SURFACE TREATMENT (CLEANING AND FINISHING)
Introduction:
A metal form may have been produced by machining, casting, forging, and other methods. These processes produce different surfaces and one or more subsequent operation is generally required to produce a desired surface. This is what is called “surface treatment (one of the process is cleaning and finishing and second another one is coating of metals). Cleaning operation is performed both preparatory to finishing operations and after finishing operations. In general, for a manufacturing product, surface finishes are used for the purpose of providing a decorative look and appealing appearance to the product surface, providing the desired colour to the surface, altering the surface properties according to the requirement, cleaning the surface of the product and protect the surface against environment effects,
For a metal surface, the cleaning and finishing is done for the following purposes
➢ To remove metal chips and cutting compounds left behind during machining operations
➢ To remove all oily and greasy substance
➢ To remove burrs and scratches produced during previous operations
➢ To remove dirt rust and similar other ill effects which are likely to hinder the adherence of the surface coating to the applied.
➢ To remove buffing and polishing compounds left over during the operations
➢ To clear the surface of any other foreign material or contaminant sticking to the metal surface
➢ To provide better appearance to the surface
➢ To make the surface suitable for taking a uniform and continuous layer of coating so as to ensure the better durability.
Selection of a Cleaning Process:
To achieve a good result after cleaning, the selection of a cleaning process is very important. If a surface is rusted heavily, a mere brushing may not be sufficient instead chemical cleaning may be an adequate method. The following considerations nay be thought of before deciding the type of cleaning operations
➢ Type and composition of the metal
➢ Type and amount of contaminant sticking to the surface
➢ The thickness of the layer of rust or scale
➢ The degree of surface cleanliness required
➢ Permissible lose of metal from the surface
➢ Shape and size of the component to be cleaned
➢ Specific tolerance for the surface finish
➢ Availability of cleaning equipments
➢ Quantum of production and cost considerations.
Depending on the above, cleaning process may be classified as
➢ Mechanical cleaning process
➢ Chemical cleaning process
➢ Ultrasonic cleaning process
➢ Flame cleaning process.
➢ Type and composition of the metal
➢ Type and amount of contaminant sticking to the surface
➢ The thickness of the layer of rust or scale
➢ The degree of surface cleanliness required
➢ Permissible lose of metal from the surface
➢ Shape and size of the component to be cleaned
➢ Specific tolerance for the surface finish
➢ Availability of cleaning equipments
➢ Quantum of production and cost considerations.
Depending on the above, cleaning process may be classified as
➢ Mechanical cleaning process
➢ Chemical cleaning process
➢ Ultrasonic cleaning process
➢ Flame cleaning process.
Mechanical cleaning process:
Mechanical cleaning process is performed by mechanical means with and without the use of any chemical cleaner primarily for the purpose of removing all classes of rust and scale from mill products, forging, castings, weldments, and heat treated parts. Scale is usually produced on a part when the metal is heated to elevated temperatures. Mechanical cleaning may also be used for decorative purposes as in polishing. The methods includes:
1. Abrasive blast cleaning
2. Tumbling and barrel finishing
3. Vibratory finishing
4. Wire brushing
5. Belt sanding
6. Buffing and polishing
2. Tumbling and barrel finishing
3. Vibratory finishing
4. Wire brushing
5. Belt sanding
6. Buffing and polishing
1) Abrasive Blast Cleaning:
This method is widely used for removing all classes of scales and rust from forging, casting, and heat-treated parts. Depending on the finish requirement, blasting may be the sole means of scale removal or it may be used to remove the major portion of scale, with pickling employed to remove the remainder. In this process, the parts are generally cleaned by the use of abrasive particles such as sand, steel grit, steel shots, impelled against the surface to be cleaned.
