STUDY OF THE CORROSION BEHAVIOR OF PAINTED STEEL WITH ELECTROCHEMICALMEASUREMENTS

Electrochemical measurements (resistance, capacitance and polarization curves) have been used to study the corrosion behaviour of painted st eel in 3% NaCl solution (usual aerated degree) at room temperature. Th e protective films were made from four resins of alkyd, acryl, epoxy-p olyamide, nitrocellulose type. The thickness of the films was 20-25 mu m. The time dependences of capacitance and resistance have been deter mined for immersion periods of up to 5000 hours. Water uptake and aver age area of conductive pathways which develop in paint film between th e substrate and the electrolyte bulk were calculated. The polarization curves were recorded for bare and painted electrodes by potentiostati c polarization (50 mV/5 min) and the characteristic polarization param eters were established. The percentage decrease of dissolution current for painted steel was calculated. The corrosion behaviour of coated s teel depends on the coating resistance, which is related to uptaken wa ter amount, conductive pathways and the state of degradation of the pa int film. The highest electric resistance is exhibited by the alkyd pa inted samples. The alkyd paint film provides a good barrier to the NaC l solution. Also, the anodic dissolution of alkyd painted steel is mor e slowly. The capacitance and resistance measurements correlated with the polarization curves offer predictable information about the perfor mances of coatings.