Corrosion

Corrosion is the term used to describe the process of the surface of metal objects getting covered by oxides (or) other salts of the metal. It is basically defined as a natural process that causes the transformation of pure metals into undesirable substances when they react with substances like water or air. This reaction causes damage and disintegration of the metal starting from the portion of the metal exposed to the environment and spreading to the entire bulk of the metal.

Corrosion is usually an undesirable phenomenon since it negatively affects the desirable properties of the metal. For example, iron is known to have good tensile strength and rigidity (especially alloyed with a few other elements). However, when subjected to rusting, iron objects become brittle, flaky, and structurally unsound.

Factors Affecting Corrosion

·         Exposure of the metals to air containing gases like CO₂, SO₂, SO₃ etc.

·         Exposure of metals to moisture especially salt water

·         Presence of impurities like salt (eg. NaCl).

·         Temperature: An increase in temperature increases corrosion.

·         Presence of acid in the atmosphere: acids can easily accelerate the process of corrosion.

Types of corrosion

Corrosion occurs in many ways depending upon the attack of the metal, by the surrounding medium. There are two types of corrosion

·         Dry corrosion

·         Wet Corrosion

Dry corrosion

Dry corrosion occurs when metals come in direct contact with atmospheric gases like O_2, CO₂ etc. Dry corrosion is hence often referred to as direct chemical corrosion or atmospheric corrosion. The rate of corrosion is faster especially near industrial areas as atmospheric air contains more corrosive gases in industrial area. Dry corrosion is then further subdivided in

·         Oxidation corrosion

·         Corrosion due to other gases

Oxidation corrosion

Oxidation corrosion is a type of dry corrosion where corrosion takes place mainly because of presence of O₂. Oxygen present in air directly reacts with metal either at low or high temperatures in the absence of moisture.

Mechanism of oxidation corrosion

Oxidation corrosion is brought about by direct action of oxygen on metals by forming oxide film. The mechanism of oxide film formation can be represented by the following reactions:

Net reaction,

In above reactions, the electrons are transferred from metal atom to oxygen. Metal losses electrons while oxygen accepts electrons forming their respective ions. These two types of ions combine together to form metal oxide layer. Nature of the oxide film formed on the surface of metal plays an important part in oxidation corrosion process. For further oxidation to continue a) the metal ion must diffuse outwards or b) oxide ion must diffuse inwards. Former occurs more readily as because of the smaller size of the metal ion.

In oxidation corrosion, the nature of the oxide film plays an important role. It may be

a) Stable A stable layer of metal oxide is formed which is impervious and this protects the metal from further corrosion. Eg :- Al, Sn. "If the volume of the oxide film is greater than the volume of the meta surface, then the oxide layer is protective and non-porous. On the other hand," if the volume of the metal oxide is less than the volume of the metal, then the layer is non-protective and porous". This is known as Pilling-Bed worth rule.

b) Unstable The oxide layer decomposes back to metal and oxygen.

c) Volatile Oxide layer vaporises as soon as it is formed. Then by the metal surface is exposed for further attack. For example, molybdenum oxide

d) Porous Oxide layer has porous or cracks, so oxygen has access to the underlying metal. Hence, corrosion continues.

Wet corrosion or electro-chemical corrosion

This type of corrosion occurs at a solid-liquid interface when the metals are in contact with moist air or any liquid medium. If two dissimilar metals are dipped in a solution, the solution act as a conducting medium between them. One of the two metals acts as the anode and the other as a cathode.

At anode: Oxidation (Corrosion) takes place i.e.

At cathode: Reduction takes place, Where electrons are consumed.

Corrosion of an anode is based on how electrons are consumed at cathode.

At cathode consumption of electron takes place either by liberation or evolution of hydrogen or by absorption of oxygen. It is also known as immersed corrosion as it occurs in metal when they are immersed or dipped in the same solution.

