A. Anodized Coatings
Anodizing is a process that is opposite to electroplating in that the part being processed is made anodic (hence, its name). The process works by anodic oxidation of the base metal. It is used mostly for aluminum, but also to some extent on magnesium and titanium.
A typical example, and the one in widest commercial use, is the anodizing of aluminum. Aluminum by itself will corrode in air, forming extensive white corrosion products. Anodized aluminum has a homogeneous, tightly adherent, and dense film that protects the underlayer of aluminum from corrosion.
A typical anodic coating is somewhat porous. It may be sealed by hot water treatment to augment corrosion protection. The building industry uses anodized aluminum almost exclusively for door and window frames on residential and industrial buildings. The porous anodic film is frequently dyed to match other trim. This dyed film is then finally sealed to provide the augmented corrosion resistance noted above.
B. Black Oxide Coatings
Black oxide coatings are used for ferrous parts. They are applied by simple immersion; that is, no electrolyte of the part is required. The solutions used are highly alkaline and usually contain nitrates or nitrites. The black oxide coating in itself is not a good corrosion barrier; it is somewhat thin and porous. The coating is frequently sealed with an oil or wax in order to extend the life of the parts. Black oxide is commonly used on non- critical areas of machine, tool, and optical parts.
C. Chromate Coatings
Chromating is a surface conversion process that is used on aluminum, brass, cadmium, copper, and zinc. It provides increased corrosion resistance to the surface of these metals. The chromate coating process is usually non-electrolytic (although one anodizing process needs a chromate bath) and occurs by simple immersion of the part in a solution of chromic acid and other salts. The chromate film is usually the last step in the plating process. This film can also provide color to the plated metal. Available chromate colors are clear, blue, yellow, green, brown, gold, and iridescent.
D. Electroless Plating
It is possible to deposit a metal on a substrate without the use of an electric current. The most common method is by a process known as electroless plating. In electroless plating the metal is reduced to the metallic state and deposited onto the substrate by the use of a reducing agent, such as sodium hypophosphite. Deposits of 0.0040 in. or greater may be obtained by this method. It should be recognized that the deposited metal contains an appreciable amount of phosphorus and is thereby different from metal deposited by a mirror-making process.
The primary advantage of electroless plating is that the deposit is not dependent upon current density, polarization, or throwing power. An electroless coating is uniform in thickness and composition without the limitations that apply to electroplating. A secondary advantage is the stability of the bath, which does not deposit metal for long periods of time unless a receptive substrate is present. This is also in contradistinction to a silvering or mirror-making process.
The most common metal plated electrolessly is nickel. Electroless nickel is used on dies, molds, valves, and other parts where a corrosion-resistant and good wearing surface is required. Although electroless nickel possesses similar properties to hard chrome, it is not to be confused with it for severe wear applications.
Electroless plating is also used as the first metallic layer for nonconductors. Plastic parts have become an important part of engineering design. A thin layer of either copper or nickel is electrolessly plated prior to normal electroplating. This makes the plastic surface electrically conductive so that the desired thickness of plate can be achieved by electroplating.
Electropolishing is a method used to remove small burrs, surface scratches, and imperfections, as well as to brighten a metal surface. Such metals as copper, steel, and stainless steel are frequently electropolished to improve their surface characteristics. Electropolishing is much less expensive than manual polishing, and it should be remembered that manual methods will remove more metal, but mayor may not provide a better surface. The depth and nature of the undesired imperfections will decide which process should be used.
Eletropolishing is done in an acidic bath and has the opposite polarity of electroplating. In electropolishing the parts being treated are made anodic. A very small amount of metal is removed, mainly the micro high current density peaks of scratches, thus improving the smoothness and brightness of the metal part.
Eletropolishing is an optimum way to pretreat parts prior to electroplating to achieve a superior finish. Usually, however, electropolishing is used as the final finish on a non-plated base metal. Used prior to electroplating it may add to the total cost to such an extent to become prohibitive.
Galvanizing is a hot-dip method of applying a coating of zinc onto ferrous substrates. The primary reason for a galvanized coating is for corrosion protection as the zinc provides a sacrificial surface to the ferrous base.
Items such as nails, reinforcing rods, anchor bolts, pipe and pipe fittings, refuse containers, and guardrails are typically galvanized. These parts require only the protection of zinc and not the additional luster that zinc electroplating can provide.
As with electroplating, any part to be galvanized must first be cleaned and activated. Galvanizing is accomplished by dipping the parts into zinc metal. At the high temperature used, three distinct zinc layers tend to form. These provide the base metal protection.
G. Phosphate Coatings
Phosphating is a process of producing an immersion coating on ferrous parts. It is done in a solution of phosphoric acid and metallic compounds at elevated temperatures. The two main types of coatings are zinc phosphate and magnesium phosphate, although iron phosphate is also used.
The phosphate coating is somewhat corrosion resistant and porous and granular in structure. It is almost always sealed with a wax or an oil to provide atmospheric protection. Due to its porous nature phosphating is also frequently used to produce a tough adherent base for painting. Phosphating is usually applied either by spray or by dipping onto articles, such as nuts, bolts, barrels, and other miscellaneous hardware.