COATING of METALS -Part-I

Post 627 –by Gautam Shah

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A coating is a thin surfacing that is applied or attached to the surface with various degrees of ‘chemical’ integration or amalgamation. Metal coatings are of four types: ‘Organic’ coatings like paints, organosols or other polymeric compounds, Non-Organic coatings of metals, Coatings of metalloids reaching to ceramic states, and Gas reaction-deposition systems.

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Horses of Basilica San Marco > Wikipedia image by Author Tteske

Metals need a coating to alter the physical and chemical properties of a surface, such as to endow desired quality of texture, colour, patterns, electrical properties, surface reactivity, strength properties, etc. Often a surface treatment is in preparation for another treatment, as a temporary or permanent application. Maintenance of specific surface properties during sub-processing, transit, fabrication, installation and repairs require special coating systems.

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Copper Bronze spouted flagon 320 BC > Wikipedia image by Rosemania

Metal items require some form of ‘surface-deep’ preparatory ‘work’ to receive the coating, but such ‘work’ for small-or-thin-body entities like plates, sheets, foils, wires, threads may involve entire mass of the body. Post such preparatory work, involving heat leaves some stresses in the item. Small or thin body items have uniform stresses. But heavy items and assembled work can have differential stresses, which affects the final coating.

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Reclining Figure : Arched Legs 1969-1970 by Henry Moore > Flickr image by Pedro Ribeiro Simoes

One of the first Metal coating realized by Man was Patina. Metal rusting is nominally a crust like degradation products, but some metals like Bronze, acquire a surface layer over a period of time. Verdigris is the natural patina. Metal artifacts exposed to different environments such air, sea-water, soils acquire patina, a layer consisting of oxides, carbonates, sulfides, or sulfates. Patinas are products of surface mass degradation, and so etch or reduce thickness of the surfaces. Some patinas, however, curtail further degradation of the surface, and so are encouraged. Patinas are often visually appealing and so desired. Effects similar to patina can be achieved by designed exposure and by treating with various chemicals. Patinas are commonly green, but may vary in colour such as of red, brown, black, blue, or gray. Its surface may be smooth, glossy, or crusty. Newly made objects are deliberately patinated to simulate the antiquity in a process is often called distressing.

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Michelangelo’s Pieta in Bronze by Ferdinando Merinelli 1932 > Wikipedia image

Patina over copper alloys, such as bronze, due to the chlorides leads to green, while sulfur compounds are brown. The basic palette for patinas on copper alloys is blue-black due to ammonium sulfide, brown-black with liver of sulfur, blue-green for cupric nitrate, and yellow-brown due to ferric nitrate. For new artefacts accelerated patination carried out by applying chemicals with heat. Colours range from matte sandstone yellow to deep blues, greens, whites, reds and various blacks. Some patina colours are achieved by the mix of pigments and chemicals. The surface is enhanced by waxing, oiling, or other types of lacquers or clear-coats. French sculptor Auguste Rodin used to instruct assistants to urinate over bronzes stored or buried in the yard. A temporary-washable patina, is produced on copper, by the vinegar (acetic acid).

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Bronze busts –with and without patina > Wikipedia image by MatthiasKabel

In architecture, metals, like copper, bronze, etc. have been used for a very long time, for wall cladding, door panelling, ceiling tiles, and roof covering. Copper provides excellent corrosion resistance. Copper surfaces form tough oxide-sulfate patina coating that protects underlying copper mass and resists further corrosion. Copper corrosion products are less toxic. Copper sheets have been used in many building to cover rounded domes, and articulated roof surfaces. Architectural copper is, though susceptible to oxidizing acids, heavy-metal salts, alkali, sulfur and nitrogen oxides, ammonia, sulfur and ammonium compounds. Brass, an alloy of copper and zinc, has good resistance to atmospheric corrosion, alkali, and organic acids.

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Thracian plaque > Wikipedia image by Ivorrusev

Metal to metal cladding systems were precursors to Plating technology. Such dual metal structures were created by beating, rolling, rivetting or co-forming. The chief purpose was to add strength to a weaker metal. Forging a metal over metal in some cases created partial homogeneity. It was easier to forge soft metals like tin, lead and copper over harder metals like Bronze, Iron etc. Tin and lead could be softened through heat and used for coating. Metal coating by cladding or sheathing, were useful for corrosion resistance, wear resistance, improved electrical/thermal conductivity and better handling (touch-feel).

