By Allen R. Jones and Donald Snyder
Atotech USA, Inc.
Rock Hill, SC

Want to learn more about Chromium Plating—the process and systems? Read "The Burn That Still Burns" a three-part article that discusses the true burn and false burn from bright nickel and chromium systems.

The electrodeposition of chromium has been used since the mid 1800s because of the deposit’s unique combination of properties that has not been matched by any other single material. Commercially, chromium deposits are typically divided into two groups, decorative and hard (functional) chromium based more upon their applications than upon the chemistry of the processes.

Decorative Chromium Plating
Decorative chromium electrodeposits, “chrome,” are almost exclusively used as protective topcoats for decorative parts. Many of the parts are fabricated from the least expensive and easiest to form material (substrate) that offers the required physical properties. Typical chrome substrates are nickel or copper plus nickel plated plastic, steel, zinc die-castings and aluminum. Stainless steel is one of the few substrates frequently plated directly with decorative chromium as well as with nickel and then chromium. Once plated, all of the parts produced from the different substrates have the same consistent, eye appealing, metallic looking decorative chrome appearance.

Decorative chromium is usually plated in the thickness range of 0.2–0.5 microns. Even though the deposits are very thin, they offer the physical and chemical properties that are required for most decorative parts. Thinner deposits are very porous and offer minimal physical properties. Thicker deposits tend to be dull and form visible cracks. The service life of chrome-plated parts can vary from untouched ceiling light fixtures in environmentally controlled rooms to truck bumpers and wheels traveling thousands of miles per year with minimal cleaning or other care. The deposit’s appearance is retained in service because of its hardness and its excellent tarnish (surface corrosion), wear, chemical and scratch resistance. However, all of these properties depend upon the chromium deposit interacting in synergy with its underlying deposit, typically nickel. Bright nickel deposits help give the chromium deposit its bluish-white color and the chromium deposits protect the nickel from dulling due to surface oxidation. While multiple or single layers of nickel are primarily responsible for protecting the substrate from corrosion, the chromium deposit helps control the corrosion rate and is responsible for maintaining the overall reflective decorative appearance during service. Nickel and especially copper plus nickel also improve the appearance of the part by leveling out the defects (scratches, pores, etc.) in the substrate. This reduces the need for expensive surface preparation of the substrate.

Hexavalent or trivalent chromium electrolyte can be used to electroplate decorative chromium. Hexavalent processes are more prevalent but trivalent processes offer distinct health/environmental/safety and productivity advantages. Hexavalent chromium ions are carcinogenic and strong oxidizers while trivalent chromium ions are neither. Also, hexavalent processes are highly acidic while trivalent are mildly acidic. Hexavalent chromium processes produce the blue-white deposit that is considered the standard for the appearance of chrome. Trivalent chromium processes can produce deposits that vary from an almost hexavalent chromium deposit appearance, metallic white, to a pewter/stainless steel appearing deposit that appears to have depth. In almost all cases, the physical properties of trivalent chromium deposits are equivalent to those of micro-discontinuous (micro-pores or micro-cracks invisible without magnification) hexavalent chromium deposits.1 Decorative trivalent chromium deposits are micro-porous as plated while pre- or post-treatments are used to produce micro-discontinuous hexavalent chromium deposits.

Compared to chromium deposits without micro-discontinuities, micro-discontinuous chromium deposits over electroplated nickel significantly enhance the protection of the part from substrate corrosion. Hexavalent chromium deposits are not plated with sufficient micro-sized pores or cracks to offer this protection. One method to increase the micro-pore count is to lightly spray the dried nickel plus chromium plated part with hard particles such as aluminum oxide. This cracks the brittle chromium deposit and produces micro-porous chromium. A more frequently used method is to deposit a customized nickel strike in between the bright nickel and the chromium deposits. If inert particles are co-deposited within this nickel strike (particle nickel deposit), the subsequent chromium deposit will contain micro-pores where the chromium does not plate over the exposed particles. Another far less common approach is to use a nickel strike that is designed to micro-crack after being chromium plated. This micro-cracks the chromium deposit. An even less frequently used method is a specially designed hexavalent chromium process that forms micro-cracks during plating. Control of this process is difficult. Trivalent chromium deposits under 0.5 microns thick are micro-porous as plated. Thicker deposits are micro-cracked.

