Operations – surface treatment of metals


Most of the large scale engineering companies would like to include surface treatment of metals as an in house operation because of the stringent quality demand the part or component may be requiring. Getting the same from outside sources may not guarantee the requisite standards. An example of this is Zinc chromate coating treatment on switchgear components, painting on certain key electrical equipment boxes in Larsen & Toubro which are taken care as an in house process. Heat treatment process is similarly taken care at Bharat Forge and at most of the automobile manufacturers. Such requirements are a must technically which form the most important factor in setting up the operations of a plant where surface / heat treatment of high quality is required. Therefore the top operations management group planning the operations must carefully consider proper planning of these aspects. In this article we have dealt in brief various surface treatment processes because of their importance in operations.

The ordinary metals and alloys which are used for industrial, domestic and other purposes, so readily corrode or tarnish when exposed to the atmosphere, that , it is necessary to treat the surface is some way that will enable the metal to resist corrosion or tarnishing for the longest possible period. Coating is used on most metal products, either for protective or decorative purposes or for both.

The key to successful surface finishing is to clean the surface first to ensure good adhesion of a coating given either by electroplating process or by painting process or by plastic coating. The various surface treatment processes are:

Cleaning of Metals

Cleaning is done before and after finishing operations. Cleaning is done to remove dirt, oil, oxides, rust and scales.

Chemical cleaning is the most popular method of cleaning. Various methods used are alkaline pickling, acid pickling, solvent cleaning, electrolyte cleaning, degreasing and ultrasonic cleaning.

Mechanical cleaning methods are used for removing rust and scales from forgings, castings, welded joints and heat treated parts. The methods used are abrasive blast cleaning, tumbling, barrel rolling, power brushing and mechanical polishing or buffing.

Finishing of Metals

The main methods of finishing metals are:

1. Metallic coatings.
2. Plastic coatings.
3. Organic finishes
4. Inorganic finishes.

Metallic Coatings:

They are applied by the process of electroplating, hot dipping such as galvanizing, colorizing, phosphate coating, anodizing and spraying of molten metals.

Metallic coating is used to provide a decorative finish, protection, against corrosion and resistance to wear. They also serve as base for painting and to provide thermally and electrically conductive surfaces.

1. Electroplating may be described as a process of covering a surface or object(usually metallic) with a thin adherent coating of the same or another metal by electrolysis process.

2. Hot dipping is a rapid, inexpensive process which forms a coating of corrosion-resistant metals on base metals by dipping in molten bath. The various kinds of coating given by hot dipping process are zinc coating on iron and steel and also tin, lead and aluminum the coatings on steel, copper and cast iron.

3. Galvanizing is a commercial term used for the process by which zinc coating is produced on iron or low-carbon steel by immersion in molten bath of zinc.

4. Tin coating is usually applied to sheet steel by hot dipping or by the electroplating process.

5. Calorizing is the commercial name for the cementation of metal surfaces with aluminum. This process is used to protect iron from oxidation at elevated temperatures.

6. Phosphate coating or Parkerizing is a process for coating the base metal with a thin phosphate coating, which serves as a base or primer before painting or enameling process on iron and steel.

7. Anodizing is an electrochemical process which improves the appearances and provides an anti-corrosion protection to parts made of aluminum.

8. Metal spraying or metallizing means to treat with, especially to coat with a metal, to impregnate with a metal or a metal compound. Metallizing is the term used to designate the process of metal spraying. Metallizing includes the preparation of the base metal, the spraying of the molten metal on the base metal and the finishing of the coated surface by grinding.

Plastic Coatings

Metal surfaces can be coated with plastic to protect them from corrosion. Chemical plant equipments, electroplating tanks, pipelines, valve bodies which are exposed to corrosive environment are coated with plastic. Normally used plastic coatings are PVC, Nylon, Polythene, Polytetra fluro ethylene (PTFE)

Organic Finishes

Organic finishing consists of coating a surface with a continuous film of an organic film-forming material such as paints, enamels, varnishes, lacquer, etc.

Inorganic finishes

Inorganic coatings are made up of refractory compounds. They include porcelain enamels and ceramic coatings, composed of inorganic mineral materials which are fused to these metals. They can be applied to both ferrous and non-ferrous surfaces.

Any of the processes or in combination may have to be installed in large organizations where the product demands precision quality and the operations management team cannot overlook the metal treatment operations. Large factory areas are allocated exclusively for surface treatment shops.