Chrome Plating

The piston rod of a hydraulic cylinder is a component that demands a number of mechanical properties that are often at odds with each other.

A piston rod must have a high tensile strength, it must have a hard surface, it must be corrosion resistant and it must be smooth. It must resist wear from the side load forces that a hydraulic cylinder will see in service. It must provide a low friction surface as it moves back and forth in the hydraulic cylinder passing over seals, wipers and bearing surfaces.

These qualities are very difficult to find in one material. A metal rod with high tensile strength will often lack surface hardness. A metal that is corrosion resistant will often lack tensile strength or is very expensive. A hard metal is often brittle or readily corrodes.

To satisfy all of these various properties and requirements, many manufacturers use chrome plating as a surface finish to hydraulic cylinder piston rods. The piston rod can thus made from high strength carbon steel to satisfy the strength requirement. High strength steel is common and relatively inexpensive. It is easy to machine and weld. Chrome plating gives this material the remaining required properties for a good hydraulic piston rod. This process is also called Hard Chrome Plating or Engineered Chrome Plating.

 

 

The machined steel piston rod is then chrome plated to give it a hard, smooth and corrosion resistant surface finish. The plating thickness is actually very small compared to the overall diameter of the piston rod, usually only about .001" to .003" in depth. This gives a very hard surface in the order of 66-70 HRC. Chrome plating on piston rods is often tested for corrosion resistance with a salt spray test that lasts for hours.

Chrome plating is a complex electro-chemical process. It involves dipping the component to be plated in a heated chemical bath of chromic acid. An electrical voltage is then applied across both the component and the liquid chemical solution. A complex chemical process then occurs that slowly applies a thin layer of chrome to the metal surface over a period of time.

Before the component can be plated, however, it must first be machined to its final condition. A piston rod will therefore be cut to length, threaded and machined so it is ready to be installed in a cylinder. It must have a smooth surface finish as the plating process is not self leveling. It will not fill in grooves, scratches, pits or other deformities. The component to be plated must also be very clean so that the plating will adhere to the surface. It is therefore manually cleaned and degreased and rinsed before plating.

The component to be plated is made cathodic. This means that it is connected to the negative terminal of an electrical power source. The cathode is often in the form of the rack from which the parts to be plated are suspended. This rack must be large and strong enough to carry the part being plated. This can be of very considerable size when manufacturing hydraulic components. The rack must also be impervious to the chemicals in the plating solution. Finally, it must also be capable of transmitting the large electrical currents used in the process.

Inside the chemical bath, positively charged anodes complete the electric circuit of the electro-plating process. When voltage is applied electrical current passes through the solution and through the components being plated. A thin layer of Chrome metal is slowly applied at a rate of about .001" (25 micrometers) per hour.

Electro-plating requires high levels of DC current. The voltage and current levels must be accurately controlled so as to produce plating of consistent quality and thickness. Providing high levels of controlled DC current is obviously expensive both in capital equipment investment, maintenance and power usage.

The plating process is in itself not a very electrically efficient process. A large portion of the current is lost in the chemical solution when it is converted to heat. Only a relatively small portion of the electrical power is actually used to deposit the chromium metal to the surface of the component being plated.

The chemical bath solution must be preheated to a temperature between 110 to 150 degrees F (43-66 degrees C). This temperature range must be maintained. Often cooling is required as the electrical process continues to heat the solution throughout the process.

In order to maintain both temperature and chemical uniformity throughout the entire plating tank, the liquid solution is mechanically agitated. This agitation process can also be achieved using fluid stirring pumps or a compressed air bubbling system.

The chemical solution used in the plating process must be continually tested for quality on an ongoing basis. This will ensure that the process maintains consistency and efficiency. Replacement chemicals are added periodically as required to the plating solution. The mechanical agitation of the solution will ensure that newly added chemicals are quickly and evenly dispersed throughout the plating tank.

Once the required thickness of plating is achieved the components are lifted from the bath solution. They must now be rinsed clean of the liquid chromic acid. This is often done with pressurized water spray. Then the parts are dried and sent to the next step of the assembly process. If the components are to be stored for a length of time before use, they are often sprayed with a corrosion resistant solution.

The chrome plating process uses a number of chemicals that are corrosive and toxic. These must be handled carefully. They come under strict state and federal government regulations. The process itself gives off fumes that are not breathable and must be ventilated and filtered. The ventilation system reclaims the chemicals and reduces atmospheric pollutants.

The final product is very much worth the complex procedure used. Chrome plating is the most common method used by manufacturers of hydraulic cylinders. Due to the expensive equipment and complex process involved, most cylinder manufacturers do not have "In House" chroming. Most "farm out" the chroming process to custom metal suppliers.

Hyco Alabama, however, has "In House" chroming. This enables us to quickly and efficiently respond to our customers needs. This critical part of the hydraulic cylinder manufacturing process is also kept under our own stringent quality control process. The result to our customers is a better product with faster delivery at a superior price.

The use of chrome plating is not without guidelines for proper use and maintenance.

Chrome plated piston rods should be kept clean and lubricated. Any chrome surface that will be exposed for a considerable length of time, such as during machine storage, should be treated with rust inhibiting solution. This solution can be made from 60% oil-based Rust Inhibitor and 40% Kerosene or Number 1 Fuel Oil. This solution should be reapplied every 6 months or after the equipment has been used and returned to storage.

Although chrome plating has a very hard surface, care should be taken not to scratch the surface. Never use sand paper or other such abrasives to clean a chromed surface. Use a solvent to remove sticky or gummy substances such as tar or paint. Use a cloth soaked in solvent to remove dirt and dust. Relubricate the shaft after cleaning or apply the anti-corrosion solution.

For substances that are very difficult to remove, use plastic or copper wool along with the appropriate solvent. Wipe off the solvent with a dry cloth after cleaning and relubricate or apply the anti-corrosion solution.

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