Galvanic corrosion or Bimetallic corrosion- When two dissimilar metals are connected in an electrolyte, the metal higher in electrochemical series undergoes corrosion. Eg:- Zn and Cu. Zn acts as anode and is protected & Cu acts as cathode.

Concentration Cell corrosion- It is due to electrochemical attack on the metal surface, exposed to an electrolyte of varying concentrations or of varying aeration. This may be the result of local differences in metal ion concentrations.

Differential aeration corrosion- The most common type of concentration cell corrosion occurs when different parts of the metal are exposed to different O₂ or air concentration. Less oxygenated part acts as anode and undergoes corrosion, whereas more oxygenated parts acts as cathode.

Protection of metal from corrosion

The various protective measures include

·         Modification of the environment

·         Cathodic protection

·         Use of protective metal coating

Modification of environment

In this method, the metals are protected from corrosion either by removal of corrosion stimulants or by the use of inhibitors. A corrosion inhibitor is a substance which when added in small quantities to the aqueous corrosive environment and effectively decreases the corrosion of the metal.

Cathodic protection

The metal to be protected is forced to behave like cathode, there by corrosion does not occur.

There are two types of cathodic protection

Sacrificial anodic protection: In this method, the metallic structure (to be protected) is connected to more anodic material. So, that the corrosion is concentrated at more active metal. The more active metal sacrifices itself by corrosion and protects the structure (cathode). The sacrificial anode are Zn, Mg, Al. This method is mainly used for protection of underground pipeline, cables, and marine structures.

Impressed current cathodic protection: In this method, an impressed current is applied in opposite direction to nullify the corrosion current and convert the corroding metal from anode to cathode. The impressed current is derived from a direct current source like a battery. Sufficient D.C current is applied to an insoluble anode, buried in the soil and connected to the metallic structure to be protected. This type of cathodic protection is applied to open water-box coolers, water-tanks, buried oil or water pipes etc.

Application of protective coatings

This method is most commonly used method to prevent metal from corrosion. As its name suggests, a coating is applied over metal so it doesn’t get corroded. It can be classified as

Coating of less active metal: Less active metals also known as noble metals protect the base metal from corrosion due to their inactivity. The protection by less active metal is satisfactory as long as the coating is perfect. A break or crack in coating may facilitate the formation of electrolytic cells due to which, more active base metal undergoes corrosion rapidly.

Coating of more active metal: Coating of more active metals like zinc, aluminium, cadmium protect the base metal from corrosion. In this case, base metal act as a cathode and coating metal becomes an anode. Thus, the two metals being in contact with the surrounding medium form a galvanic cell in which cathode remains un-attacked, while the anode corrodes.

Methods of applying metal coating

The protective metallic coating can be made by the following different methods

·         Hot dipping: Galvanizing and Tinning

·         Metal spraying

·         Electroplating

·         Metal cladding

·         Diffusion coating: Colorizing, Chromizing and Sherardizing

Hot dipping

Hot dipping method is based on the process of dipping the base metal in a molten coating metal covered by a molten flux layer. The flux cleans the surface of base metal and prevents the oxidation of molten coating metal. The base metal is of higher melting point like iron, steel, etc and the coating metal is lower melting point then base metal. Hot dipping is generally done by two methods

Galvanising

Galvanizing, or galvanization, is a manufacturing process where a coating of zinc is applied to steel or iron to offer protection and prevent rusting. In this method, steel or iron is dipped in a molten pool of zinc that maintains a temperature of around 860°F (460 °C). This molten bath begins a metallurgical bond between the zinc and the receiving metal. After the metal is pulled from the bath, it reacts to being exposed to the atmosphere, and the pure zinc mixes with oxygen to form zinc oxide. The zinc-oxide further reacts to carbon dioxide and forms zinc carbonate, which makes up the final protective coating on the material.

Steps involved in hot dip galvanizing

·         The iron or steel article to be galvanized is first cleaned with dilute sulphuric acid to remove oxide layer and impurities.

·         It is then dipped in a bath of zinc ammonium chloride solution and then allowed to dry.

·         The sheet is then dipped molten zinc.

