Chrome plating

Technique of electroplating

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This article is about a chromium electroplating technique. For other uses, see Chrome (disambiguation)

Not to be confused with Chromate conversion coating

Decorative chrome plating on a motorcycle

Chrome plating (less commonly chromium plating) is a technique of electroplating a thin layer of chromium onto a metal object. A chrome plated part is called chrome, or is said to have been chromed. The chromium layer can be decorative, provide corrosion resistance, facilitate cleaning, and increase surface hardness. Sometimes, a less expensive substitute for chrome, such as nickel may be used for aesthetic purposes.

Chromium compounds used in electroplating are toxic. In most countries, their disposal is tightly regulated. Some fume suppressants used to control the emission of airborne chromium from plating baths are also toxic, making disposal even more difficult.

Process

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The preparation and chrome plating of a part typically includes some or all of these steps:

• Surface preparation
• Manual cleaning to remove dirt and surface impurities

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• Removal of remaining organic contaminants using emulsion cleaning, alkaline cleaning, anodic electrocleaning, or solvent cleaning by immersion, spray, manual application, or vapor condensation

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• Rinsing
• Activation or electroetching
• Rinsing (not necessary if the activation and plating steps are done in the same bath)
• Immersion in the chrome plating bath, where the part is allowed to warm to solution temperature
• Application of plating current for the required time to attain the desired thickness
• Rinsing

There are many variations to this process, depending on the type of substrate being plated. Different substrates need different etching solutions, such as hydrochloric, hydrofluoric, and sulfuric acids. Ferric chloride is also popular for the etching of nimonic alloys. Sometimes the component enters the chrome plating vat while electrically live. Sometimes the component has a conforming anode made from lead/tin or platinized titanium. A typical hard chrome vat plates at about 0.001 inches (25 μm) per hour.

Some common industry specifications governing the chrome plating process are AMS , AMS , and MIL-STD-.

Hexavalent chromium

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Hexavalent chromium plating, also known as hex-chrome, Cr6+, and chrome(VI) plating, uses chromium trioxide (CrO3, also known as chromic anhydride) as the main ingredient. Hexavalent chromium plating solution is used for both decorative and hard plating, as well as bright dipping of copper alloys, chromic acid anodizing, and chromate conversion coating.[3]

A typical hexavalent chromium plating process is:

1. Activation bath
2. Chromium bath
3. Rinse
4. Second rinse

The activation bath is typically a tank of chromic acid with a reverse current run through it. This etches the work-piece surface and removes any scale. In some cases, the activation step is done in the chromium bath. The chromium bath is a mixture of chromium trioxide and sulfuric acid, the ratio of which varies greatly between 75:1 to 250:1 by weight. This results in an extremely acidic bath (pH 0). The temperature and current density in the bath affect the brightness and final coverage. For decorative coating the temperature ranges from 35 to 45 °C (100 to 110 °F), but for hard coating it ranges from 50 to 65 °C (120 to 150 °F). Temperature is also dependent on the current density, because a higher current density requires a higher temperature. Finally, the whole bath is agitated to keep the temperature steady and achieve a uniform deposition.[3]

Disadvantages

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One functional disadvantage of hexavalent chromium plating is low cathode efficiency, which results in bad throwing power. This means it leaves a non-uniform coating, with more on edges and less in inside corners and holes. To overcome this problem the part may be over-plated and ground to size, or auxiliary anodes may be used around the hard-to-plate areas.[3] Hexavalent chromium is also considerably more toxic than trivalent chromium, rendering it a major health risk both in manufacturing and disposal if not handled with care.[4]

Trivalent chromium

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Trivalent chromium plating, also known as tri-chrome, Cr3+, and chrome(III) plating, uses chromium sulfate or chromium chloride as the main ingredient. Trivalent chromium plating is an alternative to hexavalent chromium in certain applications and thicknesses (e.g. decorative plating).[3]

A trivalent chromium plating process is similar to the hexavalent chromium plating process, except for the bath chemistry and anode composition. There are three main types of trivalent chromium bath configurations:[3]

• A chloride- or sulfate-based electrolyte bath using graphite or composite anodes, plus additives to prevent the oxidation of trivalent chromium to the anodes.
• A sulfate-based bath that uses lead anodes surrounded by boxes filled with sulfuric acid (known as shielded anodes), which keeps the trivalent chromium from oxidizing at the anodes.
• A sulfate-based bath that uses insoluble catalytic anodes, which maintains an electrode potential that prevents oxidation.

