Salt spray testing in three different electrolytes
The samples were evaluated for base metal corrosion throughout the exposure according to ISO 10289. The lowest protective rating obtained for any sample was 9, meaning a total surface area of defects to be above 0 while below 0.1 %. See example of defects below.
The visual appearance of the samples after long exposure time in continuous salt spray test can be seen in the pictures below. The visual appearance of coatings 1 and 5 were seen to be most affected by the test. Coatings 2-3 showed near pristine appearance after the test. In artificial seawater, some calcium carbonate precipitate was found on the surfaces of the coatings and end-caps of the samples.
Protective rating - sensitive to number of replicates
In the table below, the protective rating for each coating was determined based on either the rating of the majority of the replicates (top table) or the lowest rating of any replicate (bottom table). The differences in pattern seen between the two tables highlight that we have a different outcome depending on the criteria we choose for treating the replicates. Careful consideration of criteria need to be taken, especially for this low number of replicates. The results also indicates that the number of replicates are insufficient for this type of test. Furthermore, the result show that there was little or no effect from running continuous salt spray test with different electrolytes for these coatings tested.
•Is it a useful test in a test-program for validation or certification purposes of hard inorganic coatings for WEC-parts such as PTO ocean rods?
Yes. It would be for purpose of quality control, because it would reveal if there would be a bad batch, or if something is wrong in the coating process, new base metal material that gives poor adhesion of coating etc.. Cracks or pores would be revealed easily.
Is 1000 hours (6 weeks) sufficient length for this type of quality control test?
Yes. Doubling the exposure length does not give any more information for these types of coating.
Is it useful to run the test with seawater electrolyte instead of or as a complement to the standardized sodium chloride?
12 weeks results basically confirms the results from 6 weeks. The salt does not affect the results significantly. NaCl would be sufficient and cheaper for this test.
Cyclic potentiodynamic polarization scans according to standard ASTM G61 revealed some differences in performance between the coatings.
Comparison of performance in different electrolytes
•One scan in artificial seawater (ASTM D1141) and one scan in natural seawater (deep seawater) from Kristineberg Research center was performed on each coating.
•That is, same standardized experiment but using more realistic electrolyte.
Artificial seawater = AS
Natural seawater = NS
Overall conclusions - Corrosion testing
The two test methods used in this project for these types of coatings, revealed that:
traditional continuous salt spray testing has its strength in quality/batch/process control and do not reveal extensive information on performance
1000 hours exposure to salt spray seem sufficient, no need to extend
Ideally a higher number of replicates than three is preferable
There is no or little information gained from using seawater electrolytes in continuous salt spray testing on these coatings.
Electrochemical testing with cyclic potentiodynamic polarization seems useful for evaluating coating protection performance towards localized attacks.
Utilizing seawater electrolytes in electrochemical tests can be of use for studies of in-depth coating performance in electrolyte more relevant to intended use.
There never only one test that can cover all aspects when verifying performance of protective measures of a component. There is a need to address and conclude a test protocol with acceptance criteria. Further work need to focus on the tests to include and finding relevant criteria based on type of protection and environment of intended use.
As these types of hard metallic coatings can be susceptible to detrimental localised attacks such as crevice corrosion, it is important to include a test to verify an acceptable resistance of the coatings. This can be important as crevices can form underneath foulants or between rod and sealing surfaces. In this project we will further work on testing the coatings according to ASTM G48 and evaluate the test method for evaluating the laser cladded coatings on rods.
Another aspect in verifying performance of the coatings are multi degradation factors. Tests are often design to test a single degradation parameter/aspect such as corrosion or fouling. In real use, multiple factors can have synergy effects. Wear and corrosion are detrimental factors alone, but can also accelerate one another and making the effect even more troublesome then when you only have one of them alone. Within this project, there is an ambition to explore accelerated tests that can incorporate multi degradation factors of rod coatings.