Accelerated corrosion testing of substrates under coatings has an inherently high level of variability. When comparing different coating systems, significant levels of variability can mask potential differences in corrosion prevention. Reducing this level of variability could significantly increase efficiency and accuracy when developing new coating systems. In this work, the effect of altering the size and shape of the scribe in method ISO 12944 will be investigated by comparing the performance of well-understood commercial coating systems. Discussion will focus on sample preparation of multiple coating types prior to corrosion testing as well as scribe creep measurement analysis afterwards.
Accelerated testing of corrosion-resistant coatings is a critical tool used in the development of protective coatings. Tests such as ASTM B1171, ISO 129442, ISO 92273, etc. are commonly used to predict the long-term viability of these coatings in exposure times of months rather than years or even decades. While this type of testing gives meaningful results and is essential to the research and development of new technology, results can often be hard to accurately analyze due to the levels of variability inherent to accelerated testing. An accelerated test typically involves applying a single coating or series of coatings to a specified substrate, often steel or aluminum, and allowing it to cure for a specified time. A defect to the coating is then typically applied via manual or automated means to expose the bare substrate to a particular environment. In ASTM B117, for example, the sample is exposed to a constant fog of neutral pH, 5% sodium chloride in water at 35°C. After testing for a certain length of time (typically dependent on the performance expectation of the coating) the sample is removed from testing and inspected visually. Methods such as ASTM D 7144 can be used to evaluate blister formation in the coating, but the main analysis of performance is typically the amount of corrosion emanating from the scribe under the coating, known as rust creep. The coating around the scribe is typically removed either physically by scraping away or chemically with paint stripper. The boundary between corroded and uncorroded substrate is measured, typically in millimeters. This measurement of scribe creep is where significant levels of variability are observed. When plotting the average scribe creep of multiple coating systems, one standard deviation above and below the mean can encompass the majority of the average bar, often making systems that could be performing differently appear statistically similar. Advanced statistical analysis programs can aid in analyzing scribe creep data and are often better suited for finding differentiation.