The trivalent chromium-plating process can plate the workpieces at a similar temperature, rate and hardness, as compared to hexavalent chromium. Plating thickness ranges from 5 to 50 μin (0.13 to 1.27 μm).[3]

Advantages and disadvantages

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The functional advantages of trivalent chromium are higher cathode efficiency and better throwing power. Better throwing power means better production rates. Less energy is required because of the lower current densities required. The process is more robust than hexavalent chromium because it can withstand current interruptions.[3]

One of the disadvantages when the process was first introduced was that decorative customers disapproved of the color differences. Companies now use additives to adjust the color. In hard coating applications, the corrosion resistance of thicker coatings is not quite as good as it is with hexavalent chromium. The cost of the chemicals is greater, but this is usually offset by greater production rates and lower overhead costs. In general, the process must be controlled more closely than in hexavalent chromium plating, especially with respect to metallic impurities. This means processes that are hard to control, such as barrel plating, are much more difficult using a trivalent chromium bath.[3]

Divalent chromium

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Divalent chromium plating is done from liquids comprising Cr2+ species. Such solutions were avoided prior to ca. , because of air-sensitivity and hydrogen evolution from aqueous Cr2+ solutions. In the s, it was discovered that CrCl2 has ca. 4.0 M solubility in water at room temperature (i.e. with H2O:Cr molar ratio around 14:1), and such liquids behave like supersaturated electrolytes with a reduced propensity toward hydrogen evolution. The best quality bright deposits are produced at relatively high current density of 20 mA/cm2.[5]

Types

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Decorative

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Art Deco portfolio with chrome-plated cover, ca

Decorative chrome is designed to be aesthetically pleasing and durable. Thicknesses range from 2 to 20 μin (0.05 to 0.5 μm), however, they are usually between 5 and 10 μin (0.13 and 0.25 μm). The chromium plating is usually applied over bright nickel plating. Typical base materials include steel, aluminium, plastic, copper alloys, and zinc alloys.[3] Decorative chrome plating is also very corrosion resistant and is often used on car parts, tools and kitchen utensils.[citation needed]

Hard

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Hard chrome plating

Hard chrome, also known as industrial chrome or engineered chrome, is used to reduce friction, improve durability through abrasion tolerance and wear resistance in general, minimize galling or seizing of parts, expand chemical inertness to include a broader set of conditions (such as oxidation resistance), and bulking material for worn parts to restore their original dimensions.[6] It is very hard, measuring between 65 and 69 HRC (also based on the base metal's hardness). Hard chrome tends to be thicker than decorative chrome, with standard thicknesses in non-salvage applications ranging from 20 to 40 μm,[7] but it can be an order of magnitude thicker for extreme wear resistance requirements, in such cases 100 μm or thicker provides optimal results. Unfortunately, such thicknesses emphasize the limitations of the process, which are overcome by plating extra thickness then grinding down and lapping to meet requirements, or to improve the overall aesthetics of the chromed piece.[3] Increasing plating thickness amplifies surface defects and roughness in proportional severity, because hard chrome does not have a leveling effect.[8] Pieces that are not ideally shaped in reference to electric field geometries (nearly every piece sent in for plating, except spheres and egg shaped objects) require even thicker plating to compensate for non-uniform deposition, and much of it is wasted when grinding the piece back to desired dimensions.[citation needed]

Modern engineered coatings do not suffer such drawbacks, which often price hard chrome out due to labor costs alone. Hard chrome replacement technologies outperform hard chrome in wear resistance, corrosion resistance, and cost. Hardness up to 80 HRC is not extraordinary for such materials. Modern engineered coatings applied using spray deposition can form layers of uniform thickness that often require no further polishing or machining. These coatings are often composites of polymers, metals, and ceramic powders or fibers as proprietary formulas protected by patents or as trade secrets, and thus are usually known by brand names.[9]

Hard chromium plating is subject to different types of quality requirements depending on the application; for instance, the plating on hydraulic piston rods are tested for corrosion resistance with a salt spray test.[citation needed]

Automotive use

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Most bright decorative items affixed to cars are referred to as "chrome", meaning steel that has undergone several plating processes to endure the temperature changes and weather that a car is subject to outdoors. However, the term passed on to cover any similar-looking shiny decorative auto parts, including silver plastic trim pieces in casual terminology. Triple plating is the most expensive and durable process, which involves plating the steel first with copper and then nickel before the chromium plating is applied.

