ASTM B117 salt spray environmental testing is the most widely recognized accelerated corrosion test used to evaluate the corrosion resistance of metallic materials, coatings, and finished components. By exposing specimens to a controlled 5% sodium chloride fog at 35°C, this standard practice provides manufacturers with a reliable, repeatable method for comparing corrosion performance across products and processes. This article explains the technical foundations, test conditions, sample types, evaluation methods, and practical applications of ASTM B117 testing.
Technical Overview
ASTM B117, titled "Standard Practice for Operating Salt Spray (Fog) Apparatus," is the foundational accelerated corrosion test standard published by ASTM International. First introduced in 1939, it has become the globally accepted benchmark method for generating a controlled corrosive atmosphere and evaluating how materials, coatings, and surface treatments respond to prolonged salt fog exposure.
The test does not simulate any specific natural environment; rather, it establishes a reproducible, aggressive corrosion condition that allows for meaningful comparative evaluation. Results from ASTM B117 testing are widely used for quality control, specification compliance, coating system qualification, and research into corrosion-resistant materials.
The key operating parameters defined by the standard include:
- Salt solution concentration: 5% sodium chloride (NaCl) by mass, dissolved in ASTM D1193 Type IV water or better
- Chamber temperature: 35°C ± 1.1°C (95°F ± 2°F)
- Solution pH: maintained between 6.5 and 7.2
- Fog collection rate: 1.0 to 2.0 mL per hour per 80 cm² of collection area
- Solution specific gravity: typically 1.0255 to 1.040 depending on referencing criteria
- Continuous atomization with no scheduled interruption of the fog cycle
The corrosive medium is atomized into a fine fog by compressed air through calibrated nozzles, and the saturated humidity within the enclosure ensures uniform condensation on all exposed surfaces. Because the environment is highly standardized, results are reproducible between properly maintained laboratories worldwide.
Testing Samples
ASTM B117 accommodates a broad range of specimen types. Proper sample selection and preparation are essential for obtaining valid, comparable results.
Metallic Coated Samples
Electroplated and hot-dip coated specimens represent one of the largest testing categories. Common examples include zinc-plated steel fasteners, zinc-nickel alloy coatings, galvanized sheet metal, cadmium-plated aerospace hardware, and tin or chromium-plated electronic contacts. Test results are frequently classified by the time to first appearance of white rust (zinc corrosion products) or red rust (substrate iron corrosion).
Organic Coated Samples
Painted, powder-coated, and lacquered panels are tested to assess barrier performance, blistering resistance, and creepage at scribe marks. Evaluation of these samples is typically conducted in conjunction with ASTM D1654, which defines rating methods for corrosion and scribe creepage of coated specimens.
Conversion Coated Samples
Chemical conversion coatings such as anodized aluminum, chromate conversion coatings, trivalent chromium passivates, and phosphate layers are validated using ASTM B117 exposure. Salt spray hours to white corrosion products are a common acceptance metric in specifications for these finishes.
Bare Metals and Alloys
Uncoated materials including carbon steel, stainless steel, aluminum alloys, copper alloys, and zinc alloys may be tested to establish baseline corrosion behavior, compare alloys, or evaluate the effectiveness of temporary protective treatments.
Finished Components and Assemblies
Real end-use products such as automotive brackets, electronic enclosures, fasteners, hinges, marine hardware, and military equipment components can be tested in their finished state to evaluate the combined performance of materials, coatings, and design features under corrosive stress.
Testing Items
The scope of ASTM B117 testing encompasses several measurable outcomes depending on the material and specification requirements:
- Time to first white rust, red rust, or other corrosion products
- Corrosion rating per ASTM D610 (rusting of steel surfaces) or ASTM D1654 (coated specimens)
- Blistering assessment per ASTM D714 (size and frequency of blisters)
- Scribe line creepage and coating adhesion loss after exposure
- Changes in appearance, gloss, color, or surface staining
- Loss of mass or dimensional change for sacrificial coatings
- Post-exposure functional performance checks for finished assemblies
Standard test durations are selected by product specification and commonly include 24, 48, 72, 96, 168, 240, 336, 500, 720, 1000, and 2000 hours. Highly corrosion-resistant coating systems, such as zinc-nickel with sealer, may be qualified for 1000 hours or more to white rust.