Some cleaning is performed by high velocity air blast, with the blast directed by hand. In many cases, an air less blast machine that cleans by impact is also used. The abrasive is fed from an overhead storage hopper to the centre of the radially rotating wheel, whereupon the metallic shot or grit is hurled in a controlled steam upon the work to be cleaned. All traces of sand , scales, oxides, and other material are removed right down to the virgin metal, providing an excellent surface for bonding final finishes.
The airless blast machine is used for cleaning engine blocks, crankshafts, castings of different shapes and size, railroad cars, car wheels, oil and gas pipes, steel strips and many other purposes.
Some cleaning is performed by high velocity air blast, with the blast directed by hand. In many cases, an air less blast machine that cleans by impact is also used. The abrasive is fed from an overhead storage hopper to the centre of the radially rotating wheel, whereupon the metallic shot or grit is hurled in a controlled steam upon the work to be cleaned. All traces of sand , scales, oxides, and other material are removed right down to the virgin metal, providing an excellent surface for bonding final finishes.
The airless blast machine is used for cleaning engine blocks, crankshafts, castings of different shapes and size, railroad cars, car wheels, oil and gas pipes, steel strips and many other purposes.
2) Tumbling and Barrel Finishing:
Tumbling, often, is the least expensive process for removing rust and scale from the metal parts. Parts configuration and size are the primary limitations of the process. Tumbling in dry abrasive (deburring compounds) is much effective for removing rust and scales from small parts of simple shape. However, tumbling
cannot descale parts of complex shape with deep recess and other irregularities uniformly. It may require several hours of tumbling, if the method is used. The addition of descaling compounds instead of deburring compounds will often decrease the tumbling time by 75%.
The operation is accomplished by placing work pieces in a drum or barrel, together with stars, jacks, and slugs or abrasive materials. The abrasive material can be sand, granite chips, and slag or aluminum oxide pallets. In operation, The barrel is rotated and the movement of the work pieces and the accompanying slugs or abrasive materials against each other produces by friction a fine cutting action which remove the fins, flashes, and scale from the products.
Barrel rolling and tumbling are similar operations, except that the barrel is loaded only 40 to 60% capacity, while in tumbling a drum is generally packed nearly full. Abrasive, such as cinders, slag, granite chips, sharp sand, are placed in the barrel with the work pieces along with water or dilute acid solution. Sometimes, mineral or scrap punching are added to the wet rolling. Hard wood dust or leather scrap are used in dry rolling to keep the work pieces separated. As the barrel turns, the mass rolls over and falls to the bottom of the barrel. This motion cuts down the surface of the parts.
cannot descale parts of complex shape with deep recess and other irregularities uniformly. It may require several hours of tumbling, if the method is used. The addition of descaling compounds instead of deburring compounds will often decrease the tumbling time by 75%.
The operation is accomplished by placing work pieces in a drum or barrel, together with stars, jacks, and slugs or abrasive materials. The abrasive material can be sand, granite chips, and slag or aluminum oxide pallets. In operation, The barrel is rotated and the movement of the work pieces and the accompanying slugs or abrasive materials against each other produces by friction a fine cutting action which remove the fins, flashes, and scale from the products.
Barrel rolling and tumbling are similar operations, except that the barrel is loaded only 40 to 60% capacity, while in tumbling a drum is generally packed nearly full. Abrasive, such as cinders, slag, granite chips, sharp sand, are placed in the barrel with the work pieces along with water or dilute acid solution. Sometimes, mineral or scrap punching are added to the wet rolling. Hard wood dust or leather scrap are used in dry rolling to keep the work pieces separated. As the barrel turns, the mass rolls over and falls to the bottom of the barrel. This motion cuts down the surface of the parts.
3) Vibratory Cleaning:
Similar like barrel rolling but instead of rolling, it is vibrated. . The operation is accomplished by placing work pieces in a drum or barrel, together with stars, jacks, slugs or abrasive materials. The abrasive material can be sand, granite chips, and slag or aluminum oxide pallets. In operation, the tub is vibrated in the range of 900 to 3600 cycles per minute. The aptitude of vibration and the frequency are governed by the size and shape of the part, material and the abrasive media. This operation is faster and less noise while comparing barrel rolling. Stage inspection can be carried out during the operation. This operation can be automated easily.