Prior to the application of chrome in the s, nickel electroplating was used. In the short production run prior to the US entry into the Second World War, the government banned plating to save chromium and automobile manufacturers painted the decorative pieces in a complementary color. In the last years of the Korean War, the US contemplated banning chrome in favor of several cheaper processes (such as plating with zinc and then coating with shiny plastic).

In , a Restriction of Hazardous Substances Directive (RoHS) was issued banning several toxic substances for use in the automotive industry in Europe, including hexavalent chromium, which is used in chrome plating. However, chrome plating is metal and contains no hexavalent chromium after it is rinsed, so chrome plating is not banned.[10]

Arms use

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Chrome-lining protects the barrel or chamber of arms from corrosion and makes these parts also easier to clean, but this is not the main purpose for lining a barrel or chamber. Chrome-lining was introduced in machine guns to increase the wear resistance and service life of highly stressed arms parts like barrels and chambers, allowing more rounds to be fired before a barrel is worn and needs to be replaced. The end of the chamber, freebore and leade (the unrifled portion of the barrel just forward of the chamber), as well as the first few centimeters or few inches of rifling, in rifles are subject to very high temperatures &#; as the energy content of rifle propellants can exceed kJ/kg &#; and pressures that can exceed 380 MPa (55,114 psi). The propellant gases act similarly as the flame from a cutting torch, the gases heating up the metal to red-hot state and the velocity tearing away metal. Under slow fire conditions, the affected areas are able to cool sufficiently in between shots. Under sustained rapid fire or automatic/cyclic fire there is no time for the heat to dissipate. The heat and pressure effects exerted by the hot propellant gasses and friction by the projectile can quickly cause damage by washing away metal at the end of the chamber, freebore, leade and rifling. Hard chrome-lining protects the chamber, freebore, leade and rifling with a thin coat of wear resistant chrome. This significantly extends barrel life in arms that are fired for prolonged periods in full-auto or sustained rapid fire modes. Some arms manufacturers use Stellite-lining alloy as an alternative to hard chrome-lining to further increase the wear resistance and service life of highly stressed arms parts.[11][12]

Health and environmental concerns

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Hexavalent chromium is the most toxic form of chromium. In the U.S., the Environmental Protection Agency regulates it heavily. The EPA lists hexavalent chromium as a hazardous air pollutant because it is a human carcinogen, a "priority pollutant" under the Clean Water Act, and a "hazardous constituent" under the Resource Conservation and Recovery Act. Due to its low cathodic efficiency and high solution viscosity, a toxic mist of water and hexavalent chromium is released from the bath. Wet scrubbers are used to control these emissions. The liquid from the wet scrubbers is treated to precipitate the chromium and remove it from the wastewater before it is discharged.[3]

Additional toxic waste created from hexavalent chromium baths include lead chromates, which form in the bath because lead anodes are used. Barium is also used to control the sulfate concentration, which leads to the formation of barium sulfate (BaSO4).[3]

Trivalent chromium is intrinsically less toxic than hexavalent chromium. Because of the lower toxicity it is not regulated as strictly, which reduces overhead costs. Other health advantages include higher cathode efficiencies, which lead to less chromium air emissions; lower concentration levels, resulting in less chromium waste and anodes that do not decompose.[3]

Maintaining a bath surface tension less than 35 dyn/cm is necessary to prevent plating solution from becoming airborne when bubbles rise to the surface and pop. This requires a frequent cycle of treating the bath with a wetting agent fume suppressant and confirming the effect on surface tension.[13] Usually, surface tension is measured with a stalagmometer or tensiometer. This method is, however, tedious and suffers from inaccuracy (errors up to 22 dyn/cm have been reported), and is dependent on the user's experience and capabilities.[14]

While they are effective for the control of toxic airborne chromium, many widely used wetting agent fume suppressants are toxic themselves because they contain perfluoroalkyl substances (PFAS), which are hazardous chemicals that can cause long-term health effects.[15] This makes electroplating one of the jobs with the highest risk of occupational exposure to PFAS, but not as high as firefighters using fluorinated aqueous film forming foams.[16] In addition to their detrimental effects on human health, PFAS are persistent pollutants that cause significant bioaccumulation and biomagnification, putting animals at the highest trophic level at the highest risk for toxic effects.[17] [18]

Mechanism of chromium electroplating

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It has been known for over a century, that chromium electroplating is relatively easy from (di)chromate solutions, but difficult from Cr3+ solutions. Several theories have been proposed to explain this finding.

