The positive and negative electrodes of DC current are respectively connected to the anode and anode of the plating bath with wires.When DC current passes through the electrolyte containing metal ions between the two electrodes, the anion and cation in the plating solution move regularly due to the action of electric field, and the anion moves to the anode and the cation moves to the cathode.This phenomenon is called" electromigration" .At this time, metal ions are reduced and deposited on the cathode to form a coating, while anodic oxidation transfers metal to ions.Of course, in addition to electromigration, ions can also be transported by convection and diffusion.

When the anion and cation reach the surface of the anion and cathode, the redox reaction occurs:
1,Cathodic reduction reaction:Meⁿ⁺+ne=Me
2,Anodizing reaction:Me-ne=Meⁿ⁺
In fact, 2H⁺+2e=H2 may occur on the cathode(it is easy to cause hydrogen embrittlement and should be avoided).OH^--4e=2H₂O+O₂

The above is the process and reaction of electroplating.But in fact, in most electroplating processes, it is not as simple as mentioned above, because the general electroplating solution is not just a simple salt electrolyte, but an electrolyte containing complexes, and the reduction reaction on the cathode is not just a simple metal ion discharge, but a chemical reduction of complex ions.This requires a series of steps to achieve.Firstly, the important complex ions in the electrolyte are transformed into surface complexes which can discharge directly on the electrode.That is, the ligands around metal ions are reorganized or the coordination number is reduced(because the ions with higher coordination number have higher activation energy, the reduction at the cathode needs to overcome higher potential barrier, while the complex ions or hydrated ions with lower coordination number have moderate activation energy, so discharge reduction reaction is easy to occur), Then the surface complex is directly discharged on the electrode.

For example:In the electrode system of alkaline cyanide zinc plating,

Zn(CN)₄^2-+4OH^-= Zn(OH)₄^2-+4CN^-(Ligand exchange)

Zn(OH)₄^2-= Zn(OH)₂+2OH^-（The coordination number decreases）

Zn(OH)₂+2e=Zn(OH)₂^2-（Adsorption）-----Electron transfer between electron and central ion

Zn(OH)₂^2-=Zn(Lattice)+2OH^-（Deletion of ligand）

Another example: in the electrode system of cyanide cadmium plating,

Cd(CN)₄^2-=Cd(CN)₂+2CN^-（The coordination number decreases）

Cd(CN)₂+2e=Cd(CN)₂^2-（Adsorption）

Cd(CN)₂^2-=Cd(Lattice)+2CN^-（Deletion of ligand）

Therefore, the whole electroplating process can be summarized into three steps:

<1>metal hydrate ions or complex ions migrate from the solution to the cathode surface.

<2>When metal hydrated ions dehydrate or complex ions dissociate, metal ions get electrons on the cathode and undergo reduction reaction to form metal atoms.

<3>The reduced atoms enter the lattice nodes.

Note:
From the perspective of electrochemical reaction as a whole, the electrode reaction consists of the following individual steps(also called sub-steps)in series:
1.Transfer of reactive particles to the electrode surface-mass transfer step in electrolyte phase;
2.The reactive particles undergo a" pre-reaction transformation process" in the liquid layer on or near the electrode surface, such as adsorption or chemical change of the reactive particles on the electrode surface-a" pre-reaction" surface transformation step;
3.Acquisition or loss of electrons on the surface of the electrode to form a reaction product-electrochemical step;
4.The reaction product undergoes a" post-reaction conversion process" in the liquid layer on or near the electrode surface, such as desorption from the surface, recombination, decomposition, disproportionation or other chemical changes of the reaction product-a" subsequent" surface conversion step;
5a.New phase formation of reaction products, e.g.bubble formation or solid-phase deposition-new phase formation step;
5b.Transfer of reaction products from the electrode surface to the solution or to the inside of the electrode-the mass transfer step in the electrolyte phase(sometimes the reaction products may also diffuse into the inside of the electrode).

Precipitation potential and complex ions

1.Precipitation potential

The potential at which metals and other substances (such as hydrogen) start to precipitate at the cathode is called Precipitation potential.