4) Wire Brushing:
Brushing is the least used for descaling parts, although it is satisfactory for removing light rust or loosely adhering scale. It is better suited for work pieces formed from tubing than for casting and forging. Manual brushing is used for removing sand from casting and removing light rust, removing loosely adhered scale from forging and for cleaning scale from weldment. Power brushing is used not only to remove unwanted material but also produces a finely finished surface. These methods are finding additional use as production tools for metal finishing operations. A major advantage of power brushing is that it can be used manual, semiautomatic or integrated methods.
Various types of brushes are used for removing unwanted materials. These include wire, fibre cord etc. Wire brushes are the best for general purposes jobs and such material as steel, stainless steel, nickel alloys, brasses, bronze and copper. Cord brushes are used with abrasive compounds for polishing, finishing and deburring metal parts. Usually the machine is of rotary type. The size range from 25mm to 500 mm in diameter with varying widths wheels to suit different shapes and size of work. The common surface speeds used vary from 1500 to 2000meter per minute.
5) Belt Sanding:
Abrasive coated belts are now widely used in modern metal industry. The belts used for polishing are coated with resins and hide glues followed by either emery, flint, garnet, aluminium oxides or silicon carbides. The advantage of using abrasive belt is the quick change that is possible from a used belt to a new one or from one grade to another. The ability of the belt to remove stock rapidly and to produce fine finishes, plus its ability to produce a fast and extreme cool cut, has led to the rapid development of coated abrasive machines. Although a very fine smooth surface is obtained, depending up on the grit and abrasive, minor scratches do remain on the surface. Another constrain is that it cannot be used for sanding deep interiors or recesses.
6) Buffing and Polishing:
THE 3/4HP BUFFING MACHINE
THE SISAL BUFFING WHEEL
The function of buffing is to produce a smooth, uniform surface with high brilliant lusture where as Polishing is usually undertaken to make metal smoother or to produce a more uniform surface. Semi automatic machines are mostly used for doing this job. These machines carry a series of polishing and buffing wheels, which can be adjusted to different positions so that all surfaces of the part can either be polished or buffed as required. The compounds and wheels selected are governed by shape of the part, material of which it is fabricated, and the appearance of the product. For ordinary polishing and buffing operations, polishing and buffing wheels are mounted on floor polishing lathes.
Commonly used polishing wheels are constructed of canvas, muslin, felt, linen or wool discs. Polishing and buffing compounds like wheels are usually divided in to two broad categories.Cutting down or removing of scratches and grains lines from previous operations
Colouring which gives the product the final bright deep lusture.Buffing compounds can either be greaseless or have a grease base. A mixture of glue base, a softening agent, and a mineral make up a greaseless compound. Grease buffing compounds use oil, tallow, and other bonds. Many abrasive compounds are used for colouring compounds, such as red rouge, crocus and white colouring compounds.
B) Chemical Cleaning:
The followings are the different types of chemical cleaning
1. Solvent cleaning
2. Alkaline cleaning
3. Acid cleaning (pickling)
4. Vapour degreasing
5. Electrolytic cleaning
They are generally used to ensure clean parts and surface before the finished is applied. They may be used individually or in combination, depending upon the material to be cleaned and the effects desired.
1. Solvent cleaning
2. Alkaline cleaning
3. Acid cleaning (pickling)
4. Vapour degreasing
5. Electrolytic cleaning
They are generally used to ensure clean parts and surface before the finished is applied. They may be used individually or in combination, depending upon the material to be cleaned and the effects desired.