·         It is then passed under a roller to make the coating uniform and remove excess of molten zinc.

·         It is then heated at a temperature of 600^o C to 700^o C and then cooled down slowly.

Advantages

·         Low installation and maintenance cost

·         Long endurance

Disadvantages

·         Galvanized coating makes containers unusable for edible stuffs.

Tinning

Tinning is the process of coating iron with molten tin, which can be used for edible applications. Tinning is the process of making tinplate, which consists of sheets of iron or steel that have been thinly coated with tin by being dipped in a molten bath of that metal. Hence the process is more precisely described as hot-dipped tin plating. This is done in order to prevent the iron from rusting.

Steps involved in process of tinning

·         The iron or steel article to be galvanized is first cleaned with dilute sulphuric acid to remove oxide layer and impurities.

·         It is then dipped in a bath of zinc ammonium chloride solution and then allowed to dry.

·         The sheet is then dipped molten tin.

·         It is then dipped in a suitable vegetable oil, to protect the hot tin coated surface against oxidation.

·         It is then passed under a roller to make the coating uniform and remove excess of molten tin.

Difference between galvanization and tinning

Galvanizing	Tinning
Process of coating steel with a thin coat of TIN to prevent it from corrosion	Process of covering iron or steel with a thin coat of ZINC to prevent it from rusting.
Tin protects the base metal iron, from corrosion due to its noble nature and higher corrosion resistance.	Zinc protects iron sacrificially. Since it is more electro-positive than iron and does not permit iron to pass into the solution.
Tin protects underlying iron till the coat is intact. Any break in coating causes rapid corrosion of iron.	In galvanized articles zinc continues to protect the underlying iron by galvanic cell action even if the coating of zinc is broken at any place
Tin coated containers and utensils can be used for storing any food stuff as tin is non-toxic and protects metal from corrosion.	Galvanized containers cannot be used for storing acidic foodstuffs as zinc reacts with food acid forming poisonous compounds.
Ideal temperature is around 250 degree C.	Ideal temperature is around 450 degree C.
Zinc chloride is used as flux.	Ammonium chloride is used as flux.

 

Metal spraying

Metal spraying, or metallizing, is the process of coating a surface with metal or alloy using spray equipment. It is used to guard metals from corrosion, employing zinc or aluminium as basic spray materials. The typical hard-facing materials used in metal spraying include cobalt, nickel with a little amount of chromium, and manganese chrome.

Steps involved in metal spraying

·         The spraying gun consists of a duct for compressed air. The coating metal which needs to be sprayed is fed into the gun in form of wire.

·         The metal wire gets melted using compressed air to form a fine spray.

·         The spray can be directed to the surface where fine molten droplets rapidly solidify and form the coating.  This coating protects the base metal from corrosion.

Advantages

Spraying can be applied to non-metal as well.

Uneven surfaces can be easily protected using metal spraying.

Disadvantages

The process is not so effective as the coating may be porous and less adherent.

Electroplating

Electroplating is basically the process of plating a metal onto the other by hydrolysis mostly to prevent corrosion of metal or for decorative purposes.

Steps involved in electroplating

·         The surface which needs to be coated is first cleaned and suspended into the electrolyte and made as cathode. The anode consists of the pure metal whose coating is desired on the article.

·         The electrolyte generally consists of a salt solution of the coating metal.

·         When electric current is passed through the solution, the metal ions from electrolyte get deposited on the article. The equivalent amount of anode gets dissolved in the form of ions and passed into the electrolyte.

·         A thin layer of superior coating metal is obtained on the cathode.

Advantages

·         Improves wear resistance.

·         Improves the thickness of the metal surface.

·         Enhancing the electrical conductivity like plating a copper layer on an electrical component.

·         Minimizing Friction.

Sherardizing

Sherardizing is a zinc diffusion coating process, which uses zinc vapor to form zinc alloys with the base material. For sherardizing it is necessary to have the source (Zn powder) close to the coating surface, because of the low partial pressure of zinc.

