Chrome Plating Process
This may be more than you ever wanted to know about industrial hard chrome plating, but we wanted you to know the extent of our chrome plating method. If you would rather, just contact us to see how this technique can be a benefit to you.
The Plating Mechanism
Chromium metal is deposited on metallic surfaces submerged in a chromic acid plating bath. Since the method is electrolysis, two electrodes exist. The part to be plated is made cathodic by connection to the negative terminal of the rectifier. Inert lead anodes are made electrically positive. When voltage is applied across the two, current flows through the solution and metal is deposited. Actual rates of deposition for commercial parts range from .0005:/hour to .003". Proper control of various plating parameters result in bright, hard, adherent deposits.
Practical industrial hard chrome plating baths now in common use have only two or three chemical constituents. Chromic acid is formed when chromium trioxide crystals are dissolved in water. Some form of catalyst is also necessary for the process to occur. The most widely used catalyst is sulfate ion, usually added to the bath as reagent grade sulfuric acid. Varying the absolute and relative concentrations of these two constituents will alter the deposit characteristics significantly. Routine bath analyses are performed at High Tech Chrome Plating's in-house lab every 15 to 30 days to ensure proper chemical levels. This routine analysis is very important in producing a bright industrial hard chrome.
As previously mentioned, direct current (DC) is used to deposit chromium metal. Since the cathodic efficiency of this method is rather low, relatively high current levels are needed. Furthermore, accurate control voltage or current is required in order to obtain consistently bright coatings of predictable thickness. In the early days of chrome plating, motor-generators were used extensively. Large electric motors were coupled to mechanical DC generators. Efficiencies were low, and the units often occupied entire rooms because of their enormous physical size. As the semiconductor devices improved in the electronics industry, the generators became less desirable compared to the various types of solid-state power supplies. Some of the newer units in use today are: saturable-core reactors, tap switch rectifiers, manual and motor driven powerstats, thyristor-based SCR models and pulse power supplies. The type used at High Tech Chrome Plating are state-of-the-art Rapid SCR controlled rectifiers.
Bussing and Other DC Conductors
Extensive bussing is provided to carry the current from the power supply to the electrified tanks. This bussing generally takes the form of multiple layers of 1/4" X 4" copper. Also, tanktop copper is permanently installed on or over the rims of the chrome plating, electroclean and strip tanks. The dimensions of this copper will vary considerably. Not only must it be sized to carry the electrical load without overheating, it also must carry the physical weight of the parts suspended from it. Horizontal crossover bars also exist which deliver power from the permanent tanktop copper bars to the center of the tank. Anodes and parts being plated require these crossover bars. However, they are not permanently mounted on the plating tank, but rather are bolted to the parts and anodes. The only other conductors required are vertical dropdown bars that suspend the parts under the solution. These bars may be supplied in aluminum, since it resists chemical attack by the chromic acid. Not unlike the tank top copper, crossover and dropdown bars must be properly sized considering amperage and weight.
Each part plated must be correctly oriented under solution. Therefore, plating racks or fixtures have several responsibilities. First of all, they must carry current from the power supply to the part. Copper and aluminum are used extensively since they are such good electrical conductors. Aluminum holds up well to non-fluoride chemistry, but copper is quickly attacked and must be coated. Racking fixtures have the additional responsibility of holding the part of proper spatial relationship with the lead anode. Sometimes, the part and anode can be mounted jointly on a common rack, then placed under the solution. Usually, the anode must be placed in the tank first, then the part is inserted near it. All racking fixtures are fabricated in-house to accommodate each part. The diversity of plated parts and the availability of metalworking equipment are factors to be considered when electing to use a hard chrome plating service.