1) Solvent cleaning:
Solvent cleaning is used on such metals as zinc, lead, and aluminium since their surfaces might be attacked by alkaline cleaners. This is primarily designed to remove oil, grease, fats, and dirt from surface for coating or electroplating. There are two types of solvents. Straight solvent and emulsifyable compounds. The straight solvents are petroleum or coal derivatives. Emulsifyable compounds are made by mixing an organic solvent with an emulsifying agent, which is soluble in water. The agent may be soap or kerosene mixture. For cleaning, parts are first wiped with a brush to remove the soil if any and then dipped in a tank containing solvent cleaner and rinsed. Then remove from the tank then wiped dry with a cloth. Work pieces with heavy deposits of oil and grease are soaked in this solution or solution is swabbed into heavy contaminated areas. They are then rinsed in hot water, preferably by means of pressure spray permitting them to be flushed away.Parts to be electroplated should afterwards be treated with alkaline cleaner to remove any organic material.
2) Alkaline cleaning:
The most common type of cleaning is with alkali. It is efficient and economical in removing oil and grease. This method cannot be used for removing rust and scale effectively. Some of the alkaline agents are sodium metasilicate, caustic soda, sodium carbonate, sodium chromate, sodium phosphate. In this process, a bath is prepared from cleaning agents. These materials are added to some type of soap to aid in emulsification. The mixture produces an alkali, which serves as the cleaning agent. The parts to be cleaned is suspended in to the hot bath. Bath is heated by an electric circuit. After cleaning, parts are thoroughly rinsed to clean off completely from any cleaning agent remaining on the surface. The process is used on all metals except zinc, lead, tin, brass and aluminium. On assemblies comprised of dissimilar metals, the presence of alkaline solution in crevices may result in galvanic corrosion, and even a trace of alkali will contaminate paint and phosphate coating.
3) Acid Pickling:
The most common method of removing unwanted pigmented compounds, which are mostly oxides of metal, is by acid pickling. Either diluted sulphuric acid, hydrochloric acid or phosphoric acid is sprayed on the part, or the part is dipped in to a tank, agitated, and then washed and rinsed thoroughly. Alkaline cleaning of the part should be used first to remove all dirt and oils in order to obtain an even removal of the oxides during the pickling process. Sometimes it is necessary to add pickling inhibitors such as detergents, liquid glycol, ether etc to decrease the action of the acid upon the metal particularly aluminium or other non-ferrous metals.
Both hot and cold baths are used. A bath made out of 10% Sulphuric acid with water is the most common solution. Pickling operation can be accelerated by heating the bath to about 700C-850C. A bath, containg 25% to 100% hydrochloric acid with water is best for steels and iron.
Both hot and cold baths are used. A bath made out of 10% Sulphuric acid with water is the most common solution. Pickling operation can be accelerated by heating the bath to about 700C-850C. A bath, containg 25% to 100% hydrochloric acid with water is best for steels and iron.
4) Vapour degreasing:
Vapour degreasing is an effective and widely used method of removing a wide variety of oil and grease. It develops a high degree of cleanliness. This is a modified version of solvent cleaning. Metal surfaces are subjected to the vapour of chemical instead of dipping.
In this process, a solution known as trichloroethylene is heated to its boiling point. A vapour is produced, and the parts to be cleaned are suspended in the vapour. The condensation of the vapour on the work removes all the oil and grease. A separate room is necessary due to pollution. This process is widely used for the materials that are prone to alkaline attack.
Vapour degreasing is particularly well adapted for cleaning oil-impregnated parts such as bearings and for removing solvents soluble oils from the interior of storage tanks.
In this process, a solution known as trichloroethylene is heated to its boiling point. A vapour is produced, and the parts to be cleaned are suspended in the vapour. The condensation of the vapour on the work removes all the oil and grease. A separate room is necessary due to pollution. This process is widely used for the materials that are prone to alkaline attack.
Vapour degreasing is particularly well adapted for cleaning oil-impregnated parts such as bearings and for removing solvents soluble oils from the interior of storage tanks.
