Chromium or chrome plating is a surface finishing process that applies a thin layer of metallic Chromium onto a metal or plastic surface, generally following application of one or more electroplated base coats to improve properties.
Chromium is a hard, corrosion-resistant metal that is white light reflective, making it a mirror finish and protective coating, when the substrate is polished.
chrome plating should be specified with great care. In particular, hardened steel parts can respond very badly to chrome plating, by becoming brittle, cracked and fragile.
Chrome plating fits in a premium position, in consumer consciousness.
It fulfills a number of roles, in consumer thinking:
- Chrome elements makes a product look higher quality
- The plating is associated with old-world values and status markers, as shiny metal was the preserve of the rich for much of human history
- Reflective surfaces are associated with ‘modern’ and ‘technology’, so chrome speaks to consumers of these things
Chromium plating technically improves the corrosion resistance of metal parts, particularly steel. It also enhances the wear resistance of metal surfaces. In machine applications it is used to apply a robust surface to moving parts, to improve hardness and wear properties of cheaper materials.
But its widest use is to alter the appearance of metal objects such as car parts, consumer electronics and wearables, kitchen appliances, and bathroom fixtures.
Chromium plating is environmentally hazardous, because of hexavalent Chromium. Handling and disposing of the plating chemicals poses hazards.
What is Chrome Plating?
The electroplating of chromium onto conductive surfaces requires an electrolytic solution containing chromium ions. In many plating processes, the metal source is the anode in the electrolytic cell, but Chromium plating works best directly from an ionic solution of chrome salts.
The Chromium ions are attracted to the cathode, which is the object to be plated, by an electric field effect resulting from the voltage between terminals.
At the cathode surface, a deionization effect releases the Chromium atom and it deposits onto the surface in a crystalline process.
This produces a plated surface that precisely copies the underlying condition, amplifying any surface defects. It is because of this effect that the substrate must carry a high polish, if the plating is to provide a mirror result.
The object to be plated is etch cleaned and rinsed, to remove any dirt, oil or other contaminants that would marr the finish or prevent adhesion/uniformity.
In most cases, for a well adhered and durable result, one or several substrate platings with other metals precede the Chromium layer.
The degreased and polished part to be plated is immersed in the electrolytic solution, wired as the cathode in the circuit which will generally use a Carbon anode. DC electrical current is passed through the ionic solution, resulting in Chromium atoms being deposited onto the part.
The thickness of the applied Chromium is controlled by adjusting the voltage and current of the cell, as well as by adjusting the concentration of Chromium ions in the electrolyte.
Types of Chrome Plating
Chromium plating comes in two basic types, with numerous small process differences. These two types are differentiated by application – aesthetic or functional.
In all forms of chrome plating, the surface of the part is prepared by grinding and polishing it to remove any surface imperfections – the level of polish defining the degree of mirror finish in the final plating.
The part is immersed in an electrolytic solution of ionic Chromium, and the cell current deposits the Chromium onto the surface of the substrate anode.
The hard chrome plating process is essentially identical to other forms of Chromium plating, but with some additional steps.
The most significant difference is that hard chrome plating uses a different electrolyte solution, containing higher concentrations of ionic Chromium and sulfuric acid. This deposits Chromium faster and with finer crystalline structure, making the final surface more wear resistant.
Hard chrome is widely used on parts that are subjected to harsh environments and/or high levels of abrasion, such as hydraulic cylinder barrels, ICE pistons and machine tool components such as slides and screw feeds.
Hard chrome plating serves to extend the life of these parts, reducing maintenance costs.
Decorative Chromium plating is an identical process to hard chrome, but it uses a lower concentration electrolyte and applies a thinner layer of chrome, in general achieving a lesser degree of surface hardening and lower abrasion resistance, but a very similar cosmetic result.
It is not possible to differentiate hard chrome and cosmetic or decorative chrome plating without destructive evaluation to measure the thickness of the application.
Decorative chrome surfaces can be adjusted from mirror finish to a fine matte finish. This was formerly done using a fine bead or grit blast to indent or abrade the surface.
However, these processes were very labor intensive and they have largely been displaced by a chemical etch stage that removes a small quantity of the plating to render the surface microscopically irregular, to a very controlled degree from slight bloom through to coarse matte finish.
It is also possible to apply a linear abraded finish to chrome plating, by fine linishing with mild abrasives.
Applications of Chrome Coating
Chrome plating has many applications, across most product sectors, due to its various beneficial properties;
Automotive and Aerospace
Chrome plating is widely (but decreasingly) used on automotive parts, to appearance and to a lesser extent to provide protection against corrosion. In many applications, stainless steels are used in place of the formerly widely used chrome plated mild steel.
Hard chrome plating is extensively used on aviation landing gear and actuator parts to improve wear properties and environmental tolerance.
The process is used on industrial equipment such as hydraulic cylinder barrels, ICE and pneumatic pistons, actuators, bearing surfaces and gears to improve wear resistance and reduce friction.
