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Surface Protection

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Steel-finishing deals with the application of protective treatments on the surface of steel items.

That is needed for avoiding atmospheric corrosion attack.

Think on how rusty your car would look without its paint system.

This branch of technology deals also with plating processes applied for aesthetic reasons or for other purposes.

Whatever treatment is selected, it must be preceded by thorough removal of contaminants likely to hinder further processing.

This is done by degreasing and cleaning to eliminate from the surfaces all traces of extraneous materials to assure successful application of required finishes.

The information covering this kind of preparation is the subject of other pages. See:

Steel-finishing is needed because usual mild or low alloy steels are readily rusting when exposed to the atmosphere, especially in presence of moisture.

For getting an idea of the measure and the success of protective processes one has only to think of the millions of cars traveling for tens of years around the world without rusting. Steel moving parts are usually dipped in oil or covered with grease that, besides lubricating, prevents the formation of corrosion or rust.

During fabrication, storage or shipping, steel surfaces may be protected with rust-preventive removable compounds made of various materials with additives and inhibitors and with variable degree of protection.

Surface Protection by Steel-finishing

The application of paints is generally performed on steel surfaces chemically treated.

Among the most used nonmetallic coating processes are phosphate coatings.

The steel surface is converted, by reaction with phosphoric acid, to an integral mildly protective layer of insoluble crystalline phosphate.

Similarly chromate conversion coatings (from reaction with chromic acid) provide some protection when applied on steel either bare or cadmium or zinc plated.

Painting for Steel-finishing means applying a thin organic coating on suitably prepared surfaces, for protection or decoration.

Paints are available in a wide range of pigments and media with large variation of characteristics and properties.

Enamels and lacquers are different classes of protective coatings for specific purposes.

Electroplating is a special type of Steel-finishing where electro deposits of various metals are applied on the surface, for a variety of purposes.

Dissolved metal ions in an electrolyte solution are moved by an electric field and are reduced at the interface such that they attach onto the steel parts connected to the cathode.

The cathode is that part of the electric circuit connected to the negative terminal of a direct current power supply.

Cadmium and zinc are used for protecting from atmospheric corrosion.

In recent years rules for environment protection imposed severe restrictions on the use and disposition of chemical solutions for Steel-finishing, considered a threat to the environment.

Steel parts submitted to Cadmium electroplating (and possibly to other metals too) should be thoroughly stress relieved before plating.

For hardened steels with hardness equal to Rockwell C 35 or higher, special procedures must be adopted.

These include a long baking period (from 3 and 24 hours) at a temperature between 175 to 200 °C (350 to 400 °F) after plating.

This treatment is intended to allow diffusion and dispersion of hydrogen atoms trapped in the plated layers.

Without it, there would be danger of Hydrogen Embrittlement of the hardened steel parts.

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Copper plating is used as first layer for other metal plating (like nickel or chrome), as a stop-off in thermal diffusion treatments (carburizing), or for decorative purposes if over-coated with a clear lacquer against tarnishing and staining.

Nickel plating, with or without underlying copper strike, is one of the oldest decorative Steel-finishing treatments.

The distribution of the nickel layer upon the substrate is strictly dependent on the density of current flowing from the cathodic surface.

At protruding areas current is higher, with consequent accumulation of excess metal, unless special provisions are put in place.

To avoid the problem of non uniform metal deposition, special non electrolytic nickel plating methods were developed, called also electroless nickel plating.

Those use an auto-catalytic reaction and provide a deposit of uniform thickness regardless of part geometry.

Two types of Steel-finishing chromium plating are in use: the decorative type, and the hard chromium type.

Because the chrome plating chemicals are very toxic, the disposal of chemical effluents is regulated almost everywhere.

For environmental protection, severe limitative requirements are now in place in the use of compounds (mainly hexavalent chromium) from the process.

Much work was dedicated to develop suitable alternatives to hard chromium plating.

Other metals like tin, lead, silver and gold can be electroplated for Steel-finishing, for decorative and other purposes.

Different coating processes include hot dipping in molten aluminum or zinc, pack diffusion, slurry process, metal spray and more.

Hot dip galvanizing of steel parts is done in molten zinc and in baths containing also other elements.

Tin is also coated by immersion or by wipe tinning.

Lead was applied in the past but is no longer approved for current use because of poisoning concerns.

Porcelain enameling and ceramic coating are additional types of Steel-finishing used in the past for a variety of household objects.

They are nowadays largely supplanted by plastic materials.

Steel-finishing instructions are an essential part of engineering drawings.

If special requirements must be observed during fabrication, they must be spelled out clearly and adequately in manufacturing documents.

Plating, Cadmium
SAE International / 01-Jul-2007 /

Plating, Nickel, General Purpose
SAE International / 18-Oct-2004 /

Plating, Electroless Nickel
SAE International / 15-Dec-2008 /

Plating, Chromium Hard Deposit
SAE International / 27-Nov-2012

Plating, Silver, Nickel Strike, High Bake
SAE International / 19-Apr-2010 /

Plating, Silver for High Temperature Applications
SAE International / 01-Feb-2008 /

Plating, Silver Copper Strike, Low Bake
SAE International / 18-Dec-2009 /

Plating, Copper
SAE International / 18-Feb-2011

Plating, Nickel Low-Stressed Deposit
SAE International / 19-Apr-2010 /

SAE AMS 2460
Plating, Chromium
SAE International / 01-Jul-2007 /

Coating, Black Oxide
SAE International / 15-Jan-2008 /

The following Video is presented as an illustration. No endorsement or recommendation are intended to the specific commercial company.

Watch the following Video on

Chrome Plating Process

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