Conforming anode plating is highly stressed at High Tech Chrome Plating. With this method, anodes are fabricated specifically for one part or group of parts with similar size and shape. The anode is built to agree with the surface contours of the part receiving the chromium deposit. During the plating cycle, the anode is positioned in close proximity to the part. In this way, accurate thickness control can be achieved which usually results in reduced secondary machining. By contrast, the widespread use of "stick" anodes permanently installed in the tank offers very little plating control. Additional benefits for conforming anode plating include higher cathodic efficiency, reduced electrical consumption, diminished cooling loads and increased rates of deposition. The anodes are sturdily built at High Tech Chrome Plating and offer years of service. Hydraulic rebuilders, machine shops, or other service-oriented companies often have a different situation. Many parts are one-of-a-kind, and turn-around time may be critical. In-house anode fabrication is valuable when these conditions exist. Depending on your corporate needs, High Tech Chrome Plating has a fixture fabrication department to construct the anodes and fixtures to fit your part.
The fumes generated during electrolysis in the chrome plating and electrostrip operations are not breathable. For this reason, complete ventilation and filtration systems are used at our plant. Hoods are positioned along one or more of the tank rims to exhaust these fumes. However, the chromic acid ventilation system utilizes extraction techniques to reclaim the acid and reduce pollutants to the atmosphere. Each chrome plating system installed complies with state and federal air standards governing the plating room and discharge to the atmosphere. All ventilation equipment is fabricated of chemical-resistant material. Hoods, ducting, elbows, transitions, laterals and fan housings are either PVC or fiberglass material. Fan impellers are plastic-coated steel. All water wash-down or reclaim plumbing is PVC.
Chrome plating is typically performed in the temperature range of 110-150 degrees Fahrenheit (43-66 degrees Celsius). In order to pre-warm the bath and part, electric immersion heaters are installed in the plating tank itself. These units have titanium or Teflon tubes, and plastic heads. Sizing varies with tank volume, but is generally calculated for an eight hour warm-up cycle. Indicating controls are used to more closely monitor bath temperature. For systems that have large power supplies, cooling is required (see "Cooling"). The temperature control then has two set points - the lower one energizes the heater's magnetic contactor, and the upper is assigned to the cooling pump.
Complex chemical reactions taking place during chrome plating make it a very inefficient process electrically. High amperages and current densities are required from the power supply, but only a small fraction of this power deposits metal. The rest is converted to (usually unwanted) heat in the bath. Cooling is needed whenever the surface and transmission losses associated with the plating tank do not exceed the heat generated during the electrolysis. If only marginal cooling loads exist, tap water pumped through a tank coil will suffice. An open loop cooling system is used to minimize water consumption. Our system includes a reservoir to store water, the tank coils submerged in the bath itself, and an external cooling tower. The tank coils are titanium heat exchangers permanently mounted in the plating tank.
Solution agitation can be beneficial in the plating tank. Both chemical and thermal uniformity is improved when the bath is mixed. This movement is especially helpful when adding replacement chemicals. During the pre warm cycle, this agitation keeps the bath from stratifying into layers of varying temperature. Agitation in the final rinse tanks will also improve the removal of liquid chromic acid from parts after plating. Although mechanical stirring of fluid pumping can be used, the best system for agitating these tanks involves air agitation. Rotary vane compressors produce the mild air which is piped to PVC distribution manifolds located at the tank bottoms.
Various pumps are used with our system. The cooling system is fitted with one or more close-coupled, centrifugal pumps. Extensive computer calculation of cooling requirements and system parameters was performed before selecting the pump with the correct head/flow curve. These calculations involve the estimation of static and dynamic losses and ideal flow rate for maximum cooling efficiency. Both centrifugal and air-diaphragm pumps are used for reclaiming rinse water. In this process, water from any given rinse tank is returned to the plating tank or rinse tank that precedes it in the chrome plating sequence. Centrifugal and gear pumps are used for spray rinsing parts immediately after plating. In this application, rinse water is pressurized and sprayed through a ring of high pressure, low volume nozzles mounted inside the spray rinse tank itself. Chemical metering pumps are used whenever small amounts of chemicals must be continuously administered to a tank or stream. Regardless of the type of pump, each unit is constructed of chemically resistant materials for its specific application.