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In decorative electroplating, bright nickel plating mostly uses a Watts-type plating bath with a pH of around 4. The Watt-type nickel plating solution is based on a solution of nickel sulfate, nickel chloride, and boric acid; by adding certain additives such as 1,4-butynediol, saccharin, and sodium dodecyl sulfate, a bright plating layer with fine crystals, good toughness, and high corrosion resistance can be obtained. However, during use, the plating solution inevitably accumulates many impurities, such as decomposition products of additives, oils, and the presence of foreign metals, which can lead to malfunctions. As the scale of electroplating expands, it is necessary to quickly and accurately identify faults that occur on site and address them promptly in order to improve production efficiency. The failure symptoms, causes, and solutions for bright nickel plating are introduced as follows. 1 Poor brightness 1.1 The coating appears foggy white 1.1.1 Causes: (1) Excessively high temperature of the plating solution (above 60 ℃); (2) pH level that is too high or too low; (3) Excessive amount of nickel sulfate; (4) Insufficient amount of boric acid; (5) Lack of the secondary brightener, butyryldiol. 1. 1. 2 Treatment method (1) Lower the temperature of the plating solution to the normal process specifications. (2) Adjust the pH value. If the pH value is too high, dilute sulfuric acid with a mass fraction of 10% is added slowly to the plating solution while stirring, in order to adjust the pH value to the specified range. If the pH value is too low, sodium hydroxide or nickel carbonate solution with a mass fraction of 5% is used to bring it back to the normal range. The pH value is related to the concentration of nickel sulfate in the plating solution; when the content of nickel sulfate is at the lower limit of the acceptable range, a higher pH value (4.5–5.1) can be used. When the nickel sulfate content is at the upper limit, the pH value should be kept within a lower range (pH = 3.8–4.1), which results in a softer nickel coating. When the pH value is too high, hydroxides (basic salts) precipitate and become trapped within the coating, often resulting in white mist and other defects; if the pH value is too low, the gloss of the coating is inferior to that at higher pH levels. (3) Appropriately dilute the plating solution and add other components. (4) Based on the analysis results, add boric acid to the process specifications. (5) Use the Hall cell test and add butyryldiol appropriately. The quality of the secondary brightener is poor; for example, using a large amount of reddish-brown acetylenediol at once can also cause a white mist to appear on the coating. Furthermore, it is also necessary to determine on which coating layer the foggy appearance occurs. If it is on the outermost coating layer and can be removed by wiping, this is often caused by impure cleaning water after nickel plating. If the foggy appearance is present beneath the nickel coating layer, it may be due to incomplete degreasing, residual polishing paste, dirt on the parts after acid washing, or copper displacement; additionally, some areas of the plated parts may dry out during transfer between different tanks. Appropriate measures must be taken to eliminate this. 1.2 Distorted coating pattern 1.2.1 Causes: (1) Use of low-quality saccharin (primary brightener); (2) Insufficient amount or poor quality of sodium dodecyl sulfate; (3) Low nickel salt content, small anode area, or passivation. 1. 2. 2 Treatment methods (1) Check the purity of the saccharin raw material. Qualified saccharin has a purity of 99%; inferior saccharin contains large amounts of white sugar that causes decomposition, leading to discoloration of the coating, and its use should be stopped. (2) Add sodium dodecyl sulfate appropriately. Generally, 1P2 (approximately 0.04 gPL) is added to the plating solution based on the amount used in its preparation, and it is supplemented as appropriate after trial plating. When the content of sodium dodecyl sulfate in the bright nickel plating solution is insufficient, not only pinholes are formed, but blooming also often occurs. Especially when the amount of brightener is low, the workpieces exhibit significant water-repelling properties after being taken out of the bath. At this time, under normal pre-plating conditions, by adding sodium dodecyl sulfate, the \"flowering\" phenomenon can be eliminated. (3) Add nickel sulfate and nickel chloride according to the analysis; check the area of the anode nickel plate and replenish nickel plates, or remove the plates for cleaning in order to activate them. Under normal circumstances, increasing (or decreasing) the amount of nickel metal anodes is used to adjust the nickel ion concentration in the plating solution; this method is more economical and stable than adding nickel sulfate, and it also benefits the quality of the coating. 