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Electroplating treatment of metal surface

🕙 2023-08-29 👆

1. Definition and Principles of Electroplating

Electroplating is a surface treatment process that utilizes electrochemical properties to deposit the desired form of metal coating on the surface of the plated piece.

Electroplating principle: In a salt solution containing the metal to be plated, using the substrate metal as the cathode, the cation of the metal to be plated in the plating solution is deposited on the surface of the substrate metal through electrolysis, forming a coating.

The purpose of electroplating is to obtain a surface layer with special properties that is different from the substrate material, and to improve the corrosion resistance and wear resistance of the surface. The thickness of the coating is generally several micrometers to tens of micrometers.

The characteristics of electroplating: The electroplating process equipment is relatively simple, the operating conditions are easy to control, the coating materials are wide, and the cost is low. Therefore, it is widely used in industry and is an important method for material surface treatment.

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2 Classification of coatings

There are many types of coatings, classified according to their performance as follows:

Protective coatings: such as zinc, zinc nickel, nickel, cadmium, tin and other coatings, used as anti-corrosion coatings for atmospheric and various corrosive environments.

Protective decorative coatings: such as Cu Ni Cr coatings, which have both decorative and protective properties.

Decorative coatings: such as Au and Cu Zn imitation gold coatings, black chromium, black nickel coatings, etc.

Wear resistant and anti wear coatings: such as hard chromium coatings, loose hole coatings, Ni Sic coatings, Ni graphite coatings, Ni PTFE composite coatings, etc.

High electrical performance coatings: such as Au coatings, Ag coatings, etc., have high conductivity and can prevent oxidation, avoiding increasing contact resistance.

Magnetic performance coatings: For example, soft magnetic performance coatings include Ni Fe coatings and Fe Co coatings; Hard magnetic properties include Co P coating, Co Ni coating, Co Ni P coating, etc.

Weldable coatings: such as Sn-Pb coatings, Cu coatings, Sn coatings, Ag coatings, etc. It can improve solderability and is widely used in the electronic industry.

Heat resistant coating: such as Ni-W coating, Ni coating, Cr coating, etc., with high melting point and high temperature resistance.

Repair coating: For some easily worn parts with high cost or machined parts that are out of tolerance, using electroplating to repair dimensions can save costs and extend service life. For example, Ni, Cr, and Fe layers can be electroplated for repair.

According to the electrochemical properties between the coating and the base metal, it can be divided into anodic coating and cathodic coating. When the potential of the coating relative to the base metal is negative, the coating is an anode, called an anodic coating, such as the zinc coating on steel; When the potential of the coating relative to the base metal is positive, the coating is cathodic and is called a cathodic coating, such as the nickel coating, tin coating, etc. on steel.

If classified according to the combination form of coatings, coatings can be divided into: single-layer coatings, such as Zn or Cu layers; Multi layer metal coatings, such as Cu Sn/Cr coatings, Cu/Ni/Cr coatings, etc; Composite coatings, such as Ni Al2O3 coatings, Co SiC coatings, etc.

If classified by coating composition, it can be divided into single metal coating, alloy coating, and composite coating.

Basic composition of electroplating solution

The main salts used for depositing metals are: single salts, such as copper sulfate, nickel sulfate, etc; Complex salts, such as sodium zincate, sodium cyanozincate, etc.

The coordination agent forms a complex with deposited metal ions, which mainly changes the electrochemical properties of the plating solution and controls the electrode process of metal ion deposition. The coordination agent is an important component of the plating solution and has a significant impact on the quality of the coating. Commonly used coordination agents include cyanide, hydroxide, pyrophosphate, tartrate, aminotriacetic acid, citric acid, etc.

The function of conductive salts is to improve the conductivity of the plating solution, reduce the voltage at the slot end, and increase the process current density. For example, adding Na2SO4 to the nickel plating solution. Conductive salts do not participate in electrode reactions, and acids or bases can also serve as conductive substances.

The pH value of buffering agents is an important process parameter in weakly acidic or weakly alkaline plating solutions. Add a buffer to enable the plating solution to have the ability to adjust its pH value on its own, in order to maintain pH stability during the plating process. A sufficient amount of buffer is required to effectively control the acid-base balance, usually adding 30-40g/L, such as boric acid in potassium chloride zinc plating solution.

During the electroplating process, metal ions are constantly consumed by anode activators, and most plating solutions rely on soluble anodes to replenish them, thereby ensuring that the amount of metal precipitated from the cathode is equal to the amount dissolved from the anode, and maintaining a balanced composition of the plating solution. Adding an active agent can maintain the anode's active state without passivation and maintain normal dissolution reaction. For example, Cl - must be added to the nickel plating solution to prevent nickel anode passivation.

Special additives are often added to improve the performance of the plating solution and improve the quality of the coating. Its addition amount is relatively small, usually only a few grams per liter, but the effect is significant. There are various types of these additives, which can be divided into:

Brightener - can improve the brightness of the coating.

Grain Refiner - can change the crystalline state of the coating, refine the grains, and make the coating dense. For example, in zincate zinc plating solution, adding additives such as epichlorohydrin and amine condensates can change the coating from sponge like to dense and bright.

Leveling agent - can improve the micro dispersion ability of the plating solution, making the micro rough surface of the substrate smoother.

Wetting agent - can reduce the interfacial tension between metal and solution, making the coating adhere better to the substrate and reducing pinholes.

Stress Relieving Agent - can reduce plating stress.

Coating hardener - can improve the hardness of the coating.

Masking agent - can eliminate the influence of trace impurities.

4 Basic steps of electroplating process

The basic steps of electroplating process include liquid phase mass transfer, electrochemical reduction, and electrocrystallization.

5 Factors affecting electroplating quality

Main salt solubility, coordination ions, and additional salts; PH value; Hydrogen evolution; Current parameters: current density, current waveform; Additives; Temperature; Stirring; Base metal: properties, surface processing status; Pre processing.

Plating method: hanging plating. Metals such as W, Mo, Ti, V that cannot be electroplated separately from aqueous solutions can co deposit with iron group elements (Fe, Co, Ni) to form alloys; Thus obtaining an appearance that a single metal cannot obtain.

Conditions for depositing alloys: At least one of the two metals can be deposited from its salt aqueous solution; The deposition potentials of the two co deposited metals must be very close.