One of the most common ways of protecting steel assets and structures is by organic protective coating systems. The performance of such protective coating systems is assessed based on results after accelerated laboratory exposure testing, where one attempts to mimic the conditions the coatings will be exposed to under in-service conditions in a significantly shorter time frame. Such testing is also how coating systems are qualified for certain corrosivity classes and durabilities, being formalized in standards and specifications such as ISO 12944-6 and NORSOK M-501 ed. 7.
Product Number:
51323-19270-SG
Author:
Andreas Løken, Anders W. B. Skilbred
Publication Date:
2023
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The present paper explores some of the inherent drawbacks of using corrosion creep as a performance indicator for corrosion protective coatings. This is shown by investigations of eight coating systems, comprised of four different coating products applied in a single coat of 250 μm (9.8 mils) and two coats of 2 x 125 μm (2 x 4.9 mils). All coatings are investigated by accelerated laboratory testing including salt spray and cyclic aging for durations of 1440 and 4200 hours. The results are then compared with the systems’ barrier properties as determined using Electrochemical Impedance Spectroscopy (EIS) measurements.
While traditional exposure testing and evaluations, including corrosion creep and pull-off adhesion, show limited differentiation in terms of the performance of the coating systems, a clear trend is demonstrated by EIS characterizations. The barrier property is shown to increase in the order A< B < C < D, also showing a significant increase upon going from a single coat of 250 μm (9.8 mils) to two coats of 2 x 125 μm (2 x 4.9 mils), even if the total thickness is unaltered.
No correlation between the barrier property and the corrosion creep or pull-off adhesion is found. In fact, the coating systems comprised of the products with the poorest barrier property were also the systems displaying the lowest corrosion creep after cyclic aging for 4200 hours with values of 2.5 and 1.7 mm (98 and 67 mils) for 2 x 125 μm (2 x 4.9 mils) Product A and B, respectively. The same products show visible degradation after exposure in the form of blistering and rusting when tested in a single coat of 1 x 250 μm (1 x 9.8 mils). The mismatch between corrosion creep and barrier property strongly highlights the limitations in using corrosion creep as a performance indicator on its own. Further, it also indicates that the corrosion creep along an artificial scribe does not reflect the protective properties of an intact coating film. It is therefore advised that EIS characterizations are done in parallel to ensure a better overall picture of the protective performance of coating systems.
