ASTM B117 Salt Spray Test: A Complete Guide
What Is ASTM B117?
ASTM B117 is the standard practice for operating salt spray (fog) test apparatus. It defines a controlled accelerated corrosion environment – a continuous fog of 5% sodium chloride solution at 35 degrees C – for evaluating the relative corrosion resistance of metallic materials, coatings, and surface treatments. B117 does not specify acceptance criteria or test duration; it defines the test environment and apparatus requirements. Pass/fail criteria and exposure hours are set by the product standard or customer specification that references B117.
ASTM B117 is the most widely referenced accelerated corrosion test method in the world. It is specified in automotive, aerospace, military, industrial coating, and electronics applications as a first-pass screening tool and for coating qualification. The test environment is reproducible across laboratories worldwide when B117 apparatus requirements are followed, making it the baseline for cross-laboratory and cross-industry comparison of corrosion protection performance.
How the ASTM B117 Test Works
Test Cabinet and Conditions
The test cabinet is a sealed chamber where atomized salt solution is maintained as a continuous fog. Temperature is held at 35 plus or minus 2 degrees C. The salt solution is 5% NaCl by weight, pH 6.5-7.2 at collection temperature. Fog is generated by a nozzle supplied with compressed air and salt solution. Collection rate is 1.0 to 2.0 mL/hour per 80 cm2 of horizontal collection area, verified by fog collector gauges inside the cabinet.
Specimen Preparation and Placement
Specimens are cleaned to remove oils and contaminants before test. Coated specimens are often scribed through the coating to the substrate to evaluate corrosion creep from a damaged area – the most common coating failure mode. Specimens are placed at 15-30 degrees from vertical, not contacting each other or cabinet walls, positioned so fog settles uniformly on test surfaces. Different materials are not tested in the same cabinet when their corrosion products could contaminate each other.
Inspection and Rating
Specimens are inspected at defined intervals for corrosion initiation, blistering, and coating failure. ASTM D1654 rates creep from a scribe line. ASTM D714 rates blistering. ASTM D610 rates the degree of rusting for painted steel. Visual ratings use photographic reference standards. The test is terminated at a defined hour limit or when specimens reach a defined failure criterion. All observations are documented with photographs.
What B117 Testing Measures
B117 measures relative corrosion resistance – how one coating or material compares to another in the same aggressive environment. It is not a simulation of any specific outdoor atmosphere; neutral salt fog at 35 degrees C does not replicate industrial, marine, or tropical environments with scientific accuracy. What it does well is discriminate between significantly different levels of corrosion protection in a reproducible way and detect failures in coating adhesion, porosity, or coverage that would not appear in short-term ambient exposure.
B117 results cannot be directly converted to years of outdoor service life without empirical correlation data for the specific material and exposure geography. The test is a quality screen and comparative tool, not a service life predictor. For applications requiring better correlation to specific environments, ASTM G85 modified salt spray tests or cyclic corrosion tests (SAE J2334, GMW14872) provide more realistic acceleration for specific corrosion mechanisms.
Also Read – ASTM B117 Salt Spray Test Procedure: Step-by-Step Overview Guide
Industry Specifications
- Automotive: GMW3191, Ford CETP 00.00-L-467, SAE J2334
- Aerospace and Military: MIL-DTL-5541 (conversion coatings), MIL-PRF-23377 (primer), MIL-PRF-85285 (topcoat)
- Industrial Coatings: SSPC-PA 1, NACE SP0188, ISO 9227 (equivalent international standard)
- Fasteners and Hardware: ASTM F1941 (electrodeposited coatings on fasteners), ISO 4042
- Electronics: IPC-TM-650 2.6.11, MIL-STD-810 Method 509
- Rating Methods: ASTM D1654 (scribe creep), ASTM D714 (blistering), ASTM D610 (rust grade)
Conclusion
ASTM B117 is a screening and comparative test, not a service life predictor. It establishes whether a coating or treatment provides meaningful corrosion protection relative to a control under a standardized, reproducible aggressive environment. The test’s value is in its universality – a B117 result means the same thing in any laboratory that follows the standard, enabling cross-supplier and cross-industry comparison. For applications where correlation to specific outdoor environments is needed, B117 is the starting point from which modified tests (ASTM G85) and cyclic tests are developed.
What does 500 hours of ASTM B117 represent in outdoor exposure? There is no universal conversion. 500 hours of B117 does not equal a specific number of outdoor service years because the acceleration factor depends on the coating system, substrate, geographic location, and the failure mode being tracked. Published correlations exist for specific systems but vary widely. B117 is best used for comparative ranking within a material family and for specification compliance, not for predicting absolute outdoor service life.
Why is the pH of the salt solution specified and how does it affect results? The pH of the 5% NaCl solution must be 6.5-7.2 at 35 degrees C per ASTM B117. pH outside this range changes the corrosion chemistry - more acidic solutions are more aggressive and produce different corrosion products than neutral fog. Municipal tap water often contains trace chemicals that shift pH; deionized or distilled water is used to prepare the solution, and pH is adjusted with acetic acid or sodium hydroxide if needed.
Can dissimilar metals be tested in the same B117 cabinet? B117 recommends against mixing materials in the same cabinet when their corrosion products could contaminate other specimens. Zinc and magnesium corrode rapidly and their ions can deposit on other specimens, accelerating or inhibiting corrosion. If different material families must be tested simultaneously, separate them with shields and ensure drainage from one group cannot contact another. Separate cabinets are preferred for materials with very different corrosion rates.
What is the difference between ASTM B117 and ISO 9227? ASTM B117 and ISO 9227 define essentially the same neutral salt spray test - 5% NaCl, 35 degrees C, continuous fog, pH 6.5-7.2. The technical requirements are nearly identical. The differences are administrative - maintained by different bodies (ASTM International vs ISO), with different format and terminology. Products sold in Europe often reference ISO 9227; US defense and automotive applications reference ASTM B117. The same equipment and solution are used for both.
How should specimens with complex geometry be fixtured in a B117 cabinet? Complex parts should be oriented so critical surfaces receive uniform fog deposition and drainage does not pool solution in unrepresentative locations. Threaded fasteners are typically tested in the as-installed orientation. Tubular or hollow parts need drainage holes or vertical orientation to prevent solution pooling inside cavities. Fixtures must be non-metallic to avoid galvanic effects. When the component has a defined in-service orientation, test orientation should replicate the service orientation.
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