1.3 The low current density area appears as a black-gray misty appearance. 1.3.1 Causes: (1) Excessive accumulation of copper and zinc metal impurities in the plating solution; (2) Presence of nitrate ions. 1. 3. 2 Treatment methods: (1) Copper and zinc impurities can generally be removed by electrolysis. First, lower the pH value of the plating solution to 3, then use corrugated iron sheet or metal mesh with an area larger than that of the anode as the cathode; electrolysis is carried out under stirring at a cathode current density of 0.2 APdm2. ·40 · March 2007 Electroplating & Pollution Control Vol. 27 No. 2 © 1994-2009 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net (2) When the mass concentration of nitrate is above 0.2 g/L, adjust the pH to 1–2 and the temperature to 70 °C; then electrolyze for about 3 hours using a cathodic current density of 1–2 APdm2, gradually reducing the concentration to 0.2 APdm2 before continuing the electrolysis until normal conditions are reached. When the NO-3 content is too high, it causes the entire coating to turn gray and black, preventing the deposition of a nickel layer. 1.4 Charring or partial charring in the high current density area 1.4.1 Causes (1) Excessive accumulation of organic impurities; (2) Too high cathode current density; (3) Poor contact between the plated part and the hook. 1. 4. 2 Treatment method (1) Treatment with potassium permanganate. First, heat the plating solution to 70 ℃ and adjust the pH value to 2.0–2.5 using dilute sulfuric acid. Dissolve the measured amount of potassium permanganate (approximately 0.3–1.0 g per liter) in hot water, and add it to the plating solution intermittently while stirring vigorously, at intervals of 15 minutes. Allow the mixture to stand for more than 8 hours. If the plating solution is red, it can be removed using hydrogen peroxide. However, there are certain types of organic substances that are difficult to remove using this method; for example, organic impurities such as animal gums can be removed by adding 0.03–0.50 g/L of tannic acid to the plating solution. After about 10 minutes, flocs will form, and after another 8 hours or more of thorough precipitation, all organic impurities can be completely removed – it is also advisable to use activated carbon in this process. (2) Reduce the cathode current density. (3) Repair or replace the hanging fixtures with new ones. 2 Roughness and burrs 2.1 Rough coating and burrs 2.1.1 Causes (1) Excessively high cathode current density; (2) Severely insufficient anode area; (3) High chloride ion content in the plating solution. 2. 1. 2 Treatment method (1) Reduce the cathode current density. (2) Increase the area of the anode nickel plate. Generally, it is advisable to maintain a surface area ratio of about 2:1 between the anode and the cathode. (3) Dilute part of the plating solution and add other components. Nickel chloride (sodium) is added to the nickel plating solution to provide the necessary chloride ions, thereby facilitating the proper dissolution of the anodic nickel plate. However, the chloride ion content cannot be too high, otherwise it will cause the anode to dissolve irregularly, increasing the amount of anode sludge and resulting in a rough and pitted coating. 2.2 Roughness and burrs in areas with high current density; in severe cases, burning occurs. 2.2.1 Causes: (1) Excessively high cathode current density; (2) Too high pH value; (3) Excessive borate content; (4) Presence of solid particles. 2. 2. 2 Treatment methods (1) Reduce the cathode current density. (2) Lower the pH value to the process specification. (3) Filter the plating solution to remove excess boric acid. (4) Inspect the filter and anode bags, and filter the plating solution to remove mechanical particulate impurities. 2.3 Burrs are present under the plated parts. 2.3.1 Causes: (1) The plating solution is cloudy; (2) The plating solution gets mixed up while handling the parts in the recovery tank. 2. 3. 2 Treatment method (1) Filter the plating solution. (2) Let the slurry to be mixed stand still, or after electrolyzing for a period of time, start plating the product. 2.4 Burrs appear on the plated parts and disappear on their own after a few plating cycles. 2.4.1 Causes: When the pH value is adjusted, there is no stirring or insufficient stirring, or cold recycled liquid (water) is added. 2. 4. 2 Treatment method: When adjusting the pH value, thorough mixing is necessary, and it is best to carry this out at the end of working hours; mixing should also be done when adding recycled liquid to ensure uniform temperature of the plating solution. 3 Pinholes and Pitting 3.1 Pinholes that penetrate to the underlying metal (such as the copper coating) 3.1.1 Causes: (1) Poor pre-treatment, with oil residues on the surface of the plated part; (2) Excessively high pH value. 