Reminder: These 6 major factors directly affect your electroplating process
The electroplating process conditions refer to the operational change factors during electroplating, including current density, temperature, stirring and waveform of the power supply, etc.
1. The effect of cathode current density
Any plating solution has a current density range for obtaining a good coating, the minimum current density for obtaining a good coating is called the lower limit of current density, and the maximum current density for obtaining a good coating is called the upper current density limit. Generally speaking, when the cathodic current density is too low, the cathodic polarization is small, the crystal grains of the coating are coarser, and the low cathodic current density is rarely used in production. With the increase of the cathode current density, the polarization of the cathode also increases (the increase of the polarization value depends on various electroplating solutions), and the crystallization of the coating becomes fine and compact; The current density cannot be too large and cannot exceed the allowable upper limit value (different electroplating solutions have different upper limit values of cathode current density under different process conditions), after exceeding the allowable upper limit value, due to the serious lack of metal ions near the cathode, a metal coating in the shape of a branch will be produced at the tips and protrusions of the cathode, or a loose coating in the shape of a sponge will be produced on the entire cathode surface. What is often encountered in production is that the phenomenon of "burning" easily occurs at the sharp corners and edges of the parts, and in severe cases, dendrites or spongy coatings are formed. In general, if the current density is too low, the cathodic polarization is small, and the formation speed of the crystal nucleus is slow. However, the growth rate is fast; if the current density continues to increase, the cathodic polarization will gradually increase, and the cathode overpotential will also continue to increase. It becomes thinner and thinner; when the current density continues to increase to a certain value (the upper limit of current density), a charred coating appears, showing a loose spongy, or a rough coating with abnormal color. This is because when the current density is too large, there is a serious lack of discharge metal ions near the cathode, resulting in the rapid precipitation of hydrogen, which causes the pH value to rise rapidly, and metal hydroxides or basic salts are formed on the surface of the cathode to be attached to the coating. , forming voids. Pockmarks. Loose and charred, etc. Within the normal current density range, increasing the current density can obtain a more detailed coating, and can also speed up the deposition rate and improve labor productivity. The size of the current density range of the bath is usually determined by the properties of the bath, the concentration of the main salt, the bath temperature, and the stirring.
2. Effect of plating solution temperature
When other conditions remain unchanged, increasing the temperature of the solution usually speeds up the cathodic reaction rate and ion diffusion rate, and reduces the cathodic polarization. Therefore, the crystallization of the coating will become coarser. However, it cannot be considered that increasing the solution temperature is unfavorable. If it is properly coordinated with other process conditions, increasing the solution temperature will also achieve good results. For example, increasing the temperature can increase the upper limit of the allowable cathode current density. The increase of the cathode current density will increase the cathodic polarization to make up for the lack of temperature rise. This will not only not make the coating crystal coarse, but also speed up the deposition rate. , Increase productivity. In addition, it can improve the conductivity of the solution, promote the dissolution of the anode, improve the current efficiency of the cathode (except for chrome plating), reduce pinholes, and reduce the internal stress of the coating. Raising the bath temperature will reduce the cathodic polarization and lead to the coarsening of the coating crystals. This is because the discharge metal ions have greater activation ability when the temperature of the plating solution is high, which reduces the electrochemical polarization; in addition, the increase of the temperature increases the ion diffusion rate due to thermal motion and reduces the concentration polarization. . The combined result is a reduction in cathodic polarization during electrodeposition. In fact, increasing the temperature usually increases the upper limit of the current density, and at the same time, due to the increased solubility of salts, a higher concentration of the plating solution is allowed, so that a larger current density can be used. And increasing the current density can improve the cathodic polarization, which is conducive to the formation of fine-grained coatings, so as long as the coordination is appropriate, increasing the temperature of the plating solution will also help to form a good coating. In addition, increasing the temperature has the advantages of improving the conductivity of the plating solution, promoting the dissolution of the anode, reducing the pinholes in the coating, and reducing the internal stress of the coating. Therefore, electroplating can be performed at a high temperature within the operating temperature range. Some types of plating require heating to obtain a qualified plating layer, and some types of plating must work at a certain temperature. It is absolutely necessary to make up for the lack of performance of the plating solution by heating or cooling. Taking bright nickel plating as an example, when the temperature is below 40C, even if a brightener is added, it is difficult to obtain a bright coating. However, when the plating solution is heated to above 50C, a very bright nickel plating layer can be obtained.
