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Operational points of acid-plated zinc-nickel alloy for cast
Pubdate date:2019-10-17Views:0
Cast iron is widely used in the automotive, aerospace, shipbuilding and other industries due to its good wear resistance, low cost, simple process, high production efficiency, good vibration damping, low notch sensitivity and easy cutting. However, the surface of the cast iron is easily rusted, difficult to handle after rusting, and has poor decorative properties, and needs to be improved by subsequent treatment to improve its surface properties. Electroplating can effectively improve the appearance and corrosion resistance of cast iron parts, and has become a hot spot for popularization and application.

    1. The special characteristics of iron castings and plating countermeasures

    Compared with other substrates such as rolled and forged steel, the casting process of cast iron parts is special, the structure is complicated, and electroplating is relatively difficult. For the particularity of cast iron parts, the plating countermeasures taken are:

    (1) The surface of cast iron parts has high carbon content, and a large amount of sand or carbon silicon compound remains on the surface. Hydrogen has a low overpotential on carbon, which is easy to precipitate and generate a large number of bubbles [1]. It is difficult to reach the potential of metal deposition at a given current. The plating layer is not easy to grow, so the electroplated zinc-nickel alloy of the cast iron piece can only select the acid system with high current efficiency (the current efficiency of the acid zinc-nickel alloy is about 90%, and the alkaline zinc-nickel alloy is only about 50%). The structure of cast iron parts is generally complicated, so a plating solution with good dispersibility and good covering ability is also used.

    (2) The structure of cast iron substrate is loose and porous, there are a lot of sand holes and shrinkage holes, the surface is rough and uneven, so the surface area of ​​micro-plating is much larger than the macroscopic area, which causes the actual current density to be much smaller than the apparent current density, so it is not only With an acid plating system, the initial current should be 1 to 2 times larger than other steel substrates. The surface is activated by a large current shock to make the loose surface covered by the dense coating [2].

    (3) The surface of the cast iron part will form a non-conductive oxide film during the high temperature casting process, which hinders the growth of the zinc-nickel alloy coating. Therefore, the scale of the surface must be completely removed before plating. The general processing of cast iron parts is: casting-shot blasting-machining. Shot blasting is an important processing step for electroplating of cast iron parts. It mainly uses sand blasting machinery to hit the surface, which not only removes the scale, but also causes the high carbon layer formed during casting to fall off, resulting in microscopic roughness, which is beneficial to improve the bonding strength of the coating. The quality directly affects the quality of the plating of cast iron parts.

    (4) During the processing, a large amount of grease, lubricating oil and cryogen will remain on the surface of the cast iron, as well as residues from the mold. The sand and iron scraps left by the sandblasting will be affected by external influences during storage to form an oxide film. Insoluble contaminants, and a large amount of graphite on the surface of the cast iron itself, so the cast iron parts need to be pre-treated by special pickling and ultrasonic cleaning to form a clean, uniform wetting, good conductivity, easy to plate surface.

    2. Iron casting process

    At present, most of the cast iron parts adopt the acid galvanizing process with relatively simple process and equipment, but the anti-corrosion ability of the galvanized layer is poor, which can not meet the high corrosion resistance requirements of the automobile and other industries. Although the zinc-nickel alloy coating can greatly improve the corrosion resistance, However, due to the complicated process of acid electroplating zinc-nickel alloy, the process control is difficult, and it has been difficult to promote the application on a large scale. In recent years, with the rapid development of chemical and mechanical manufacturing processes, the process performance of acid-plated zinc-nickel alloys for cast iron parts has been greatly improved, and the control of production processes has become increasingly mature, which has become a research hotspot in recent years and has been gradually promoted on a large scale.

    There are two main methods for electroplating zinc-nickel alloys in cast iron: acid galvanizing + alkaline electroplated zinc-nickel alloy, or direct acid-plated zinc-nickel alloy. In order to solve the problem that the casting cannot be directly alkaline-plated, the acid galvanizing base is used, but in the case of alternating hot and cold, the bonding force between the double plating layers is not good, it is easy to fall off, and the process is more complicated, the thickness and the nickel content. Such quality control risks are greater. The characteristics of different plating processes are shown in Table 1.



    3. Pickling electroplated zinc-nickel alloy process

    3.1 Process

    Ultrasonic cleaning - washing - hot dip - water washing - pickling - washing - ultrasonic cleaning - washing - electric degreasing - washing - 40% (volume fraction) hydrochloric acid activation - washing - plating - washing - 2% (volume fraction) nitric acid Light-washing-passivation-washing-closing-leveling-drying.

    3.2 pre-plating treatment

    3.2.1 Ultrasonic cleaning

    The structure of the cast iron is loose, the porosity is high, and the insoluble pollutants such as dust and sand remaining on the surface are difficult to be completely removed by the general cleaning method. Ultrasonic cleaning uses a powerful shock wave to penetrate into the pores of the workpiece surface to thoroughly clean the surface of the cast iron part [3]. As shown in Fig. 1a and Fig. 1b, after ultrasonic cleaning, the surface of the cast iron part is relatively clean.



