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Production process status and Fault treatment of Tungsten Alloy Electroplating

  • Mosten
  • 22 Jul

Tungsten alloy electroplating is a new type of electroplating which takes into account the dual properties of wear resistance and anticorrosion. It is environmentally friendly and has no harm to operators and high production efficiency. In recent years, it has gradually gained the attention of the mechanical industry. From the research room of a university to the national 12th five-year Plan development and promotion project, the application of tungsten alloy multicomponent electroplating has gone through more than ten years. At first, it was used to replace decorative set of chromium, but when trying out this technology, it was found that the processed products were more suitable for functional electroplating. Tungsten alloy electroplating was first used in hydraulic equipment and petroleum machinery, and then gradually expanded to other industries. At first, the problem is how to move from laboratory to mass production, how to move from formula to application, which is a long-term running-in process.

1·Formulation and Application of several typical Tungsten Alloy plating Bath

The production process of the entire tungsten alloy must be described in detail, and it is necessary to start with the formula. At present, in the market, the more common is the "nickel-tungsten-phosphorus", the formula control is simple, the solution is stable, meets the high requirement of the current environmental protection, therefore, the formula is used for experimental processing of each industry product at the beginning. * The "iron-nickel-tungsten" formula is considered to be more wear-resistant, and when the performance of the nickel-tungsten phosphorus is difficult to meet the requirements of the product under certain application conditions, the method is replaced by the plating. The "nickel-tungsten" or the "alkali-nickel-tungsten-phosphorus" has more application in the inner wall processing, and has the advantages that the dispersion capacity is higher than that of the other, but also has certain technical difficulty in the maintenance, and the application is not wide. In addition, some people have made nickel-tungsten-copper, nickel-tungsten-magnesium and the like, and the difficulty of the visible-modulation formula is not large. The sustainability and stability of the formulation, as well as the difficulty of maintenance and control, determine the use of these formulations in mass production.

1.1 Iron, nickel and tungsten The formula is composed of nickel sulfate, sodium tungstate, ferrous sulfate and coordination agent at 70 ~ 75 °C, pH7.0~7.5.. First, nickel sulfate was added, dissolved into the corresponding coordination agent, then poured into ferrous sulfate solution with appropriate amount of reductant, then sodium tungstate solution with another corresponding coordination agent was poured into sodium tungstate solution with another corresponding coordination agent. The required brightener and low foam wetting agent were added, and the plating solution was obtained by constant volume. When the beaker experiment was carried out, the 2L solution worked continuously for 72 hours and the corresponding consumed metal was added continuously. The properties of the processed test pieces did not change, but the white precipitation appeared after storage, which was suspected to be a compound formed by trivalent iron, sodium tungstate and coordination agent. The difficulty of this formula lies in the control of trivalent iron. The diameter of beaker is small and the contact surface between solution and air is small. Compared with 2L solution, the oxygen production in electrolysis is also less, and the oxidation effect on ferrous sulfate is not obvious, but after a long time of work, trivalent iron begins to accumulate and precipitates naturally after a period of time. In order to add continuously, a suitable proportion of reductant and coordination agent must be prepared to ensure the stability of ferrous sulfate, resulting in excessive addition of the two. The excessive reduction agent directly affects the arrangement of elements and the proportion of metals in the coating, and the higher concentration of iron ion coordination agent not only reacts with trivalent iron to produce precipitation, but also easily reacts with sodium tungstate to produce precipitation. It is very difficult to control the stability of iron, nickel and tungsten. The preparation of three kinds of metals, two kinds of coordination agents and one kind of reductant and brightener is very complex. In the production control, the analysis of reductant and brightener can not reach the degree of coordination. It is also found that the concave and convex joint of the profiled workpiece will be peeled in the case of two different coordination agents, so the unified coordination agent must be used, but the wear resistance of the workpiece decreases after the replacement of the coordination agent. In industrial production, the groove surface and oxygen production are large, so it is difficult to ensure that the precipitated particles are completely undoped in the coating, thus affecting the stability of the coating performance. For enterprises accustomed to the traditional mode of production, great efforts should be made to control the production of this plating seed, and only strict management can ensure the continuity of production.