3. 1. 2 Treatment methods: (1) Identify the causes in the preceding processes prior to nickel plating, and take appropriate measures such as improving oil removal, rust removal, cleaning, and pre-plating activation processes. (2) Lower the pH value to the process specification. If the pH value of the plating solution is too high, basic salts will precipitate near the cathode; this helps hydrogen bubbles to remain on the surface of the cathode, resulting in defects such as pinholes in the coating. 3.2 There are many large pinholes and pitting, which are distributed fairly evenly. 3.2.1 Causes: (1) Solid particles and suspended matter in the plating solution; (2) Excessive accumulation of organic impurities. 3. 2. 2 Treatment methods (1) Filter the plating solution to remove particulate impurities and suspended solids. (2) Remove by referring to the above relevant treatment methods. 3.3 The pinholes and pitting are small and evenly distributed. 3.3.1 Cause: The content of sodium dodecyl sulfate in the plating solution is too low. March 2007 Electroplating and Environmental Protection, Volume 27, Issue 2 (Total Issue 154) ·41· © 1994-2009 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net 3.3.2 Treatment methods: Add sodium dodecyl sulfate appropriately in accordance with the process specifications. 3.4 Pinholes and pitting appear in a ringworm-like pattern, mostly located beneath the plated surface. 3.4.1 Causes: Excessive accumulation of iron impurities in the plating solution. 3. 4. 2 Treatment method: The most effective way to remove iron impurities is to use 2–4 mL of hydrogen peroxide with a mass fraction of 30% to oxidize ferrous iron in the plating solution to ferric iron; then, a sodium hydroxide or nickel carbonate solution with a mass fraction of 5% is used to raise the pH value to 5.5–6.0. The mixture is left to stand for more than 8 hours so that Fe3+ converts into Fe(OH)3 precipitate, which is then removed by filtration. If production cannot be halted, the electrolytic method can be used: by increasing the cathode area and applying an electrolytic current density of 0.1 APdm2, treatment for a certain period of time can alleviate the problem. 3.5 Pinholes and pitting on the edges of the plated parts as well as on the side facing the anode 3.5.1 Causes (1) Excessively high cathodic current density; (2) Excessive accumulation of metal impurities; (3) Too low borate content. 3. 5. 2 Treatment methods (1) Reduce the cathode current density. (2) Remove by referring to the above relevant treatment methods. (3) Add boric acid based on the results of chemical analysis. If the boric acid content in the plating solution is too low, it will inevitably cause the pH value to rise, leading to the formation of hydroxides that deposit together with the nickel layer, resulting in pinholes and pitting in the plating layer. The main reason for the formation of pinholes and pitting in a bright nickel plating layer is the evolution of hydrogen at the cathode during plating; this hydrogen adheres to the surface of the part being plated, hindering the deposition of the plating metal. If hydrogen bubbles remain on the plated surface for a long time, pinholes are formed; if they stay there for a short time, pitting occurs. Therefore, pinholes and pitting often mix together. 4 Poor adhesion 4.1 The entire coating peels off from the substrate 4.1.1 Causes: (1) Improper pre-treatment of the workpiece; (2) Prolonged cathodic degreasing or chemical etching of steel parts, resulting in hydrogen absorption by the substrate, with hydrogen escaping after electroplating. 4. 1. 2 Treatment methods: (1) Strengthen the degreasing, derusting, and cleaning processes of the workpiece before pre-plating to ensure a clean surface of the substrate. (2) Appropriately shorten the time for cathodic electrolytic degreasing and pickling to prevent hydrogen absorption in the workpiece matrix. 4.2 Bubbling and peeling of the nickel plating layer 4.2.1 Causes (1) Bubbling occurs when the insulating coating on complex parts or fixtures is damaged, allowing liquids to get trapped; (2) An excessive amount of sodium dodecyl sulfate in the plating solution. 4. 2. 2 Treatment methods: (1) For complex parts, it is necessary to thoroughly clean the solution attached to them during processing; repair any damaged insulation on the fixtures. (2) 3 gPL of powdered activated carbon was used to thoroughly treat the plating solution in order to remove excess sodium dodecyl sulfate. According to literature, when oil contaminants enter the plating solution, the directional arrangement of sodium dodecyl sulfate molecules allows them to form an adsorption layer around the oil, enclosing it. At this point, the oil ends up trapped between the hydrophobic groups inside the micelles, resulting in spherical or layered micelles. As the concentration of sodium dodecyl sulfate increases, the number of these oil-absorbed micelles also increases; when they become trapped in the plating layer during the electroplating process, bubbles are formed. 