The Protection of Metal
The previous two decades have witnessed an explosion in the use of metal for many applications ranging from the simplest hand tools to parts for the Space Shuttle. Metals play an increasingly important part in our daily lives. However, the increased use of metals has highlighted their major disadvantage, they corrode! It is estimated that we spend over 300 Billion Dollars per year in the battle to fight corrosion.
Historically, man has employed three different mechanisms to stop corrosion:
(1) barrier coatings
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. To use our raincoat analogy, if you do not button your raincoat, you get wet! Please refer to the photographs accompanying this article. The aluminized epoxy mastic, epoxy polyamine and the red lead/linseed oil primer coatings are representative of barrier coatings. These panels have been subjected to 1500 hours of ASTM B117 Salt Fog Test where a solution of salt and water is misted onto the panels to accelerate corrosion. You can readily see the coating breakdown and, where these panels have been scribed to bare metal, ferric corrosion has occurred.
(2) inhibitive primers
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. Refer to the Zinc Chromate Alkyd Primer in the accompanying photo for an example of the protection afforded by an inhibitive primer.
(3) the use of zinc anodes
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 its 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 its 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 ° 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.
Shortly after World War II, British researchers found that a coating could be devised which had a high loading of pure zinc dust (typically 95% zinc by weight in the dried film) and a special binder that does not insulate the zinc particles either from each other or from the base substrate. Such a coating would react according to the galvanic principle. Thusly, ZRC Galvanizing Compound was born!
Initially, the protection provided by ZRC is wholly that of galvanic action. As the zinc is sacrificed, zinc corrosion products are formed through the reaction of zinc ions with moisture and carbon dioxide. Zinc hydroxycarbonates and other zinc salts form in the film making the coating denser and thereby reducing its conductivity. The anodic action of the zinc continues until the film is converted into a dense, impervious barrier, resistant to weather, water and fume attack. However, if the coating is damaged, fresh zinc metal is available around the edge of the damaged area to provide renewed galvanic action. Thus, the protection afforded by ZRC is twofold; galvanic protection and barrier protection. Please refer to the accompanying photo of ZRC which illustrates its protective qualities. Note that there is no corrosion in the area which has been scratched down to expose bare metal!
ZRC exhibits several advantages over conventional hot-dip galvanizing techniques, as follow:
The hot-dip process involves large tanks of molten zinc. Most companies do not have a zinc tank in their back yard, so, they have to ship their metal parts to the galvanizer where they may wait in line with other jobs. Once dipped, the parts are shipped back. ZRC, on the other hand, can be easily applied like a paint by spray, brush or roller by anyone, anywhere on their own schedule. In the plant, on the jobsite, there is no limitation on ZRC's use and no large shipping bills!
The hot-dip galvanizing process is done at 850 ° F. This high temperature can warp and embrittle thin gauge metals. ZRC is applied at ambient temperature with no heat! No warpage, ensuring a good fit of parts during erection, and no breakage.
Hot-dip galvanizing, being a fused metal coating is very hard. This can be very useful in some applications. However, the flip side of the coin is that this hardness also equates to poor impact and bend resistance. If you were to bend or impact a galvanized steel item, chances are great that the zinc coating will chip and break off, exposing the underlying steel to corrosive attack. ZRC coated steel can be bent and impacted with absolutely no damage to the zinc surface, even when bent over 180 ° . What does this mean to you? If your fabricated item needs to be adjusted for fit at the jobsite (bent), ZRC offers the best chance for coating success, thereby reducing expensive downtime that could occur should you need to order new hot-dip galvanized items to replace the broken ones.
We happen to think that gray is a beautiful color. However, as unbelievable as it may seem, there are those who disagree and specify that their zinc coated surface is to be topcoated with a pretty color. In order to topcoat a hot-dip galvanized surface, you must first spend some considerable time preparing the zinc surface to accept the topcoat. With ZRC, you apply the topcoat directly over the ZRC surface without any additional preparation. Saves you time and money!
When you ship your metal items to a galvanizing plant, you have no control over the quality of your zinc coating. To be fair, some galvanizers do a great job and there should be no concern over the quality. However, over the years we have been called to repair many poorly galvanized jobs. When you apply ZRC yourself; in your own plant or on your jobsite, you completely control the quality of the ZRC application.
In short, we think that ZRC zinc-rich coatings provide the best answer to your corrosion protection needs!