Micro-discontinuities in the chromium deposit spread the corrosion potential over the chromium and underlying nickel deposits, with the chromium the cathode and the nickel exposed through the micro-discontinuous sites, the anode. Because of the very large number of nickel sites, the anode current at any one location on the nickel is greatly reduced, which retards the rate of corrosion at any one site. North American automotive decorative chromium specifications require nickel plus micro-porous chromium for all exterior applications because of its increased corrosion resistance while maintaining the after-corrosion appearance of the chromium deposit. These specifications do not allow micro-cracked chromium deposits but they are permissible in other areas of the world. The importance of micro-discontinuous chromium and the required physical properties of the underlying nickel deposits are reviewed in ASTM B 456. ASTM specifies a minimum of 30 micro-cracks per mm or 10,000 micro-pores per sq cm.²

Chemistry
Even though there are several different families of decorative chromium processes each offering specific advantages, they are extremely versatile, which blends the properties of one process into the advantages of another process. However, platers might prefer one process compared to another because it offers advantages for their particular operation. Decorative chromium processes are most easily divided based upon their catalyst system, Table I. Chromium trioxide, CrO₃, commonly referred to as chromic acid (the hydrated form of CrO₃) is used to supply the hexavalent chromium ions. The sulfate is commonly controlled by addition of sulfuric acid (H₂SO₄). Proprietary additives are usually used to control the fluoride catalyst. Proprietary trivalent chromium processes are used in almost all cases because of the complexity of the solution formulation.

The hexavalent chromium processes listed in Table I can be further divided into self-regulating processes and those regulated by analysis. Self-regulating processes use additives that dissolve at the level required to maintain the proper catalyst concentration for the process. Increased temperatures raise catalyst levels resulting in improved plating rates. The analytically controlled processes use soluble additives that contain a specific amount of catalyst. Catalyst adds based upon analysis or amp-hours offer the advantage of varying their concentration to alter the bath performance.

Some of the advantages and disadvantages of hexavalent and trivalent chromium processes are listed in Table II. Other than the type and amount of catalysts used, most hexavalent chromium processes have similar operating conditions, Table III. This table also contains some general operating conditions for trivalent chromium processes. Some of the more specific factors within each process family also can be listed.

Single-Catalyst Hexavalent Chromium Processes
All chromium processes use less than 100% of their cathode efficiencies for chromium deposition. Single-catalyst processes have about 12% chromium plating efficiency. The remaining current goes to the formation of hydrogen and trivalent chromium, Cr³. The chromium deposition efficiency increases proportionally with chromic acid concentration, up to 250 g/liter and decreases thereafter. A CrO₃/SO₄ ratio of approximately 100:1 is common. Chloride in any hexavalent chromium process will over catalyze the process and adversely affect the appearance of the deposit.

Co-catalyzed Hexavalent Chromium Processes
The inclusion of the second catalyst, fluoride, can increase the average cathode efficiency to about 22%. The efficiency increases with increasing chromic acid concentration up to 300 g/L. A CrO₃/SO₄ ratio of about 190:1 is common. These processes are less sensitive to current interruptions and can plate over more passive substrates with reduced defects. They also have an increased plating speed, better coverage, wider bright range, and more tolerance to impurities. However, they will lose their fluoride catalyst if heated over 66°C. Also, the fluoride catalyst etches steel substrates resulting in iron contamination of the plating solution. An increase in rectifier voltage is required if metallic impurities (Ni, Fe, Cu and Cr³) are permitted to build over 7.5 g/L. Because of the second catalyst and the need for additional analysis, these processes are more expensive to operate than single-catalyst processes.

Tri-catalyzed Hexavalent Chromium Processes
These processes are similar to co-catalyzed processes except for the addition of a third catalyst, a proprietary organic catalyst. This improves the cathode deposition efficiency and chromium coverage in the low-current-density areas. This process is the most expensive decorative hexavalent process to operate but the improved low current density coverage justifies the cost in some operations.

Trivalent Chromium Processes
Platers switch to these processes primarily for increased productivity and improved environmental/health/safety factors. The improved coverage and throwing powers (similar to nickel), faster plating speed, the lack of burning and white wash, and a complete tolerance to current interruption all lead to reduced rejects and, in many cases, more parts on the plating rack. These factors along with a simplified method to maintain the purity of the solution and to waste treat it, if necessary, makes it less expensive per part to plate even though the operating solution is more expensive to purchase. These processes are similar to nickel processes in that they have to be analyzed and maintained more frequently than hexavalent chromium processes. The slight difference in deposit appearance and the need to passivate some thin nickel (under 6 microns) parts are cited by some as reasons not to convert to trivalent chromium processes.

Specialty Processes
Dark gray to black chromium deposits are available from both hexavalent and trivalent chromium processes. The hexavalent processes are used for solar collectors and both are used for decorative applications. Other than the additives used to darken the deposits, these processes operate similarly to others in the same family of processes. Hexavalent “cold chromium” (16 –21°C) processes are used where plating efficiency is less important than coverage in the very low-current-density areas. They are more difficult to operate and use different sulfate (450:1 for CrO₃:SO₄) and fluoride concentrations than typical hexavalent chromium processes. Barrel chromium processes are also available by using special hexavalent formulations in specially designed barrels.

Figure 1. Photomicrographs of anodically etched chromium deposits from (a) high-efficiency etch-free, (b) fluoride, and (c) conventional baths. The samples were plated under typical conditions for each bath. The cross sections were polished prior to etching.