Steps involved in Sherardizing

·         The iron articles to be coated are first cleaned.

·         The articles are then packed with zinc dust and zinc powder oxide in a steel drum.

·         The steel drum is provided with electrical heating arrangement to raise the temperature. The steel drum is rotated by means of a motor.

·         During this period, zinc gets diffused into iron forming Fe-Zn alloy at the surface which protects the iron surface from corrosion.

Advantages

·         The coating of metal is uniform

·         No change in dimension of articles.

Metal Cladding

Metal cladding is a type of protective coating, where the protective material such as metal powder or foil is bonded to a substrate by applying heat and/or pressure.

Steps involved in metal cladding

·         The base metal to be protected is sandwiched between two sheets of coating metal.

·         This cladded metal is then passed through two heavy rollers maintained at high temperature.

·         The sandwiched metal becomes cathodic with respect to base metal so that the electrolytic protection is provided. All corrosion resistance metal can be used as a cladding material.

Advantages

·         It increases the strength of the base metal.

·         The cladding metal also provide electrolytic protection to base metal.

Disadvantages

·         Only plain surfaces can be protected using metal cladding.

·         Base metal cannot be completely prevented from corrosion using metal cladding.

 

Paints

Paints are mechanical dispersion mixture of one or more pigments in a non-volatile film forming material (usually a thinner). Paints are used for decorative purpose as well as protective coating over metal. Paint needs time to dry out. To accelerate the drying, small amount of dryers are also added to the paint.

Characteristics of a good paint

Fluidity: Paint should be fluid enough so that it easily spreads over the entire surface to be protected.

Coverage: Paints should have good covering capability.

Durability: A good paint should be durable and long lasting

Adherence: A good paint should adhere to the surface to which it has been applied.

Appearance: The paint film should look glossy and shinny.

Flexibility: The paint should not get cracked on drying

Constituents of paints

Paints is a composition of various materials. But some constituents are essential for a paint. They are discussed below:

·         Binder

·         Pigment

·         Filler/ Extender

·         Volatile organic compound (Thinner)

·         Driers

·         Additives (Anti skinning agents, Anti settling agent, Plasticizers, fire retardants etc

Binder

Binders are usually resins or oils but can be inorganic compounds. Binder is the actual film forming component & absolutely required ingredient of any paint. It consists of a resin and a solvent thinner. It is the part which solidifies to form the dry paint film when the solvent evaporates. They are non-volatile & mainly polymers of various types. Different resins form dry film on the substrate in different manners.

Function of binder

·         Binds pigment, fillers & additives together

·         Imparts adhesion & strongly

·         It influences gloss, durability, flexibility and toughness

Pigments

Pigments are finely ground inorganic or organic powders of higher refractive index (> 1.5). Higher the RI, more the light is bent & greater the opacity. Good opacity has good lighting absorbing & /or scattering properties. The colour of pigment depends on the composition of the pigment used. For example, White lead and titanium oxide is are white pigments whereas red lead, ferric oxide, etc are red pigments. Good pigments are opaque, non toxic and chemically inert.

Function of pigment

·         It provides opacity and colour to paint ilm.

·         Give strength to the paint film.

·         Provides protection to paint by reflecting harmful ultraviolet light.

·         Increases weather resistance of paint film

Extender / Filler

Non-expensive commonly natural inorganic materials added to the paint in order to increase its volume. Extenders are mainly inorganic substances & do not provide colour to the paint but added to improve adhesion, ease of sanding and film strength. As they are cheap in comparison to prime pigments, they reduce overall cost of the paints.

Function

·         thickens the film

·         Increases volume, paint film thickness

·         reduces cost of the paint

·         imparts toughness, abrasion resistance & texture

·         Control consistency

Widely used fillers: Calcium carbonate, Gypsum, Ground silica, Barytes, Slate powder, French chalk, china clay, asbestos, silica, mica, whiting etc.

