Hard Chrome Plating Clean Bill of Health

Surface engineering is a hot topic, with applications throughout a wide range of industrial sectors. Engineering surface treatments and coatings can bring specialist properties of corrosion and wear resistance, without compromising the substrate characteristics. New processes are emerging quickly, with innovations and developments sometimes outpacing any proven industrial demand or performance demonstration. There is physical vapour deposition, chemical vapour deposition, implantation, a bewildering array of thermal spray processes, fused coatings, new variations on diffusion processes, all striving for the ultimate hard, tough, wear resistant, low friction and anti-corrosion surface.

And yet, for many applications, such a coating already exists, its qualities too long taken for granted or even overlooked, and its development too long neglected. Hard Chrome Plating brings a wide variety of desirable properties and characteristics; properties that many engineers require of their components and products.

But, because if its use of hexavalent Cr, the process has now come under the scrutiny of the environmental legislators, so that this invaluable technology is under imminent threat. It is a situation that A T Poeton Ltd, one of the UK's foremost surface engineering companies, is determined to address through their Low-Effluent Chrome Plating Project.

Hard Chrome plating is an electrolytic process utilising a chromic acid-based electrolyte. The part is made the cathode and, with the passage of a DC current via lead anodes, chromium metal builds on the component surface.

A wide variety of parts can be coated, from textile guide pins to large steel mill rolls; it requires only the appropriate jigging, a large enough bath, sufficient lifting capacity, and adequate power sources. For the engineer, some of the attractive properties of hard chrome plating include:

It is ultra hard; 850-1000Hv (65-70Rc), harder than most industrial abrasives and steel counterfaces. Unusually, it combines this hardness with a degree of toughness, so the deposit can stand up to high stress contact.

It gives superb substrate adhesion; greater than 10,000 psi. Before plating, substrates are subjected to rigorous cleaning, so that the surface to be coated is completely free from contaminants. But the ultimate adhesion is achieved in the plating bath itself, with an initial reverse voltage surface etch which removes any last vestiges of surface contamination.

It can be applied to a wide variety of substrates; encompassing a range of engineering or highly alloyed steels (including stainless steels and cast irons), lightweight aluminium alloys or titanium alloys, copper alloys and bronzes, as well as nickel-based alloys.

It has a bright, attractive finish; it responds well to grinding and polishing, providing ideal surfaces for dealing with delicate products (like textiles and paper), for giving low friction, and for high precision parts like lathe beds.

The deposition temperature is low; and with de-embrittlement treatment has little effect on substrate properties. Provided that high strength steels are stress-relieved before plating and heat-treated again afterwards (usually for a few hours at about 200ºC), there will be few problems with hydrogen embrittlement.

It can be applied to a wide range of geometries. With careful attention to anode design and jigging, chrome plating can be applied evenly to flat surfaces, to fine bores, even with large aspect ratios, to cylindrical surfaces like shafts or rollers, and to holes and close tolerance assembly areas.

The plating is stable and noncorrodable; it is suitable for most organic acids and gases (not chlorine), excellent in hot oxidising or reducing air, in commodities like beer, sugar, brine, coal gas, cyanides, fruit acids, molten glass, glue, milk, oils and fuels.

It gives ultra high metal-to-metal sliding wear resistance; up to 100 times less wear than given by through-hardened, induction hardened, carburised or nitrided steels in dry sliding situations, and greatly superior to electroless nickel.

The exceptional dry sliding wear characteristics are now being exploited to the full by its rapidly increasing use on moulding dies for automotive parts. A 25 tonne cast iron body panel die represents a huge investment by a car manufacturer, and the life can be extended many times by the use of hard chrome plate, particularly on deep-drawn aspects of the panel, where material is stretched and pulled across the die surface under very high loads.