This is effective as a final cleaning process for removing oil and grease from machined surfaces when extreme cleanliness is required. It is almost always used for final cleaning of steel parts prior to electroplating.
In this process, an alkaline cleaning solution is used with electric current passing through the bath in which the part to be cleaned is one electrode.. This causes the emission of oxygen at the positive pole and hydrogen at the negative pole. The material from which the part is made and the cleaning action desired determine whether the part should be made the anode or cathode. Parts from soft material like lead, zinc and tin must necessarily be cleaned cathodically because they would be badly etched if cleaned anodically. Anodic cleaning is receiving favour because if work piece is anode, oxygen will have evolved at its surface and the bubbles of oxygen formed on surface. While bursting these bubbles will have a rubbing effect thereby removing the dirt. To avoid explosion, no chloride is used and the sop content is kept very low. Most of the materials can be cleaned by this method.
In this process, an alkaline cleaning solution is used with electric current passing through the bath in which the part to be cleaned is one electrode.. This causes the emission of oxygen at the positive pole and hydrogen at the negative pole. The material from which the part is made and the cleaning action desired determine whether the part should be made the anode or cathode. Parts from soft material like lead, zinc and tin must necessarily be cleaned cathodically because they would be badly etched if cleaned anodically. Anodic cleaning is receiving favour because if work piece is anode, oxygen will have evolved at its surface and the bubbles of oxygen formed on surface. While bursting these bubbles will have a rubbing effect thereby removing the dirt. To avoid explosion, no chloride is used and the sop content is kept very low. Most of the materials can be cleaned by this method.
C) Ultrasonic cleaning:
When ultrasonic vibrations of sufficient power level are transmitted in liquid, cavitations takes place. This action in liquid has two effects: bulk, which refers to those effects within the liquid, and surface, which occurs between the liquid and the solid. Many organic compounds are broken down by cavitations. Thus the dirt and grease clinging to solid articles in an ultrasonic cleaning tank are ripped apart and emulsified.
Frequencies of approximately 30,000Hz are characteristic of ultrasonic cleaning. Typical fluids are water to which has been added a detergent or solvent such as cyclohexane and trichlorothylene.
Ultrasonic cleaning is, however, more expensive than other methods because of initial cost of the equipment, and higher maintenance cost. Typical applications in which ultrasonic methods have proved advantages are as follows
∙ Removal of tightly adhering or embedded particles from solid surfaces
∙ Removal of fine particles from powder metallurgy parts
∙ Cleaning of small precision parts, such as those for cameras, watches, microscope components
∙ Cleaning of parts made of precious metals
∙ Cleaning of parts with complex configuration
A typical ultrasonic cleaning facility is composed of a generator for producing a electric energy, a transducer for converting the electric impulses into high frequency sound waves, and a tank for holding the cleaning fluid into which the transducer transmit its sound energy.
Frequencies of approximately 30,000Hz are characteristic of ultrasonic cleaning. Typical fluids are water to which has been added a detergent or solvent such as cyclohexane and trichlorothylene.
Ultrasonic cleaning is, however, more expensive than other methods because of initial cost of the equipment, and higher maintenance cost. Typical applications in which ultrasonic methods have proved advantages are as follows
∙ Removal of tightly adhering or embedded particles from solid surfaces
∙ Removal of fine particles from powder metallurgy parts
∙ Cleaning of small precision parts, such as those for cameras, watches, microscope components
∙ Cleaning of parts made of precious metals
∙ Cleaning of parts with complex configuration
A typical ultrasonic cleaning facility is composed of a generator for producing a electric energy, a transducer for converting the electric impulses into high frequency sound waves, and a tank for holding the cleaning fluid into which the transducer transmit its sound energy.