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Gold foil processing at a workshop in Kanazawa Japan > Wikipedia image by Eckhard Pecher

Some of the simplest methods of sheathing used Gold, Silver and their amalgams with mercury. Gold, Silver, Tin and mercury based amalgams were used as liquidized coatings, where as Silver and Gold were fused as thin sheets or foils. Tin coatings were used for mirror making.

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METAL COATINGS

METAL COATINGS

Post 438 ⇒   by Gautam Shah 

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Metals are coated for many different purposes, and with metals, alloys, non metal substances (carbon, phosphorus, sulfur, selenium, and iodine) and polymeric materials (plastics, elastomers, etc.). Metal coatings provide specific surface properties, like, rust inhibition, preventing tarnishing by weather, improve surface conductance or resistance, for surface alloying, for surface hardening, for pseudo metalizing (providing appearance or behavioural qualities of another metal), for surface alteration like adding gloss or textures, providing insulation, wear resistance, and for imparting colours. Some of the Coatings are discussed here.

Dyed Anodizing of Aluminium

CONVERSION COATINGS are chemical surface treatments which provide temporary corrosion protection to the surface and create an appropriate substrate for subsequent treatments or coatings. There are two main processes: Phosphating and Chromating.

Parkerizing, bonderizing, phosphating, or phosphatizing

Phosphate coatings are mainly applied to ferrous and zinc, and to some extent for aluminium, tin and cadmium metals. It is mainly used as a pretreatment for painting work of automobiles and sheet metal components. The applied coating is thin and porous. It offers mechanical keying to the paint film. The electrical insulation or inertness restricts the corrosion breaks. The phosphating treatment consists of immersion, brushing or spraying of hot dilute ortho-phosphoric acid solution. Similarly Zinc phosphate coatings are also used.

Stamping steel with zinc chromate conversion coating

Chromating or chromate coating is depositing a chromium oxide layer over a metal surface to enable the metal to react with the oxide layer. The process is used, mainly to increase the corrosion resistance of metals such as aluminium, magnesium, tin, zinc and cadmium, and also increase the tarnish resistance of copper and silver. Pure Aluminum is very soft, so its alloys (mixtures) with copper and other metals were used to make a lightweight but strong parts for aircraft. Chromium metal coatings were used to reduce the corrosion. It is used for hardware items and tools.

Zinc coating

DIFFUSION COATINGS are also known as cementation coatings. A part of the coating interacts with the substrate and forms an alloy with it. Cementation coating process is very similar to carburising of iron to produce a surface-hardened steel (iron is heated with carbon particles for the diffusion to occur). Common diffusion processes are Aluminizing (calorising), Chromising and Sherardising (zinc cementation coating). There are other diffusion coatings such as Siliconising and Borating. Hot dip galvanizing, tinning and terneplating also involve alloying between coating and substrate, but the process differs from that of diffusion coating. Diffusion coatings are created by, the gas-phase at high temperatures by exposing the substrate to a volatile (usually halide) compounds of the coating metal, and through a solid-phase by packing the substrate metal in a container with the powdered coating metal and heating them together.

Aluminized Steel for Car muffler (silencer)

Aluminizing can be performed by both solid state and pack diffusion. Aluminizing may also be performed by heating ferrous metals in an inert, or reducing atmosphere containing dry gaseous aluminium chloride at 700-1100°C. It is also possible to produce aluminium diffusion coatings by coating the ferrous surface with aluminium by hot dipping (or with aluminium powder) and then heat treating to permit diffusion to occur.

Zinc coated steel 1 cent coin

Chromising is application of chromium on Steel, for enhanced oxidation, corrosion and wear resistance. For gas phase chromising the articles are heated in a powdered mixture of chromium, alumina or kaolin and an ammonium halide in a hydrogen atmosphere at 1000-1100°C. It is also possible to chromise steel by heating it in a fused salt bath of chromium chloride, chromium metal, barium chloride and sodium chloride in an argon atmosphere.

Inside Part of Tin plated Can

Sherardising is zinc diffusion coating of steel. Sherardising is applied on nuts, bolts, screws and chains. The articles are packed into a drum containing blue zinc powder (zinc containing 5-8% zinc oxide) and alumina or sand, the drum is heated to 360-430°C and rotated. Sherardised coating is a hard matt grey finish of uniform thickness.