Different metals have different precipitation potentials, and metals with positive precipitation potentials can be precipitated on the cathode preferentially.That is to say, at the cathode, the reaction with higher precipitation potential will occupy a dominant position.

For example, the potential of depositing AL must be lower than that of precipitating H by more than 0.8 V, so in aqueous solution of AL, only H ₂ can be obtained on cathode

Another example is that the precipitation potential of Zn is higher than that of Cu, The precipitation potential of Pb is low.In Zn plating, if there are metal ion impurities such as Cu and Pb in the plating solution, they often precipitate on the cathode preferentially than Zn, which destroys the regular deposition of Zn plating layer and leads to the thickening and blackening of Zn plating layer.But sometimes we can use this rule to remove metal ion impurities with positive precipitation potential by electrolysis.

On the anode, Reactions that require the smallest positive potential mainly occur.For many metals, Anodic reaction is its own dissolution, but if the potential is high enough, other oxidation reactions may occur.For example, when the potential of Cu anode is raised to about 2V higher than the potential of Cu dissolution, water will be oxidized to oxygen.However, this potential is impossible on Cu anode.

Hydrogen overpotential:The difference between the electrode potential of hydrogen evolution and its equilibrium potential at a given cathode current density is greater than the hydrogen overpotential.The higher the hydrogen overpotential, the more difficult it is to evolve hydrogen, and vice versa.

Cathodic polarization plays a very important role in the quality of coating.In the electroplating bath with very small polarization, the coating is very rough, even spongy.Only when the cathode polarization is large can the high-quality coating be produced.In the process of electrodeposition, the purpose of fine crystallization is mainly realized by improving cathodic polarization.The degree of cathodic polarization is large.Relatively speaking, the nucleus formation speed of electrodeposition is faster than that of nucleus formation, and the grain size of the coating is fine.

2.Complex ions

As we have shown above, the precipitation of metal ions on the cathode is not just a simple ion discharge reaction, but an electrochemical reduction of complex ions.

Many ions, such as Ni²⁺，Fe²⁺，Co²⁺_{, etc., can form stable complexes with a certain amount of water, such as [Ni(H2O)6]²⁺},_{[Fe(H2O)6]²⁺,[Co(H2O)6]²⁺, which are different from ionic compounds and covalent compounds, and are characterized by containing metal ions and multiple ligands in their composition A body (other ion or atom) that is bound by a pair of electrons provided separately by a ligand that is common to all.}

However, most metals have weak complexing ability of hydrated ions, so it is not enough to use water as complexing agent for electroplating.In order to get an ideal coating, complexing agent with strong complexing ability and other additives and brighteners must be added.

The activity of complex ions is much lower than that of metal ions hydrated alone, so their standard potential will move to a more negative value, which may be very large, thus changing the position of metals in the so-called electrochemical order, which is very important for alloy electroplating we will talk about later.

For example, in simple salt solution, The standard potential of Zn is 1.1 V less than that of Cu, and 0.8 V less than that of Cu in cyanide plating solution, but when the content of cyanide increases, the potential of Cu will become further negative (because the complexes of Cu change into complexes with higher coordination number [Cu (CN)₃]²⁺ and [Cu (CN)₄]_{³⁺, but Zn has little change, and finally the potential of Cu becomes the same as or even more negative than Zn, so it is possible to plate Cu-Zn alloy.}

Electricity
The most important complex ion used in Cd, Au, Ag and Zn plating is cyanide ion.Because most cyanide complexes are decomposed by acid and release highly toxic hydrogen cyanide, the plating solution must be alkaline.In addition, as mentioned in the above example, There are usually several different complexes in cyanide plating bath, However, the discharge reduction reaction of complex ions does not occur on several complex ions, Generally speaking, The deposition occurs on the complex ions containing the least coordination of cyanide, This is true even if its relative concentration is very low (but in actual electroplating, due to the high current density used, not only the coordination ions with low coordination number can discharge, but also the coordination ions with high coordination number can discharge at the potential far away from the equilibrium potential, that is, when there is a high overpotential).

