The Precautions of chrome plating

- Mar 22, 2018 -

1) Improve the plating adhesion Due to the poor dispersion ability and depth capability of the chrome plating electrolyte, the leakage plating phenomenon may occur for some parts with complicated shapes.

In the case of hard chrome plating, the plating peeling phenomenon often occurs due to poor bonding force. In the production operation, the following measures can be adopted.

①Inrush Current For some complicated parts, in addition to the use of a pictorial anode, a protective cathode and an auxiliary anode, it is also possible to impact the part for a short time with a current several times higher than the normal current density when the part enters the slot. Increased, a rapid deposition of a layer of chromium on the surface of the part, and then returned to normal current density plating.

Inrush current can also be used for hard chromium plating of cast iron parts. Due to the large amount of carbon in the cast iron parts, the overpotential of hydrogen precipitation on carbon is low. In addition, there are a lot of pores on the surface of cast iron parts, making the real surface area much larger than the apparent area. If the plating is applied at the normal current density, the true current density is too small and there is no deposition of metallic chromium. Therefore, when cast iron pieces are plated with hard chrome, an inrush current must be used to increase the cathode polarization.

②Anode etching (etching) When the alloy steel with a thick oxide film on the surface and the hard chrome plated with a high carbon steel or chrome plating on a chrome plating layer with a longer power off time are continuously chrome plated, the part is usually used as an anode for a short period of time. Etching treatment electrochemically dissolves the oxide film and forms a microscopically rough surface.

③Stepped type feeding alloy steel containing nickel and chromium, the surface of which has a thin and dense oxide film, hard chromium plating will affect the binding force of the coating and the substrate, for this reason, the first plating in the chromium plating solution The anodic etch is performed, and then the part is turned to the cathode, and the current is controlled several times less than the normal value. The general voltage is controlled to about 3.5V so that only the hydrogen evolution reaction occurs on the electrode. Since the initial ecological hydrogen atom has a strong reducing ability, it can reduce the oxide film on the metal surface to metal, and then use a stepped current in a certain period of time (such as 20 ~ 30min), and gradually increase the current until the normal processing conditions. plating. As a result, electroplating is performed on the surface of the activated metal to obtain a plating layer with good adhesion. In addition, in the process of hard chrome plating, it sometimes encounters midway power failure. At this time, a thin film oxide layer will also be formed on the surface of the chrome plating layer. If direct current is applied to the plating, there will be peeling of the plating layer. The "type of power supply", so that the surface can be activated, and then transferred to normal plating.

④Preheating before plating For large hard chrome plating, the workpiece must be preheated prior to plating. Otherwise, it will not only affect the bonding force of the chrome plating layer but also the temperature of the plating solution, so the large parts should be pre-plated before plating. Heat for a few minutes to equalize the temperature of the substrate and bath. The bath temperature change is preferably controlled within ±2°C.

2) Hydrogen removal after plating

Because the current efficiency of chrome plating is low, a large amount of hydrogen is precipitated on the cathode. For hydrogen-producing steel parts, hydrogen removal should be performed at a temperature of 180 to 200° C. after plating for 3 hours to avoid hydrogen embrittlement.

3) Influence of Impurities in Bath and Removal of Common Harmful Impurities in Chromium Plating Electrolyte Metal ions such as Fe3+, Cu2+, Zn2+, Pb2+, Ni2+, and Cl-, NO3-.

The metal ions mainly originate from the dissolution of the metal that is not covered by the chromium layer, and the parts that fall into the plating tank are not salvaged and dissolved in time, and the anode is etched. When the metal ions are accumulated to a certain content, the chrome plating process will be greatly affected, such as the reduction of the bright range of the coating, the decrease of the dispersion ability of the electrolyte, and the deterioration of the conductivity. The tolerance of the plating solution to impurities increases with the increase of the chromic anhydride concentration, so the low concentration plating solution is extremely sensitive to impurities. When the bath contains more than 15 to 20 g/L of Fe3+, more than 5 g/L of Cu2+, and more than 3 g/L of Zn2+, the bath must be processed. The use of low current density processing can receive a certain effect.

