A quantitative real-time evaluation of rust creep propagation in coating systems exposed to field testing and cyclic ageing test

This study investigated the rust creep propagation of two different coating systems, an epoxy primed (EP/PUR) and a zinc-rich epoxy primed (ZnEP/PUR) coating system exposed to the field and the accelerated cyclic ageing test. Non-destructive scanning acoustic microscopy (SAM) was used to assess the coating samples and detect rust creep propagation. The results showed a linear increase in rust creep propagation over time in all coating systems exposed to the field and the cyclic ageing test. The EP/PUR coating system exhibited a rust creep propagation rate of 0.228 mm/week under field exposure and 0.380 mm/week in the cyclic ageing test, while the ZnEP/PUR coating system showed rates of 0.024 mm/week under field exposure and 0.292 mm/week in the cyclic ageing test. Notably, the rust creep propagated through filiform corrosion mechanism under field exposure for both coating systems and that was not observed in the cyclic ageing test. The reliability of evaluating the anti-corrosive performance and rust creep growth around a scribe of different anti-corrosive coating systems exposed to the accelerated cyclic ageing test, as defined in ISO 12944-9, is questionable. The study highlights the benefits of applying non-destructive scanning acoustic microscopy for rust creep propagation investigation capturing information that may be missed by the traditional standardised method.