During electroplating, faults are caused by many and complex factors, such as phase loss, short circuit, or component damage in the power supply equipment, and quality variations in raw and auxiliary materials, which are sometimes difficult to identify and are unavoidable objective factors. However, neglecting process specifications and operating procedures is a man-made factor that can be avoided. In production, among many faults, the latter are always more than the former.
Fault examples: Poor deposit brightness
(1) Poor brightness of zinc-iron alloy deposits is common. The factors causing it are relatively few, mainly due to reduced consumption of brightener. Appropriate replenishment can improve brightness; if the effect is not good, continue adding. Once brightener is overdosed, other faults can occur. Pay attention to adding less but more frequently, and the fault can be eliminated.
(2) The concentration of main salt or conducting salt can also affect deposit brightness. It is best to analyze the main components of the bath before adjustment.
(3) Poor deposit brightness also indicates a low cathodic current density. Properly increasing the current density can eliminate the fault.
(4) Many impurities in the bath increase the solution resistance and also affect deposit brightness. Brightener contains effective components that sequester copper and lead impurities, so brightness reduction caused by many impurities is rare. Once it occurs, the solution is turbid and the deposit is rough; purify the solution.
Mottled deposits
During electroplating, mottling on workpieces is a common fault. Many factors cause mottled marks, so they should be treated differently.
(1) Substrate defects, such as corrosion spots, cause mottled deposits on those areas; in severe cases no deposit forms. This fault cannot be solved by electroplating.
(2) Oil or rust not completely removed from the workpiece causes mottling, peeling, or no deposit on those areas. Strengthening degreasing and derusting can avoid it.
(3) After acid pickling, workpieces are not cleaned thoroughly before entering the plating tank; residual acid on the workpiece causes mottling. Strengthening cleaning can eliminate it.
(4) After plating, if workpieces are not cleaned thoroughly before passivation, the passivation film at residual bath spots tends to mottle. Strengthening cleaning can remove it.
(5) If additives are not diluted, they enter the bath and appear milky before dissolving and dispersing, causing mottled deposits. Dilute additives and add under stirring to avoid this.
(6) If workpieces are hung too densely, the deposit in mutually shielded areas has no brightness and appears mottled like dark and light sides. Reduce workpiece loading to avoid the fault.
(7) If the spacing between anodes exceeds 300 mm, power lines cannot reach the gap between two anodes, causing a misty whitish mottling of dark-and-light pattern. Keeping anode spacing at 300 mm prevents this.
(8) Poor contact between the anode copper hook and the busbar causes local poor conductivity on the workpiece and easy mottling. Move the anode to make contact and the fault is eliminated.
(9) If anode length is insufficient, and workpieces extend beyond the anode length, power lines cannot reach the overhanging areas, which also easily causes misty mottling.
Cloudy, gray, or black deposits
This fault mainly occurs in low current density areas, has many causes, and occurs frequently.
(1) The main salt concentration, whether too high or too low, is prone to cause this. If too high, the deposition rate is fast, the deposit brightness is good, and high current density areas are not burned, but the deposit in low current density areas is cloudy, gray, or black. If too low, deposition rate and brightness decrease. Analyze the bath to adjust, or perform a Hull cell test. Do not blindly add main salt; a slight carelessness makes the fault hard to remove.
(2) If conducting salt concentration is low, the deposit in low current density areas is not only cloudy, gray, or black, but the voltage is high and current cannot be increased; if current is increased slightly, high current density areas burn and brightness is low. Analysis and adjustment or Hull cell test can eliminate it.
(3) If make-up additive concentration is low, throwing power and covering power decrease, and the deposit in low current density areas becomes cloudy, gray, or black. If judged accurately, proper replenishment can eliminate it; do not over-add.
(4) Many foreign metal and organic impurities in the bath. If they are impurities such as copper and lead, additives have a sequestering function and no treatment is needed. Generally, it is caused by excessive decomposition products of additives. At this time, the solution is somewhat turbid; activated carbon should be added and removed by filtration.
Rough deposits
(1) More solid particle impurities in the solution, mainly from insoluble substances produced by anode dissolution. Organic decomposition products of additives are best treated with activated carbon and filtration.
(2) The Fe3+ content in the bath increases. Fe3+ is generated from oxidation of Fe2+ in the main salt. During the oxidation reaction, H+ in the solution is consumed and decreases, pH increases, and Fe3+ changes from colloidal state to insoluble hydroxide solid particles. These are adsorbed on the deposit, causing roughness. When Fe3+ is high, the solution becomes turbid and slightly brownish-yellow; in severe cases it becomes brick red. At this time, add 0.5–1.0 g/L reducing agent to reduce Fe3+ to Fe2+; the solution becomes clear again and the fault is eliminated.
(3) The content of additive (carrier brightener) is too low. Replenish make-up additive at 1/2 of the amount for a newly prepared bath.
(4) The relative distance between the anode copper busbar and cathode copper busbar is too close, resulting in a distance between zinc plates (anodes) and racks (cathodes) less than 150 mm. Change the rack design to suit it.