Galvanizing is a molten zinc dipped coatings on steel. Steel can also be coated by zinc through electro plating. Such a coating, if it is cut or scratched, then the zinc flows to decay preferentially to the steel and provide continuous protection.

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Corrugated Zinc-Iron sheets

VAPOUR DEPOSITION coatings are formed, by ‘condensation of metal vapour originating from molten metal, high voltage discharges between electrodes (cathode sputtering), or from chemical means such as hydrogen reduction or thermal decomposition (gas plating) of metal halides’ A thin specular coating can be achieved on metals, plastics, paper, glass and fabrics by means of vaporization.

Tin coated punches of steel

Siliconising is carried out by heating steel in contact with silicon carbide powder in an atmosphere of silicon tetrachloride vapour. It case hardens the steel for high surface hardness and wears resistance. Siliconising is done to refractory materials.

Silver art work

METAL CLADDING and PLATING are used for coating, cladding or plating a metal surface with another metal. Hot-dipped coatings of low-melting metals are used for steel articles. Electro plating is used for plating Zinc, Nickel, Gold, Silver, Chrome, Tin, and Nickel-Cobalt alloy through Barrel plating, Rack plating, Strip plating processes. Electro plating nominally provides a very glossy surface, but by reversing the current in electrodes during the final stage a matt finish can be achieved. Other processes very close to this category include Electroless plating, hot dipping, metal spraying, powder spraying, vacuum metalizing and anodizing,

VITREOUS ENAMEL COATINGS often called ceramic coating are glassy but noncrystalline surfacing. A slip is applied by dipping or flow coating, and fired or sintered to form a vitreous coating. Dry enamelling is used for castings, such as bathtubs. ‘The casting is heated to a high temperature, and then dry enamel powder is sprinkled over the surface, where it fuses.’

Ceramic enamel coated steel cooking pot

POWDER COATINGS use polymeric materials such as acrylic, polyester, and epoxies, which are heated and sprayed or sprayed over a heated object. Powders are also applied to electrically charged materials which are attracted to and adhere to the substrate until it can be transported to an oven.

Powder coating

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COATINGS -surface finishing technologies

COATINGS -surface finishing technologies

Post 238 ⇒   by Gautam Shah 

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Coatings are thin surfacing. A coating is mainly created through a change in the physical state of a material (change of a phase); first from higher or solid to liquid (or even gas) for the purpose of application, and than reconversion to a solid (or even a very heavy liquid phase such as ‘creamy or pasty’).

Enamelling Minakari work India

At an application stage, a lower phase helps in many ways:

  • Easy, uniform and thin spread over the surface.
  • Efficient dispersion of costly or rare constituents.
  • Easy and thorough mixing of constituents such as film forming materials and other additives, during production and application.
  • Less energy is required for application.
  •  A controlled rate of deposition.

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We tend to identify Coatings as Paints and Varnishes. Coatings as technology, however, include diverse methods of surface formation. The most primitive coatings were daubing of clay over walls and floors to create a smooth surface. The next major use was finishing the ceramics in pre and post firing processes. Coatings of oils and waxes were applied on materials like leathers, furs, and fabrics, to make them, water and fungus resistant, soft and supple. Lime and other minerals were used as colourant through coatings. Starch, gums, and other protein materials were used as stiffening or bodying coatings. Application of melted tin over metal surfaces is very ancient coating system. Ceramic slips were applied to create glaze over pottery items. Metal oxides were fired over metal surfaces to form ceramic enamels. Coal tar and Bitumen have been used in waterproofing applications. Metal plating, metallizing and surface alloying are fairly ancient technologies of coatings.

Enamelled Tumbler

Metal coating, enamelling, Minakari, etc. Ceramics or metal objects are coated with heat fusing coats of metallic and non metallic compounds. Vitreous enamel coatings are glassy, but non-crystalline coatings are matt. In a wet application a slip is prepared through a water suspension of crushed glass, flux, suspending agent, refractory compound and colouring agents or opacifiers. The slip is applied on a raw product or half-baked product by dipping or flow coating. It is then fired at temperatures for the slip to fuse and flow into a glassy surface. The Dry process involves spraying the enamel powder onto a heated product, to allow the slip to fuse. Enamelling has been used as a coating system for iron surfaces in sign boards, utility items like dishes, tumblers, cooking ranges, hospital wares, ceiling panels, partition panels etc.