    3.2.2 Hot Dip Removal

    Hot-dip oil directly determines the success or failure of pre-treatment and electroplating of cast iron parts, because acid and water are incompatible with the surface of the grease. Incomplete degreasing will not only lead to subsequent pickling failure, but also form a non-uniform wetted surface. Cause plating defects. How to evaluate the effect of hot-dip oil removal on the rough surface of cast iron parts is also difficult, and it can generally be judged by evaluating the corrosion. The evaluation methods and steps are as follows: (1) After hot-dip degreasing, the parts are removed from the hanger and completely immersed in water for cleaning; (2) the cleaned workpiece is pickled and then ultrasonically cleaned; (3) The workpiece was placed in the air for 30 min to detect corrosion. As shown in Figure 2, the uniform corrosion of the surface of the cast iron parts indicates that the degreasing is relatively thorough. After an effective degreasing process, the surface of the cast iron piece forms a uniform wettable surface as shown in Figure 1c. The main factors affecting the degreasing effect are temperature, time, agitation, alkali content and surfactant. Too low a temperature, too short a time, insufficient agitation, and too low a surfactant content may result in incomplete degreasing. The alkali is generally selected from a small amount of NaOH. When the content is too low, the degreasing is not complete, and the content is too high, which will damage the substrate and affect the plating quality.



    3.2.3 pickling

    Pickling is the key to ensure good adhesion of the coating. As can be seen from Figure 1d, its main function is 4 points: (1) removing oxide scale and rust on the surface of the casting; and (2) removing iron chips remaining after surface processing; (3) Corrosion of a part of the iron matrix, so that the carbon particles are exposed on the surface, so that the subsequent carbon particles are detached; (4) the micropores on the surface are converted from the closed state to the open state, which facilitates subsequent cleaning and electroplating. Pickling is generally carried out with dilute hydrochloric acid for 5 to 10 minutes. The time is too short to achieve the effect of pickling; the time is too long, the surface will expose too much carbon, affecting the quality of plating.

    3.2.4 Ultrasonic cleaning

    After pickling, a large amount of carbon particles and graphite remain on the surface of the cast iron, which reduces the overpotential of hydrogen, resulting in a large amount of hydrogen evolution and hindering metal deposition, so the surface carbon must be removed. As shown in Fig. 1e, the surface carbon particles and graphite are peeled off by ultrasonic vibration cleaning (non-chemical process) to expose the surface of the electroplatable metal. The ultrasonic frequency is generally 25~40kHz/min, and the frequency is too high or too low will affect the decarburization effect. The surfaces that were ultrasonically cleaned and not ultrasonically cleaned were significantly different, as shown in Figure 3. The surface that is not ultrasonically cleaned is darker, probably due to the presence of a large amount of black carbon particles and graphite on the surface, while the ultrasonically cleaned surface is metallic. Ultrasonic cleaning is one of the most important processes that must be used in the pre-treatment of cast iron parts, which directly determines the success or failure of electroplating of cast iron parts.



    3.2.5 electric oil release

    The electric de-oiling is to further clean the surface of the cast iron piece, in particular to completely remove the fine impurity particles which have not been dissolved in the previous process, so as to form a more uniform and wet surface as shown in Fig. 1f to improve the bonding force of the plating layer. Cast iron parts generally use cathodic electric de-oil, the cleaning agent uses alkaline chelate, the counter electrode is generally low carbon steel, and the current is about 5.5A/dm2. Care should be taken to control the time and current of the cathode to de-oil the oil to avoid hydrogen embrittlement and affect the strength of the cast iron.

    3.3 plating

    The electroplated zinc-nickel alloy of cast iron is mainly composed of sulfate system and chloride system. The chloride electroplating system has good conductivity, good dispersibility, high current efficiency, fast deposition rate, low hydrogen embrittlement, good brightness and easy operation at room temperature. Other advantages have been studied [4-5]. Most of the electroplating production lines currently put into actual production use a chloride system. In terms of electroplating equipment, acid electroplated zinc-nickel alloys are more complicated than alkaline electroplated zinc-nickel alloys. As shown in Fig. 4, the alkaline electroplated zinc-nickel alloy generally adopts a zinc anode, and nickel is added by a solution, and the acid electroplated zinc-nickel alloy generally adopts two sets of soluble anodes, namely a zinc anode and a nickel anode, and the area ratio of the two is generally ( 4:1)~(8:1), and two rectifiers are used, and the current ratio is generally (4:1)~(8:1). The recommended plating system is: KCl210~250g/L, ZnCl240~70g/L, NiCl2·6H2O90~120g/L, H3BO320~30g/L, suitable amount of complexing agent, brightener and corrosion inhibitor.



