The salt spray test: how does it work and what does it tell you?
In the Test explained series we look at one test at a time: why you do it, how it works and what you can do with the result. This time: the salt spray test. How well does your product resist rust?
Why do we do this test?
Metal products that are exposed to moisture, dirt and salts in use will eventually rust. A coating slows this process down. But how much more resistant to rust is your product with a particular coating?
We find out with the salt spray test.
What can you do with the result?
The result helps you assess long-term corrosion resistance, and so make the right choices in design and surface treatment.
Note: what this test does not tell you
- No conversion into years. There is no fixed factor for converting the test duration into years of service.
- Less suitable for galvanised steel. For galvanised steel, cyclic corrosion tests are more suitable.
Which standards do we use?
The salt spray test is usually carried out to ISO 9227 or ASTM B117.
- ISO 9227. This standard specifies the apparatus, the reagents and the procedure for the neutral salt spray test (NSS), the acetic acid salt spray test (AASS) and the copper-accelerated acetic acid salt spray test (CASS). These are used to assess the corrosion resistance of metallic materials, with or without permanent or temporary corrosion protection.
- ASTM B117. This American standard is virtually identical to ISO 9227, with a few minor differences, including the positioning of the samples and the quality of the salt used.
These standards describe how the exposure must take place, not how the assessment must be carried out, how long the test should last or which acceptance criteria you apply.
How do we carry out the test?
The test samples are sprayed with a sodium chloride solution for a period agreed in advance. There are three variants: the neutral salt spray test, the acetic acid salt spray test and the copper-accelerated acetic acid salt spray test.
The acetic acid test is mostly carried out on aluminium. We only use the copper-accelerated test in very specific cases, mostly on aluminium or electroplated products.
- 1. Sample preparation. We label the samples and measure the coating thickness. If required, we make a scribe through the coating down to the metal substrate. This allows us to determine the undercreep afterwards.
- 2. Exposure. The samples are exposed to the test environment for the duration agreed in advance. Where possible, we place the samples at an angle of 15 to 30° from the vertical. This is important so that the salt solution can run off.
- 3. Assessment. At the end of the test, we carefully rinse the samples with water, after which they can be assessed.
The most common mistakes with this test
It may seem simple: put the samples in the cabinet and let it run. Yet things often go wrong because of a lack of knowledge and experience.
- Poor choice of samples. Testing is often done on panels. That does not show how the coating performs on edges, welds and other potentially weak spots. How we do it: where possible, we test on the product itself, or on a scale model.
- Wrong test duration. The test duration is determined by the corrosivity category and the expected service life. Completely wrong choices are often made here, leading to incorrect conclusions. How we do it: we calculate the right test duration based on specialist knowledge and experience.
- Lack of proper acceptance criteria. Carrying out a test without acceptance criteria is pointless. It is important to state clearly which requirements the coating must meet once the test is complete. How we do it: depending on the base material and the application, we draw up suitable performance requirements.
How do we assess the result?
What we look at: there is almost always a visual assessment. We record where defects appear and after how long. We look mainly at corrosion, blistering and delamination from the scribe made beforehand.
How: a visual assessment can simply be described in words, but it can also be formally quantified using standards.
Criteria: as with the test duration, we agree with you in advance which requirements must be met once the test is complete. This is often based on company-specific standards or, for example, Qualicoat, Qualisteelcoat, NORSOK or ISO 12944.
How do we know the test is right?
The conditions in the cabinet must be equally aggressive every time, and the assessment must be carried out in the same way, whichever lab technician does it. That is why we monitor the following.
- Corrosivity check. Mass loss due to corrosion is the most important indicator of how well the cabinet is working. That is why we regularly test bare reference samples and determine their mass loss with an analytical balance.
- Salt concentration and pH. We measure and record the salt concentration and the pH of the spray solution several times a week.
- Collected solution. We determine how much solution settles on the products and how it is distributed throughout the cabinet.
- Temperature. The temperature of the cabinet is controlled and logged automatically, so that we can trace how the exposure went if necessary.
Quality you can demonstrate
M2LAB is accredited to ISO/IEC 17025. What does that mean? An independent body, in the Netherlands the Dutch Accreditation Council (Raad voor Accreditatie, RvA), has established that we carry out our tests competently, impartially and reproducibly. It is not a one-off stamp: the RvA returns regularly to assess this again.
The most important proof: round robin testing
In a round robin, several laboratories receive the same material and carry out the same test. The results are then compared. If our result differs, we find out why and adjust our method. This demonstrates that our results match those of other labs.
What else we do
- Calibration. We regularly compare our measuring equipment with ISO 17025-traceable references.
- Fixed method. Every test follows a defined, proven method in accordance with the standard.
- Qualified people. Only demonstrably trained staff carry out tests and assessments.
- Measurement uncertainty. We know how accurate our result is and take that into account.
- Audits. We carry out internal audits. We investigate errors and complaints down to the root cause.
For you, this means: a test report you can build on, even if a discussion arises later.
Curious how the coating on your product performs?
We test to the standard that suits your application, and explain the result clearly.
M2LAB investigates the service life of coated products: corrosion, sunlight, chemicals, mechanical stress and visual appearance.