Driers

Depending upon the nature of the solvent and film thickness, the drying process may take as long as several hours. Thicker the film, longer the drying time. If the drying process is artificially accelerated, there may be problems with adhesion between the protective film and the metal surface. Metallic salts of Lead, Manganese, Cobalt, etc. of organic acids are generally used as drier.

Purpose

·         to accelerate the drying process.

Examples: Lead acetate, Cobalt octate, Manganese octate, Litharge, Red lead, Lead octate, Manganese dioxide, Zinc sulphate, etc.

Additives

Additives are small amounts of different chemical substances improving or modifying the paint properties. Added to a paint in amounts 0.001% & ≤ 5% & have a profound influence on physical & chemical properties of the paint.

Function

·         Prevent clustering of pigments

·         Texturizers impart textures to the coatings.

·         Antifreezers helps to withstand exposure

·         Pigment stabilizers improve pigment stability

·         Fire retardant properties

Methods of application of paints

Paint is applied to the surface through several methods. Some of them are discussed below:

Roller coating: Roller coating method is used when the material to be painted is in form of flat sheets. Roller is dipped in paint and rolled over the material which needs to be painted. During the passage, the articles are painted uniformly.

Brushing: An easy method of applying paint is brushing. Brush is dipped in paint and then the brush is applied on the surface. A non-viscous and covering paint is must to apply it on material. A good brush should have flexible yet rigid bristles. Brush gives a smooth finish on the surface. Paints who are more viscous are first mixed with a thinner before getting applied.

Spraying: This method involves the application of paint using spray gun. Spraying method is very useful for uneven surfaces. The process is quick but more paint is required as compared to other methods. The spray can be directed to the surface where fine molten droplets rapidly solidify and form the coating.  In recent advancements, paint is made of negative charge and the article to be painted is made positive. So, when paint is sprayed, negatively charged particles of paint is attracted to positively charged articles and hence less paint is wasted.

Dipping: Dipping is the simplest method of application of paint. The article to be painted is simply dipped in paint and then the article is taken out and paint is dried. The major disadvantage of dipping is that thickness of applied paint is not uniform.

Tumbling: Tumble spray involves placing components in a specially designed, hexagonal shaped unit. A controlled spray of atomized coating is then applied to parts as the unit rotates. The hexagon design of the unit is important as the components are flipped and rotated more than a simple cylindrical unit could. Once the coating is applied, parts are loaded in baskets and baked in a batch oven.

Chemical resistant paints

Chemical resistant coatings are designed for metal, concrete floors & walls, Wood & Fiber to prevent the abrasion from harsh chemicals including the acids and several other abrasive materials. Chemical resistant paints protect different surfaces from chemical. The chemical resistance of a coating will depend on number of criteria such as

·         The type of chemical used and its concentration

·         The duration for which the article has been exposed to paint

·         Chemical resistant paints are required in nuclear facilities, liquid spillages, storage of high temperature chemicals, where enhanced lubrication is required, etc.

Heat resistant paints

Heat resistant paints are designed to withstand high working temperature on commercial and industrial metallic surfaces and structures from heat treatment applications. For example, chimneys of furnaces, incinerators operate at very high temperature. This type of surface requires a paint which does not breakdown at high temperatures. Heat resistant paint does not deliver fire retardant properties but still can be used in industrial applications. Generally high resistant paints are applied using spraying.

Cellulose Paint

Cellulose paints are highly toxic, flammable and very fast drying. It is one of the industrial paints. Its raw material consists of the reaction of cellulose obtained from cotton and wood with alcohol and acids. Cellulose-based paints are not widely used in construction compared to synthetic and water-based paints. The reason for this is that the harmful chemicals it contains and the thinner used in thinning is cellulose-based.

Luminous Paint

Luminous paint or luminescent paint is paint that exhibits luminescence. In other words, it gives off visible light through fluorescence, phosphorescence, or radioluminescence. There are three types of luminous paints: fluorescent paint, phosphorescent paint and radio luminescent paint.