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METAL COATINGS

Conversion coatings are chemical surface coating treatments which provide some degree of corrosion protection to the surface, mostly very temporary and an appropriate substrate for subsequent treatments or coatings. Two principal processes are used, namely Phosphating and Chromating.

Pre paint dipping coating

Phosphating: Phosphate coatings are applied to ferrous metals and zinc surfaces. The phosphate coating is an excellent pretreatment before painting, and as a base for impregnation with enamels, paints, lacquers, oils and waxes. The electrical inertness of the coating helps in localizing corrosion breaks. A light or rapid phosphate treatment is used when a paint film is to be applied but a heavy phosphate coating is used when the coating is intended to function as a barrier film and carrier for oil impregnation. Phosphate coatings may be produced by immersion, brush application or spraying.

Zinc phosphate coatings: Zinc phosphate coatings are smooth and fine grained, and are excellent surface preparative treatments prior to painting, for improved adhesion.

Chromating: Chromating process is used to increase the atmospheric corrosion resistance of metals such as aluminium, magnesium, tin, zinc and cadmium primarily, although it is sometimes also used to increase the tarnish resistance of copper and silver.

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DIFFUSION COATINGS

Diffusion coatings are also known as cementation coatings, differ from other forms of metal finishing in that, part of the coating interacts and alloys with the substrate. Cementation coating process is very similar to carburising of iron to produce surface-hardened steel, where iron is heated with carbon particles for the diffusion to occur. Common processes that fall into this category are: aluminizing (calorising), chromising and Sherardising (zinc cementation coating). Hot dip galvanizing, tinning and aluminizing as well as terneplating also involve alloying between coating and substrate. Diffusion coating can be made in two ways, namely in the gas phase at high temperatures by exposing the substrate to a volatile (usually halide) compound of the coating metal and through a solid phase by packing the substrate metal in a container with the powdered coating metal and heating them together.

Aluminizing: Aluminizing can be done by both solid state and pack diffusion. This surface alloy however, is porous and brittle so the parts are heated in air at 815-980° C for 12 to 48 h, to allow Interdiffusion of aluminium and iron to occur.

Chromising: Chromising is formation of a diffusion coating on iron or steel by chromium to create a surface with enhanced oxidation, corrosion and wear resistance.

Sherardising: Sherardising is zinc diffusion coating of steel, most widely applied diffusion treatment. Commercially sherardising is less important than hot dipping or galvanizing treatments, and electro deposition of zinc. Sherardising is though very useful for treating nuts, bolts, screws and chains. Treated articles are characterized by high abrasion resistance and uniformity of coating thickness irrespective of the article shape. The sherardised coating is a hard matt grey finish.

Galvanising: Galvanising is a term commonly given to molten zinc dipped coatings on steel. Steel can also be coated by zinc through electro plating. The steel is cleaned and then dipped in heated hydrochloric acid which produces iron chloride on the surface of the metal that can act as a flux. However if the coating is cut or scratched, then the zinc flows to decay preferentially to the steel and provide continuous protection.

Vapour deposition: A thin specular coating is formed on metals, plastics, paper glass and even fabrics through a deposition by means of vaporization. Coatings formed by condensation of metal vapour originating from molten metal, high voltage discharges between electrodes (cathode sputtering), or from chemical means such as hydrogen reduction or thermal decomposition (gas plating) of metal halides.

Siliconising of steel by done by heating in contact with silicon carbide powder in an atmosphere of silicon tetrachloride vapour. The treatment primarily provides high surface hardness and wear resistance.

Steel bluing

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SPRAYED METAL COATINGS

Sprayed coatings are formed by the impingement and subsequent flattening of large numbers of molten metal particles on the surface. The deposit has high intrinsic tenacity due to mechanical interlocking. The deposits are laminated or stratified since they are built up in layers. Sprayed metal coatings have a matt finish and are porous and as a result function as good substrates for paint films. Metal spraying is possible at very low and short duration of temperature so that metals, plastics, rubbers and even paper can be sprayed without damage.

Chromed Plastics

There are basic Four types of spray metal processes: 1. Metals in powder form are heated to liquefy during spraying, 2. Hot liquidized metals are atomized during spraying, 3. Metal powders are sprayed on heated objects, and 4. Objects are coated or stuffed with metal powders, and then sintered.

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