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Describe the fundamentals of metal electrodeposition and the main stages of the formation of the metal deposit during electrolysis.





Describe the fundamentals of metal electrodeposition and the main stages of the formation of the metal deposit during electrolysis.

Electrodeposition is the formation of a metal coating on the surface of the base material, which takes place due to the electrochemical reduction of metal ions from the electrolyte solution. Appropriate technology is often called electroplating. In addition to producing metal coatings or foils, electrochemical reduction is used to extract metals from their ores (electrometallurgy), to manufacture products with precisely defined shape and dimensions (electroforming, electroforming). In most cases, the metal precipitate is crystalline, which is why the process is called electrocrystallization. Electrolyte is an ionic conductor in which metal-containing particles are in the form of a solution or a melt. The solvent is usually water, although recently

Some specific processes also use organic solvents and other ionic liquids. The process of metal extraction actually consists in immersing the object (electrode) in a vessel (electrolyzer, galvanic bath, electrochemical cell) containing electrolyte and counter electrode, after which the electrodes are connected to an external source

Electric current. The object to be coated is connected to the negative pole. As a result, the electrode recovers the ions to the metal, which forms a precipitate on the surface.

Electrolysis is the passing of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent, producing chemical reactions at the electrodes and separation of materials.

The main components required to achieve electrolysis are:

An electrolyte: a substance, frequently an ion-conducting polymer that contains free ions, which carry electric current in the electrolyte. If the ions are not mobile, as in a solid salt then electrolysis cannot occur.

A direct current (DC) electrical supply: provides the energy necessary to create or discharge the ions in the electrolyte. Electric current is carried by electrons in the external circuit.

Two electrodes: electrical conductors that provide the physical interface between the electrolyte and the electrical circuit that provides the energy.

Electrodes of metal, graphite and semiconductor material are widely used. Choice of suitable electrode depends on chemical reactivity between the electrode and electrolyte and manufacturing cost.

First law of electrolysis.

In 1832, Michael Faraday reported that the quantity of elements separated by passing an electric current through a molten or dissolved salt is proportional to the quantity of electric charge passed through the circuit. This became the basis of the first law of electrolysis:

m = k \c or m=eQ

where; e is known as electrochemical equivalent of the metal deposited or of the gas liberated at the electrode.

Second law of electrolysis

Faraday discovered that when the same amount of current is passed through different electrolytes/elements connected in series, the mass of substance liberated/deposited at the electrodes is directly proportional to their equivalent weight.


Describe the fundamentals of electroplating. Give classification of galvanic coatings.

Electroplating is a process that uses electric current to reduce dissolved metal cations so that they form a thin coherent metal coating on an electrode. Electroplating is primarily used to change the surface properties of an object (e.g. abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc.)

The process used in electroplating is called electrodeposition. It is analogous to a galvanic cell acting in reverse. The part to be plated is the cathode of the circuit. In one technique, the anode is made of the metal to be plated on the part. Both components are immersed in a solution called an electrolyte containing one or more dissolved metal salts as well as other ions that permit the flow of electricity. A power supply supplies a direct current to the anode, oxidizing the metal atoms that it comprises and allowing them to dissolve in the solution. At the cathode, the dissolved metal ions in the electrolyte solution are reduced at the interface between the solution and the cathode, such that they "plate out" onto the cathode. The rate at which the anode is dissolved is equal to the rate at which the cathode is plated, vis-a-vis the current through the circuit. In this manner, the ions in the electrolyte bath are continuously replenished by the anode.

The cations associate with the anions in the solution. These cations are reduced at the cathode to deposit in the metallic, zero valence state. For example, for copper plating, in an acid solution, copper is oxidized at the anode to Cu2+ by losing two electrons. The Cu2+ associates with the anion SO42− in the solution to form copper sulfate. At the cathode, the Cu2+ is reduced to metallic copper by gaining two electrons. The result is the effective transfer of copper from the anode source to a plate covering the cathode.

The plating is most commonly a single metallic element, not an alloy. However, some alloys can be electrodeposited, notably brass and solder.

Many plating baths include cyanides of other metals (e.g., potassium cyanide) in addition to cyanides of the metal to be deposited. These free cyanides facilitate anode corrosion, help to maintain a constant metal ion level and contribute to conductivity. Additionally, non-metal chemicals such as carbonates and phosphates may be added to increase conductivity.

