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Pure water immersion proposed for faster coating screening
A study on the effect of NaCl concentration on anticorrosive coating performance reveals that high salt levels inhibit oxygen transport through dense coatings, potentially masking true barrier quality – and proposes pure water immersion as a more discriminating rapid screening method.
Assessing the anticorrosive performance of coatings under realistic service conditions remains a complex task, as environmental corrosivity varies considerably. The concentration of oxygen permeating through a coating is believed to be one of the critical factors influencing the corrosion process at the substrate. A new study has now systematically investigated how different NaCl concentrations affect the protective performance of the same coating system – with surprising implications for established testing methods.
The researchers found that increasing NaCl concentration actually prolonged the time until substrate corrosion and coating blistering occurred. Analysis of coating water absorption rates, electrochemical impedance spectroscopy (EIS) data and actual substrate corrosion conditions revealed that NaCl acts through a triple mechanism: it reduces the water absorption rate of the coating, lowers the amount of dissolved oxygen in the solution and inhibits oxygen transmission within the coating itself.
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EIS limitations exposed for high-performance coatings
A particularly noteworthy finding concerns the limitations of EIS as a diagnostic tool for dense, high-performance anticorrosive coatings. Even when substrate corrosion had already initiated, the impedance value |Z| at f = 0.01 Hz remained at approximately 10¹¹ Ω·cm², suggesting that EIS alone cannot reliably reflect oxygen transfer conditions in intact barrier coatings. This observation raises important questions about the interpretation of impedance data in accelerated testing of premium coating systems.
Based on these findings, the authors propose pure water immersion as an alternative rapid screening method. By significantly increasing the oxygen concentration permeating through the coating, this approach provides a more discriminating assessment of the protective quality of different coating systems.
Rethinking accelerated corrosion testing protocols
The study challenges conventional assumptions about salt spray and saline immersion testing by demonstrating that higher NaCl concentrations can paradoxically improve apparent coating performance by suppressing oxygen transport. For formulators and testing laboratories evaluating high-performance anticorrosive coatings, these results suggest that current standard test protocols may benefit from complementary pure water immersion assessments to obtain a more complete picture of barrier quality.
Source: Song, S. & Wang, Z., Refining coating anti-corrosion assessment via NaCl concentration-dependent performance analysis. Progress in Organic Coatings 109936 (2025).
