The Limitations of ASTM B117 Salt Spray (Fog) Test in Coating Evaluations
We know evaluating “or equal” coatings can sometimes be daunting. Using ASTM test methods to evaluate equals has become popular, unfortunately, many of these tests have been misused as marketing tools. To use the results of ASTM test methods to evaluate equivalent coatings, you must make sure that the required tests are relevant to the subject service environment and actually correlate to longer coating service life
ASTM B117 Standard Practice for Operating Salt Spray (Fog) Apparatus
In this blog we will discuss one of the most commonly specified and controversial test methods – ASTM B117 Standard Practice for Operating Salt Spray (Fog) Apparatus.
Significance and Use of ASTM B117 Salt Spray Test
We can look to the text of the standard to determine the purpose for which this test method was designed. Under Section 3, “Significance and Use,” the standard states:
3.1 This practice provides a controlled corrosive environment which has been utilized to produce relative corrosion resistance information for specimens of metals and coated metals exposed in a given test chamber.
3.2 Prediction of performance in natural environments has seldom been correlated with salt spray results when used as standalone data.
3.2.1 Correlation and extrapolation of corrosion performance based on exposure to the test environment provided by this practice are not always predictable.
3.2.2 Correlation and extrapolation should be considered only in cases where appropriate corroborating long-term atmospheric exposures have been conducted.
3.3 The reproducibility of results in the salt spray exposure is highly dependent on the type of specimens tested and the evaluation criteria selected, as well as the control of the operating variables. In any testing program, sufficient replicates should be included to establish the variability of the results. Variability has been observed when similar specimens are tested in different fog chambers even though the testing conditions are nominally similar and within the ranges specified in this practice.
Limitations of ASTM B117 Salt Spray Test for Coating Comparison
The standard is very clear that the intent is only to compare the result of specimens that are prepared the same way and exposed in the same chamber together. This standard was never designed and is not equipped to compare the relative corrosion resistance of different coatings exposed in different chambers under different conditions.
Industry Insights and Critiques of ASTM B117 Salt Spray Testing
The coating industry has known this for many years. In 1995, The Society for Protective Coatings (SSPC) published a paper that concluded, “It is obvious that ASTM-B117 Salt Spray (Fog) Testing and similar methods offered a rapid but unreliable means for predicting coating behavior.”
In 2010, the JPCL published an article by Mike O’Donoghue, V.J. Datta, Mike Winter, and Carl Reed entitled “Hubble, Bubble, Test and Trouble: The Dark Side of Misreading the Relevance of Coating Testing” that provides insight as to why ASTM B117 is an unreliable means for predicting coating behavior.
Finally, Dr. Stuart Croll of North Dakota State University said of ASTM B117 in his paper entitled “Don’t Pass the Salt, ”This is a zombie test method! Meaning one that should have died many years ago, but it keeps being brought back to life by those who can benefit from their results of this test!”
The Continuing Misuse of ASTM B117 Salt Spray Test in the Coating Industry
It has been known in the coatings industry for well over two decades that the Salt Fog test is not a reliable way to compare coatings; however, we still see it being specified.
ASTM B117 FAQs
What is ASTM B117?
ASTM B117 is a standard practice for operating salt spray (fog) apparatus, providing a controlled corrosive environment to produce relative corrosion resistance information for metals and coated metals. ASTM B117 aims to offer a rapid, standardized method to evaluate the performance of coatings under specific conditions.
What is ASTM B117 salt spray test?
The ASTM B117 salt spray test is a method designed to create a controlled corrosive environment using a salt fog chamber to evaluate the corrosion resistance of metal and coated metal. Despite its widespread use, it has been recognized that results from this test do not reliably predict performance in natural environments.
What industries use ASTM B117?
ASTM B117 is widely used across various industries where evaluating the corrosion resistance of coatings is crucial, including automotive, aerospace and marine sectors. These industries use the test to assess the durability and performance of protective coatings and materials.
Why does ASTM B117 continue to be used?
Despite its flaws, ASTM B117 remains widely used because it's easy to perform and it’s cost-effective. Many companies continue to rely on it for quick and comparative testing due to its long history in the industry.
Where can I find more information about using ASTM testing to evaluate product equals?
For more in-depth information on using ASTM testing to evaluate product equals, you can watch our recorded webinars: Evaluating Product Equals – More Is Better, Right? and ASTM Material Specifications and ASTM Standard Test Methods, Aren't They the Same? These webinars provide valuable insights into the nuances of ASTM testing and how it can be applied to product evaluation.
For more information on how Sherwin-Williams Water & Wastwater anti-corrosive coatings solutions can protect your assests, contact a Sherwin-Williams Coatings Specialist today.