electroplating and electroless plating corrosion engineering

8/4/2019 Electroplating and Electroless Plating Corrosion engineering

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ByAli Dad Chandio


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Electrochemical and chemical techniques

In electrochemical (electroplating) and chemical techniques, several effects may beutilized.

To deposit a metallic coating or to either deposit or set a non-metallic coating onthe surface of a metal, alternately, to polish or clean (pickle) a metal surface, thiseffect is electrochemical reduction (for electrochemical and conversion coatings,

electrolytic polishing and etching) or chemical reduction (for chemical andconversion coatings, and chemical polishing and etching).

In the case of paint coatings, other chemical reactions are utilized. Coatingsobtained in this way exhibit properties that are superior to those of the coatedmetal. These properties include: corrosion resistance, wear resistance and some

physico-chemical ones, like color, luster and reflectivity.

Coatings are most commonly produced by the bath technique, i.e. immersion in anelectrolyte, a chemical bath, paint or sol, by spraying and, less frequently, bytampons, centrifugal rinsing or spreading.


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Electrolytic deposition (electroplating) enables the creation of metal or alloy

coatings, as the result of reduction, by electric current at the cathode, of ions of the

coating metal from electrolyte solutions.

The obtained coatings may be single layered, with a thickness of 0.3300 m or

multi-layered.

The most frequently used coating metals (in descending order ) are: chrome, nickel,

zinc, tin, cadmium, copper, lead, silver, gold, rhodium, palladium, platinum,

ruthenium, iron, cobalt, indium, as well as alloys: Sn-Pb, Sn-Ni, Sn-Cd, Zn-Ni, Cu-

Zn, Ni-Fe, Ni-Co, Ni-P, Co-P, Co-W, brasses, used single or in combination with

other coatings, mainly for corrosion protection, and for decorative purposes.

In some branches of technology, e.g. repair of machine components, intermediate

layers in electronics, special military applications, electroplating is complementary

to tampon deposition (instead of an electrolytic bath, a fabric tampon dipped in it or

a saturated solid is used).


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The tampon technique allows selective deposition of microcrystalline

metal or alloy coatings on fragments of even very big objects.

With some modifications of the electrolyte (aqueous solutions ofalkalis, acids and salts) as compared to immersion techniques, thistechnique also enables the obtaining of coatings with special propertieson both metallic and non-metallic substrates, e.g. ceramic, glass,plastics.

Besides a 10-20 times shorter deposition time, lower cost and materialas well as energy economy, the tampon technique yields coatings

which are harder than those obtained in the immersion technique and

have a disordered or amorphous structure with a lower hydrogencontent.

Finally, the tampon technique also enables the deposition ofcomposite ceramic-metal coatings.


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Chemical deposition (electroless) enables the obtaining of metal oralloy coatings on metals or alloys, as the result of exchange, contact or

reduction with, or without the participation of a catalyst. Exchange and contactis used for the deposition of tin coatings; exchange, contact and reduction withthe utilization of a catalyst is the technique used for copper and nickelcoatings.

Contact and reduction, but without the catalyst is used for the deposition ofsilver coatings, while gold is deposited by means of exchange and reductionwith a catalyst. In most cases deposition is by chemical reduction without acatalyst (e.g. by sodium hypophosphite) in baths or by spraying.

It is applied for the deposition of coatings, mainly nickel, on substrates which

are otherwise difficult to electroplate (complex shapes, slender long holes, etc.)and on other coating metals: Ag on Cu, brasses or non-conductors, such asglass and plastics, Au on Cu or brasses, Co and Cu on plastics; Pd, Pt, Sn on Cuor on Al; Bi on steels; Ag on glass. Layer thickness is 5-20 m. The mainpurpose of these applications is enhancement of corrosion resistance orcreation of contact layers on cast iron and steel (Ni) prior to coating with

enamel.


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Conversion deposition is an artificially induced and controlledprocess of metal or alloy corrosion by chemical or electrochemicaltreatment.

It is result is the formation on the surface of a coating which ispractically insoluble in water or in the triggering environment, tightlybound to the substrate material and exhibiting dielectric properties.

It is composed of compounds of substrate material with the reagentsolution, in which object are immersed or which is sprayed on theobjects. It may be a chromate, phosphate, oxide, oxalate or a other

coating.

Depending on the type of bath used and on the substrate material (e.g.aluminum, zinc, cadmium, steel, copper and its alloys, magnesiumalloys, silver, etc.) the coatings may have different compositions, color

and properties.


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The thickness of coatings ranges from several to several hundred micrometers.

Conversion coatings are applied for corrosion resistance and:

to improve adherence of paint coatings to steel, zinc and aluminum,

to improve the properties of other coatings,

to activate the diffusion of nitrogen to steel, facilitate cold deformation ofsteel (broaching, extrusion, pressing), electrical insulation of substrate,

to reduce friction (lubricating coatings),

to enhance esthetic value (decorative coatings).

Broadest application is found by phosphate coatings (coatings of steel sheetprior to painting) and oxide (oxidizing of machine components, tools andfirearms).


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Polishing - finishing treatment, carried out with the purpose of

obtaining smoothness and luster of the object surface, andaccomplished in an electrolyte or a chemical bath.

It consists of selective dissolution of peaks of microasperities, whileleaving microrecesses practically unchanged:

chemical polishing (electroless polishing, chemical brightening) brightening and partial polishing, carried out by treatment of metalsand alloys, most frequently aluminum and its alloys, in bathscontaining oxidizing agents (primarily acids, like orthophosphoric,nitric, sulfuric, acetic), in order to achieve an attractive appearance;


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electrolytic (electropolishing, electrochemical polishing) - smoothing (dissolution ofasperities of heights greater than 1 m) and brightening (reduction of asperities from

above 1 m to below 0.01 m), carried out by treatment of metals and alloys (mainlyaluminum), with appropriately selected electrolytes and current conditions.

The polished object is the anode.

The treatment does not alter the state of residual stresses in the surface layer.

It is used to obtain high luster or as preparation of the substrate for protective-decorativeprocesses (e.g. electroplating).

Etching - removal of layers of scale, rust, oxides or alkaline salts from the surface ofmetals and alloys, carried out before final pickling and deposition of electroplatedcoatings.

It can be carried out by:

chemical means (electroless) - by immersion in acidic solutions, reactingwith metaloxides,

electrolysis - in an electrolytic process, where the metal may be pickled by the anode orthe cathode.


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Chemical setting enables the production of paint coatings out of materialdeposited by any chosen technique, as the result of:

oxidation at ambient or elevated temperature, upon contact with oxygen from the air -by spontaneous oxidation or oxide polymerization of the filmogenic substance (drying oilor the product of its initial transformation);

reticulation,without the participation of oxygen, of chemosetting resins at ambienttemperature, due to polymerization, polycondensation or polyaddition, under theinfluence of reagents (catalysts, resinous co-reagents or other macromolecularsubstances).

Gelling or formation of coatings by the sol-gel technique - a low temperature synthesisof coating material by way of the sol1) colloidal solution and multiphase gel2).

In stricter terms, this is a process of formation of sol, its subsequent transformation intogel and final treatment of gel.

The sol is a homogenous solution of an easily soluble precursor (e.g. aloxyl derivatives) inan organic solvent, mixed with a reagent, e.g. with water [15, 16]. After acid treatment(e.g. by water with HCl), the sol is transformed into gel by polycondensation.

electroplating and electroless plating corrosion engineering

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