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TECHNICAL MEMO #11: Setting standards for salt spray testing.
Though it is difficult to equate performance in salt spray testing to service life, the salt spray testing of panels as defined in ASTM Specification B117 remains the most commonly used gauge of long-term life of painted metal surfaces. While its ability to measure finish quality in terms of real-world service life can be challenged, salt spray testing is generally accepted as a practical means of drawing conclusions as to comparative quality between tested finishes. It can be used effectively to evaluate performance of different metal substrates, pretreatment systems, or finishing processes on a comparative basis. It can also be used effectively to monitor performance of a given finishing line over time, although as a pure quality control tool it suffers from the time lag in obtaining results from the tests. Salt spray testing can also be a helpful troubleshooting tool, though difficulty in interpreting the cause of any weakness or failure during tests, plus the time lag mentioned above make this frustrating.
There are some things one should understand about the salt spray test itself when setting salt spray standards or attempting to interpet results of salt spray tests. While ASTM Specifications are very explicit about design and construction of salt spray cabinets, and attempt to clearly define all conditions related to conducting salt spray tests, it is rare that any two different salt spray cabinets will yield results that consistently agree. Despite one's best efforts, the results from any particular cabinet may also vary over time. Particularly critical items to control during testing are exposure time, salt content of the condensed fog, temperature, amount of salt fog produced, and exposure angle of the parts. If the parts to be tested are scribed, this should also be done in a very consistent manner. In short, every effort should be made to insure that only one variable is permitted, that being the target for evaluation. Since minor variations in one parameter (such as a five degree difference in exposure angle or a little extra paint film thickness) can completely skew the results of a salt spray test, this point is hard to overemphasize. Enough panels should be tested to allow discarding any that appear to give results radically inconsistent with the majority if so desired.
It should also be noted that a salt spray test by nature includes a humidity exposure test. In interpreting or rating salt spray tests, I believe that a distinction should be drawn between the corrosion attack on the metal substrate, representing pure salt spray failure, and the adhesion loss (often without visible substrate corrosion) that can occur due to humidity failure. I recommend this distinction because these two types of failures frequently have totally different root causes. In addition, humidity failures can occur so quickly and be so catastrophic in nature as to preclude any meaningful salt spray test results.
Provisions of ASTM Specification B 117 adequately define the design, construction, and maintenance of the salt spray cabinet, and the conduct of the test itself. Preparation and rating of test panels is covered in various ASTM Specifications. We recommend that all aspects of conducting the test be defined so as to comply with ASTM Specifications. Preparation of test panels should be done so as to closely represent the most predominate (or critical) production parts. The method of scribing the panels should be specified. We recommend that the ASTM recommendations as to rating of tested panels be augmented by drawing a distinction between corrosion creepage and adhesion loss due to humidity failure. Specifications should also cover any tests to be run prior to salt spray testing, such as film thickness, pencil hardness, impact adhesion, or conical mandrel bend test, and specify any results from such tests that would preclude salt spray testing or cause the results to be questioned.
ASTM Specifications never define what constitutes a pass or failure. Since this amounts to setting quality standards, no one should allow any outsider to define these parameters unless they are willing to give up control of their quality standards.
Failure is only defined by specifying both a minimum performance level to be attained and a specific amount of exposure time. Unless failure is clearly defined, exposure time is meaningless. Rating frequency should be specified, and failure defined in terms of corrosion creepage from the scribe line within the specified exposure time.
Arriving at the specifications outlined above is always a judgment call, but will be heavily influenced by the end use of the product, industry and competetive standards, and the purpose of the salt spray testing. Other considerations may have varying amounts of influence. Marketing will usually require that standards be set that are at least competitive within the industry, and may tend to favor specifications set with longer exposure times and looser definitions of failure, since most people tend to discuss salt spray only in terms of time. Shorter exposure times with more stringent definitions of failure are usually preferred by quality control, since quicker test results are easier to interpet and react to, and will catch problems quicker. Salt spray failures are generally front end loaded in terms of exposure time. A standard of 250 hours exposure with zero creepage allowable may provide as tough a standard as a 1000 hour test allowing 1/4 inch creepage.
Standards should be set so that they can be met most of the time, but will not be met if anything in the finishing system deteriorates. An occasional, marginal failure probably only verifies that standards are set correctly. There is no reason not to set multiple standards correlating to the various rating periods during the test.
Standards should be adjusted when they become invalid. For example, a major improvement in the finishing system should probably result in revised salt spray standards.