Buried steel structures such as pipelines or tanks are usually protected from corrosion by a combination of organic coatings and cathodic protection (CP). In this configuration, the coating must be compatible with CP and able to sustain ageing under the induced environment. Indeed, detrimental interaction between CP and organic coatings can be observed; either by the induced cathodic disbondment, or the chemical degradation due the local environment, such as high alkalinity and oxygen radicals at the vicinity of open defects.
This was confirmed through a recent study, in where a 30 years old disbonded Fusion Bonded Epoxy (FBE) coating under CP. The coating collected close to an open defect and characterized by electrochemical impedance spectroscopy (EIS), showed significant aging. Even though the possible chemical aging of polymeric matrix of coatings under alkali conditions is known, such mechanism is not fully studied, especially on commercial industrial coatings containing fillers and additives in addition to the polymeric matrix.
In the present work, a commercial liquid epoxy coating was subjected to chemical ageing in alkaline environment at different pH and temperatures. The free coating film degradation was studied by EIS, gravimetry, and spectroscopic investigation (Infrared and Raman). The results showed a clear impact of the alkaline pH and the temperature on the coating with a significant drop of coating resistance over time (RC). The RC decrease is expected to further accelerate the ageing through an increase CP current demand and related local alkalinity. Thus, the proposed chemical method appears relevant and shows the necessity to consider such specific ageing for coatings qualification and improvement.