The ASTM B117 standard, officially titled "Standard Practice for Operating Salt Spray (Fog) Apparatus," is the most widely recognized and used specification for conducting salt spray (fog) tests. Here's a detailed overview:
Purpose and Scope
- Objective: To evaluate the corrosion resistance of materials, coatings, and finishes under controlled laboratory conditions.
- Applications: Commonly used for metals, plated parts, painted surfaces, and protective coatings in industries like automotive, aerospace, marine, and construction.
Key Components of ASTM B117
- Test Chamber Conditions:
- Temperature: Maintained at 35°C (95°F).
- Humidity: Saturated atmosphere (100% relative humidity) inside the chamber.
- Salt Solution: A 5% sodium chloride (NaCl) solution in deionized water, with a pH between 6.5 and 7.2 (neutral).
- Test Duration:
- Typically ranges from 24 hours to 1,000+ hours, depending on the material and desired corrosion resistance level.
- Common benchmarks include 24, 48, 72, 96, 240, 500, or 1,000 hours.
- Sample Preparation:
- Samples must be clean, free of contaminants, and prepared according to specific guidelines (e.g., masking of non-tested areas).
- Placement in the chamber ensures uniform exposure to the salt spray.
Test Procedure
- The chamber continuously sprays the salt solution using atomizing nozzles, creating a fine mist that settles on the samples.
- Samples are inspected periodically (e.g., every 24 hours) for signs of corrosion, such as:
- Rust formation.
- Blistering or cracking of coatings.
- Loss of adhesion.
Evaluation Criteria
- Visual Assessment: Samples are compared to standards or photographs to rate corrosion severity.
- Quantitative Methods: Measurement of corrosion product weight loss or coating degradation (e.g., using imaging software).
- Failure Criteria: Defined as the onset of specific corrosion types (e.g., 5% red rust on steel).
Limitations of ASTM B117
- Accelerated vs. Real-World Corrosion: The test accelerates corrosion but does not fully replicate dynamic real-world conditions (e.g., temperature fluctuations, UV exposure, pollutants).
- Comparative Only: Results are most useful for comparing materials under identical test conditions, not predicting exact real-world lifespans.
Related ASTM Standards
- ASTM G85: Covers modified salt spray tests, including:
- Prohesion® Test: Cyclic wet/dry conditions with lower pH.
- Acetic Acid Salt Spray (AASS): More aggressive corrosion for decorative coatings.
- Copper-Accelerated Acetic Acid Salt Spray (CASS): For aluminum and anodized finishes.
- ASTM D1654: Standard test method for evaluating corrosion resistance of painted coatings.
Industry Applications
- Automotive: Testing car parts, fasteners, and underbody coatings.
- Aerospace: Assessing durability of components exposed to salt and moisture.
- Marine: Evaluating materials for boats, offshore equipment, and coastal infrastructure.
- Construction: Testing metal finishes on bridges, buildings, and infrastructure.
Importance of Compliance
- Quality Control: Ensures products meet minimum corrosion resistance standards.
- Product Reliability: Helps manufacturers validate coating performance and durability.
- Regulatory Compliance: Required for certifications in many industries (e.g., automotive, defense).
Example Use Case
A manufacturer of outdoor lighting fixtures might use ASTM B117 to test the corrosion resistance of a new powder-coated finish. If the coating shows minimal corrosion after 500 hours, it indicates suitability for coastal or high-humidity environments.
In summary, ASTM B117 is a cornerstone standard for corrosion testing, providing a controlled and repeatable method to evaluate material performance in salt-laden environments. While it has limitations, it remains a critical tool for industries requiring durable, corrosion-resistant products.