Plumbing and Kitchen/Bathroom Fixtures and Appliances
Chrome plating is near universal on plumbing fixtures such as faucets, showerheads, and handles to improve their appearance and cleanability and to protect against corrosion.
Chrome plating of furniture components is used to imply a modern and attractive appearance.
Electronics and Consumer Products
Chrome plating is used on consumer electronic enclosures for its durable and attractive surface qualities.
The Chrome Plating Process
There are a range of possible steps in preparation for chrome plating onto metals, and the selected process can be shorter or longer, depending on the the quality and durability of the outcome that is required for the application:
- The first step is to strip the component down to bare metal to allow the electroplated layers to adhere fully and durably. Various levels of processing are applied, depending on the starting state of the parts. Surface preparations may include sandblasting, immersion in hot caustic soda and potentially reverse electrolysis to remove existing plating (in the case of repairs). This stripping removes oil, rust and any other contaminants, to prepare for the application of the electroplated layer(s). Finally, parts are rinsed in deionized water to remove any stripping chemistry that remains.
- Once the part is clean and decontaminated, it must be polished to a mirror finish. This is often a manual process, but larger parts can be electropolished as a first step, before final hand polishing.
- Parts are racked and prepared for electroplating, usually by hanging them from Copper hooks or wired which are connected to the electroplating anode.
- Higher quality chrome plating requires a Copper plating stage as the first application. The Copper gives very high adherence to steel and brass surfaces, reducing later peel and separation risk. The initial Copper layer also helps in reducing surface imperfections and provides an easier surface for additional polishing (in steel parts). The Copper electroplating process generally uses Copper sulfate solution as the electrolyte, with a Carbon cathode and the metal part as anode in a cell.
- The racked parts are removed from the bath, water rinsed and air dried.
- A polishing stage is sometimes added, after the Copper plating, for the highest quality finish.
- Next the second electroplating process is performed, using Nickel. A Nickel sulfate plating cell applies a fine layer of Nickel, which is a good smoothing/hardening agent for the surface and it bonds well to both Copper and Chrome, without any diffusion.
- Racked parts are again rinsed and air dried.
- Another polishing stage is sometimes added, after the Nickel plating, which can improve the luster and quality of finish.
- The final stage is electroplating in a chromic acid cell with an application of 12-volt to the electrodes. The duration of this stage and the concentration of the chromium salts and sulphuric acid in the bath define the thickness of the applied plating.
Chrome plating onto plastics has other stages before the electroplating process.
- Plastics are initially etched in a chromic acid-based solution to enhance their adhesive capabilities.
- Residual chromic acid is then rinsed from the plastic parts.
- Next, parts are sprayed with a solution consisting of palladium and tin salts, to act as catalyst for the next step.
- Parts are then electroless coated with Copper or Nickel, catalyzed by the Palladium/Tin application.
- Parts can then be electroplated with Chromium in a similar way to the above stages.
The Benefits of Chrome Plating
Benefits of chrome plating include:
- Corrosion resistance, allowing parts that would be environmentally affected to serve for longer.
- Durability, improving the resistance to wear, scratching and other damage, extending functional lifespan.
- Aesthetic appeal, rendering objects with a shiny, polished appearance that enhances aesthetics.
- Hygienic and cleanable surfaces result from chrome plating.
- Chrome plating can improve the heat resistance of surfaces, toughening surfaces working in high temperature environments.
The Main Industrial Applications of Chrome Plating
Chrome plating improves hardness, chemical resilience and durability of machine surfaces, while also helping in cleaning and damage and wear identification, as failure in the Chrome will generally expose the dull Nickel or Copper sub-layers.
Optimal wear/durability and corrosion resistance means parts last longer, even under severe wear/sliding contact use. Corrosion resistance can be equally valuable, making parts suitable for use in harsh environments.
Hard chrome is an idea surface for a wide range of industrial applications, including:
- Hydraulic cylinder barrels and pistons
- Automotive and mechanical components, including wear surfaces on Aluminum alloy parts
- Rollers, molds, dies and other metal forming surfaces
- Hard chroming can be used for added toughness in plastic mold tooling, on bearing AND molding surfaces
- Press tooling
- Mining and agriculture equipment for harsh, abrasive environments
- Shafts and rotors for pumps
- Shuttles, guides and bobbins used in textile making
- Rollers and paper handling parts in printing
How to Remove Chrome Plating
To remove or strip chrome plating from metal, either use mechanical abrasion/grinding or it can be done using a reverse electrolysis approach, in which the part to be de-plated is made the ANODE in an electrolytic cell using a strongly caustic electrolyte, which ionizes and dissolves the plated metals
Kemal’s Chrome Plating Services
Kemal has been handing clients hard chrome and decorative chrome plating needs for many years and we have a wealth of experience that is at your disposal.
Whether you’re needing turbine blades to deliver extended life, or decorative plating consumer product parts, if you need harder wearing actuator shafts or chromed plastic decals for a wearable consumer product, Kemal has the knowledge and skill to help you, at competitive pricing.
We will be pleased to hear from you, to get an understanding of your chrome plating needs and show you how we can assist.