In the complex, the central ion occupies the central position, and the ligand is located around the central atom.Its specific structural characteristics have great influence on the performance of the plating bath, which is one of the important research topics for electroplating workers.In addition, the nature and deposition mechanism of complex ions, especially many complex plating solutions (such as pyrophosphate plating solutions), have not been fully understood up to now.

Discharge position and crystal growth of metal ions

Metal ions deposited on the cathode can get a uniform coating, but from the microscopic point of view, the probability of ion discharge on the surface is different.The activation energy of discharge on the crystal plane is the lowest, The activation energy of edges, kinks and holes is higher, Therefore, the probability of metal ions discharging directly into the lattice at the growth point is relatively small, but first discharging on the plane to form adsorbed atoms, and then obtaining less energy to move to nodes, edges, steps or other irregular parts through diffusion, and enter the metal lattice at these places.

When these growth nodes spread along the crystal plane, monatomic growth layers connected by microscopic steps are formed, and they grow until they meet adsorbed impurities and gather into clusters to form multilayered growth steps and macroscopic steps.The microscopic step may be the outcrop of the original screw dislocation or other defects, or it may be formed by two-dimensional crystal nuclei caused by impurities.

The crystal structure of the electrodeposited layer depends on the crystalline phase characteristics of the deposited metal itself.

If the lattice of the substrate metal and the coating is similar in geometry and size, the substrate structure can be extended invariably.This growth type is called epitaxy.

If the crystal structure of the coating is far from that of the substrate, The grown crystal will be the same as the matrix structure at the beginning, and gradually change to a self-stabilizing crystal structure.However, if there are some adsorbent substances on the formed surface, the crystal lattice will change, and these adsorbent substances will be entrained into the deposited layer, which will prevent the normal crystal lattice growth or inhibit the grain growth.

High residual stresses often exist in electrodeposited layers, which may be caused by the mismatch of lattice parameters or the inclusion of foreign materials.Inclusions may be oxides, hydroxides, sulfur, carbon, hydrogen, etc.These impurities prevent the formation of normal lattices.

In an ideal situation, the interface between the electrodeposited layer and the substrate is in complete contact.At this time, because the first layer of deposited atoms is bound by the crystal of the substrate, the bonding strength is very close to the strength of the substrate metal itself.That is to say, theoretically speaking, the combination between the coating and the substrate is quite firm.However, the actual situation is not the case, such as the usual plating skin phenomenon is caused by poor process.

Factors affecting coating quality

There are many factors affecting the quality of the coating, but the most important ones are:

1.Influence of plating solution

<1>Main salt solubility:The higher the main salt solubility, the smaller the concentration polarization, the lower the crystallization nucleation rate and the coarser the microstructure.This effect is more obvious in single salt plating bath where electrochemical polarization is not significant.The dispersion ability of dilute solution is better than that of concentrated solution.

<2>Effect of ion matching:The ion matching makes cathodic polarization stronger, so the coating is denser, the dispersion ability of the plating solution is better, and the leveling ability is higher.

<3>Effect of additional salt:In addition to improving the conductivity of plating bath, it can also Enhancing cathode polarization ability is beneficial to obtain fine-grained coating.

2.PH value

The pH value of plating bath affects the discharge potential of hydrogen, the precipitation of alkaline inclusions, the composition of complexes or hydrates and the adsorption degree of additives.However, the influence degree of various factors is generally unpredictable, and the best PH value is often determined by experiments.

The anode and cathode efficiency can be known by measuring the pH value of the plating bath.

When the anode is insoluble, the metal ions in the plating bath will gradually decrease and become acidified at the same time.Because the following reaction will occur at this time:

2H2O=O2+4H+++4E-or 4OH-=O2+2H2O+4E-(in alkaline bath);

Both reactions cause acidification.

On the other hand, the precipitation of hydrogen on the cathode will alkalize the solution:2H++2e=2OH-+H2

Therefore, in the electroplating process, if the pH value rises, it means that the cathode efficiency is lower than the anode efficiency; The anode efficiency is lower than the cathode efficiency if the pH value decreases.

