Since 1994 when NORSOK M-501 "Surface preparation and protective coating" was introduced there have been three major coating failure incidents on the Norwegian Continental Shelf. It is instructive to look into the coating failure mechanisms for these three cases and compare that with the NORSOK M-501 coating pre-qualification program. Why did the pre-qualification testing not reveal these failures and what can be learned for future testing? The three cases are (1) painting of thermally sprayed aluminum (TSA) (2) cracking of a rapidly curing coating system and (3) topcoat flaking of two-coat paint films.
Failure of painted TSA was the first case. The installations were actually built before the NORSOK M-501 standard was introduced but a test similar to the pre-qualification test was still performed. In the field rapid corrosion of the TSA under the paint caused massive coating failure after just a few years of operation. Investigations showed that the TSA was attacked by a corrosion mechanism resembling crevice corrosion. It is interesting to note that the testing performed showed significant corrosion from scribe but the confidence in the coating system was so high that the test results were disregarded.
In the second case rapidly curing coatings were introduced to increase productivity in the yard. Already in the yard a few cracks were found in the coating and after relatively short time in the field severe cracking was reported. The coating cured faster than the solvent evaporated. The solvent that evaporated after formation of a solid film resulted in a significant level of internal stress since the film was unable to shrink laterally due to adhesion and crosslinking. Also here the coating system did not pass the pre-qualification test though the test did not reveal cracking as prime cause of failure. Corrosion creep was slightly higher than the pre-qualification limit. However the coating system was accepted as an exception due to the benefit of the rapid production rate and the marginal failure in the test.
Topcoat flaking of a two-coat system is the last major coating failure case. In this case the coating passed the pre-qualification without any sign of flaking. The exact mechanism for the flaking has not been revealed later but loss of adhesion is always important in flaking incidents. Corrosion of the zinc in the zinc rich primer seemed to contribute to the adhesion loss but other factors may also have been important.
In general NORSOK M-501 has fulfilled its purposes and in two of the three cases of major coating failure the coating system did actually not pass the pre-qualification testing. The failures have only involved System 1 (atmospheric) in M-501. For Systems 3B (tanks) 4 (floors) 5 (passive fire protection) and 7 (submerged) that also require pre-qualification no major incidents of coating failure have been reported. The fact that parts of NORSOK M-501 are referred to by other industries as well is another indication for the success and significance of the standard.