray (fog) testing. Where ASTM B117 uses a continuous neutral salt fog at 35 degrees C, ASTM G85 introduces five annexes that modify the chemistry, pH, cycling, and temperature of the test environment to better simulate specific corrosive conditions. Each annex targets a different corrosion mechanism – acidic industrial atmospheres, automotive cycling, aerospace alloy corrosion, marine environments, and SO2-polluted air. ASTM G85 results are not directly comparable to B117 results or to each other across annexes.
The modifications in G85 reflect the understanding that neutral salt fog accelerates general corrosion of zinc and steel coatings but poorly predicts performance in environments with acid rain, sulfur dioxide, or wet-dry cycling. The annexes were developed to improve correlation between laboratory exposure and field performance for specific material-environment systems.
ASTM G85 Test Annexes
Annex A1 – Acetic Acid Salt Spray (AASS)
AASS modifies the neutral salt solution with glacial acetic acid to bring the pH to 3.1-3.3. The acidic fog better simulates industrial and urban atmospheres with acid rain. It is more aggressive than B117 toward anodised aluminium, zinc, and decorative chrome finishes. AASS is widely specified for architectural hardware, automotive trim, and anodised aluminium components. It is also the basis for CASS (copper-accelerated acetic acid salt spray) when copper chloride is added.
Annex A2 – Cyclic Acidified Salt Fog
Annex A2 uses a cyclic exposure combining acidified salt fog, dry-off at elevated temperature, and high humidity. The cycling introduces a wet-dry mechanism that drives coating delamination more representative of outdoor weathering than continuous fog. It is used for automotive coatings and painted steel components where cyclic exposure correlates better with in-service performance than continuous fog.
Annex A3 – Seawater Acidified Test (SWAAT)
SWAAT uses synthetic seawater acidified with acetic acid at 49 degrees C. The elevated temperature and seawater chemistry target aluminium alloys in marine and aerospace applications. It is the most aggressive of the G85 annexes and is specified for heat exchanger tubes, marine aluminium structures, and aerospace skin materials where resistance to seawater pitting is critical.
Annex A4 – SO2 Salt Fog Test
Annex A4 adds sulfur dioxide gas to the test chamber along with the salt fog, simulating industrial atmospheres with SO2 pollution. It is used for coated steel and galvanised products in industrial environments. The combined action of SO2 and chlorides accelerates coating breakdown and corrosion product formation. IEC 60068-2-52 and ISO 3231 cover related SO2 corrosion test methods.
Annex A5 – Dilute Electrolyte Cyclic Fog/Dry Test
Annex A5 uses a dilute salt solution with cyclic fog and dry periods at controlled humidity, without acid modification. It was developed for aluminium alloy corrosion testing where neutral B117 is insufficiently discriminating. It is referenced in aerospace aluminium alloy specifications for ranking alloy and temper corrosion resistance.
Also Read – ASTM Environmental Testing Methods: Temperature, Humidity & Exposure Standards
How ASTM G85 Testing Works
Specimens are prepared per the applicable annex. They are placed in the test cabinet at the specified angle (typically 15-30 degrees from vertical), and cabinet conditions are set per the annex: solution chemistry, pH, temperature, fog rate, and cycling schedule. Specimens are inspected at defined intervals for blistering, rust creep, and rating per ASTM D1654 and other specified criteria.
Cabinet qualification follows ASTM B117 equipment requirements – fog collection rate, temperature uniformity, and solution chemistry are checked at test start and periodically during extended exposures. Test duration ranges from 96 hours for routine quality checks to 1,000+ hours for material qualification programs.
Industry Specifications
- Automotive Coatings: SAE J2334, GMW14872, Ford CETP 00.00-L-467
- Aerospace Aluminum: ASTM G85 Annex A3 (SWAAT), MIL-DTL-5541, AMS 2770
- Architectural and Hardware: ASTM G85 A1 (AASS), AAMA 2604/2605
- Industrial Coatings: ASTM G85 A4 (SO2), ISO 3231, SSPC-PA 1
- Corrosion Rating: ASTM D1654 (scribe creep), ASTM D714 (blistering), ISO 4628 series
- Cabinet Equipment: ASTM B117 (equipment requirements apply to G85 cabinets)
Also Read – Accelerated Corrosion Testing: Salt Spray, Cyclic, and Humidity Methods
Conclusion
ASTM G85 extends salt spray testing beyond the neutral fog of B117 into environments that better match specific field conditions. The appropriate annex depends on the material, coating system, and service environment. AASS for anodised aluminium and decorative coatings, SWAAT for marine and aerospace aluminium, cyclic annexes for automotive painted steel, SO2 fog for industrial coatings. No annex is universally more aggressive – each is more severe for the mechanism it targets.
What is the purpose of ASTM G85 testing? ASTM G85 testing is performed to evaluate the corrosion resistance of metals, coatings, and surface treatments under modified salt spray conditions. It simulates more realistic environmental exposure—such as acid rain, industrial pollution, or marine environments—compared to the standard salt spray test (ASTM B117).
How does ASTM G85 differ from ASTM B117? While ASTM B117 employs a continuous neutral salt fog environment, ASTM G85 introduces modifications such as acidic solutions, SO₂ exposure, or cyclic wet/dry conditions. These variations make ASTM G85 more representative of actual service conditions, thereby providing a more accurate prediction of real-world corrosion behavior.
How long does an ASTM G85 test take? The duration varies depending on the annex used and product specification requirements. Typical tests range from 24 hours to over 1000 hours, with periodic inspections for corrosion initiation, coating failure, or surface changes.
Can ASTM G85 results be correlated to outdoor exposure years? No direct, universal correlation exists. Acceleration factors vary by material, coating system, geography, and the specific G85 annex used. G85 results are most reliable for comparative ranking of materials and coatings within the same test program. Automotive OEMs have developed correlations between their cyclic tests and proving ground exposure, but these are system-specific and should not be generalized.
How is corrosion extent rated after G85 exposure? ASTM D1654 rates creep from a scribe line. ASTM D714 rates blistering by size and frequency using a photographic reference scale. ISO 4628 Parts 2, 3, and 4 cover rusting, blistering, and cracking ratings for painted surfaces. The applicable rating method is specified by the referencing standard or customer specification. For uncoated metals, mass loss or pit depth measurement may be more appropriate.
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