This month the questions being answered by our corrosion technology experts relate to accelerated weathering testing of maintenance coatings and selection of the CP system

Question:

When carrying out accelerated testing to determine the best maintenance coating system to protect a North Sea offshore structure for >5 years, which (cyclic) test provides the best correlation with exposure in this environment? CL

Answer:

Before considering which test gives the best correlation with exposure in an offshore marine environment one needs to appreciate that applying maintenance coatings in such an environment will usually be done under sub-optimal circumstances. Bearing in mind that the air humidity should be <85% and the temperature of the substrate should be at least 3°C above the dew point, then considering that the average annual humidity in Aberdeen is 85%, with only April to June usually below 85%, anytime outside of this period is unlikely to meet the required conditions without using habitats.

Most epoxy paint systems require time to cure and this creates an additional constraint for offshore coating work given the restricted workspace. Typical cure times for epoxy coatings at 15°C, the average temperature offshore Aberdeen, are 7 to 8 hours, which adds to the time pressure for offshore work.

Much offshore coating work is now done using rope access teams. This adds another level of difficulty in obtaining good surface preparation and coating application. If one was to have a truly representative test then the application of the coating for testing should be done by someone hanging from a rope whilst avoiding salt spray!

At least one N. Sea Operator recommends using ISO 12944-9* for the testing of maintenance coating systems to obtain a correlation with practice. This standard superseded ISO 20340:2009 in 2018, and seems to have gained some credibility with discerning Operators. Experience indicates, taking into account the constraints of obtaining a good coating application in an offshore environment, the testing specified in this standard, and in particular the 4200 hour cyclic ageing test, provides a robust assessment of a maintenance coating system. For coating systems that pass the testing specified in the standard, providing they are correctly applied, one can generally expect to get at least 5 to 7 years’ service for a 3-coat system.

With the increasing use of surface tolerant coating systems it’s important to ensure that qualification to ISO 12944-9 is done with the same degree of surface preparation as that applied in practice, although this is not strictly in accordance with the standard which requires Sa2½ minimum surface preparation. However, the standard does allow this option if “agreed between the interested parties”. Also be aware that single and dual coat systems may not have the same level of UV protection. Although UV exposure does form part of the ISO 12944-9 cyclic ageing test protocol, one may not get the same number of years’ service as with a 3-coat system.

And last but not least, it is recommended to always ask for independent testing if there is not a substantial track record for a given coating system. Steve Paterson, Arbeadie Consultants Ltd.

*ISO 12944-9:2018 Paints and varnishes – “Corrosion protection of steel structures by protective coating systems – Part 9: Protective paint systems and laboratory performance test methods for offshore and related structures”