An earlier view suggested, that an active Cr3+ species (perhaps, with a ligand rather than water) forms initially from electroreduced Cr6+.[19][20] This active Cr3+ species can be reduced into metallic chromium relatively easy. However, the "active Cr3+" also undergoes within less than 1 second a transition into "inactive Cr3+", which is believed to be a polymeric hexa-aqua complex.[21] Some complexes of Cr3+ with ligand other than water can undergo relatively fast electroreduction to metallic chromium, and they are used in chromate-free chromium plating methods.[22][23]

A different school of thought suggests, that the main problem with chromium plating from Cr3+ solution is hydrogen evolution reaction (HER), and the role of chromate is to scavenge H+ ions in a reaction that competes with H2 evolution:

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Cr2O72- + 14H+ + 6e- &#; 2Cr3+ + 7H2O

The shine of plated chrome depends on whether microscopic cracks in the plating are visible on the surface. The dull appearance of some chrome layers is due to continuous cracks that propagate through the whole plated metal layer, while bright deposits appear in the case of small microcracks that are confined to inner depth of the deposit. This HER side-reaction mechanism seems more acceptable by the electrochemistry community at present. Methods of plating chromium from Cr3+ solutions that rely on reversed current pulses have been commercialized (allegedly, to reoxidize the H2).[24] [25] [26]

See also

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References

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Further reading

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• SAE AMS
• SAE AMS
• SAE AMS - Plating, Chromium

Figure 7 - C.G. Spring and Bumper Co. advertisement in .

Trivalent chromium solutions

The modern period of trivalent chromium plating could be said to have begun with a series of four patents20-23 filed in - covering work by J.F.K. McCullough and B.W. Gilchrist of the Ternstedt Manufacturing Company. These were an incomplete teaching or description, and no article was published. The basis was soluble, crystalline chromic chloride (CrCl3&#;6H2O) with various salt additions, and soluble chromium anodes. The chromic acid formed at the anodes was reduced with oxalic acid and hydrochloric acid to trivalent chromium.

The time of plating is not given anywhere, but the adhesion was poor and could only be improved by cutting the time down to a few seconds, according to the impression gained by the writer. At any rate, it seems that the process must have failed, as nothing more was heard of it.

The writer picked up an impression somewhere (a few tests in ) that good results could be obtained with the violet sublimed anhydrous CrCl3 either due to its purity or to its particular combination of impurities. This is insoluble, but dissolves readily with the evolution of heat if reduced a little bit, or if a little chromous chloride (CrCl2) is added. Some further tests in November and March dispelled the idea, and only black or blackish, peeling plates were obtained.

T. H. Webersinn (Fig. 8) was an inspired, indefatigable researcher in the s both in Waterbury and at Columbia University. His work on trivalent solutions was mainly from -35. He felt very strongly that the trivalent process he developed was a good advancement in the art and should be offered for commercial use. However, management applied a very strict performance standard and would not promote his process unless he could identify a specific improvement over ordinary hexavalent baths. While he matched the performance of hexavalent solutions as to appearance, speed of plating, plate distribution and so on, he failed to demonstrate any specific superiority. His best solution, K-100, apparently dated September (Fig. 9), had the following composition:

Potassium chrome alum, 420 g/L

Potassium fluoride, 40 g/L
Chromic acid, 15 g/L
Potassium hydroxide, 60 g/L
Chromium oxalate, 180 g/L

Figure 8 - T.H. Webersinn (-) in .

Figure 9 - Samples plated by T.H. Webersinn in trivalent chromium solution in .

- Samples plated by T.H. Webersinn in trivalent chromium solution in .

Chromic acid was found to be beneficial to this oxalate solution, but more than about 10-20 g/L tended to oxidize the oxalate radical. It becomes something of a moot question whether this is a trivalent or a dilute hexavalent solution. The good color of the deposits obtained tends to favor the concept of deposition from the small amount of chromic acid present.