Finishes of metals
The principal types of finishes applied to metal products are
1. Organic finishes
2. Metallic coating
3. Plastic coating
4. Inorganic finishes
5. Vacuum metallizing
The principal types of finishes applied to metal products are
1. Organic finishes
2. Metallic coating
3. Plastic coating
4. Inorganic finishes
5. Vacuum metallizing
1) Organic finishes:
Organic finishing consists in coating a surface with a continuous film of an organic film-forming material. This film may be applied for protecting the surface from corrosive influences, for enhancing the appearance, or a combination of both but it does not allow the holding of close dimensional tolerance and it has only average resistance to abrasion. Its resistance to elevated temperature is also poor. This family of organic coating includes paints, enamels, varnishes, lacquer, shellac, and rubber base coating, with vehicles of synthetic resins, rubber, linseed and tung oil.
Method of application:
Organic finishes are applied by brushing, spraying, electro coating, dipping and centrifugal process. In these methods, a prime coat is first applied, followed by a light sanding to form a surface for good cohesion.
Organic finishes are applied by brushing, spraying, electro coating, dipping and centrifugal process. In these methods, a prime coat is first applied, followed by a light sanding to form a surface for good cohesion.
a) Brushing: -
Brushing is performed by means of a brush made out of a combination of long and short animal hairs or fibre with their tips splitted into many parts, which is fixed on a wooden handle. Brushing requires the greatest amount of labour and the least amount of material. This is a slow and manual operation. For a good result, it require highly skilled labourer. This process is widely used for painting manually the steel structure, cabinets, furniture, steel doors and windows, grills and other fittings in building work, repair and maintenance work.
b) Spraying: -
Spraying with a spray gun is rapid and gives a smooth coat but is wasteful of material. While this method is used to a great extent for the industrial applications and does produce good result with skilled operator, it is a costly operation due to high labour cost today. The method consists of breaking up the liquid coating into a fine spray and throwing it on to the work surface with high velocity. The disadvantage of this process is that the paint stick on to the near by surfaces and waste of paint. The cost of spray painting by industry can be reduced considerable by the use of automated spray-painting equipment. It can be air-spraying, hydraulic-spraying, electrostatic-spraying, steam-spraying.
Spraying with a spray gun is rapid and gives a smooth coat but is wasteful of material. While this method is used to a great extent for the industrial applications and does produce good result with skilled operator, it is a costly operation due to high labour cost today. The method consists of breaking up the liquid coating into a fine spray and throwing it on to the work surface with high velocity. The disadvantage of this process is that the paint stick on to the near by surfaces and waste of paint. The cost of spray painting by industry can be reduced considerable by the use of automated spray-painting equipment. It can be air-spraying, hydraulic-spraying, electrostatic-spraying, steam-spraying.
c) Electro-coating: -
In electro-coating, the product to be painted is an electrode of a high-voltage circuit. A grid, which is positioned close to and between the spray nozzle and the part, is used as the other electrode. A dc current applied to the two electrode and the part is sprayed through the grid. The paint particles pick-up ions and become charged electrostatically.
d) Dip coating: -
One of the most economical methods of painting is dip coating. Dipping can be done by hand or automatically on conveyors. The part must drain easily and must not accumulate paint, which will make a wavy surface instead of producing a clean and even surfaces. Some of the advantages of this process are
∙ It is an economical method
∙ It can be used for any complex shapes
∙ It can be used for small parts without loss of paint
∙ It can be very well used for coating of primers and paints effectively
Some of the disadvantages are
Selective painting cannot be done as the parts are dipped in the paint
Very thin film of paint cannot be done
One of the most economical methods of painting is dip coating. Dipping can be done by hand or automatically on conveyors. The part must drain easily and must not accumulate paint, which will make a wavy surface instead of producing a clean and even surfaces. Some of the advantages of this process are
∙ It is an economical method
∙ It can be used for any complex shapes
∙ It can be used for small parts without loss of paint
∙ It can be very well used for coating of primers and paints effectively
Some of the disadvantages are
Selective painting cannot be done as the parts are dipped in the paint
Very thin film of paint cannot be done
d) Dip coating: -
One of the most economical methods of painting is dip coating. Dipping can be done by hand or automatically on conveyors. The part must drain easily and must not accumulate paint, which will make a wavy surface instead of producing a clean and even surfaces. Some of the advantages of this process are
∙ It is an economical method
∙ It can be used for any complex shapes
∙ It can be used for small parts without loss of paint
∙ It can be very well used for coating of primers and paints effectively
Some of the disadvantages are
Selective painting cannot be done as the parts are dipped in the paint
Very thin film of paint cannot be done
e) Centrifugal finishing: -
This process is primarily used for small parts. Such parts are placed in a strongly made wire basket, dipped and then put in the centrifuge and whirled. This throws the excess paint from the surface of the parts. The parts are finally removed and hung to dry.