4.3 Peeling of the coating to reveal the underlying copper layer 4.3.1 Causes (1) After the workpiece is coated with copper (including via cyanide plating or acid copper plating), if it is not cleaned properly or is left exposed to air for an extended period, oxidation occurs on the surface; (2) Excess saccharin in the plating solution (resulting in the flaky shedding of the nickel layer); (3) Excessive butyrdiglycol (leading to the chunky shedding of the nickel layer). 4. 3. 2 Treatment methods: (1) Strengthen the cleaning process after copper plating and the activation step before nickel plating; minimize the time that the copper-plated parts spend in contact with air. (2) Electrolytic treatment at low current density, or adsorption using activated carbon. (3) Remove by referring to the above relevant treatment methods. 4.4 The coating peels off at the edges. 4.4.1 Causes: (1) High tensile stress; (2) Excessive cathode current density; (3) Presence of large amounts of iron impurities and organic substances. 4. 4. 2 Treatment methods: (1) Adding stress-reducing agents, such as appropriately increasing the saccharin content, can impart compressive stress to the coating and enhance its ductility. (2) Reduce the cathode current density. (3) Remove by referring to the above relevant treatment methods. 5 High brittleness of the coating 5.1 Causes (1) Excessive accumulation of metal impurities, especially chromate ions; (2) Too low borate content; (3) Excessive amount of (secondary or primary) brighteners; (4) Too high pH value; (5) Too low temperature of the plating solution; (6) Excessively high cathode current density. 5.2 Treatment Methods (1) To remove hexavalent chromium, sodium thiosulfate or ferrous sulfate is generally used. ·42· March 2007 Electroplating & Pollution Control Vol. 27 No. 2 © 1994-2009 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net For example, when the mass concentration of hexavalent chromium is 0.5 gPL, adding 0.16 gPL of ferrous sulfate raises the pH to 6; at a temperature of 70 °C and with stirring, Cr3+ can be precipitated. After filtration, the pH is adjusted to meet the process specifications. (2) Add boric acid to bring it within the normal range based on the analysis results. (3) Electrolysis using a low current density or activated carbon adsorption. (4) Lower the pH value to the process specification. (5) Raise the plating solution temperature to the process specification. (6) Reduce the cathode current density. 6 Poor coverage ability 6.1 No coating in areas with low current density 6.1.1 Causes: (1) Effect of hexavalent chromium; (2) Excessive amount of (secondary/primary) brighteners; (3) Too low pH value; (4) Poor contact between the hanging fixtures; (5) Excessive organic impurities; (6) Poor pre-treatment; (7) Too low chloride ion content. 6.1.2 Treatment method: (1) Remove by referring to the relevant treatment methods mentioned above. (2) Electrolysis using a low current density or activated carbon adsorption. (3) Raise the pH value to the process specification. (4) Repair or replace the hanger with a new one. (5) Remove by referring to the above relevant treatment methods. (6) Check the surface cleaning condition after copper plating in the previous step, and thoroughly clean it. (7) Add nickel chloride to the process specifications based on the analysis results. Chlorides can reduce anodic passivation, allowing the nickel anode to dissolve properly, and improve the electrical conductivity of the plating solution as well as its covering and dispersing capabilities. 6.2 Failure to apply chromium plating, or local areas of the chromium coating turning yellow or showing mottling: (1) Passivation of the nickel coating surface; (2) Adsorption of brighteners on the nickel layer or the presence of deposits; (3) Excessive metal impurities in the nickel plating solution. 6. 2. 2 Treatment methods: (1) For plated parts that have been left for an extended period of time after nickel plating, they can be treated by immersing them in a sulfuric acid solution with a mass fraction of 10%, or by chromium plating after immersion in chromic acid. (2) Appropriately reduce the brightener; treat with activated carbon if necessary. In winter, it is difficult to clean with cold water in order to prevent condensates from forming on the surface due to the liquid remaining on the workpiece. Therefore, after nickel plating, it should first be washed in warm water, and then chrome plating should be applied after activation, which will eliminate the mottling phenomenon. (3) Remove by referring to the above relevant treatment methods.