Low-temperature iron plating refers to the iron plating process performed by asymmetrical alternating current at a lower temperature (starting plating at 4 ° C). The principle is that the two half-waves of the alternating current are not equal by a certain means. After the energization, the larger half-wave makes the workpiece have a cathode polarity and is plated with a plating layer; the other smaller half wave makes the workpiece have an anode polarity. Part of the plating is electrolyzed, which gradually thickens the coating. The low-temperature iron plating equipment is simple, the obtained coating layer is uniform, the surface is smooth, and the bonding strength is high.
1 low temperature iron plating features
(1) The low-temperature iron plating layer has high hardness, and the hardness range can be adjusted by the process control, and the Rockwell hardness can be up to 63-65.
(2) The iron plating layer is integrated with the matrix body, and the bonding strength between the plating layer and the substrate is high. The ferrite of the iron-plated layer was observed to be continuously distributed with the matrix ferrite.
(3) The surface of the coating has micro-cracks and is in the form of a mesh. Has good oil storage performance and good wear resistance.
(4) The deposition rate of low-temperature iron plating is fast, about 0.6-1.0 mm/h, and the thickness of the coating can reach 2 mm at a time. In addition, the iron plating cost is low and non-toxic, so it can be widely used.
2 Development of low temperature iron plating technology
The earliest studies on iron plating in foreign countries began in 1846, and were initially applied to the surface of printed lead plates for iron plating [2]. The use of a thin layer of iron on the lead plates and copper plates used in the printing industry improves the wear resistance, extends its service life, and prevents the action of certain printing pigments. At that time, the method of sulphate iron plating was adopted. The deposition rate of the iron plating solution was very slow and the application surface was narrow. In 1908, some scholars began to study the chloride plating process. As a result, the coating obtained at room temperature was hard and brittle, with high internal stress, poor bonding force and easy to fall off, so it was only suitable for high temperature plating. The plating system has been improved to form a high-temperature chloride DC plating process, and has been widely used in the repair of wear parts; however, high-temperature iron plating requires complex heating and heat preservation equipment, and the plating liquid is highly volatile, and Due to the acidic solution, the environmental pollution is very serious, and the hardness and wear resistance of the coating are poor [1]. In the late 1960s, due to the development of electronic technology, some countries used asymmetric electroplating power in the iron plating technology [2], which made the chloride plating technology develop, and the hardness and wear resistance of the coating were also obvious. Improvement. In recent years, most countries have transferred iron plating research to amorphous coatings, corrosion resistant coatings and magnetic coatings.
In the 1960s, China introduced DC-plated iron technology from the Soviet Union to repair mechanical parts that were out of tolerance due to wear and tear [3]. At that time, the temperature of the plating solution was generally above 80 °C, and the composition of the plating solution was difficult to control. The acid mist generated during the anodic etching and electroplating process before plating had a serious influence on the personnel and the environment, and the hardness of the plating layer was low (cloth). The hardness is 200-400), the wear resistance is poor, and the scope of use is greatly limited.
In 1975, for the first time in China, an asymmetric AC continuous flow iron plating process was introduced. At the same time, in order to make the coating layer and the substrate firmly bonded, the anode was etched and activated by using 30% by mass of sulfuric acid before plating, because the plating temperature was only 25 ~50 ° C, it is called low temperature iron plating process [4]. The process equipment is simple and conforms to China's national conditions. Its coating has a Rockwell hardness of 50, good wear resistance, a deposition rate of 0.15~0.30mm/h, a current efficiency of over 90%, and a plating thickness of 2.0. ~3.5mm, and can be continuously plated; the plating liquid raw materials are cheap and easy to purchase, and the cost is low, which has caused widespread attention. The plating solution and coating properties have been studied, and it has been developed and applied in China very quickly. Because the treatment liquid is easily damaged by the anodic etching with sulfuric acid, the treatment liquid is likely to cause harm to the operator and the environment. Soon after, the iron plating process directly with DC current and the symmetrical alternating current activation in the plating tank after the corrosion of hydrochloric acid occurs Symmetrical AC electroless plating without sulfuric acid anode etching. However, due to some key technical problems, especially the poor stability of the process, the low bonding strength between the iron-plated layer and the substrate has not been well solved, resulting in a decrease in the quality of the iron-plated workpiece. Therefore, in the late 1970s, the iron plating process was once low, and many manufacturers were forced to stop production.