Testing Methods
ASTM B117 prescribes strict procedural controls to ensure consistency and reproducibility. A typical test sequence proceeds as follows:
First, specimens are cleaned according to the applicable material specification, avoiding solvents or abrasives that could alter the test surface. Samples are then positioned inside the chamber on non-corrosive racks, tilted at an angle between 15 and 30 degrees from vertical so that condensate drains freely rather than pooling on the surface. Samples must not contact each other, and condensate from one specimen must not drip onto another.
During exposure, the chamber is operated continuously at 35°C, and the salt solution is atomized to maintain the required fog collection rate of 1.0 to 2.0 mL per hour per 80 cm². Collection devices placed within the chamber are checked at least every 24 hours to verify that the collected solution maintains a sodium chloride concentration between 6% and 8% by mass when measured by titration or density, and a pH between 6.5 and 7.2.
Upon completion of the specified exposure period, specimens are carefully removed, gently rinsed or dried as required by the evaluation protocol, and examined. Corrosion assessment is performed according to the relevant evaluation standards referenced in the product specification.
Several related test methods extend the basic B117 protocol for more aggressive or specialized environments:
- ASTM G85 Annex A1: acetic acid salt spray test (fog at lower pH)
- ASTM G85 Annex A2: cyclic acidified salt fog with SO2 (SWAAT) for aluminum alloys
- ASTM B368: copper-accelerated acetic acid salt spray test (CASS) for decorative plating
- ISO 9227: the international equivalent standard covering neutral, acetic, and CASS spray methods
Applications
ASTM B117 testing is embedded in quality systems across nearly every industry where corrosion performance is critical:
- Automotive: qualification of fasteners, brake components, chassis hardware, and body-in-white coating systems against OEM salt spray requirements
- Aerospace: validation of cadmium replacements, anodized structures, and plated landing gear and engine peripherals
- Marine and offshore: assessment of hardware, fittings, and protective coatings exposed to chloride-laden atmospheres
- Electronics and telecommunications: verification of connector platings, enclosure finishes, and PCB surface treatments
- Military and defense: conformance testing for hardware procured under defense specifications citing salt fog exposure
- Construction and infrastructure: evaluation of rebar coatings, structural fasteners, curtain wall components, and galvanized products
- Consumer products: durability confirmation for appliances, bicycles, tools, and outdoor equipment finishes
Procurement documents, industry standards such as ASTM B633 for electrodeposited zinc coatings, and international automotive specifications routinely cite minimum ASTM B117 exposure hours as pass/fail acceptance criteria.
FAQ
What is the difference between ASTM B117 and ISO 9227?
ASTM B117 and the neutral salt spray (NSS) method of ISO 9227 specify essentially identical test conditions, including 5% NaCl solution, 35°C temperature, and collection rate of 1-2 mL per 80 cm² per hour. ISO 9227 additionally covers acetic acid and CASS variants, while ASTM B117 focuses on the neutral fog method with related accelerated methods defined in ASTM G85 and ASTM B368.
Does ASTM B117 correlate directly with real-world service life?
No. ASTM B117 is a comparative, accelerated test and does not predict corrosion behavior in a specific service environment. The standard itself states that results should not be treated as a direct correlation to service life. Cyclic corrosion tests such as ASTM D5894 or SAE J2334 often provide better correlation with outdoor exposure for coated steel products.
What solution is used in the test?
A 5% sodium chloride solution prepared in water meeting ASTM D1193 Type IV purity or better. The sodium chloride must be essentially free of nickel and copper impurities and contain less than 0.1% total impurities by mass when dried, with limits on iodide content.
How long should a sample be tested?
Exposure duration is defined by the applicable product specification rather than by ASTM B117 itself. Common durations range from 24 hours for basic passivates to 1000 hours or more for high-performance zinc alloy coating systems.
What are typical pass criteria?
Acceptance criteria vary by specification but commonly specify no red rust, no white rust, or a minimum corrosion rating (for example, ASTM D610 Rating 8 or higher) after the required exposure period. Scribe creepage limits may also apply for painted specimens.
How should samples be oriented inside the chamber?
Specimens must be supported at an angle between 15 and 30 degrees from the vertical plane, positioned so they do not contact each other and so that condensate does not drip from one sample onto another, ensuring uniform exposure across all test surfaces.
