A question that we are often asked about salt spray testing is “How does it relate to years of corrosion resistance?” The short answer is that salt spray testing does not directly correlate to corrosion in the real world.
There have been numerous attempts over the last 50 years to link salt spray testing to reality, but as far as we are aware, the relationship is still relatively weak. A common misconception is that salt spray testing is used to predict service life, whereas its actual purposes are:
- looking for regressions in production processes (coating was passing a salt spray test, now it isn't)
- finding design flaws – these often reveal themselves quite early on in salt spray testing (for example, an area that might not be coated sufficiently or the coupling of dissimilar metals)
- highlighting the areas most at risk of corrosion
- establishing how the part will behave while corroding
Generally recommended salt spray times are set down in national and international standards, either for the product, the material or the coating. However, these generally do not come with an expected service life either.
There are number of reasons that make it difficult to predict service life using salt spray testing.
Corrosion Rate of Environment
The environment that a part is in service in determines how quickly it will corrode. For example, a zinc plated article would on average corrode 88% faster in an urban atmosphere compared to a rural atmosphere (based on data from ASTM B633-13 X1).
Corrosion Behaviour
The reason that neutral salt spray testing is useful is that it is a relatively fast test (compared to real weathering). Since the corrosion is happening faster, that changes the nature of it. It doesn't have wet and dry cycles like the real world. It is exposed to a higher concentration of corrosion promoters. It does not benefit from being washed down by rain. It is only exposed to two very well controlled agents: water and sodium chloride (salt), whereas in reality, multiple corrosion mechanism may be in action and be promoted by a variety of agents.
Something that takes a decade is clearly going work differently to something that takes a couple of weeks.
Efforts have been made, particularly in the automotive industry to produce a more representative test. They have developed tests that are generally cyclic, which are well suited to their particular coating schemes, but not necessarily for other applications.
So What Do I Do?
Firstly, seek guidance from national, international and industry standards. These may already establish salt spray requirements for a product, material or coating and this is a good starting point.
Do trials on existing parts, where the service life is known from experience and try to find a rough relationship.
Undertake salt spray testing to indicate potential weak points in your design.
Establish with your suppliers a baseline of performance and specify this in a standard to prevent regressions – where possible sticking to common salt spray exposures and multiples of 24 hours.
More Info and Specialist Advice
Please email us if you would like advice on salt spray testing. This page just gives general suggestions and our specialists will be able to provide more in-depth information.
Disclaimer
This page is provided for information only, it should not be considered advice and we cannot accept any responsibility or liability for your use of the information on this page. The information on this page is used and relied on at your own risk and you bear the sole responsibility for any outcomes. E&OE.