Decorative Chromium Plated With Related Information General
The most important issue in the decorative chrome plating process is that the surface of the material to be plated should be clean. For this, the material to be plated (mold, shaft, etc.) is firstly subjected to a series of cleaning operations. In the cleaning process of the material, it is subjected to alkaline hot bath degreasing in order to remove organic contaminants on the surface. The working principle of this bath is the process of saponification organic impurities with determined alkalinity. The active ingredients of the alkaline hot bath degreasing are Na3PO4, NaOH, and Na2CO3. The bath temperature runs between 60-70oC. The material is kept in this bath for 10 to 15 minutes and then washed with pure water. After the alkaline oil removal process, the electrolytic degreasing is also used for cleaning the material. This process operates in 1-15 volts, and 200-300 ampere current and the fine oils on the material are cleaned. Although Na3PO4, NaOH, and Na2CO3 are used in electrolytic degreasing, some emulsifiers and inhibitors are used in addition to these substances. Electrolytic degreasing takes as little as 2-3 minutes, and the operating temperature is equivalent to the normal ambient temperature. After washing process, plating process is performed. Information about the order of plating operations and how they occur are given in the following sections.
A) Cyanide Copper Plating:
The person employed in the cyanide copper plating process must begin knowing the instructions for the use of the cyanide. The reason is that a dangerous situation may arise as a result of careless working with cyanide. Therefore, the material safety data sheets of cyanide compounds should be explained in detail to the employees and the employer by an occupational safety specialist. Afterward, these measures should be printed out and hung in a visible part of the tank where the employee works with cyanide. Otherwise, if cyanide enters the human body, poisoning, and even further considerations, may result in fatal accidents.
The main purpose of cyanide copper plating is to bring the surfaces of metals that are less resistant to acid to the level where plating can be performed. The plating takes place in an alkaline environment around Ph 10, 3-5 volts, and 250-300 amperes current. The active ingredient used in the plating is sodium cyanide (NaCN) and copper cyanide (Cu(CN)2). Besides these active ingredients, moisturizers and polishers are also used.
The chemical reaction in the tank is as follows:
Cu(CN)2 + 2NaCN –> Na2 Cu (CN)3
Nonsalable –> Solvable
A certain amount of cyanide is necessary to dissolve the metal salt in the tank. Excess cyanide in the medium is called "free cyanide," and this free cyanide helps to plate the copper in the medium more strongly and easily.
The free cyanide present in the reaction is very effective at plating copper on the metal surface, and the amount of free cyanide present in the environment varies as the metal surface is plated. Therefore, if you want to have a good plating result, the amount of free cyanide in the environment should be checked at regular intervals and kept at the controlled level.
In copper plating baths, ammonium thiosulphate and lead tantalate are used as plating agents in addition to plating active substances. Lead tantalate is a very good brightener. However, unwanted peeling may occur on the plated metal surface if not used under control. For this, it is necessary to keep the lead in the medium at the optimum level. The determination of excess lead in the medium is done by the Hull Cell test.
The pH control and free cyanide control in cyanide copper baths are very important in terms of the quality of the plating. If the pH falls too much, the brightness property of the bath falls; on the other hand, if the free cyanide in the environment increases too much, this decreases the cathode efficiency and the decrease of cathode efficiency decreases the plating quality. Other factors such as temperature, current density, and the effect of the polisher are also important in itself.
B) Acidic Copper Plating:
The main purpose of Acidic Copper Plating is to make the surface smoother and brighter before the nickel plating on the metal. Active ingredients of the acid copper bath are copper sulfate (CuSO4) and sulfuric acid (H2SO4). In addition to active substances, special carriers and brighteners are used. Since sulfuric acid is used in the bath, the work is carried out in an acidic environment. Baths below pH 1 are used. As anode, electrolytic copper anodes should be preferred. Another consideration to be taken when using the anode is the anode-cathode ratio. The anode is made of copper metal, and the cathode is made of material to be plated. For a quality coating, the anode-cathode ratio should be 1 (anode/cathode) = 1) because the excess of anode causes extra copper sulfate to pass into the solution, which reduces the acidity of the bath and causes the precipitation of copper sulfate.
The working temperature of the acidic copper bath is between 22 and 4800C. As the bath temperature increases, the dissolving rate of copper sulfate will increase, and the plating will be better. Electric immersion heaters can be used to increase the temperature. The current density range in the working bath is 1-4 A/dm2, as well as the working voltage of the anode cathode, is between 0.5 and 3. Different tanks are used in acidic copper plating. These are ceramic tanks, plastic tanks, glass tanks, and plastic-lined tanks.
C) Bright Nickel Plating:
The purpose of bright nickel plating is to prevent corrosion on the metal surface and to provide decorative properties to the metal. Nickel sulfate, nickel chloride, and boric acid are used as the active ingredient in bright nickel plating. In order to adjust the pH balance of the bath, sulfuric acid and ammonia are used. Nickel mine is used as the anode. The factors affecting the plating quality of the nickel bath are the temperature of the bath, concentration of metal salt, pH, and contaminants. The effect of bath temperature on the plating is in two forms. The first is that if the temperature is low, the plating has a hard and brittle structure, while if the temperature is high, this increases the concentration of metal salt, which is, in this case, intended in contrast to the other case. The reason is that the more the metal salt concentration increases, the less the fragility of the coating decreases, and thus better results are obtained in terms of plating.
D) Chromium Plating:
The purpose of chromium plating is to increase corrosion resistance by supporting nickel plating. From a decorative point of view, the appearance of the plated metal is brought to a more natural color, and a visually richer product is manufactured.
The raw material used in nickel plating is chromic acid (H2CrO4). In addition to the active ingredient, sulfuric acid and catalysts are used in company-specific products. 7% of alloy lead is used as anode.
The purpose of using sulfuric acid in chromium plating is to facilitate the transmission of chromium to the metal surface by acting as a catalyst. The tanks used in chromium plating baths should be preferred by taking into account the corrosive properties of chromic acid. The tanks that show the best resistance against the corrosive properties of chromic acid are the lead tanks that contain 6% Sb (antimony).
The preparation of the chromium plating bath is as follows: first, add the necessary amount of water to the tank, then add the chromic acid and then the sulfuric acid, and mix with a polypropylene rod. The factors that must be kept under control for the quality of chrome plating are current density, temperature, and sulfate ratio. If you want to have the best plating result, the bath temperature should increase or decrease by up to 1.5oC.
Click here for more details about Chromium Plating: Chrome Plating
More detailed information about the nickel plating process will be available in our next article.
Decorative Chrome Plating Process Steps
- 1. Ultrasonic degreasing
- 2. Rinsing with pure water
- 3. Electrolytic degreasing
- 4. Rinsing with pure water
- 5. Rinsing with 3% H2SO4
- 6. Rinse with pure water
- 7. Cyanide Copper Plating
- 8. Rinsing with pure water
- 9. Rinsing with 3% H2SO4
- 10. Rinsing with pure water
- 11. Acidic Copper Plating
- 12. Rinsing with pure water
- 13. Bright Nickel Plating
- 14. Rinsing with pure water
- 15. Chromium Plating
- 16. Rinsing with pure water
- 17. Drying
- 18. Packaging