An article introducing the corrosion test campaign
Piston rods for offshore use often have hard metallic coatings for corrosion protection in this harsh environment. Traditional electroplated coating techniques are being challenged by new emerging application solutions such as high speed laser cladding. Methods and protocol for testing and verification of these emerging coatings in ocean energy solutions need to be implemented to deal with the lack of experience from the field.
In the De-Risk project we are investigating the usefulness of two different corrosion test methods. First method is the commonly used continuous salt spray test according to standards ISO 9227 and ASTM B117. While being a powerful and robust test for quality control purpose, it has received some criticism for the lack of correlation with performance in environments for intended use. However, it is common to see coating suppliers showing perfect performance of their corrosion protection after 1000 hours exposure in this test. Here, we tested five different coatings in a continuous salt spray test for a total duration of 2000 hours. Coating 1 was electroplated, the remaining coatings were laser cladded. We also performed the same test in three different electrolytes: sodium chloride according to standard, artificial seawater, and natural seawater. This was done to see if the seawater electrolytes could add any additional information on performance in more relevant electrolyte composition and pH.
Figure showing a picture of a coated rod sample to the left and a salt spray chamber with multiple samples.
The second method is an electrochemical technique according to standard ASTM G61. It is a standardised method for conducting cyclic potentiodynamic polarisation measurements for localized corrosion susceptibility of iron-, nickel-, or copper-based alloys in a chloride environment. This test method provides an indication of the susceptibility to initiation of localized corrosion, given by the potential at which the anodic current increases rapidly. The more noble this potential, the less susceptible is the alloy to initiation of localized corrosion. Important to note is that while the results of this test are not intended to correlate in a quantitative manner with the rate of propagation that one might observe in service when localized corrosion occurs, it can indicate a ranking of the performance of different coatings. As in previous test, both sodium chloride and seawater electrolytes were utilised to study the corrosion protection performance dependency of electrolyte.