It provides superb abrasion resistance, even under high contact stresses. It gives a very low wear rate (at least 100 times better than hardened steel or electroless nickel) with abrasive products like textiles, paper and food stuffs. With thick deposits, it gives excellent wear resistance under high stress contact against sand, coal, cement, limestone, glass fibres, etc.

With abrasion being the most common and destructive wear process found in industry, hard chrome has the potential to solve many problems in pumps , valves, bearings, etc. throughout a wide range of industries; including pharmaceutical, chemical, oil and gas, textiles, printing, food, mining, and countless others.

And it produces very low friction against polymers, carbons and graphites. This makes it ideal for textile applications or for seal faces, as well as for components like vane pumps. It is the perfect counterface for PM and other polymers which rely on establishing a transfer-film for their best performance.

Hexavalent chrome is toxic and, in the absence of proof to the contrary, is considered carcinogenic. The risk is attached to people working near the electrolyte and not to the plated product itself (unlike cadmium plate). Chrome released from conventional effluent treatment plants is in a much reduced toxicity trivalent form and, increasingly, the UK and European legislators are tightening the discharge limits to the environment. The effect on chrome plating has been predictable, with a rising cost of effluent treatment and a concern amongst the users that either cost or availability of this invaluable coating process will be threatened. Some are looking for alternative coatings or treatments, but the unique combination of properties afforded by chrome plate is hard to match.

The appropriate way forward is not to abandon the process in the face of such concerns, but to confront the environmental aspects of the process and eliminate the discharge problem at source, aiming towards the goal of zero-effluent plating. To this end, a forward thinking project, headed by A T Poeton Ltd, has been cosponsored by the Departments of the Environment and Trade and Industry. Ale team of industrial supporters (see front cover) includes users and suppliers, and the research programme is aided by the expertise of the department of Chemistry at Portsmouth University.

The intention is that, by early 1998, there will be a fully operational zero-discharge demonstration unit at the Poeton Gloucester site. It will be available for viewing and assessment by plating companies and users alike to spread the word that environmentally friendly chrome plating can be a reality.

ZERO DISCHARGE PLATING

Ate project, which has been underway since September 1996, has already revealed how current working practices, many being a consequence of historical complacency, can be revolutionised. By changes in design of the plating vat, the acid vapour extraction and filtration system, and to the rinsing and masking procedures, it will be possible to recycle nearly all the electrolyte back to the bath. That is hexavalent chrome which currently ends up in the foul sewer or in the exhaust from the stack.

There is also the attractive possibility of saving energy and water. In existing plating systems, extraction systems effectively displace expensively heated air from the plating shop every few hours. Design innovations can greatly reduce these losses and more than return any increased investment that is required in the new plant. With improved rinsing, water uses can be reduced. Electrolyte replacement costs can be virtually eliminated.

One major obstacle to the cause of zero-discharge chrome plating arises from the practice of recycling. Inevitably, the concentration of trivalent chrome and metallic ions like iron, copper and aluminium in the electrolyte will rise steadily. Normally, they are diluted by frequent bath replenishment with new acid.

One major aspect of the project is being directed at correlating these ionic concentrations with key aspects of the plating quality; those properties of wear, corrosion and finish that the user will need to see maintained. A clear link has been established between the levels of such contaminants and aspects such as pitting, throwing power and hardness.

With knowledge of the maximum permissible levels of contamination, the final challenge is to remove them at a rate that can keep pace with production and without the need for large capital and operational costs. The merits of Ion Exchange are already well established, but the project team is looking hard at the alternative of dialysis. If the problems of membrane degradation and extraction efficiency can be solved, there is a real possibility of low cost, low maintenance acid regeneration.

Chrome plating has had a bright past; its future can be even brighter. It is now up to the plating industry to be more pro-active, to bring out and sell the benefits of a coating which can out-perform most other surface treatments and can be applied economically and effectively. 'Mere is a need to look now at new investment in environmentally friendly plating, so that the future restraints on discharges become an irrelevance; keep the hexavalant chrome where it is needed - in the plating vat.