Webersinn no doubt got a good deal of his inspiration from the publications of Mazzucchelli,24,25 who found that ammonium chromium oxalate, Cr(C2O4NH4)3&#;H2O gave a much better solution than any other trivalent salt. However, he did not get as good results as those obtained in the commercially used hexavalent solutions in Italy. Nevertheless, Mazzucchelli prepared his crystalline ammonium chromium oxalate by pouring 2 molar chromic acid solution into 6 molar oxalic acid and heating until crystallization occurred. He used a two-compartment cell, but frequently found Cr+6 in the catholyte.

At the same time, Britton and Westcott found better results with oxalate than with acetate or tartrate.26 Charles Kasper of the Bureau of Standards published an outstanding review in on deposition from both chromic and chromous solutions.27 The writer's reviews in and give some references to the use of chromium ammonium oxalate in brush plating.16,17

S. Eguchi in Japan has also, to some extent, confirmed Webersinn's work with oxalate, but again the distinction between hexavalent and trivalent solutions tends to become blurred or lost and quantitative investigation increasingly difficult.28-32 The language barrier is enough of a problem, but for some unaccountable reason, Chemical Abstracts will not refer to the Journal of the Metal Finishing Society of Japan as such, but insists on calling it Kinzoku Hyomen Gijutsu - a constant handicap. Eguchi typically may use 200 g/L chromic acid solution to which he adds 75 g/L (NH4)2SO4 and 500 g/L oxalic acid (H2C2O4&#;2H2O) and lets this mixture stand overnight at room temperature to get a solution with a major part of the chromic acid converted to trivalent chromium, but about 24 to 76 g/L of chromic acid remaining.32

A brief paper in cast the shadow of some things to come. Wade and Yntema33 found best results from a solution of 0.5M chromic sulfate containing 4 moles of ammonium sulfate and 0.1 mole/L of ammonium oxalate. Citric acid also had beneficial effects. Organic acids and ammonium sulfate raised the pH at which basic chromium salts precipitated, and thus widened the plating range.

The U.S. Bureau of Mines began a long-range investigation of the electrowinning of chromium metal in , primarily to utilize low-grade domestic chromium ore, but also to study the properties of high-purity chromium metal. This investigation has already been referenced in the last three editions of Modern Electroplating.15-17 At the same time, approximately, the Electro Metallurgical Company division of Union Carbide Corporation made a detailed study of the methods and procedures developed by the Bureau of Mines and added what appeared to be the best of them to its own conceptions and developments. The confluence of these two powerful streams of investigation broke down many stubborn obstacles and was rewarded with the creation of an outstandingly fortunate and successful development.

A report34 on the electrolysis of chromic sulfate solutions cited an efficiency of 45 percent for the reduction of Cr+3 and was followed by pilot-plant operations claiming an efficiency of 60 percent.35 Electrolyte modifications and improved cell design were discussed in a third report.36 Subsequent papers37-40 addressed developments leading to a commercial plant designed to produce up to 6 million lb./year of chromium. It went on stream in with an average plant efficiency of 45 percent. Good descriptions of this were given by F.E. Bacon.41-43 Electrodeposition of chromium from trivalent solutions was also reported in Japan44-49 and Russia.50

Jangg and Burger51 studied chromium deposition on mercury cathodes, and obtained as high as 68 percent efficiency from a 6 g/L solution of Cr+3 sulfate containing 50 g/L ammonium sulfate at 30°C by raising the pH to 3.2. With 4.0 g/L chromium as Cr+3 chloride at 30°C and pH 1.5, they obtained 70 percent current efficiency at the higher current densities. The importance of black, spongy deposits of low overvoltage in their work is discussed elsewhere.52

In , Bharucha and Ward of the British Non-Ferrous Metals Research Association (BNFMRA) published information on a trivalent chromium plating process using an unusual solution with 50 percent by volume dimethylformamide (DMF) as the solvent for chromic chloride.53 The research was also sponsored by the International Lead Zinc Research Organization (ILZRO), New York, NY, and the results were patented.54 The remarkable feature was that efficiencies in the range of 40 to 50 percent were obtained. These efficiencies were confirmed in two articles published in .55,56 The second article is particularly helpful, and discloses that some Cr+3 is reduced to Cr+2, which remains in the solution until re-oxidized and is not further reduced to metal. This creates a reducing condition in the solution that makes metal contamination much less of a problem. The final concentration of DMF selected was 400 g/L, or about 40 percent by volume.