Fluorescent paint

Fluorescent paints 'glow' when exposed to short-wave ultraviolet (UV) radiation. These UV wavelengths are found in sunlight and many artificial lights, but the paint requires a special black light to view so these glowing-paint applications are called 'black-light effects'. Fluorescent paint is available in a wide range of colours and is used in theatrical lighting and effects, posters, and as entertainment for children.

Phosphorescent paint

Phosphorescent paint is commonly called "glow-in-the-dark" paint. It is made from phosphors such as silver-activated zinc sulphide or doped strontium aluminate, and typically glows a pale green to greenish-blue color. The mechanism for producing light is similar to that of fluorescent paint, but the emission of visible light persists long after it has been exposed to light. Phosphorescent paints have a sustained glow which lasts for up to 12 hours after exposure to light, fading over time.

Radio luminescent paint

Radio luminescent paint is a self-luminous paint that consists of a small amount of a radioactive isotope (radionuclide) mixed with a radio luminescent phosphor chemical. The radioisotope continually decays, emitting radiation particles which strike molecules of the phosphor, exciting them to emit visible light.

Emulsion paint

Emulsion paint is water-based paint, that consists of pigment, emulsifier, coagulant, and water. It's called emulsion because the pigments are dispersed in water emulsified with an emulsifying agent. Emulsion paints are quite popular for their durability, easy-to-use features, and low cost. Emulsion paint consists of tiny polymer particles within which the pigments are trapped. The particles are suspended in water, then as the paint dries the particles fuse together creating a film of paint on the wall. Once this happens the polymer can’t be resuspended in water, which is why you can’t wash a water-based emulsion paint back off the wall once it has dried. Emulsion paints are free from hazard and have no objectionable odour.

Metal Paints

Metal paints are paints which easily adhere to metal surfaces. Paint coating on metal surfaces is applied on bodies for protection and decoration. The coating on metal may be of galvanic type or barrier type.

Cement paint

Cement paints are prepared by mixing white cement with colouring pigments, hydrated lime and fine sand. Cement paints are available in form of powder of particular colour. The dispersion medium may be water or oil depending on the purpose of coating. Before applying cement paint, a primer coat consisting of dilute solution of sodium silicate and zinc sulphate is necessary. Cement paints have water proofing capacity and give a stable and decorative film.

Water paints or distempers

Distemper paint is an ancient type of paint made of water, chalk, and pigment. It is bound with either an animal glue or the adhesive qualities of casein, a resin that comes from solidified milk. The basic constituent of Distemper Paint is chalk, lime, and water. Such kind of paint can be applied directly on cement walls without any other coating on them without using the primer. They are a cheaper option and they stay good for more than 3 to 4 years.

Varnishes

The Varnish is a transparent, hard, protective finish or film primarily used in wood finishing and also for other materials. Varnish is a homogeneous colloidal solution of natural or synthetic resins in oils or thinners or both. It enhances and gives comfort to the grain of the wood and is resistant to impact, heat, erosion, water, and alcohol. It can be used as a top coat over a painted surface.

Characteristics of good varnish

·         It should be dry quickly

·         On drying it should form a hard, tough and durable film.

·         It should have good weathering properties, resist abrasion and wear well.

·         It should be able to retain its colour and shine.

·         It should be uniform and pleasant looking on drying.

Constituent of varnish

The ingredients of varnish are:

Resins

Solvents

Driers

Resins

Commonly used resins are copal mastic, amber gum and lac. Quantity of varnish depends much upon the quality of resin used. Copal is considered to be the best, toughest, hardest and is very durable for external work.

Solvents

These must suit the resins used. Boiled linseed oil is used to dissolve copal or amber, turpentine oil for common resin or mastic, methylated spirit for lac. Wood naphtha, because of its offensive smell is not suited for superior works and is used only for cheap varnish.

Driers

These should be added only in small quantities as an excessive injures varnish and impairs its durability. Litharge or lead acetate are the commonly used driers in varnish added to accelerate drying process.