(5) Main salt zinc chloride content is too high; properly dilute the solution and correspondingly supplement conducting salt and brightener.
Blistering and peeling of deposit
(1) The problem lies in pretreatment. Strengthening degreasing and pickling, and checking workpieces before entering the tank can remove it.
(2) Overdosing brightener; or drying temperature after zinc plating and passivation is too high, causing powdery detachment of the deposit.
Burned deposit
(1) The conducting salt content is low, deposition rate is low, and brightness is poor. If current is slightly high, deposits in high current density areas burn. Properly adding conducting salt eliminates the fault.
(2) Cathodic current density out of control. Because a silicon rectifier power supply cannot maintain constant voltage and constant current, this mostly occurs in sudden changes in AC power. This is an unavoidable temporary phenomenon and quickly returns to normal current. Another situation is caused by improper operation, increasing the current too much; just appropriately reducing the current eliminates it.
(3) Workpieces are removed from the tank with power on; places touching zinc plates are burned and blackened at random positions. As long as the power is turned off before removing workpieces, this fault will not occur.
Turbid, slightly brown, or brick-red solution
Zinc-iron alloy plating solution sometimes becomes turbid or slightly brown, and in severe cases brick red, which is unavoidable. It is caused by Fe2+ being oxidized to Fe3+ in the solution, ultimately leading to rough deposits.
When the bath becomes turbid, one possibility is high conducting salt concentration. After properly adding water and stirring, the turbid solution immediately becomes clear. Another is that after adding water, it remains turbid. This is the colloidal state in the process of Fe2+ oxidized to Fe3+, making the solution turbid and then slightly brownish red. Dissolve 0.5 g/L reducing agent in water and add to the solution; after Fe3+ is reduced to Fe2+, the solution becomes clear again.
Coating pinholes and streaks. Pinholes and streaks in the coating may appear simultaneously or separately, and the main cause is additives. Additive-related causes fall into two cases: one is an exceptionally bright coating; the other is very poor coating brightness. The former is due to excessive brightener, while the latter is due to insufficient carrier brightener. In the former case, when brightener is excessive in the solution and dilution with water does not help, the zinc plating process commonly uses hydrogen peroxide to remove it, which not only oxidizes and decomposes the brightener but also oxidizes iron impurities in the solution and removes them by filtration. The zinc-iron alloy process, however, must not be treated with strong oxidizing agents such as hydrogen peroxide or potassium permanganate; it can only be treated with a large dose (5–7 g/L) of powdered activated carbon, which both delays production and increases cost. Therefore, the brightener should be added frequently and in small amounts to avoid this problem. In the latter case, when the carrier brightener is insufficient, add it gradually in small amounts until the fault is eliminated, and never over-add. In this process, the carrier brightener is incorporated into the make-up agent, so replenishing it is all that is needed to eliminate the fault.
Because pinholes are tiny and difficult to see, they can easily be missed with even the slightest carelessness. Streaks resemble gas flow and occur mainly when the current is relatively high and the make-up agent concentration is too low, resulting in a large amount of hydrogen evolution and streak-like gas flow. They are more common in high current density areas, though they may also appear in other areas occasionally. Properly reducing the current density can eliminate them.
Pinholes and streaks also occur when the pH is too high. The chloride zinc-iron alloy plating bath is weakly acidic. Since Fe²⁺ is readily oxidized to Fe³⁺, the pH rises and the acidity of the solution decreases during oxidation. When the pH rises above 5, hydroxides form, creating pinholes or streaks.
Yellowing and blackening of the coating. The yellowing and blackening of the zinc-iron alloy coating in high current density areas are mainly caused by the bright dip solution in post-treatment. The mass fraction of nitric acid in the bright dip solution is 0.1%–0.3%, one-tenth of that used for bright dipping in zinc plating. Such a trace amount of nitric acid cannot perform bright dipping; it only cleans and removes an extremely thin organic film on the coating surface, thereby maintaining good adhesion between the coating and the passivation film. Because the coating contains iron, yellowing readily occurs when the mass fraction of nitric acid exceeds 0.6%; when it reaches 2%, the coating turns black. If yellowing occurs, shorten the bright dipping time to 1–2 s. If the yellow color is relatively deep, replace the bright dip solution.
Abnormal color of the color passivation film. The normal color of the color passivation film on zinc-iron alloy coatings is a uniform bright multicolor. If the color is abnormal, that is, the passivation film in high current density areas is darker than in other areas, creating a contrast. The darker the color, the greater the contrast, ranging from greenish to dark green, ink green, and even black. This is caused by the high iron content in the coating in high current density areas. Therefore, 3–5 min before the parts are removed from the tank, reduce the current density to about 1 A/dm², and the color difference defect will be eliminated.
Conclusion. In summary, most fault factors are caused by deviations from process specifications and operating procedures during production. Therefore, it is necessary not only to strengthen awareness of process specifications but also to strictly implement operating procedures, so that many electroplating faults can be prevented before they occur.