2) Metallic coating:
Metallic coatings may generally be applied by electroplating, hot dipping such as galvanizing, calorizing, and phosphate coating or anodizing and spraying of molten metal. They are used to provide a decorative finish, protection against corrosion, and resistance to wear. They also serve as base for painting to provide reflectant surface and to provide a thermally or electrically conducive surface.
Metallic coatings may generally be applied by electroplating, hot dipping such as galvanizing, calorizing, and phosphate coating or anodizing and spraying of molten metal. They are used to provide a decorative finish, protection against corrosion, and resistance to wear. They also serve as base for painting to provide reflectant surface and to provide a thermally or electrically conducive surface.
a) Electroplating: -
Electroplating may be described as a process of covering a surface or object usually metallic with a thin adherent coating of the same or other metal by electrolysis. The form and details of the original part are retained.
Electroplating may be described as a process of covering a surface or object usually metallic with a thin adherent coating of the same or other metal by electrolysis. The form and details of the original part are retained.
Essential elements of plating process are the cathode, anode, electrolyte and direct current at low voltage. The article (work piece) is connected electrically to the cathode bar, and on the anode bars are suspended plates of metal to be plated. The solution itself contains dissolved salts of the metal to be plated (the coating metal). In order to increase the conductivity, other chemicals that will ionize highly are added (such as sulphuric acid to an acid copper plating bath). As the current is passed through the circuit, metallic ions migrate to the cathode and, upon losing their charge, are deposited as metal upon it. The current density largely determines the rate at which the metal is deposited. Plating metals are chromium, nickel, copper, zinc, cadmium, and tin. The precious metals, such as silver, gold, platinum and rhodium are also applied for plating. Chromium plating is widely used because of its pleasing appearance and its resistance to corrosion and wear. Aqueous solutions, called electrolyte, used in electroplating are designated as acid, neutral and alkaline baths. The acid bath is used extensively for low cost plating. The neutral bath primarily for plating of nickel, and the alkaline bath transmit a dense, fine grain deposit that may be made highly reflective and smooth.
b) Hot-dipping: -
This is a rapid, inexpensive process, which allows to form coating of corrosion-resistant metals into base metals by dipping in molten bath at less cost than by electroplating. The process is widely used for zinc coating on iron and steel. Tin, lead and aluminium coatings are applied to the base metals in addition to zinc. Base metals are restricted to the materials with higher melting temperature such as cast iron, steel and copper. The coating of sheet, wire and pipe is done on continuous basis whereas other shapes have to be immersed in the bath in batches.
This is a rapid, inexpensive process, which allows to form coating of corrosion-resistant metals into base metals by dipping in molten bath at less cost than by electroplating. The process is widely used for zinc coating on iron and steel. Tin, lead and aluminium coatings are applied to the base metals in addition to zinc. Base metals are restricted to the materials with higher melting temperature such as cast iron, steel and copper. The coating of sheet, wire and pipe is done on continuous basis whereas other shapes have to be immersed in the bath in batches.