The BNFMRA made an agreement with Albright and Wilson Ltd. in for the joint development and commercial exploitation of the DMF process.57 Difficulties and production experiences do not seem to have been publicized. Outdoor corrosion tests were reported in in which directly plated zinc-die-cast parts failed by dulling more rapidly than the same parts plated with conventional Cr+6 baths.58 On the other hand, much later tests in with both chloride and sulfate DMF baths gave better results in obtaining inclusions of Al2O3 and Cr3C2 particles in the Cr+3 deposits than in deposits from Cr+6 baths.59

J.E. Bride was employed at Battelle Memorial Institute from -56 and did some work on Cr+3 plating in this period. Then from -72 he worked at Diamond Alkali Company, later Diamond Shamrock Corporation, the latter part of this period being devoted to developments in Cr+3 plating. In he went with E. I. du Pont de Nemours & Company, Inc., and it took over the dossier of patents he had developed with some collaborators and added them to some of its own.60-74

At the end of he published a comprehensive article.75 DuPont offered these patented procedures to the trade but it is not clear just what they may have contributed to Cr+3 technology. Presumably they became part of the effort of Albright and Wilson Ltd. to establish the process, perhaps with formate or glycollate as a complexing agent for Cr+3.

In mid- and a new process, Alecra 3, was introduced, and in the process of doing this, Crowther and Renton76 and Chalkley, Crowther and Renton77 published some discussion of past failures and problems. There was burning at high current densities, and inability to plate into recessed areas. The coverage at low current densities was unacceptable, and compared unfavorably with that of existing hexavalent electrolytes. After a limited time the formation of a little hexavalent chromium caused the plating speed to fall off slowly to zero, and the solution could not be restored by sulfur dioxide treatment. In a later process78 it was found possible to reduce hexavalent chromium to Cr+3 by adding hydrogen peroxide and removing excess peroxide by heating. This, of course, would oxidize any Cr+2 present. The same would probably also be true of any hypophosphite present, the use of which was mentioned in several patents.79-80 It appears that ferrocyanide was added to these baths to remove trace metal contaminants.81 An important patent basic to this process should also be listed.82 It covers the use of formate or acetate together with a bromide and other useful constituents.

The Harshaw Chemical Company entered into an agreement with Albright & Wilson Ltd. in to distribute the new process in North America.83 The process is also sold by Waldberg SA in France. No doubt there are many other international sales arrangements.

The properties of trivalent chromium deposits are distinctly different from those from Cr+6 solutions. They generally have a darker color due to impurities and resemble the appearance of stainless steel. The deposits are intensely microporous with more than 1 million pores per cm2.78 At thicknesses of more than 2.5 μm (0. in.) they become dull, and over 15 μm (0. in.) powdery. The structure is weak and friable, and somewhat lacking in adhesion in greater thicknesses, and is consequently not considered for wear resistance.

While there is better coverage around holes and on underneath surfaces in practice, many recesses were more difficult to cover than expected, and doubts were raised as to the validity of Hull Cell work.76-77 However, the charts that were frequently drawn of plating rate vs. current density contained an element of sophistry and were a little misleading. Thus the plating rate could not remain constant while the current density increased unless there was a substantial decrease in current efficiency at the same time. The writer published some current efficiency/ current density curves from Huba and Bride's patent67 to show the lack of reproducibility of supposedly identical solutions, presumably due to impurities.

This difference was shown by Roubal,84 indicating the failure of chromium to deposit at a lower current density from Cr+3 solutions than from Cr+6. Again, Moritz indicates plating to a lower current density with Cr+6 than with Cr+3 by means of a current efficiency/current density chart.85

A further paper by Barnes, Ward and Carter86 reports good results for even thin deposits of decorative trivalent chromium plate for outdoor corrosion resistance directly on zinc. If the very small pores in trivalent chromium plate are plugged with dried and insoluble corrosion products the performance is good. On the other hand, in accelerated tests with continuous wetting with a highly conducting electrolyte, corrosion may continue unchecked and cause undercutting of the chromium.

Another report on corrosion characteristics was published by Snyder.87 Gianelos contributed three reviews of recent experience.88-90

Roubal84 reported a careful investigation of the Gyllenspetz and Renton patent.82 This was extended by throwing-power measurements with the now-old-fashioned Haring-Blum cell. An extensive study of trivalent chromium publications was reported by J. Datta91 from a dissertation submitted to TH-Aachen in Germany. Unfortunately, a more detailed publication has not appeared. A preliminary report is given on a citrate bath developed by Datta.