3. The effect of stirring
Stirring will accelerate the convection of the solution so that the consumed metal ions near the cathode can be replenished in time and the concentration polarization of the cathode can be reduced. However, after stirring, the upper limit of the allowable cathode current density can be increased, so that the phenomenon of crystal coarsening caused by the reduction of cathodic polarization by stirring can be overcome. The use of stirring can achieve higher current density and higher current efficiency. A tight and fine coating is obtained. For some bright baths, such as bright sulfate copper plating and bright nickel plating, stirring can also improve the leveling of the coating. In some cases, streaks or orange peel can also be eliminated. The stirring plating solution must be filtered regularly or continuously to remove various solid impurities and dross in the solution, otherwise, it will reduce the bonding force of the coating and make the coating rough, loose, and porous. The use of stirring can increase the current density range, improve the electroplating efficiency, improve the dispersion ability of the plating solution, and improve the quality of the coating. Agitation is commonly used in bright nickel and copper plating processes. Stirring can strengthen the convection of the plating solution and reduce the thickness of the diffusion layer so that the discharge metal ions on the cathode surface can be quickly replenished during electrodeposition, and the concentration polarization can be reduced. At the same time, stirring can increase the upper limit of the allowable current density, increase the operating current density, and increase the cathodic polarization. At present, the commonly used methods for stirring the plating solution include moving the cathode, stirring with compressed air, and circulating the plating solution. Cathode movement has lateral movement and vertical movement. Compressed air stirs violently, which can float the solid particles deposited on the bottom of the tank and disperse them into the plating solution. Therefore, when using compressed air to stir the plating solution, a continuous filtration device is generally required. Otherwise, the floating solid particles will cause the coating to be rough or burrs. For some plating solutions that are easy to interact with oxygen and carbon dioxide in the air, such as iron plating and sulfate tin plating, it is not appropriate to use this type of stirring.
4. The influence of power
The power sources commonly used in electroplating production are rectifiers and DC generators, and various current waveforms can be obtained according to the number of phases of the AC power supply and the rectifier circuit. For example, single-phase half-wave, single-phase full-wave, three-phase half-wave, and three-phase full-wave, etc. The practice has proved that the current waveform has an influence on the crystal structure of the coating, the dispersion ability, coverage of the brightness plating solution, the alloy composition, and the consumption of additives, so the selection of the current waveform should be paid attention to. At present, in addition to the general direct current, periodic commutation current and pulse current can also be used according to actual needs.
The periodic commutation current periodically changes the direction of the DC current. That is, during electroplating, the direction of the DC current is forward for a period of time, and reverse for a period of time. The current is to use the plated part as the anode. The sum of a period of forwarding electroplating time and a period of reverse stripping time is one cycle time (tk+ta=T). It has been proved by practice that cyclic commutation current is applied to cyanide copper plating and fluoride silver plating. The quality of the coating obtained is much better than that obtained with ordinary direct current, which is due to the reverse stripping. When the plating can be removed The resulting inferior coating can reduce or eliminate the roughness and burrs on the coating; at the same time, it can also make the tip and edge of the plated part with thick coating thickness, and remove more coating during stripping. The coating thickness is uniform and the leveling is good. In the application of periodic reversing electroplating, it is best to perform cathodic electroplating in the parts tank first to prevent the plated parts from being used as anodes when there is no coating, causing the base metal to corrode and contaminate the plating solution.
A pulsed current is one in which a unidirectional (cathode) current is periodically interrupted by a series of open circuits (no current passing through it). The difference between it and the commutation current is that the plated part is not used as the anode, but the power supply is stopped intermittently. Due to the intermittent interruption of the current, the cathode potential changes periodically with time. DC rectifiers are commonly used in electroplating power supplies, and the selection of different rectifier power supplies also affects the quality of electroplating.
(1) The influence of power supply. The power of the electroplating power supply must be able to withstand the current required by the parts being processed and can work continuously without failure. Generally, the total current required is calculated according to the surface area of the product to be plated and the normal current density range of the planted species, plus a certain insurance factor to determine the size of the rectifier power supply. Select the power supply and voltage range for different plating types. The general electroplating tank voltage is below 6v. Due to changes in anode passivation and solution conductivity (effects of concentration, temperature, stirring, etc.), the cell voltage will fluctuate, sometimes around 10V. Therefore, the conventional electroplating power supply voltage should be in the range of 0~12V. For processes such as chrome plating and aluminum anodizing, the power supply voltage should be 0~24V or higher.
(2) The influence of the power waveform. Electroplating generally uses a DC power supply, but there are often errors in the understanding of DC. It is not that the current from the rectified power supply is a stable DC. All have a certain pulse, and the magnitude of the pulse depends on the rectifier circuit and device used. Usually, the power supply suitable for electroplating should be a three-phase bridge rectifier and a filtered line. Especially for chrome plating, smooth DC is beneficial to the chrome plating process. Because the current efficiency of chrome plating is only 10% to 15%, if there are more pulses in the DC current, the current efficiency will be further reduced, and its dispersion ability will also decrease, so the plating quality cannot be guaranteed. AC and DC superimposed current is a power supply that superimposes AC and DC together for electroplating. At present, it has been used in pyrophosphate copper plating and copper-tin alloys can obtain a coating with fine crystals and better gloss, and can also expand the range of cathode current density.
