Multiple thermal immersion ageing of commercial epoxy coatings

Abstract

Purpose

The purpose of this paper is to evaluate the degradation of two commercial epoxy coatings with different pigments recommended for marine immersion, in a proposed short time accelerated aging cycle, correlating the electrochemical, thermal and superficial characterization.

Design/methodology/approach

An immersion accelerated ageing method in an aggressive environment with increasing temperature has been described: samples of two films applied on AISI 1010 steel and films free of substrate of two different pigment coatings were submerged in hot brine, maintaining them for 24 h at 25°C, then increasing daily in 20°C increments to 85°C; followed by cooling to 25°C. The applied samples were evaluated in situ electrochemically using electrochmical impedance spectroscopy (EIS) (while they were in the heated environment) and the free films were submitted to differential scanning calorimetry (DSC) and scanning electronic microscopy (SEM) at the beginning and end of each temperature stage.

Findings

Results from the EIS tests mainly showed that the protective properties of both coatings (as indicated by coating capacitance and coating resistance values) decreased (when the temperature was increasing, but at the end of the cycle, the trend of both parameters was to reach their initial values, indicating that the change was physical (and reversible). However, after several cycles the trend of properties was to decrease, indicating then that a chemical damage (irreversible damage) had been caused by aging. The DSC technique showed that the coatings were plasticized after the immersion, due to the change in glass‐transition (Tg), and SEM revealed the presence of micro cracks after several cycles, confirming that mechanical degradation had taken place due to hot immersion.

Practical implications

Ageing cycles of this type could be applied to organic coatings, perhaps varying the aggressive environment and ageing time according to the hardness and chemical resistance of the coatings, as well as the Tg temperatures of the resins employed.

Originality/value

The conditions employed on the aging cycle were suitable to cause physical and mechanical damage to the films and to enable comparative performance testing to be evaluated quickly, and to promote chemical degradation. All results obtained by the three techniques were correlated satisfactorily and could be used to characterize the type of damage sustained by the coatings.

Citation

Angeles, M.E., Magaña, C.R. and Rodríguez, F.J. (2009), "Multiple thermal immersion ageing of commercial epoxy coatings", Anti-Corrosion Methods and Materials, Vol. 56 No. 3, pp. 154-161. https://doi.org/10.1108/00035590910955513