Blister formation is usually the first sign of degradation of any corrosion protection coating. It is therefore advisable to study the behavior of this coating material under conditions that could eventually cause the film to detach in the form of blisters.
This test is usually performed as follows. First, holes are drilled in the substrate where the bubbles were formed before the liquid paint coats the sample surface. The hole is plugged with a material, such as Teflon, that does not adhere to it during the painting process so that the plug can be easily removed after film formation. The hole can also be created after spraying using the spark erosion technique.
Separation of the membrane can be initiated and supported by hydrostatic pressure supplied by wellbore fluids (oil, mercury, etc.) or pressurized air. "In any case, pressure is the main measure of the progress of the debonding process. To obtain adhesion values, either as maximum stress or as adhesion energy (work of adhesion), it is necessary to monitor the height and diameter of the bubbles.
A relatively low-magnification optical system is sufficient for this purpose. From these geometrical data, together with the tensile modulus of the film and its thickness, a critical pressure value can be calculated. It is the pressure that causes the growth of the blisters, so the pressure can be used as a basis for determining the adhesion strength. The preparation and measurement procedures of the samples are relatively complicated, but the basic information generated by the test results on the mechanism of blister formation actually makes up for this shortcoming.
