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Surface Treatment Of Metals

Author : Date : 9/11/2012 3:58:55 AM

There is a wide variety of techniques for finishing the surfaces of metal products so that they resist corrosion, fit better and look better.Some products are treated by a sequence of several of these techniques. 

Before any of these techniques can be applied, the products must be thoroughly cleaned. A number of methods of cleaning are used, individually or in sequence. They include mechanical grinding, brushing and polishing (which produce metallic or oxidic dust-aluminium dust may be explosive), vapour degreasing, washing with organic grease solvents, “pickling” in concentrated acid or alkaline solutions and electrolytic degreasing. The last involves immersion in baths containing cyanide and concentrated alkali in which electrolytically formed hydrogen or oxygen remove the grease, resulting in “blank” metal surfaces that are free from oxides and grease. The cleaning is followed by adequate rinsing and drying of the product.

Proper design of the equipment and effective LEV will reduce some of the risk. Workers exposed to the hazard of splashes must be provided with protective goggles or eye shields and protective gloves, aprons and clothing. Showers and eyewash fountains should be nearby and in good working order, and splashes and spills should be washed away promptly. With electrolytic equipment, the gloves and shoes must be non-conducting, and other standard electrical precautions, such as the installation of ground fault circuit interrupters and lockout/tagout procedures should be followed. 


Treatment Processes


Electrolytic polishing

Electrolytic polishing is used to produce a surface of improved appearance and reflectivity, to remove excess metal to accurately fit the required dimensions and to prepare the surface for inspection for imperfections. The process involves preferential anodic dissolution of high spots on the surface after vapour degreasing and hot alkaline cleaning. Acids are frequently used as the electrolyte solutions; accordingly, adequate rinsing is required afterwards. 


Electroplating is a chemical or electrochemical process for applying a metallic layer to the product-for example, nickel to protect against corrosion, hard chromium to improve the surface properties or silver and gold to beautify it. Occasionally, non-metallic materials are used. The product, wired as the cathode, and an anode of the metal to be deposited are immersed in an electrolyte solution (which can be acidic, alkaline or alkaline with cyanide salts and complexes) and connected externally to a source of direct current. The positively charged cations of the metallic anode migrate to the cathode, where they are reduced to the metal and deposited as a thin layer (see figure 82.6). The process is continued until the new coating reaches the desired thickness, and the product is then washed, dried and polished. 


In electroforming, a process closely related to electroplating, objects moulded of, for example, plaster or plastic are made conductive by the application of graphite and then are connected as the cathode so that the metal is deposited on them.


In anodization, a process that has become increasingly important in recent years, products of aluminium (titanium and other metals are also used) are connected as the anode and immersed in dilute sulphuric acid. However, instead of the formation of positive aluminium ions and migrating for deposition on the cathode, they are oxidized by the oxygen atoms arising at the anode and become bound to it as an oxide layer. This oxide layer is partially dissolved by the sulphuric acid solution, making the surface layer porous. Subsequently, coloured or light-sensitive materials can be deposited in these pores, as in the fabrication of nameplates, for example. 




Surface treatment of metals involves a multiplicity of processes entailing a broad range of potentially toxic exposures, most of which can be prevented or controlled by the diligent application of well-recognized preventive measures.