ASTM G85 Modified Salt Spray

ASTM G85 describes five modified salt spray tests, each simulating a particular type of corrosion process that neutral salt fog cannot adequately simulate. In these tests, the solution is made acidic, SO2 is added, the electrolyte is diluted, or an alternating wet/dry exposure period is used. MatX Lab performs tests on coated steel, aluminum alloy, and plated assemblies according to ASTM G85.

What Is ASTM G85?

ASTM G85 is the standard practice for modified salt spray (fog) testing. Where ASTM B117 defines one continuous neutral fog, G85 defines five alternatives in separate annexes:

1. Annex A1, acetic acid salt spray. Solution acidified with acetic acid, used on decorative chromium and some plated coatings.
2. Annex A2, cyclic acidified salt spray. Acidified fog run in cycles rather than continuously.
3. Annex A3, acidified synthetic seawater, cyclic. Known as SWAAT, using synthetic seawater acidified to a low pH with alternating fog and high-humidity soak stages. Common in aluminum and aerospace specifications.
4. Annex A4, sulfur dioxide salt spray. Salt fog with SO₂ introduced, representing industrial atmospheres.
5. Annex A5, dilute electrolyte cyclic fog and dry, using a weak salt and ammonium sulfate solution cycled with drying stages and widely used on organic coatings.

Cycles of wetting and drying in the cyclic annexes are significant because corrosion in use occurs due to wetting and drying of surfaces. The drying step concentrates and stresses the film, which does not occur continuously under fog.

ASTM G85 Scope and Applications

The practices apply to metals and to the coatings on them. Annex choice follows the mechanism you need, and picking the wrong annex is the most common way these programs waste time.
There are four applications of the G85:

1. Specification conformity. Compliance with a specific annex and duration, usually Annex A3 for aluminum alloys and Annex A5 for paint systems.
2. Comparison of paints. Ranking of organic paint systems according to Prohesion, since neutral fog provides a poorer correlation with outdoor performance.
3. Evaluation of aluminum alloys. Examination of exfoliation and intergranular attack according to SWAAT, where neutral fog provides poor differentiation.
4. Simulation of industrial atmosphere. Use of the sulfur dioxide annex when exposure includes acidic pollutants.

Continuous neutral fog runs under ASTM B117. Condensing humidity without salt falls under ASTM D2247, and light plus water exposure for coatings falls under ASTM D7869. As with B117, G85 sets no exposure duration and no acceptance criteria, which stay with your specification.

ASTM G85 Test Procedure

The annex decides the chemistry and the cycle. The mechanics of racking, collection, and rating stay much the same across all five.

Limitations: Results from different annexes are not comparable, and a Prohesion result says nothing about how the same panel would perform under SWAAT. Cyclic exposure tests are more time-consuming in practice than the hours involved imply, since drying periods count in the overall time as well. Acidic treatments damage chamber hardware and increase maintenance demands; therefore, chamber availability becomes a practical consideration when scheduling the tests. Correlation with maintenance services remains coating-dependent even in the case of cyclic annexes, and thus an outdoor panel will have to be used if there is any intention of substantiating the life claim. Specimens of aluminum exposed in SWAAT show drastic forms of failure that are hard to evaluate in terms of rating, but should instead be described.

ASTM G85 Specimen Requirements and Test Conditions

Confirm the annex in writing at quotation. A program run under A5 when the specification called for A3 produces data that cannot be used, and the exposure time is gone.
Assemblies with crevices, a combination of metals, or blind holes do not respond to cyclic exposure like flat panels, as the solution accumulates when wet and becomes concentrated when dry. Specify the surfaces to consider, and we’ll orient the parts.

ASTM G85 Test Results and Reporting

Your report includes the annex, the actual cycle carried out, and the total exposure carried out, including all the chamber logs made using the program.
Ratings appear in the form your specification uses: scribe creep in millimeters under ASTM D1654, rust grade under ASTM D610, blister size and density under ASTM D714. Photographs accompany each specimen at every inspection point.
The failure modes are noted along with the numerical score. There are four failure modes – exfoliation, intergranular attack, filiform corrosion, and general blistering, which all indicate different origins, and the numerical score without the accompanying explanation is misleading.

ASTM G85 FAQs