March 2011 Archives

The most frequently used industrial plating for corrosion protection purpose are zinc (galvanizing) and tin plating. The zinc plating is used for corrosion protection of steels in natural environments such as air and water, and tin plating for the cans in the canning industry.
The zinc plating is the best known of plating less noble than steel. As shown in [Fig.1], this plating forms a cell with the steel being the positive and zinc being the sacrificial negative pole, protecting the steel from corroding. This is the same in principle as the sacrificial positive pole Corrosion protection scheme. The zinc plating film will eventually dissolve away and the steel will begin to corrode, therefore the corrosion resistance is proportional to the thickness of the zinc cell. This is in wide use as both hot dipped galvanizing and electroplating.

The representative example of electro-zinc plated sheet steel is the Surface Treated Sheet Steel. It has precisely controlled thickness and excellent machinability, and often painted and used for automotive bodies, home appliances, equipment cabinetry, and construction material. For the automotive applications, zinc-nickel and zinc-iron alloy plating with further increased corrosion resistance are used.

Tin plating with pin holes and scratches in air and water forms a cell with the tin film being the positive pole since the tin is more noble than the steel, and does not offer the sacrificial Corrosion protection effects. It fares relatively well indoors but the same cannot be expected in environments where water contacts occur. However, the zinc would become of the negative pole when in contact with food and protects the steel from corroding in food canning applications. This is because the dissolved tin and certain elements from the food combine and form some complex compounds. Currently, steel sheets with plating film coated with highly anti-corrosive synthetic resin is used for food canning applications.

Hot dipping or thermal spraying must be used for aluminum plating since it cannot be electroplated. In water, aluminum plating is more noble compared to the steel base and a sacrificial Corrosion protective effect cannot be anticipated. Aluminum plating is typically used on high temperature applications such as stoves and chimneys as a heat resistant material.

There are corrosion protection methods of covering the surfaces of metal objects with other metals with higher corrosion resistance properties called Plating, Thermal Spraying, Diffusion Coating, and Metal Cladding, and are used based on the applicable corrosion environment the metals are placed in. Various surface coating are used for home appliances, automobile, communication equipment, architecture, chemical plants, marine vessels, bridges, and ground embedded objects.

Most metal coatings are "plating". They are either "hot dip plating" where metal objects to be plated are dipped in hot melted coating metal, or "electrolytic plating" where electrical current is applied on the subject metal as an anode in a electrolyte solution containing plating metal ions.

In addition to the plating mentioned above, there are others where melted metal is sprayed onto metal surfaces as Thermal Spraying, and Cladding where different metals are welded, explosion welded, or rolled together.

Normally, industrially processed plating has pin holes and scratches occurring from usage, and the metal is exposed at these locations. The behaviors of corrosions occurring would differ whether the plating metal is either more noble than the base metal or not. Nickel, silver, copper and chromium are more noble than steel, and cadmium and zinc are less noble than steel. Aluminum and tin can be either depending on the environments.

With the pin holes on noble metal coat film as shown in [Fig.1], a battery is formed with the base steel being the negative pole, and corrosion will occur.

With this type of plating, the pin holes cannot be completely eliminated although the thickest possible film is applied to reduce the porosity. Therefore the usage is limited to ornamental decorating purposes in relatively mild corrosive environments, and not for corrosion protection purposes. Chrome plating is often used as noble metal plating but is not at all effective in environments with water due to the cracks in its film. For office equipment and automotive applications, chrome plating over copper or nickel base plating is often used to improve wear resistance characteristics. An automotive bumper plating example is shown in [Fig.2].

Painting has been in long use for isolating metal surfaces from various corrosive environments. Painting here means to coat metal surfaces with paint and create layers to protect the metal surfaces from corrosive environments.

Painting is applied to virtually everything from ground structures such as bridges, buildings, towers, chemical plants, and marine structures such as shore protections, off-shore structures, as well as various vehicles and home appliances. It is said that 60% of all corrosion protection costs are spent on painting.

Paints are comprised of mixtures of insoluble pigment particles suspended in "vehicle" components that form the coating layers. Generally, solvents are added to the former to give fluidity. This is applied on metal surfaces and let naturally or heat evaporate the solvents for the coating layer to form by chemical reactions.

Natural oils such as linseed and tung oils are customarily used as the "vehicle'. These belong in the drying oil category where oxidation and polymerization occur and harden when exposed to air.

The pigments added to the vehicle are: titanium oxide for white, carbon black for black, and Prussian blue for blue coloring. For the adjustments of glossiness, strength, and volume of the coatings, barium sulfate, calcium carbonate, clay, and diatomaceous earth are used.

The most important Corrosion protect element is the Corrosion protect pigment. Lead tetroxide Pb3O4 and zinc chromate ZnCrO4 are historically being used. They function as corrosion inhibitors by dissolving into water that passes through the coating layer.
The important trait for these pigments would be the dissolvability in water. If the dissolvability is insufficiently low the corrosion protection would be ineffective, and if the dissolvability is excessively high the premature runoff would occur.

Oil based corrosion protection paints have been in wide use. These are comprised of lead tetroxide added to natural vehicles such as linseed oil, and were commonly used due to good economy offered. But recent advancements in high polymerization technology resulted in more use of excellent synthetic resin based paints. The current mainstream choice are phthalic acid, chlorinated rubber, urethane, epoxy, and vinyl chloride resins.

Paint coating method is the easiest and applicable to virtually all shapes and sizes and relatively lower in cost compared other methods. However, the Corrosion protection effectiveness can vary largely depending on the coating application techniques used. This means that it is relatively difficult to always maintain the quality level at a constant.

The quality variations may come from application specific coating layer formation processes, base metal preparation condition where the surfaces are more prone to result in rusts as well as some conditions that may prohibit the coating from adhering perfectly. The pre-coating preparation largely affects the final outcome.