Electrochemical measurements to evaluate the damage due to abrasion on organic protective system

Organic coatings are the most commonly used system for protection of metals from corrosion. In several applications organic coatings have to show, in addition to the protection properties and a good aesthetic appearance, good resistance to impact and abrasion. In fact, the mechanical damage can remarkably decrease the protection properties, even in the case of a very protective organic coating.

To evaluate the abrasion resistance of organic coatings the Taber Abraser test is frequently used. The mechanical damage is determined by measuring the mass loss without considering the form of damage (morphology and influence on corrosion protection performances).

The aim of this work is to evaluate the degradation of protection properties, caused by abrasion in Taber tests, using electrochemical methods.

Polyester powder coated steels were studied. Several parameters were considered such as the number of cycles, hardness of abrasive grinders and imposed weight.

By considering the resistance and capacitance of the organic coatings, obtained by fitting the electrochemical impedance data, it is possible to evaluate the trend of damage evolution as a function of the number of cycles and of the test parameters.

These EIS technique can distinguish the slight difference in aggressiveness of the two types of grinders used. The CS10 grinders produce debris which show the tendency to stick to the grinder itself and to the paint, reducing the abrasive action. Nevertheless, in this case the abrasion of the coating appears more uniform. In contrast, CS17 grinders increase the tendency to produce localised defects, which reduces the performance of the paint.

A different degradation rate and morphology were observed: an initially high damage value, followed by a decrease was observed due to both the presence of debris and the progressive efficiency loss of the abrasive wheels. For this reason, polishing of the grinder after every 1000 cycles is necessary to maintain a constant abrasion efficiency.