Classification of galvanic coatings

Considering the requirements that are imposed on the performance characteristics of parts, galvanic coatings can be conditionally divided into three types:

- Protective-decorative galvanic coatings (used to impart decorative and protective properties to surfaces at the same time);

- protective electrolytic coatings (used to protect parts from corrosion in various corrosive environments);

- Galvanic coatings for special purposes (used to give the surface of the metal certain special properties, such as magnetic, hardness, wear resistance, electrical insulation, etc.). Also, specialty galvanic coatings can be applied to restore worn parts

Depending on the mechanism of protective action, all galvanic coatings are divided into: cathode and anodic. Compared with the potential of the protected metal, anodic coatings always have more electronegative, and cathodic coatings have more electropositive potential. For example, in relation to steel, cadmium and zinc are anodic coatings, and gold, nickel, silver, and copper are cathodic.

The mechanism of the protective effect of electroplating largely depends not only on the nature of the metal, but also on the composition of the operating environment.


Chemical oxidation

Chemical oxidation is carried out by processing the product in solutions (melts) of oxidants (chromates, nitrates, etc.). With this method, the surface of the product is passivated or applied to protective and decorative layers. For ferrous metals, chemical oxidation is carried out at a temperature of 30 to 100 ° C in alkaline or acidic formulations. For acid oxidation, a mixture of several acids is used, for example, nitric (or orthophosphoric) and hydrochloric acid with some additives (Ca (NO3) 2, Mn compounds). Alkaline oxidation is carried out at temperatures slightly higher, about 30 - 180 ° C. Oxides are added to the formulation. After the oxide layer is applied, the metal products are well washed and dried. Sometimes the finished coating is oiled or further processed in oxidative solutions.

Protective layers obtained with the use of chemical oxidation have less protective properties than films obtained by anodizing.

Thermal oxidation

Thermal oxidation is the process of formation of an oxide film on a metal at elevated temperatures and in oxygen-containing (perhaps water vapor) atmospheres. Thermal oxidation is carried out in heating furnaces. When thermal oxidation of low-alloy steels or iron (the operation is called blasting), the temperature is raised to 300-350 ° C. For alloyed steels thermal oxidation is carried out at higher temperatures (up to 700 ° C). The duration of the process is about 60 minutes. Very often thermal oxidation is used to create an oxide layer on the surface of silicon products. Such a process is carried out at high temperatures (800 - 1200 ° C). Oxidized silicon products are used in electronics.

Microarc oxidation

Microarc oxidation (MDO) is a method of obtaining multifunctional oxide layers. Microarc oxidation - marching from anodizing. It allows to apply layers with high protective, corrosion, heat-resistant, insulating, decorative properties. In appearance, the coating obtained by the microarc method is very similar to ceramics.

Now this is one of the most promising and in-demand methods of applying oxide layers, because Allows to apply heavy-duty coatings with unique characteristics.

The process of microarc oxidation is, in most cases, carried out in weakly alkaline electrolytes with pulsed or alternating current. Prior to the coating, no special surface preparation is required. The peculiarity of the process is that. That energy is used from electrical microdischarges, which move randomly along the surface being treated. These microdischarges have a plasmachemical and thermal effect on the coating and electrolyte. The oxide layer is approximately 70% formed deep into the base metal. Only 30% of the coating is completely outside the product.

The thickness of the coatings obtained by the microarc method is about 200 - 250 μm (thick enough). The temperature of the electrolyte can vary from 15 to 400 ° C, and this does not have a special effect on the process.

The electrolytes used do not have a harmful effect on the environment and their service life is very long. The equipment is compact, does not take up much space and is simply in use.

The dissipation capacity of the electrolytes used is high, which makes it possible to obtain coatings even on difficult-to-relieve parts.

Microarc oxidation is used to form coatings mainly on magnesium and aluminum alloys.


Answer: 2 sec


Answer: 20,4 sec


36. Calculate the thickness of the zinc coating, which was obtained at the next conditions: time of the process = 20 min, surface S = 0,80 dm2, current density j = 3,2 A/dm2 and the cathode current efficiency = 84.4% for zinc.