    3.4 post processing

    After electroplating of cast iron parts, dilute nitric acid (about 3%) is generally used for chemical polishing, which can improve the brightness and flatness of the coating and improve the appearance and firmness of the passivation film. The surface of the cast iron is porous, and the acid and alkali solution is easily left in the hole, which not only causes the coating to be black and dark, but also greatly reduces the corrosion resistance of the coating. Therefore, the post-plating cleaning of the cast iron is very important. It is usually washed alternately with hot water and cold water for 2~3 times, and then passivated. When the hot water is cleaned, the plating solution that has penetrated into the pores of the cast iron may overflow due to thermal expansion, and the opposite is the case when the cold water is washed, that is, the cold water enters the pores to prevent the surface from being whitened. Note that the water temperature of hot water cleaning should not be too high, generally 80 ° C; the time should not be too long, usually 1~2min, otherwise it will form an oxide film on the surface of the coating. The first hot water tank is easily contaminated by the plating solution and must be replaced frequently. . As with other substrate-galvanized zinc-nickel alloys, the coating must be passivated and sealed to improve corrosion resistance, improve appearance, and achieve uniform color and gloss in uniform silver and black.

    3.5 Causes and Countermeasures of Common Faults

    3.5.1 Plaque black spots

    (1) The pre-treatment is not clean, and the residual oil and other contaminants in the micropores ooze out after drying, contaminating the coating, and the pretreatment should be strengthened.

    (2) The passivation solution has impurities, and impurities should be removed or the passivation solution should be replaced.

    (3) The plating solution has impurities, and the plating solution should be strengthened.

    (4) After the plating, the cleaning is insufficient, and the plating solution remains in the hole, and the residual liquid migrates to the surface under a certain humidity, and the plating layer is corroded to generate black spots. The plating should be strengthened after cleaning.

    3.5.2 The coating is dark

    (1) The passivation time is too long and the passivation time should be shortened.

    (2) The hot water cleaning time after plating is too long, and the appropriate cleaning time should be selected.

    (3) Insufficient additives should be supplemented with additives.

    3.5.3 plating has pinholes, pitting

    (1) There are too many holes in the surface of the substrate, which causes the substrate to be too rough, and the casting process should be improved.

    (2) If the substrate is shot blasting, the parameters such as shot peening pressure, time and the material, size and hardness of the pellet should be adjusted.

    (3) Pre-treatment is not clean, contaminants or graphite are present on the surface, and pre-treatment should be strengthened.

    (4) The plating solution contains solid impurities, and the filtration treatment of the plating solution should be strengthened.

    3.5.4 Coating bubbling and peeling

    (1) The pretreatment of the substrate is not thorough, and the surface remains oxide film, graphite and other impurities, and the pretreatment process should be strengthened.

    (2) The cleaning temperature after electroplating is unreasonable, excessive thermal expansion and contraction, and the appropriate hot and cold water cleaning temperature should be selected.

    (3) The concentration of the pickling solution is too high, or the cathodic electrolysis time is too long, the current is too large, resulting in serious hydrogen evolution by pretreatment, and the gas expands when drying after electroplating to form bubbles. The pickling concentration should be appropriately lowered to adjust the time and current of the cathode electrolysis.

    (4) There are many impurities such as organic matter in the plating tank, and the plating solution should be strengthened.

    3.5.5 plating is not plated locally

    (1) The structure of the part is too complicated, or the design of the hanger is unreasonable, and the workpiece partially forms a current dead zone. The hanger should be adjusted to optimize the structural design.

    (2) The initial current density is too small, and the initial current density should be appropriately increased.

    (3) Improper pre-treatment, oily, graphite, oxide scale residue on the surface, resulting in the inability to form a coating. The degreasing, pickling, and ultrasonic cleaning processes should be strengthened.

    3.5.6 The coating is brittle

    (1) The nickel content of the coating is too high, and the anode area and current density ratio should be adjusted.

    (2) There are many organic impurities in the plating solution, and impurities in the plating solution should be removed.

    (3) If the bath temperature is too high, the bath temperature should be appropriately lowered.

    4. Conclusion

    Although the casting of cast iron parts is difficult, the process is relatively complicated, and the production process has many problems. However, by adopting automatic control equipment and strengthening the process control of each process, stable batch production can be realized. Although the domestic process of acid zinc-nickel alloy for cast iron parts has been studied earlier [6-7], the actual application progress is relatively slow. In recent years, with the development of the automobile industry, acid zinc-nickel alloy has been applied to automobile brake calipers on a large scale, and some zinc-nickel alloy electroplating lines with high degree of automation have appeared. However, the technology is still monopolized by foreign plating liquid manufacturers, and the cost is relatively high. High, so domestic electroplating manufacturers need to continuously study the practicality of domestic electroplated zinc-nickel alloy process, improve the automation degree of domestic electroplating equipment by optimizing the plating system to improve quality stability and reduce production costs.
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