3.Influence of current parameters

<1>Current density:Each plating solution has its best current density range, which should be determined according to the composition of electrolyte, solubility of main salt, pH value, temperature and stirring conditions.Increasing solubility of main salt, heating up and stirring can increase the upper limit of current density.Low current density, small cathodic polarization, coarse crystallization of coating, even no coating; With the increase of current density, cathodic polarization increases and the coating becomes fine; However, if the current density is too high, the crystals will grow rapidly along the power line direction to the inside of the electrolyte, resulting in nodulation and dendritic crystals in the coating, even burning; When the current density is extremely high, hydrogen evolution is strong on the cathode surface, the PH becomes larger, and the alkali salt of metal will be mixed in the coating, which will make the coating black; In addition, with the increase of current density, the anode is sometimes passivated, resulting in the lack of metal ions in the plating bath.

<2>Current waveform:The effect of current waveform on coating quality is very obvious in some bath.

① Three-phase full-wave rectifier is equivalent to voltage-stabilized DC, and has little effect on the microstructure of the coating.

② Single-phase half-wave will make Cr plating layer produce dull black-gray color.

③ Single-phase full wave can make Cu and Cu-Sn alloy coatings bright.

④ Periodic commutation current:

When electroplating some metals(such as Cu, Ag), periodic commutation current can make the coating crystallize finely, the surface is smooth, the current density can be increased, and the deposition speed can be improved.However, it cannot be used indiscriminately, and in some cases it is even harmful.(For example, in acidic bath solution, for castings with grooves, it is harmful and useless to adopt periodic commutation current process).

4.Effect of additives(including complexes, levelers, brighteners, etc.)

Adding a small amount of certain substances in the electrolyte can obviously improve the structure of the coating and make it flat, bright and dense.These substances are called additives.

There are two main ways for additives to act in plating bath:① forming colloid to adsorb on metal ions, hindering metal ions from discharging and increasing cathodic polarization; Adsorption on the cathode surface hinders the discharge of metal ions on the cathode surface, or hinders the diffusion of discharged ions, which affects the deposition and crystallization process and improves the cathode polarization.

The additives have the functions of leveling, brightening, wetting and eliminating internal stress according to their different properties, thus improving the microstructure, surface morphology, physical, chemical and mechanical properties of the coating.

5.Effect of temperature

With the increase of temperature, the diffusion accelerates and the concentration polarization decreases.At the same time, with the increase of temperature, the dehydration process of ions is accelerated, the surface activity of ions and cathodes is enhanced, and the electrochemical polarization is also reduced.Therefore, with the increase of temperature, the cathodic polarization decreases and the crystallization of the coating is coarse.

Stirring can reduce cathodic polarization and make grains coarser; However, the current density can be increased, thus improving the production efficiency.In addition, stirring can also enhance the effect of leveling agent.

7.Effect of base metal on coating

<1>Effect of base metal properties:Adhesion of coating It is closely related to the chemical properties and crystal structure of the matrix metal.If the substrate metal potential is negative to the deposited metal potential, it is difficult to obtain a well-bonded coating, even can not be deposited.If materials(such as stainless steel, aluminum, etc.)are easy to passivate, it is difficult to obtain high adhesion coating without special activation measures.When the matrix material matches the crystal structure of the deposited metal, it is beneficial to the epitaxial growth at the initial stage of crystallization, and it is easy to obtain a coating with high adhesion.

<2>Influence of surface machining state:The surface of plated parts is too rough, porous and cracked, and the coating is also rough.Black spots, bubbling and peeling will occur in pores and crack areas.Graphite on the surface of cast iron can reduce hydrogen overpotential, and hydrogen is easy to precipitate in graphite, which hinders metal deposition.

8.Effect of preprocessing

Before electroplating, it is necessary to finish and clean the surface of the plated parts, remove burrs, sand inclusion, residue, grease, oxide scale and passivation film, so that the base metal exposes a clean and active crystal surface.Only in this way can we get a sound, dense and well-bonded coating.Improper pretreatment will lead to skin peeling, peeling, bubbling, burr, hair and other defects.

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