3) Plastic coating: -
Metal surfaces that need protection from corrosion can be coated with plastics because of their excellent anti-corrosive characteristics for a wide variety of corrosive environments. Chemical plant equipments, tanks, pipelines, valve bodies are the common places where protection from corrosive environment is achieved by plastic coating the metal parts. Plastic coated metals also find wide application in cans for foodstuffs, water-treatment plants decorative metal furniture. In general, the coatings have good resistance to corrosion coupled with good adhesion, high resistance to abrasion and attractive appearance.
Metal surfaces that need protection from corrosion can be coated with plastics because of their excellent anti-corrosive characteristics for a wide variety of corrosive environments. Chemical plant equipments, tanks, pipelines, valve bodies are the common places where protection from corrosive environment is achieved by plastic coating the metal parts. Plastic coated metals also find wide application in cans for foodstuffs, water-treatment plants decorative metal furniture. In general, the coatings have good resistance to corrosion coupled with good adhesion, high resistance to abrasion and attractive appearance.
4) Inorganic coatings: -
Inorganic coatings are made up of refractory compounds. They include the porcelain enamels and ceramic coatings composed of inorganic mineral materials, which are fused to these metals. They may be readily applied to both ferrous and non-ferrous surfaces. These coatings provide excellent resistance to corrosion at elevated temperatures. They also provide good appearance and resistance to abrasion and maintain close tolerance.
Inorganic coatings are made up of refractory compounds. They include the porcelain enamels and ceramic coatings composed of inorganic mineral materials, which are fused to these metals. They may be readily applied to both ferrous and non-ferrous surfaces. These coatings provide excellent resistance to corrosion at elevated temperatures. They also provide good appearance and resistance to abrasion and maintain close tolerance.
5) Vacuum metallizing: -
Basically it is a process of vapour coating or vapour deposition. Any metal can be coated on a base material. This process is very effective for depositing a thin film of a metal compound. The commonly used coating metals are aluminium, chromium, silver, nickel, gold, platinum and germanium. The most base materials are different metals, ceramics, glass, plastics, paper and cloth. The parts to be metallized and the coating materials are placed in a high vacuum chamber. The coating material is heated electrically. During this process, the material vapourized and leaves the surface of the coating material in atomic form. These vapours strike the surface to be coated and condensed there. The parts are slowly rotated to expose the entire surface to vapour. The condensed metal vapours are deposited over the surface to form highly pure and extremely thin film of the coating material.
Basically it is a process of vapour coating or vapour deposition. Any metal can be coated on a base material. This process is very effective for depositing a thin film of a metal compound. The commonly used coating metals are aluminium, chromium, silver, nickel, gold, platinum and germanium. The most base materials are different metals, ceramics, glass, plastics, paper and cloth. The parts to be metallized and the coating materials are placed in a high vacuum chamber. The coating material is heated electrically. During this process, the material vapourized and leaves the surface of the coating material in atomic form. These vapours strike the surface to be coated and condensed there. The parts are slowly rotated to expose the entire surface to vapour. The condensed metal vapours are deposited over the surface to form highly pure and extremely thin film of the coating material.
Advantages:
∙ A very small amount of material is consumed as a very thin film is coated.
∙ Coating can be done on extremely costly material like gold, platinum etc
∙ Highly reflective and decorative surfaces can be produced on metals, plastics, textiles etc quite easily and cheaply.
∙ Can be effectively used for decorative coatings on items like household goods, souvenirs, gift items, and artificial jewellery
∙ For providing functionally effective coating for corrosion and thermal resistance.
∙ A very small amount of material is consumed as a very thin film is coated.
∙ Coating can be done on extremely costly material like gold, platinum etc
∙ Highly reflective and decorative surfaces can be produced on metals, plastics, textiles etc quite easily and cheaply.
∙ Can be effectively used for decorative coatings on items like household goods, souvenirs, gift items, and artificial jewellery
∙ For providing functionally effective coating for corrosion and thermal resistance.
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