After entering the 80s of the last century, Dong Wensheng et al [5-6] studied a non-anodized iron plating process and successfully applied to the repair of mechanical parts (such as crankshafts, shafts) and flat parts, coatings and The base body is firmly bonded, and the joint strength of the iron plating layer can be determined by the pull-out method to reach 360 MPa. A stable and reliable coating can be obtained by plating on various iron-based materials (including carbon steel, cast iron, cast steel, and alloy steel). The elimination of the anode etching process before plating not only greatly reduces the pollution to the environment, but also reduces the labor intensity, and the yield rate reaches over 99%, which indicates that the domestic iron plating technology has developed to be stable and reliable. Practical stage.
3 Research status of low temperature iron plating
In recent years, with the increasing social needs, coupled with the deepening of the research on iron plating theory, low-temperature iron plating technology has also been continuously developed, and its application fields have been continuously expanded. The early domestic iron-plating repairs focused on crankshafts and other shaft components used in agricultural machinery and automobiles, and have now entered the diesel engine crankshaft and steam turbine rotor for repairing ships, diesel locomotives, oil fields, mines, and metallurgical machinery. The stage of large parts such as cylinder liners and crossheads.
At present, a major research direction of domestic low-temperature iron plating technology is to strengthen the coating and improve its thermal stability. Among them, Song Xiufu [7] obtained a superior composite coating by adding serpentine nanoparticles to the plating solution, and the microhardness of the coating reached about 6000 MPa. Yang Sen, Liu Yi et al [8-11] obtained a composite coating with a microhardness of 8340 MPa by adding rare earth lanthanum to the plating solution, and studied the pH value, current density, temperature and the quality of iron in the bath in detail. The effect of concentration on low temperature iron plating process and performance.
Song Bangcai et al. [12] studied the brush iron plating process, and obtained a good iron plating layer under the optimized plating solution formula and process conditions. The preparation of the plating solution by using iron filings and ferric chloride has the advantages of low cost, fast preparation rate, high hardness of the obtained coating layer and good bonding force. Huang Xiaoting et al [13] developed a formula for brushing iron plating solution. The maximum thickness of the coating obtained in the optimized plating solution is 1 mm, the Rockwell hardness is between 40 and 50, and the bonding strength and oxidation resistance are good. Zhang et al. [14] studied the effect of periodic reverse pulse plating on the properties of iron plating. The results show that at the same current density, the periodic reverse pulsed iron coating can penetrate 1.6cm into the copper ring, the plating strength is 38kN, the hardness is high, the toughness is good, the elongation is 3%, the current efficiency is high, and the deposition efficiency is 86%. , deep plating ability.
4 development prospects of low temperature iron plating
There is still a large potential market for iron plating technology. In general, most of the parts that need to be plated are often the outer surface of the parts; however, the inner surfaces of parts such as cars, marine cylinder liners, crankshafts, etc. are often subject to more severe wear and a large number. Domestic companies that use iron plating to repair important parts and components have a considerable scale.
It can be foreseen that further improving the corrosion resistance, heat resistance and wear resistance of the iron plating layer is the main development trend of low temperature iron plating technology. Adding an appropriate amount of alloying elements to the plating solution is one of the effective ways. In addition, there is a lot of advantages in the non-etching low-temperature iron plating technology, so there is a lot of room for development in both theoretical research and practical application. As long as the relevant technical problems are solved, the iron plating technology will have a wider industrial application prospect.
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