ASTM B117 Salt Spray Testing: What the Standard Actually Requires

ASTM B117 Salt Spray Testing: What the Standard Actually Requires

If you specify or run corrosion tests, you have almost certainly seen a requirement for ASTM B117. It is the neutral salt spray (fog) test that has been around since the 1930s and remains the default method for comparing the corrosion resistance of coatings, platings, and base metals. But despite its familiarity, B117 is frequently misapplied, and results are often over-interpreted.

This article walks through the core requirements of ASTM B117, the practical details that determine whether your test is valid, and the common traps that lead to misleading data. If you are new to the standard or need to audit an existing test procedure, this is the reference you want.

What ASTM B117 Defines and What It Does Not

ASTM B117, officially titled Standard Practice for Operating Salt Spray (Fog) Apparatus, is a test method for creating a controlled corrosive environment. It describes the apparatus, the reagent, the exposure conditions, and the procedure for running a continuous salt fog exposure. The standard does not specify pass/fail criteria, exposure durations, or acceptance limits — those come from the product specification or a separate standard (e.g., ASTM D1654 for evaluating painted specimens).

In other words, B117 tells you how to run the test, not what the result means. A common mistake is to treat a 500-hour B117 result as a direct predictor of service life. It is not. B117 is a comparative test: it lets you rank materials or coatings under a highly accelerated, constant salt fog environment, but the correlation to real-world atmospheric corrosion is often poor, especially for cyclic or wet/dry conditions.

The standard applies to ferrous and non-ferrous metals, and to organic and inorganic coatings. It is widely referenced in automotive, aerospace, and fastener specifications, often with a required duration and a maximum allowable rating for rust or blistering.

Key Requirements in the Standard: Chamber, Solution, and Conditions

ASTM B117 sets out specific tolerances that you must meet for the test to be valid. The main clauses are in Sections 4 through 9 of the current revision (B117-19). Here are the critical ones:

• Test solution (Section 6): The salt solution is 5 ± 1 parts by mass of sodium chloride (NaCl) per 95 parts of distilled or deionised water. The NaCl must be essentially free of copper and nickel, and the total impurities must not exceed 0.3%. The pH of the collected solution, not the reservoir, must be between 6.5 and 7.2 at 25 °C (77 °F). Adjust with dilute hydrochloric acid or sodium hydroxide if needed.
• Chamber temperature (Section 7): The cabinet's exposure zone must be maintained at 35 ± 2 °C (95 ± 3.5 °F). This is the temperature inside the fog, not the wall of the chamber.
• Fog collection rate (Section 8): For a horizontal collection area of 80 cm², each collector must gather between 1.0 and 2.0 mL of solution per hour over an average of at least 16 hours. This is a measure of fog density and uniformity.
• Air supply (Section 5): The compressed air must be free of oil and dirt, and it must be humidified before entering the chamber to prevent evaporation of the fog droplets. The pressure is typically regulated to 10–25 psi, depending on the nozzle type.
• Specimen placement (Section 9): Specimens must be positioned so they do not shield each other, and the angle of inclination from vertical should be between 15° and 30°, unless otherwise specified. The top surface of the specimen should be exposed to the fog.

These numbers are not suggestions. A test run at 33 °C or with a fog collection rate of 0.8 mL/h is not a valid B117 test, and any results are questionable.

Setting Up the Test Correctly: Practical Steps

Getting a valid B117 run is as much about preparation as it is about the chamber. Here is a practical checklist that I use when setting up a new test or auditing an existing one:

1. Clean the specimens – Degrease and clean per the applicable product specification. Oily fingerprints alone can cause false corrosion spots.
2. Mask or protect edges – If you are testing a coating, cut edges and scratches may be the failure point. Use a suitable stop-off lacquer or tape, unless the spec says otherwise.
3. Verify the solution pH – Measure the pH of the collected solution after a short warm-up, not the make-up tank. Adjust if outside 6.5–7.2.
4. Calibrate the chamber – Run the chamber empty for 24 hours and check temperature and collection rates at multiple points. Adjust baffles or nozzles to achieve uniformity.
5. Position specimens correctly – Avoid contact between specimens and any metal support. Use plastic or glass racks. Ensure the fog can reach all surfaces.
6. Monitor continuously – Log temperature and collection rates daily. A single drift above 37 °C can invalidate the entire run.
7. Document everything – Record the start and stop times, solution preparation, and any deviations. This is essential if the test is ever challenged.

If you are running a long test (e.g., 500 or 1000 hours), check the collection rate every 24 hours. The standard allows the fog to be interrupted only for inspection or for adding solution, but the interruption must be kept to a minimum.

Common Pitfalls and How to Avoid Them

Even experienced labs make avoidable errors. Here are the most frequent issues I see in B117 testing:

• Using tap water – The standard requires distilled or deionised water. Tap water introduces ions that change the pH and can cause spurious results.
• Ignoring the collected solution pH – The reservoir pH is not what matters. The fog that condenses on the collection funnel is what you must measure, and it can drift from the reservoir value.
• Overcrowding the chamber – If specimens are too close, they shield each other and the fog distribution becomes non-uniform. The standard requires that no specimen should be directly above another.
• Misreading the collection rate – The 1.0–2.0 mL/h range is per 80 cm² collection area. If you use a smaller funnel, you must adjust the expected volume. Many labs fail here.
• Assuming B117 equals cyclic corrosion – B117 is a constant fog test. It does not replicate wet/dry cycles, UV, or temperature changes. For those, look at ASTM G85 (which includes cyclic salt fog) or ASTM G154 for UV.

Another subtle issue is the salt purity. The standard limits the total impurities in the NaCl to 0.3%, and specifically restricts copper and nickel. Copper contamination alone can accelerate corrosion on aluminium and steel, giving you a false failure.

Interpreting Results: Ratings and Reporting

B117 itself does not provide a rating system, but it is often paired with ASTM D1654 (Standard Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive Environments) for evaluating scribe creep and blistering. For bare metals, you might use ASTM G1 for preparing and cleaning specimens after exposure, and then measure mass loss or pitting depth.

When you report a B117 result, always include:

• The exposure duration (e.g., 96 hours)
• The temperature and collection rate data
• The specimen type and surface preparation
• The rating method used (e.g., ASTM D1654 rating number)
• Any deviations from the standard

Without this context, a “500-hour B117” statement is meaningless.

When to Use (and Not Use) B117

Use B117 when you need a quick, comparative screening of coatings or materials under a standardised salt fog. It is excellent for quality control, batch-to-batch comparison, and verifying that a process change did not degrade corrosion resistance.

Do not use B117 to predict service life in a specific environment. For more realistic testing, consider cyclic corrosion tests like ASTM G85 (Prohesion or cyclic salt fog) or ISO 11997, which include wet/dry transitions and are often better correlated with outdoor exposure.

Also, B117 is not suitable for testing materials that are sensitive to chloride stress corrosion cracking, unless the specimen is specifically designed for that purpose. In such cases, use a dedicated standard like ASTM G36 for boiling magnesium chloride.

Takeaway

ASTM B117 is a straightforward but unforgiving standard. The tolerances are tight, and small deviations can invalidate your results. The key to success is to follow the exact clause requirements for solution, temperature, and collection rate, and to document everything.

If you are specifying a B117 test, make sure your product specification includes a clear acceptance criterion. If you are running the test, invest time in calibrating your chamber and verifying the fog collection rate before you start. A little extra effort upfront saves you from defending an invalid result later.

When in doubt, consult the latest revision of ASTM B117 and any companion standards like ASTM D1654 or ASTM G85. They are there to keep your corrosion testing honest.