Zn

j=3.2 A/dm2

Ƞ=84,4%

M(Zn)=65\mol

F=26,8 A• h

ρ(Zn)= 7 133 kg/m3

τ=20 min = 0,33 h

d=?1

d= j • q • Ƞ • τ / ρ

q=M/nF

q=65/2*26,8 =1,21 g/A• h

d=3.2 A/dm2• 1,21 g/A• h • 84,4 • 0,33 h / 7 133 g/dm3=0.01512 dm = 1,512 mm

Answer: d = 1,512 mm


37. Calculate the thickness of the nickel precipitate formed within 20 minutes, if the current efficiency is 90%, the current density is 3A/dm2. The density of nickel is 8902 kg/m3.

Ni

j=3.0 A/dm2

Ƞ=90%

M(Ni)=58,7g\mol

F=26,8 A• h

ρ(Ni)= 8902 kg/m3

τ=20 min = 0,33 h

d=?

d= j • q • Ƞ • τ / ρ

q=M/nF

q=58,7/2*26,8 =1,01 g/A• h

d=3.0 A/dm2• 1,01 g/A• h • 90 • 0,33 h / 8902 g/dm3=0,0101 dm = 1,01 mm

Answer: d = 1,01 mm


38. Calculate the thickness of the gold coating on the wedding ring, which was obtained at the next conditions: time of the process = 10 min, current density j = 1,2 A/dm2 and the cathode current efficiency = 95%. Ring has internal diametr: 1sm 8 mm, external diametr: 1sm 10 mm, width: 1sm

Au

j=1.2 A/dm2

Ƞ=95%

M(Ni)=197 g\mol

F=26,8 A• h

ρ(Au)= 19 300 kg/m3

τ=10 min = 0,166 h

d=?

d= j • q • Ƞ • τ / ρ

q=M/nF

q=197/1*26,8 =7,35 g/A• h

d=1.2 A/dm2• 7,35 g/A• h • 95 • 0,166 h / 19300 g/dm3= 0,00721 dm = 0,73 mm

Answer: d = 0,73 mm


Describe the fundamentals of metal electrodeposition and the main stages of the formation of the metal deposit during electrolysis.

Electrodeposition is the formation of a metal coating on the surface of the base material, which takes place due to the electrochemical reduction of metal ions from the electrolyte solution. Appropriate technology is often called electroplating. In addition to producing metal coatings or foils, electrochemical reduction is used to extract metals from their ores (electrometallurgy), to manufacture products with precisely defined shape and dimensions (electroforming, electroforming). In most cases, the metal precipitate is crystalline, which is why the process is called electrocrystallization. Electrolyte is an ionic conductor in which metal-containing particles are in the form of a solution or a melt. The solvent is usually water, although recently

Some specific processes also use organic solvents and other ionic liquids. The process of metal extraction actually consists in immersing the object (electrode) in a vessel (electrolyzer, galvanic bath, electrochemical cell) containing electrolyte and counter electrode, after which the electrodes are connected to an external source

Electric current. The object to be coated is connected to the negative pole. As a result, the electrode recovers the ions to the metal, which forms a precipitate on the surface.

Electrolysis is the passing of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent, producing chemical reactions at the electrodes and separation of materials.

The main components required to achieve electrolysis are:

An electrolyte: a substance, frequently an ion-conducting polymer that contains free ions, which carry electric current in the electrolyte. If the ions are not mobile, as in a solid salt then electrolysis cannot occur.

A direct current (DC) electrical supply: provides the energy necessary to create or discharge the ions in the electrolyte. Electric current is carried by electrons in the external circuit.

Two electrodes: electrical conductors that provide the physical interface between the electrolyte and the electrical circuit that provides the energy.

Electrodes of metal, graphite and semiconductor material are widely used. Choice of suitable electrode depends on chemical reactivity between the electrode and electrolyte and manufacturing cost.

First law of electrolysis.

In 1832, Michael Faraday reported that the quantity of elements separated by passing an electric current through a molten or dissolved salt is proportional to the quantity of electric charge passed through the circuit. This became the basis of the first law of electrolysis:

m = k \c or m=eQ

where; e is known as electrochemical equivalent of the metal deposited or of the gas liberated at the electrode.

Second law of electrolysis

Faraday discovered that when the same amount of current is passed through different electrolytes/elements connected in series, the mass of substance liberated/deposited at the electrodes is directly proportional to their equivalent weight.








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