1.2 Acid nickel tungsten phosphorus



The formula consists of nickel sulfate, phosphite, sodium tungstate and a single ligand, without the need for brightener. Working temperature 65 ~ 70 °C, pH2~3. The preparation method is simple. The sufficient coordination agent is added to sodium tungstate solution, then poured into ferrous sulfate solution, and then the phosphite can be added. When using Ni-W-P solution to do beaker experiment, only need to cooperate with a certain low-level wetting agent, * coordination agent is not conducive to control, will cause blackening or rough. The formula is very simple from preparation to production management, it is not necessary to stabilize the solution by excessive coordination agent, and the metal is not oxidized in aqueous solution, and no compounds affecting the performance are formed in the process of electrolysis, so it is easy to be accepted by the industry in the early stage of tungsten alloy electroplating. But there are still many problems. First, the amount of coating determined by the concentration of each element is still the focus of debate among the major entrustors. Second, the causes of peeling, hemp spots and pinholes are not in line with the previous views put forward by experts, and took a long time to explore. Thirdly, the effect of Ni-W-P ratio and technological conditions on wear resistance was gradually confirmed after obtaining considerable industry support, and there is still room for further improvement of wear resistance. Nickel, tungsten and phosphorus are stable and popularized, but there are doubts about wear resistance. In fact, the wear resistance of nickel-tungsten phosphorus coating is comparable to that of chromium plating, but because the hardness can only reach 900HV. when used in the environment of high movement rate and pressure, the coating will still deform and damage.



1.3 Alkaline nickel-tungsten phosphorus and nickel-tungsten



These two formulas were developed after customers doubted the performance of acidic nickel-tungsten phosphorus. The tungsten content of alkaline nickel-tungsten phosphorus is high, which is highly respected by the entrusted processing party, or is a selling point of tungsten alloy electroplating process. Customers think that the tungsten content is high, the amount of wear-resistant metal crystals is high, but it is not. In the experiment, the higher tungsten content will appear the phenomenon of sand formation of the coating, that is, too many particles and insufficient adhesion. This still needs to be verified in experiments and production. The formula of alkaline nickel-tungsten phosphorus is composed of nickel sulfate, sodium tungstate, phosphite and coordination agent. the choice of wetting agent depends on the electroplating mode of the product, which is the same as the preparation method of acidic nickel-tungsten phosphorus. The difference is that it is necessary to add a large number of alkaline coordination agents and ordinary alkaline solutions to increase pH to 7.0 ≤ 7.5, and the working temperature is 65 ~ 70 °C. This formula has the same characteristics as acid Ni-W-P. The metal is not oxidized and the coordination agent is more reasonable. However, the accumulation of organic matter in long-term production will also affect the plating rate, but there will be no insoluble precipitation, and the treatment is simple. The formulation of alkaline nickel-tungsten is composed of nickel sulfate, sodium tungstate and coordination agent, which is a simple formula in tungsten alloy electroplating system. The preparation method is no different from the above, and the working environment is the same. There is also the problem of organic matter accumulation in long-term work, and the treatment method is the same as alkaline nickel-tungsten phosphorus. The advantages of these two plating seeds are dispersion and leveling ability. In general, acidic nickel-tungsten phosphorus and iron-nickel-tungsten completely cover the metal surface more than 30 μ m, while nickel-tungsten and alkaline nickel-tungsten phosphorus can be completely covered at 20 μ m. In general, corrosion resistance is first required in inner hole electroplating. Iron, nickel and tungsten are not suitable, while acidic nickel-tungsten phosphorus is limited by process, alkaline nickel-tungsten phosphorus and nickel-tungsten become. 1.4 from the point of view of application, there is no doubt about the corrosion resistance of electroplating tungsten alloy products. The further study of improving wear resistance is different from that of chromium and cadmium because of the properties of metal and alloy grains. Although the wear resistance of the coating can not be evaluated by hardness, it can be compared that the hardness of the traditional chromium coating can reach 1100 HV. only iron, nickel and tungsten can reach it, and most of the other tungsten alloy coatings are about 900HV. Although it can be roughly similar to chromium plating in application, it is found that the wear resistance of tungsten alloy coating is only 2% of that of chromium plating after a simple dry friction comparison of production sampling. Another difficult problem to solve is that the wear resistance of the coating electroplated with the same formula in different working environments is very different, which requires an exact theoretical explanation, but it is difficult to rely entirely on the theory to create the experimental conditions. For simple nickel-tungsten, after all, it is two kinds of metals, reasonable control of the ratio directly affects the quality of the coating. Some technical promotion units often mention formula in the promotion of tungsten alloy electroplating technology, but it is not. According to the proportion of metal in the coating content, the blending agent can be easily obtained, and it can be said that the current formula is almost a semi-open secret. To sum up, it is not difficult to prepare and apply the above formula or other nickel-tungsten plating seeds. What really hinders the development of technology is the crisis of trust that can not be strictly managed in large-scale production environment, resulting in unstable product performance. At present, most investors in tungsten alloy industry are used to the traditional electroplating dirty production mode, and the quality instability occurs from time to time. The high raw material cost of tungsten alloy electroplating is also a reason that hinders the development of process. However, it is very meaningful to analyze water consumption, power consumption and social cost comprehensively, and tungsten alloy electroplating is listed in the 12th five-year Plan of the country.