Different kinds of Varnishes:

Based on the different solvents used, varnishes are classified under the following categories:

Oil Varnish

These are made by dissolving hard resins like amber or copal in oil. They are slow to dry but are hardest and most durable of all varnishes. They are suited for being used on exposed surfaces requiring polishing or frequent cleaning and for superior works.

Turpentine Varnish

These are made from soft resins like mastic, common resin is dissolved in turpentine oil.

Spirit Varnish

Varnishes in which spirit is used as a solvent is known as spirited varnish or French Polish. Shellac is dissolved in spirit and the product is applied in a thin layer. This varnish gives a transparent finish thus showing the grains of the timber. These however, do not weather well and as such are used for polishing wood work not exposed to weather.

Water Varnish

They consist of lac dissolved in hot water with borax, ammonia, potash or soda just enough to dissolve the lac. Varnish so made withstands washing. It is used for painting wall paper and for delicate work.

Japans

Japans are pigmented varnishes which are added to paint to give it a good colour and lustre. Japans are generally used to paint bicycles and electrical devices. Japans are usually of two types: Printer’s Japan and decorative Japans. Printer’s Japans consists of resin dissolved in drying oil containing drier and thinner. Printer’s Japans make paint more lusturous. Decorative Japan is prepared by heating linseed oil and lead oxide. This results in a solid mass which is known as lead oil. This lead oil is then mixed with asphaltum and thinner to get decorative Japans.

Enamels

Enamel is a type of paint which is traditionally oil-based, although variations like water-based or latex-based enamels are readily available in the market. Enamel gets air-dried hard and used as protective as well. Enamel is considered best for the materials which are of regular use and thus enamel helps to protect the wear and tear products or the things that are subject to remain outside the shelter and faces every type of climatic conditions for example cars. Thus, enamel is considered best for materials like furniture, vehicles or any kind of wood or metal.

Constituents of enamel

Pigments: Generally, titanium dioxide and calcium sulphate is used as pigments for enamels. Pigments as generally white. But when coloured pigments are used, they are referred to as Japans.

Vehicles: Vehicles make enamels of glossy finish. The vehicle of enamel is either oil and resin or only resin. Vehicles increase the viscosity, gloss and colour of enamel.

Driers: Commonly used driers are naphthenates or resonates of copper, zinc, etc.

Thinners: Thinners used are turpentine, acetone, etc.

Lacquers

Lacquer is a type of hard and usually shiny coating or finish applied to materials such as wood or metal. It is most often made from resin extracted from trees and waxes. Lacquer is a substance that provides a strong and shiny finish. The main objective of using lacquer is to provide a highly shiny and glossy coat on the surface of the wood or any object that is being coated. Lacquer is not a durable substance, and it is only used for its appearance.

Constituents of liquor

Cellulose derivative: Cellulose derivatives give durability and water resistance property to the film.

Resins: Resins are used to provide adhesion, thickness and gloss to film. Resins generally used in lacquers are copal, alkyd, etc.

Solvents: The solvent is used to dissolve the film forming material. The solvents used are esters, ketones, alcohols, etc.

Plasticizers: They reduce the brittleness of liquor. This improves the adherence and durability of plasticizers. Commonly used plasticizers are castor oil, chlorinated diphenyls, etc.

Diluent: Diluents are generally coal-tar products which reduces the viscosity as well as cost of the liquor. Benzol and naptha are used as diluents.

Difference between enamel and lacquer

Parameters of Comparison	Lacquer	Enamel
Definition	Lacquer is a coating solution that is primarily used to give a shiny coating to surfaces	Enamel is a hard paint solution that is used for coloring as well as to give a hard coating on the surface
Durability	Lacquer is not durable and long-lasting	Enamel is a durable and long-lasting coating
Drying time	Lacquer dries very quickly, right after application	Enamel takes a longer time to dry
Thinner	Specific lacquer thinners are used	Spirits are used as thinner agents while applying enamel
Demerit	Lacquer develops bubbles if not applied properly	Enamel does not have such bubble formation