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Chromium Plating

Chrome plate continues to play an important role in a number of key manufacturing areas, and Poeton expect this to continue well into the future. We are therefore committed to maintaining our substantial facility to offer hard chromium plating as a key part of our comprehensive metal finishing service. Poeton is registered with the appropriate authorities and is licensed to this effect, having invested to ensure better than full compliance with statutory obligations.

Equally, we are committed to investigate alternatives, coatings that can match the performance of hard chrome plate without the accompanying environmental problems. To this end, Poeton play a key role within the aerospace industry and the European Community, participating in research and development initiatives to investigate new processes.

Certainly, careful evaluation of applications can sometimes permit the use of alternative coatings, many of which are also carried out by Poeton. Whilst some of these can match the wear resistance of hard chrome plate, and others the corrosion protection, it is extremely difficult to find a cost-effective replacement that provides the unique combination of wear, corrosion, hardness, aesthetics, surface finish and substrate bonding. Hard chrome plate is a unique coating, one that will not be easily replaced.

Some Important Points about Chrome Plate

Hard chrome plate is a silver coloured electroplated coating, deposited from a hexavalent chrome electrolyte. It can be applied to all common engineering alloys, with high levels of adhesion to the substrate, achieved by reverse-etching prior to deposition. This level of atomic bonding distinguishes hard chromium plate from alternatives like thermal spraying, where the bond is purely physical.

Chrome plate is hard - around 1000Hv - and it combines this property with extreme toughness, again distinguishing it from alternatives like electroless nickel, where the hardness comes with a degree of brittleness. Hence, hard chrome plate out-performs its potential alternatives in wear resistance, either due to abrasion or metal-to metal sliding. (Many of the test results can be seen in the product pages on this website.)

Compared to its possible replacements, chrome plating remains the most effective coating for many applications. It is inexpensive, can be performed in large batches and is a reliable. It's a process that has endured for over one hundred years, providing hard, wear resistant coatings for a wide range of industrial applications, and it was the first coating process offered by Poeton when the company began in the late 19th century.

Uses

In general, hard chrome plate is applied for wear resistance or salvage. For the latter, the worn part is cleaned up, by machining or grinding, and then brought back to oversize with chrome plate, prior to final grinding. Typical uses are:

Thickness

The minimum thickness for a viable, wear-resistant coating of hard chrome plate is 12.5µ (0.0005 inch), a deposit that will be smooth and bright, and should require no post-finishing. For arduous, high load applications, layers up to 250µ (0.010 inch) might be required, post-ground to a pre-determined final size.

With thin coatings, designers can plan for an accurate coating thickness, with even distribution, for instance, adding up 25µ to the diameter of a shaft. For thicker coatings, the part must be prepared under-size, plated to over-size, and ground back to its required dimensions.

Variations

Standard hard chrome plate (Poeton Apticote 100N) is naturally micro-cracked – a network of ultra-fine cracks (less than 1µ wide) in a ‘crazy-paving’ pattern. They are an inevitable consequence of stresses in the coating, and are beneficial to lubricant retention on the surface.

If the substrate requires exceptional corrosion protection, one option is to apply a ‘thin, dense chrome’ (Poeton Apticote 100C), where the electrolyte and temperature are adjusted to produce a crack-free deposit. It is thinner and softer than conventional hard chrome. For additional wear protection, conventional hard chrome can then be applied on top. Alternatively, the hard chrome plate can be undercoated with a thick layer of electrolytic nickel, providing a reliable corrosion barrier. (Note that chrome plate and nickel plate are not sacrificial coatings, like zinc or cadmium; if they are breached, substrate corrosion can occur unchecked.)

Poeton Precision Hard Chrome Plating is a specialised variant, where the deposit thickness and coverage is controlled to close tolerances. It requires special anodes, cathode extensions and jigging, and eliminates the need for final grinding. The subsequent coating is harder, has superior substrate bonding, gives more wear resistance and requires shorter production times.

Flash hard chrome plating is a thinner variant (about 10µ), with limited covering power, applied only for mild wear situations.

Substrates

Hard chrome plating can be applied to cast iron, all grades of steel, most aluminium alloys. Non-ferrous metals such as copper and bronze are ideal substrates.

Substrates can be cast, rolled or extruded. However, materials with near-surface porosity or imperfections should be avoided. Plating on hardened steels is more difficult, and isolated work-hardened areas can cause irregular coating.

High Tensile Steels

Most steels are readily electroplated with chromium and require no heat- treatment. But heat-treatment of high strength steels after plating may be required to minimise the effect of hydrogen embrittlement and/or any reduction in fatigue strength.

Surface finish

The component finish prior to plating should ideally be 0.2µ Ra (ground) or 0.8µ Ra (turned), although polished surfaces can be successfully plated. Rougher surfaces may cause pitting in the coating. In general, the coating finish will be slightly rougher than that of the original substrate. Hard chrome can be polished to a high finish – 0.02µ Ra or better.

Process control

Hard chrome plate is susceptible to pitting and softening if the electrolyte and plating parameters are not optimised. In the Poeton Apticote 100 process, great attention is paid to detail:-

Environmental aspects

A hard chromium plated component presents no significant hazards. The metal is non-toxic, and can be ground, lapped or polished without any unusual precautions.

The potential environmental problems occur with the hard chrome plating process itself, and the use of the hexavalent chrome electrolyte (chromic acid), so the perceived hazards are to the chrome plating operators and to ground water, as traces of the electrolyte (typically as rinse effluents) are released. So there are strict controls on all aspects of the chrome plating process, covering allowable discharges to air and ground water, and operator exposure. Specifically, because hard chrome plating is a relatively inefficient process, there is considerable evolution of hydrogen above the bath. This buoyant gas carries up a mist of hexavalent chrome, presenting an inhalation hazard, so that regulations demand the use of bath lip extraction (through scrubbers and filters) as well as suppressants.

Hexavalent chrome is considered to be toxic and carcinogenic, and there are some claims of a higher level of cancers amongst chrome plating operators, evidence from a time when chrome plate was widely used in the automotive industry, with high throughput and operator exposure. However, this evidence is by no means conclusive, and it is for such reasons that hard chrome plating will be re-evaluated under the new EU 'REACH' initiative. This will seek to determine hard evidence of any harmful effects of hexavalent chrome (and hundreds of other industrial chemicals) and then draw up legislation that will dictate their future permitted uses.

In contrast, trivalent chrome, used to deposit thinner, softer decorative chrome plating, is non-toxic. Efforts to produce a thick, hard chrome plate using this more benign chemistry have been on-going for years and it may yet be that a viable hard chrome coating can be produced, negating the need to seek alternatives through other coating processes.

Contact us for complete information regarding EU directives on the use of chrome plating, Health & Safety information and reputable sources of independent advice.

Can I Specify chrome plating?

Yes. Hard chrome plate is for aircraft, defence, offshore, oil and gas, nuclear, hydraulics, pneumatics and many other industries. There are many areas in which use of chrome plate is restricted, such as food handling, medical and household goods.

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