Schloetter obtained an early patent92 in which he added ammonium thiocyanate to chromic chloride baths. He thought chromium anodes could be used and dissolved directly in the form of chromic salt, but in the writer's experience this never occurs and chromium anodes only dissolve directly as chromic acid. Levy and Mornyer93-94 also used thiocyanate to get better chromium deposition from trivalent baths.

Another new Cr+3 process was developed in England by IBM United Kingdom Laboratories Ltd. and is being sold by W. Canning Materials Ltd. It has a base of thiocyanate complexant and a number of patents have been obtained. A two-compartment cell is used; two articles95-96 describe the working and advantages.

Still another new process is described by Tomaszewski and Fischer of Udylite.97 Professor Tajima remarked in the discussion that Cr+3 is not as corrosion resistant as Cr+6 and is grayer in color. The Toyota Company set up a pilot plant for the trivalent system but did not adopt it for production. Trivalent chromium is not now in use in Japan on either automobiles or cameras.98

French Center of Information

As an aftermath of World War II, the French government established a Center of Information on Hard Chromium with Paul Morisset as director. It operated effectively for 32 years, though its output was almost exclusively in the French language.

In Morisset published a little-known paperback book of 220 pages reviewing industrial applications of hard chromium. In , he published an excellent general textbook of 477 pages on chromium plating. This was translated into English and supplemented by Oswald, Draper and Pinner to make a 611-page book that appeared in . This latter book became something of a "bible" that can be found in the hands of many older chromium platers, but is otherwise not very well known. It is said to have been translated into Russian and Japanese, but, unfortunately, the English edition went out of print in a few years.

Morisset published a handsome, 895-page, bound book, Chromage dur et decoratif, in , but again it was not translated into English. Thus, by a sequence of events, we lack a high-grade, well-documented, comprehensive text of chromium plating technology in English. The second edition of the German book by Weiner in appeared in English translation by A. Walmsley in , but this is a somewhat modest, albeit well-organized, text of 224 pages.

The only other text with a good bibliographical background is the eighth edition of Gmelin's Handbook of Inorganic Chemistry (in German, with English notes), System No. 52, "Chromium," Part A, Section 2, p. 473-622 (). This gives numerous references on specific subjects but is already 20 years out of date [in - Ed.]. It seems the only possibility of a good, up-to-date reference text on chromium deposition in English would be a multi-author effort of some kind.

Chromium-plated tin cans

If we go into a supermarket today and examine the tops of tin cans, we find that they fall into two classes - those with the bright, white metallic appearance of reflowed tin plate, and those with a clean, bluish, steely look. The two finishes are used interchangeably, sometimes on parts of a can, sometimes on complete cans. The amount of each finish is about half of the total number of cans. The cans that give the appearance of clean, bare steel are chromium plated by special procedures.

This is a revolution in can-making that was started in Japan about 20 years ago by H. Uchida and co-workers at the Fuji Iron and Steel Company Ltd. under the name Cansuper.99,100 The process was promptly adopted by American can makers, and beverage cans made in this manner were introduced on a large scale in . A review was given elsewhere.17 The product has been dubbed tin-free steel (TFS) in view of its major function of replacing the scarce and expensive metal tin (Fig. 10). The chromium coating is extremely thin, less than 0. in., or 0.05 μm, and is applied to the rapidly moving steel strip in a few seconds or even fractions of a second. It serves to promote adhesion of the coating of baked lacquer that follows, and to prevent filiform or underfilm rusting.

 

Figure 10 - Chromium-plated beverage can produced by Weirton Steel Co. late in .

We have here an example where a thin coating is better than a thick coating. This is beyond the ken of practically all platers, who spend their lives being told to "put on a thicker coating." A μm is about 10,000 Angstroms (0. in.), and if we take one-tenth of this, or Angstroms, as an upper coating thickness, we are dealing in coatings about 300 or 400 atoms thick at the most. Films thinner than this can be called "thin films" and are common in electronic work. Our contact with electronic work has to do with "thick films" such as plating through-holes of printed circuits, mostly for contact purposes. Since electroplating by its very nature has to start with thin films only a few atoms thick, it is an ideal way to produce thin films of all sorts that have properties different from those of bulk material. It might pay to study books such as those of Chopra.101,102 On page 266 of the recent book for example, we learn that extremely thin films of indium or tool steel have a lower coefficient of friction than ordinary thicknesses.