Metal impurities can be removed by treatment with a strong acidic cation exchange resin. In order to reduce the oxidative destruction of the ion exchange resin by the chromium plating solution, the plating solution should be diluted to 80 g/L or less before being processed. Because the strong acid cation exchange resin is more expensive, it sometimes converts the spent plating solution to other uses, such as passivation solution, to reduce the production cost.

The newly prepared chrome plating solution generally has a voltage of 3 to 5 V. If the concentration is high, the voltage should be lower. If the voltage is found to be greater than the aforementioned value, the bath may contain impurities. Cl- from the bath solution make-up water, part of the washing water, etc. brought in, or hydrochloric acid etched clean after the dirty into. Too much Cl- will degrade the ability of dispersion and depth of the bath, graying, roughening, and even spotting of the coating, and may also cause corrosion of the substrate and the lead anode. To eliminate excessive Cl-, the bath can be heated to 70°C, electrolysis at high current density, oxidation at the anode to chlorine, and precipitation. However, this method consumes large amounts of energy and the effect is not ideal. It is also possible to add appropriate amounts of silver carbonate to produce silver chloride precipitates. Although this method works well, the added silver carbonate can also react with chromic acid to form silver chromate deposits. Not only did the silver salt consume too much, but it also lost chromic anhydride, increasing the production cost. The best way is to minimize the Cl-into. Therefore, it is best to use deionized water for the replenishing bath, and dilute sulfuric acid for the weak etch before plating. When hydrochloric acid must be used, the cleaning is enhanced. NO3 is the most harmful impurity, even if the content is low, it will make the coating gray, tarnish, and corrosion of the lead lining and lead anode of the plating tank. The NO3-removal method is: electrolytic treatment with 1 A per liter of electrolyte. If the plating solution contains a large amount of NO3-, the sulfate in the plating tank is first removed with BaCO3, and then electrolysis is conducted at a high current of 65-80°C to remove the nitrate ions at the cathode to NH3.

4) Suppression of chromium fog

In the chrome plating process, due to the use of insoluble anodes, the cathode current efficiency is very low, resulting in the precipitation of a large amount of hydrogen and oxygen. When the gas escapes from the liquid surface, it has a large amount of chromic acid and forms chromium mist to cause serious pollution. Currently there are two ways to suppress chromium fog.

The floating body method puts foam pieces or pieces into the liquid surface of the bath. These floating bodies can block the escape of the chrome mist.

However, when the part is out of the slot, the operation is inconvenient. In addition, the chromic acid oxidation ability is very strong, and the crushed pieces are etched, so that the decomposition products are accumulated in the plating solution, which also affects the coating quality.

②Adding foam inhibitors A foam inhibitor is a surfactant that lowers the surface tension of the bath and creates a stable foam layer that covers the bath surface. General surfactants are not stable at higher temperatures and in the presence of strong oxidants, but fluorocarbon surfactants are stable in the above medium. It has been reported that there are many kinds of chromium mist inhibitors, of which the best one is a long-chain fatty organic compound containing a polar group, such as sodium perfluorooctane sulfonate [CF3(CF2)6CF2SO3Na] The most typical one is that when adding 0.2-0.5 g/L per liter of plating solution, good results can be achieved. China has trial-produced potassium perfluoroalkyl ether sulfonate, abbreviated as F-53 chromium mist inhibitor, and its addition amount in the chromium plating solution is 0.04~0.06g/L. When used, the F-53 is first made into a paste with water, diluted with water, boiled and dissolved for a moment, transferred to a chromic tank heated to 50-60°C, and the insoluble F-53 can not be directly poured into the plating tank.

The foam layer formed by the chromium mist inhibitor in the plating solution is tightly covered on the surface of the plating solution. When hydrogen and oxygen with chromic acid precipitate, it collides with the foam layer on the surface, and many tiny chromic acid mists combine to form larger The droplets, due to the action of gravity, will return to the bath when they rise to a certain height, while the hydrogen and oxygen continue to rise until they leave the liquid surface, thus eliminating the gas and effectively suppressing the chromium fog.

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