Measurement of cathodic disbonding using a scanning acoustic microscope

It is shown that scanning acoustic microscopy provides a nondestructive, precise, and rapid method for the in-situ monitoring of cathodic disbonding. It provides an excellent technique for comparing the rate of disbonding or organic coating/substrate systems that have been subject to a variety of treatments to retard disbonding. The authors describe the details of the measurement process. Results are presented for polybutadiene films on steel substrates that have different surface treatments. Surprisingly, the disbond results in a nonuniform attack of the polymer, appearing as a series of isolated sites that have been designated microblisters. These microblisters develop into circular zones and grow in diameter as a function of time. Taken together, the microblisters form a propagation front approximately parallel to the scribe initially placed in the coating. The data show that the movement of the front after an initiation period is proportional to the square root of the time. An interpretation of the results is presented