It is known that thin metal filaments, or whiskers, grow to very small dimensions hardly greater than the thickness of thin films, and have unusual electrical and mechanical properties. Thus, Arnold103 measured the conductivity of tin whiskers and found that it corresponded to the conduction of more than 1 million amperes per square inch cross section. Bourgoin104 made bismuth filaments that showed a Type III superconductivity at room temperature.

 

Improved throwing power

Improved throwing power for chromium plating solutions has been a goal of many researchers since .109 During this author's four-year collaboration with Professor D.J. Kenney and a graduate student, Kris Komolboon, at the University of Detroit, it was demonstrated that a chlorine-containing compound formed by adding a small amount of HCl to a standard chromic/ sulfuric acid bath significantly improved throwing power and extended the plating range into the low-current-density areas of Hull Cell panels. Curves 1 and 2 in Fig. 11 show the distribution of chromium on Hull Cell panels obtained with the chlorine compound. Curve 3 shows the inferior distribution on a panel plated in a standard solution. The chloride addition, which results in the formation of chromyl chloride (CrO2Cl2) also improves the brightness of the deposit.

Figure 11 - Figure from patent application on use of chloride compound in chromium plating bath. Curves 1 and 2 show good Cr distribution; Curve 3 shows inferior distribution from standard bath, all for 5-hr deposits.

- Figure from patent application on use of chloride compound in chromium plating bath. Curves 1 and 2 show good Cr distribution; Curve 3 shows inferior distribution from standard bath, all for 5-hr deposits.

Chromium unpopular?

It has been said that beauty is in the eye of the beholder, but when automotive designers ask the public to gasp in admiration over black bumpers on a black car, we suspect the presence of quite a few individuals with tongue in cheek. A good technical review of the current situation was given by Pegram under the title "Bright Chrome Is Dead - Long Live Bright Chrome!"105 After discussing such subjects as alternative finishes, the genuine alternative, the genuine article, and nickel is so expensive, Pegram concludes: "Re-birth or no re-birth, long live bright chrome!"

When nickel plating was introduced in -70, good quality was maintained on the whole, and it quickly had a surge of popularity bringing with it the introduction of dc generators around -76 to displace batteries as the source of power. When the New York, Chicago and St. Louis Railway was built in -82 from Buffalo to Cleveland, Chicago and St. Louis, it became known as the The Nickel Plate Road in the Cleveland and especially the Sandusky, Ohio, area. An account of many of the 32 ways that were suggested as the source of this name in a history of the railroad106 gave as the best one that "the company had the cash on hand to pay as the road was built, the term being borrowed from the barkeepers' slang, that the fellow who paid for his drinks on delivery and never had his score chalked, was a nickel-plated fellow."

The early popularity of "chromium plated" as synonymous with "durable, long-lasting, untarnishable" and so on was mentioned in discussion of the early days. More recently, an author discussing restored antique automobiles called them "Chrome Dreams,"107 and a account108 of General Motors as the equivalent of the 14th largest nation in the world is called "Chrome Colossus." One cannot consider the adjective "chrome" as anything but a flattering one.

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About the Author

Dr. George

Dubpernell

 

Dr. Dubpernell, now retired and a resident of Huntington Woods, Mich., has spent 60 years in the field of chromium plating. His early experience in electroplating began in and included work with lead (Detroit Battery Co.), zinc (Harris Zinc Process Co.), copper and nickel (C.G. Spring and Bumper Co.), and cadmium (The Udylite Corp.). His first introduction to chromium plating was in . Nine years later he joined United Chromium, which later became Metal and Thermit Corp., then M&T Chemicals Inc. Dr. Dubpernell studied and worked under Dr. Colin G. Fink at Columbia University, where he received his M.S. degree in chemical engineering, and earned his Ph.D. at the University of Michigan. Dr. Dubpernell has been granted at least 10 patents and has published many papers. He is a national honorary member of the AES, a charter member of the Institute of Metal Finishing, and an emeritus member of the American Chemical Society and The Electrochemical Society. *Written at the time this paper originally was published.   

 

 

 

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