Manufacturing: Surface Finishing

Introduction

Improving the appearance of a finished product for purely aesthetic reasons can be important because it often increases the salability of the product. The increased product performance and safety provided by proper edge and surface finishing is also important. The removal of burrs and sharp edges improves safety for both the worker and product user by eliminating the possibility of cuts and making parts easier to handle. For critical components, the surface condition and edge geometry can be a major influence on component performance and durability.

From strictly an engineering point-of-view, surface finishing is primarily good for one thing: preventing corrosion. Historically, man has employed three different mechanisms to stop corrosion: (1) barrier coatings, (2) inhibitive primers and (3) the use of zinc anodes.

Barrier coatings are simply paints applied to the surface of metal to create a "Barrier" or "Wall" between the metal and the corrosion-initiating exterior atmosphere. To protect yourself on a rainy day, you put on a raincoat to keep you dry. The paint of a barrier coating acts as a raincoat for the steel. This method is only partially effective since barrier coats are not completely impermeable to moisture and will eventually breakdown, allowing the corrosion process to begin, and will only protect as long as the coating is intact. If a barrier coating is scratched or damaged in some way exposing the underlying metal, corrosion begins.

Corrosion-inhibitive primers employ special pigments which provide corrosion protection through their ability to release inhibitive ions which are carried to the metal surface as water penetrates the coating. At the metal surface, these ions modify anode and/or cathode reactions, and force the steel's potential to corrode into a passive mode. These coatings are somewhat effective but give limited service life and will allow corrosion to occur at damaged areas, like barrier coatings.

Back in the 1700's a man named Luigi Galvani discovered that if you place two dissimilar metals in direct, electrical contact with each other and subject them to an electrolytic solution, ions from the least noble metal go into solution, liberating electrons and causing a current flow into the more noble metal preventing it's ions from going into solution. The process described, which became known as "Galvanizing" (aptly named for Mr. Galvani), employs the use of zinc as the anode, or least noble metal. The zinc slowly releases it's ions causing the current to flow into the metal it's applied to. The "hot-dip" galvanizing process, where iron or steel is dipped into molten zinc at 850 degrees F, was born by a French chemist named Melovin in 1742. Since then, hot-dip galvanizing has been considered by many to be the epitome of corrosion protection.