5. Influence of base metal on coating quality
(1) The influence of the properties of metal materials. Whether the combination of the coating metal and the base metal is good is closely related to the chemical properties of the base metal. In a certain electrolyte, if the potential of the base metal is negative than that of the coating metal, it is not easy to obtain a well-bonded coating. Like steel parts in sulfate copper plating electrolytes, iron has a negative potential than copper. When steel parts are placed in the plating solution, copper ions will be replaced and attached to the surface of the parts. The displacement plating layer is loose and has poor bonding force, which affects the bonding between the plating layer and the base metal. There are also some metals with very negative potential, such as zinc, aluminum, etc., which are very active and have a greater tendency to replace. For electroplating on such metals, special pretreatment (pre-dip galvanizing, etc.) must be carried out before plating. In addition, some metals such as stainless steel, chromium alloy, and other metals with passivation properties are easy to form an oxide film on the surface. When electroplating on such metals, it is difficult to obtain and matrix without special activation treatment. Combined with strong plating.
(2) The influence of processing properties before plating. The processing state before plating and the preparation work before plating have a very important influence on the quality of the coating. The surface of cast iron parts is often uneven and porous. Electroplating on such a surface is often easy to obtain a rough and porous coating. Moreover, the graphite in pig iron has a low hydrogen overpotential, and hydrogen is easily precipitated there, which hinders the deposition of metals and even cannot obtain a uniform and continuous coating. It is difficult to obtain high-quality electrodeposited coatings on parts with machining defects (cast pores, rough surfaces). Because of the coating deposited on the surface of this part. After a certain period of time, black spots appear on the surface, also known as flooding or bleeding.
(3) The surface state of the base metal. Since the overpotential of hydrogen on the rough surface is smaller than that on the smooth surface, hydrogen is easy to precipitate on the rough surface, and the coating is not easy to deposit. Therefore, reducing the roughness of the base metal can often improve the coverage. Another example is that the base metal contains impurities with a small hydrogen overpotential (such as carbon impurities in cast iron), and hydrogen is easy to precipitate on these impurities, and the coating is difficult to deposit. If the overpotential of hydrogen on the base metal is smaller than the overpotential on the coating metal, more hydrogen will escape when it is just entering the bath for electroplating. If the coating is plated locally at this time, then the part where the coating is plated first will have less hydrogen evolution and high current efficiency, which will reduce the dispersion ability. At this time, in order to obtain a uniform and continuous coating, a short-time high current density "shock" is often used at the beginning of the power-on, so that the surface of the base metal is quickly plated with a layer of metal with a large hydrogen overpotential, and then according to the normal regulations. Electroplating at a high current density can eliminate the adverse effects of the base metal on the dispersion ability and coverage ability.
6. The influence of geometrical factors on the quality of electroplating
Geometric factors include the shape and size of the plating tank; the shape and configuration of the anode; the shape of the hanger and the shape of the parts to be plated.
(1) Plating tank. The design of the plating tank is determined according to the output, size, and shape of the parts to be plated.
(2) Hanging tool. The influence of the hanger on the quality of electroplating is very large, but it is most easily overlooked. The conductive cross-section of the hanger should be calculated and allocated according to the total current intensity of the plating tank and the number of hangers. The specific form of the hanger should also be determined according to the shape of the part. The hanger should be made of copper or brass. This can increase the electrical conductivity. Judging from the current passing through the 1mm2 wire, the copper is 24A/mm2, the brass is 5A/mm2, and the iron is 3A/mm2.
(3) Anode. The best way to load the anode is to use a titanium basket, which can ensure that the surface area of the anode does not fluctuate greatly. In addition, the dissolution utilization rate of the anode can be greatly improved. The surface area of the anode should be 1-2 times that of the cathode, and the area of the anode should be larger in special cases. Wire electroplating is more suitable to keep the area of the anode more than 20 times larger than that of the cathode, because the working current density is large, and the anode is easily passivated. When using a hanging anode, do it in strips. And keep the hook in a good conductive state. The purity of the electroplating anode should be above 99.99%, otherwise, the impurities in the plating solution will affect the quality of the electroplating layer. Insoluble anodes (such as iron plates, stainless steel plates, etc.) can also be used if necessary.
(4) The shape of the plated parts. The shape of the plated part is one of the important factors that determine the difficulty of electroplating processing. Barrel plating is possible for simple parts, such as standard or small parts. For strips, inclined horizontal suspension can be used. However, for special-shaped pipe fittings, self-hole parts, etc., corresponding hangers should be designed, and some auxiliary anodes should be set up to solve the problem of coating distribution on the inner wall of deep holes. When hanging, try to keep the protruding part away from the anode. Use the parts themselves to block each other. When necessary, a protective cathode is used to prevent the protruding parts of the plated parts from being "burned" due to the "tip discharge effect". For each part to be plated, the plated surface area should be calculated to determine the magnitude of the current given during the electroplating process.
(5) Geometric factors of electrodes and plating tanks. The shape and size of the electrodes, the distance between the electrodes, the position of the electrodes in the plating tank, the shape of the plating tank, etc., all affect the uniform distribution of the coating on the cathode surface. In order to improve the uneven current distribution on the electrodes caused by this, auxiliary cathodes and pictographic anodes are often used in electroplating production, and the distance between the cathode and anode is appropriately increased.
