Coatings

COATINGS FOR CORROSION RESISTANCE

COATINGS TEMINOLOGY

Barrier Coatings- coatings which act as a barrier to the environment, for example epoxy coatings on reinforcing steel excluding moisture from contact with the metal surface. These coating are incapable of electrochemical reaction. Defects in these coating will lead to corrosion.

Conversion Coatings – coatings which react with the substrate to form stable, non-reactive products, for example chromate conversion coatings which establish iron, zinc, or aluminum chromates on the surfaces of these metals.

Sacrificial Coatings – coatings which dissolve preferentially to protect the underlying metals, fro example, zinc, aluminum or cadmium on steels, or zirconium nitride on 304 stainless steel.

Decorative Coatings – coatings which improve the appeal of a piece of metal., for example gold plating of brass, chrome plating of steel, titanium nitride on 304 stainless steel. The coatings are usually noble or cathodic to the underlying metal.

Reactive Coating – coatings which react to produce a secondary product which protects the underlying metal, but does not involve the metal in the reaction.

AUTOMOBILE COATINGS.

            Automobile steels are usually coated in several processes. In the first step a sacrificial coating consisting of a mixture of aluminum and zinc is placed on the steel by hot dipping. A second coating using a phosphate is applied to help paint adhesion. Phosphate based crystals less then 5mm in size are deposited to help the next layer adhere to the steel. An electrophoretic epoxy coating is then applied above the phosphate, followed by a primer layer which may be zinc rich and so provide some sacrificial protection. Above the primer layer a color coat is applied, followed by a clear coat.

CHROME PLATING.

            There are different types of chrome plates, decorative or soft chrome and hard chrome for wear resistance. The decorative chrome is a multiplayer process, often with a copper layer initially plated onto the component. The soft copper layer can be polished to leave a mirror finish, so any surface defects in the component such as small scratches or dings can be leveled out. The next layer is a nickel layer followed by the top chrome layer.  The component needs to be polished as a final operation. Chrome plate often contains cracks. One theory is that the chrome deposits in the hydride form which then transforms to the elemental chrome of a smaller volume. Tensile stresses build up from the volume difference and the plate cracks. One way around this is to “pulse plate” which greatlky reduces the number of cracks in the coating by  reinitiating the palte process with each pulse on a microscopic level.
Hard chrome is often applied directly onto steel and needs some protection such as oil. Hydraulic components are often hard chrome plated as it provides a good surface for seals and a good wear surface to face up to bushings.
ANODIZING.

Anodizing is an anodic process which utilizes the ability of a metal to passivate. Anodizing artificially increases the thickness of the passive layer and so imparts protection. Aluminum alloys are probably one of the best  known materials to anodize. A sulfuric or chromic acid bath is used for aluminum alloys. The component is the anode and a cathode is provided.  A passive layer builds up with time. The layer can be artificially colored through the use of dies in the baths. A sealing process is needed for aluminum alloys, which is either a boiling water dip or a chromic acid dip to seal pores in the passive coating. The structure of anodized aluminum is thought to consist of hollow hexagons. Titanium and its alloys can also be passivated with colors than depend on the thickness and type of oxide produced. In the mid seventies, passivated titanium jewelry was popular. 

CONVERSION COATINGS.

            Conversion coatings are  coatings applied by dipping the metal in a tank or spraying in which a chemical reacts with the surface of the metal to form a corrosion resistant surface. Usually, conversion coatings are very thin. The best example is chromate conversion coatings applied to aluminum alloys. In this process, a surface cleaning step, followed by a rinse precedes  deoxidation, then rinse,  followed by the conversion coating, rinse and then drying. This is a wet process, where the metal to be protected is dipped in successive tanks. The temperature, time and pH of the tanks should all be monitored to control the process and eventual quality.

ENVIRONMENTAL ASPECTS OF CHROMATES.

            It should be noted that there is a wide ranging effort to replace chromate conversion coatings, as there are strict limits on the personal exposure limits (PEL) and the DoD is trying very hard to replace environmentally unfriendly chromates. So far the effort has met with limited success and the search is still proceeding for a suitable replacement. Indeed, for many wet coatings environmental issues are a prime driver in new technology. The effluent from plating shops has to meet rigid standards to avoid polluting waterways such as rivers and estuaries. Today a considerable amount of capital investment is electroplating lines for chromium for example is expended in ensuring that the fluids used in the plating meet the necessary standards, both local and Federal.

CONDUCTIVE POLYMERS.

            These are polymers that conduct electricity. They have been found to provide corrosion resistance. They can be applied by the usual techniques such as spraying, dipping or spin coating. The usual polymers are polyaniline, polypyrole, and others.The techniques to produce conductive polymers is very specialized and more wok is required before the corrosion protection by these materials is fully understood.

RUST CONVERTERS.

            These materials react with an existing layer of rust to transform it to a stable compound on the surface that is also effective in excluding the environment. Early ones were based on tannic acid which formed tannates on the surface from rust.