PAINTSQUARE BLOG

Tap ... tap … tap...

Between the years of 1660 and 1664, my ninth great-grandfather Andries Rees was repeatedly dragged before the magistrate in the Dutch colony of Nieuw Amsterdam. The charge was tapping on Sunday.

Employed as a soldier by the Dutch West India Company, Andries arrived in the colony around 1650. His tapping establishment was located just below the fortified wall that we now know as Wall Street.

Sometime after the Dutch capitulation to the British in 1665, Andries and his wife Celitje gave up tapping and soldiering and moved up the Hudson River Valley.

The verdant valley was populated with Dutch farming families named Roosevelt, Van Buren, Fonda, etc.—the descendants of whom would become American presidents and movie actors. My more pedestrian Rees ancestors eventually Anglicized the family name to the like-sounding Race.

Tinkering and Tapping

My own tapping tradition began on a Sunday in the cargo hold of a Greek-owned oil tanker. It was my first day on the job as a consultant, and my new boss was tinkering with a procedure that we would soon refer to as the tapping test.

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Shatter patterns result from the coating failing in a brittle manner, due to a sharp impact.

To perform the tapping test, we used a painter's 5-in-1 tool to tap the painted surface. Tap, tap, tap. What we were looking for was brittle failure of the coating.

Brittle coating failure speaks to the mechanical properties of the coating. The test can also be used to evaluate the relationship between the mechanical properties of the coating and the surface preparation.

The more brittle the coating and the smoother the steel substrate, the more likely it is that the tap test will cause the coating to delaminate. Brittle coating failure is typically a response to an external force, such as impact or flexure, applied to the coating.

Brittleness Basics

The degree of brittleness of a coating often depends on formulation factors. Specific coating types are known to be more brittle than others. Plasticizers are typically added to coatings to compensate for an inherent brittleness.

Brittleness can also be a function of the conditions under which a coating is cured. Curing epoxy coatings at low temperatures or with poor ventilation may result in a film with poor mechanical properties. Often times, the poor mechanical properties manifest as film disbondment.

More often than not, these disbondment failures are subsequently evaluated using the standardized pull-off adhesion test. This can lead to confusion, because the results are often uniformly excellent or randomly poor/excellent. How come?

There are two problems with using tensile adhesion tests if the underlying cause of failure is a brittle coating.

First is the fact that a tensile adhesion test is not intended to evaluate the effects of shear force. If the test is done correctly, the force is applied in the vertical direction with little or no shear. Mechanical stresses applied to a coating in service are just not well replicated by a tensile adhesion test.

When a steel substrate flexes, the force applied to the coating is parallel to the surface. Random poor/excellent tensile adhesion results often result when inferior test equipment or poor technique result in the sporadic introduction of shear stress in addition to tensile stress.

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Save the other kind of time-honored tapping for after the coating testing is over.

The second reason why pull-off adhesion may fail to identify or diagnose brittle coating failure is the fact that the force is applied slowly. Brittle failure is initiated when the stress is applied rapidly, such as when a stone impacts the coating or when the substrate flexes.

Failure Finesse

And hence the tap test, an ingenuous non-standardized method that can be used to rapidly assess the mechanical properties of a coating.

In the hands of an experienced evaluator, it can be used to identify incipient brittle failure or to diagnose one possible cause of a coating disbondment failure. If brittle failure has been diagnosed, then the investigator will know what factors need to be addressed in their failure analysis.

Just don't confuse the test with the recreational tapping that my ancestor performed as a public service to the residents of Nieuw Amsterdam.

ABOUT THE THE BLOGGER Tim Race Tim Race began his career in paint technology with the U.S. Army Corps of Engineers, where he was nicknamed “Paint Dude” by his future wife. He later served as a consultant, working for Corrosion Control Consultants and Labs and independently. Race received the SSPC Technical Achievement Award in 1998 and was named an industry Top Thinker by JPCL in 2012. After 30 years in the industry, he is now enjoying retirement and letting his wife do the heavy lifting. SEE ALL CONTENT FROM THIS CONTRIBUTOR