Bridging Laboratory Testing and In-Service Coating Performance for Structural Aerospace Applications

Abstract

In the aerospace industry, toxic and carcinogenic chromate-based inhibitors are still widely used in coatings to protect structural components of aircraft throughout their entire lifespan. Alternatives currently lack proven long-term performance, partly because accelerated ageing tests may provide a qualitative ranking of alternatives, but fail to accurately predict service lifetime in their real-world application. However, the reasons for these discrepancies between laboratory-based test results and in-service performance remain insufficiently understood.

This dissertation aims to deepen the understanding of the factors influencing coating degradation and their underlying mechanisms, both in practical applications and test environments. Such knowledge is essential for developing improved test methods capable of reliably comparing the performance of chromate-containing coatings with alternative systems. These advancements could significantly accelerate the development process of new coatings driving innovation in the paint and coating industry.

The study consist of two separate research tracks: (i) forensic research into the degradation mechanisms of aircraft components after long-term in-service use and (ii) experimental research into degradation mechanisms in test environments. Each track focuses on two aspects: (i) corrosion and inhibitor action on aircraft metal alloys and (ii) coating degradation.

The forensic analysis examined four aircraft components that had been in-service for over 35 years, using visual inspection, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results showed that large areas of the coated components remained well-protected throughout the entire service life. However, three specific forms of degradation were identified: (i) erosion at the tip; (ii) corrosion around rivets and (iii) corrosion near fasteners at the leading edge. These findings demonstrate that even chromate-based coatings may not sustain the provision of long-term corrosion protection in complex multi-material areas.

Further forensic analysis focused on the protective mechanisms and degradation factors of the original coatings using electrochemical impedance spectroscopy (EIS), SEM and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). Results confirmed that chromate-containing coatings are exceptionally effective; after more than 35 years of service, they outperformed some newly applied systems. This superior performance was attributed to chromate adsorption on corrosion products like aluminium hydroxide, which increased the pore resistance of the coating. Simultaneously, it was found that the polymers in the original coatings had degraded due to thermal oxidation. Temperature increase due to exposure to sunlight caused oxidation in the polymer, which accelerated moisture uptake. This, in turn, led to faster inhibitor leaching, compromising the coating barrier properties.

The experimental study compared two chromate-based coatings with two alternatives under various exposure condition, including a cyclic salt spray test (CSST), outdoor exposure and flight tests. Results revealed that the corrosion and inhibition mechanisms observed in the CSST did not align with those observed in flight tests. These differences were attributed to variations in time of wetness (TOW) during the relative humidity (RH) cycles, temperature fluctuations, differences in the type of deposited substances (such as salt) accumulating at the test specimens and excessive electrolyte exposure in CSST. Furthermore, galvanic coupling at fasteners was difficult to prevent leading to accelerated corrosion. Chromate-based systems provided partial active corrosion inhibition around fasteners, while alternative systems failed. However, the alternative systems offered improved corrosion resistance between aluminium-coated surfaces coupled with carbon fibre-reinforced polymer (CFRP). This improvement is due to the novel polymer formulation in the alternative systems, increasing their barrier properties as compared to the legacy polymers used in chromate-based systems.

Further experimental analysis on coating degradation under different exposure conditions, using EIS, SEM and ATR-FTIR, revealed that hydrolysis and thermal oxidation were the primary causes of polymer degradation during flight tests, with inhibitor leaching playing a comparatively minor role in the coating degradation. In contrast, inhibitor leaching was the dominant degradation factor in CSST and outdoor tests, significantly accelerated by UV radiation and excessive electrolyte exposure.
The study also highlighted the important role of the anodized oxide layer in coating systems. In chromate-based coating systems, chromate adsorption onto aluminium hydroxide within the pores of the anodized oxide layer, increase corrosion resistance, whereas in alternative systems, only the polymer inside the pores provides additional protection.

This dissertation provides valuable insights into factors for improving artificial ageing tests. Integrating thermal oxidation, increasing TOW during RH cycles and reducing electrolyte exposure into test protocols can enhance the predictive value of these tests. Additionally, incorporating complex material combinations with fasteners into updated sample configurations is considered crucial for realistic testing. These improvements can lead to more effective evaluations of alternative coating systems, accelerating the development and implementation of sustainable alternative coating systems.