Hard Chromium Plating
Hard (functional) chromium coatings are typically thicker than 1.2 um and in some applications can be more than 100 um thick. Chromium deposits impart functional qualities of wear and corrosion resistance, lubricity and release properties to a wide range of substrates. Substrates that are chromium plated include hardened and unhardened steels, tool steels, stainless steels, cast iron and aluminum. Chromium coatings allow the use of lower cost or lighter materials substrates, also the unique engineering properties of electrodeposited chromium that cannot be attained by substrates. Chromium coatings are very cost effective compared to many coating processes.

Chromium coatings resist corrosion in various environments.³ They are especially resistant to corrosion in oxidizing environments. Chromium coatings are used in the automotive, agricultural, chemical, petroleum, and manufacturing industries. For more severe applications duplex coatings are used. First a layer of nickel or electroless nickel is applied then chromium is plated. Some duplex coatings consist of a chromium layer that is plated and ground and then another chromium layer.

Figure 2. The chromium deposit thicknesses were 25 mm (1 mil) and the corrosion testing was conducted according to ASTM B117. Samples were electroplated from etch-free, fluoride, and conventional baths, under optimum conditions for each chemistry. The samples were tested as plated without any post finishing.

Functional hexavalent chromium plating has less environmental impact than many plating processes.⁴ Environmental metrics have been used to quantify the impact of plating processes on the environment. These metrics consider both a processes impact on the environment and the productivity of the process. Functional hexavalent chromium processes had first or second best metrics in half of the rated metrics. Various environmental controls prevent hexavalent chromium air and water emissions. The metallic chromium coating is not toxic and is not allergenic as are some of the proposed alternative coatings.

Chemistry
There are three predominant hard chromium plating processes: conventional, fluoride (mixed-catalyst), and etch-free high-efficiency. All of the baths contain chromium (VI) oxide commonly referred to as chromic acid (CrO₃) and sulfate. The sulfate and fluoride compounds act as catalysts. The etch-free high-efficiency bath contains a non-halide catalyst. Chromium cannot be electrodeposited from an aqueous CrO₃ solution unless one or more catalysts are present. Depending on which catalysts are present and the plating parameters, between 10 and 30% of the cathodic current will be used to reduce hexavalent chromium (Cr⁶⁺) to chromium metal. Their deposit properties and operating conditions are summarized in Table IV. The etch-free high-efficiency process will be referred to as the etch-free process. Typical deposit structures after anodic etching are shown in Figure 1.

Figure 3. (left) Taber Abraser, dry abrasive wear with 1kg load on CS-10 wheels after 10,000 cycles. The results are the average of three tests. Figure 4. (right)Falex, lubricated sliding wear with 340 kg load for five hours. The results are the average of three chromium plated test pins against 1137 vees.

Attributes of the different processes are shown in Table V. Each item is ranked for each process. The ranking values are -, 0 and +, the terms indicate that the process or deposit is inferior, average, and superior to the other processes, respectively. For some shops or plating jobs some attributes will be more important than others. The etch-free process has the most beneficial attributes. These attributes are discussed in detail below.

Corrosion Resistance
Corrosion resistance of a chromium plated part is influenced by the substrate quality, pretreatment, plating and post treatment. The optimum plating process should produce the least amount of nodules and the most microcracks for the best corrosion resistance. The microcracks of etched samples are shown in Figure 1. These figures show that as the microcrack density increases the depth of the etched cracks decrease. Corrosion resistance of identical samples plated under optimum conditions for each of the three baths is shown in Figure 2. The improved corrosion resistance of the etch-free process is due to a higher microcrack density and less nodular deposit.⁵

Figure 5. Variation of efficiency and microcrack density as a function of ratio (sulfate) for conventional and the etch-free processes.

Wear Resistance
Wear resistance is related to the deposits' hardness and toughness. Abrasion and sliding wear resistance data is shown in Figures 3 and 4. The etch-free process contains the smallest grains or crystallites of the three types of deposits and wears the least of the three processes.6 Wear is also related to the microcrack density and oxygen content of the deposit.

Operating window
The operating window is defined by the ranges of temperature, current density, and composition in which acceptable deposits are plated. Conventional chemistry has the smallest operating window, the fluoride process has an intermediate operating window, while the etch-free process has the largest operating window. The fluoride process has limited practical temperature range due to the volatility of the fluoride at higher temperatures. Figure 5 depicts the variation of efficiency and microcrack density as a function of ratio (sulfate concentration varied), compared to values obtained at a 100:1 ratio, for conventional and the etch-free processes. The conventional process has higher variations since it is a single-catalyst system.

Deposition Rate
The deposition rate is a function of the cathodic efficiency and the current density. Figure 6 shows the deposition rates for the three processes. At the same current densities the etch-free process is up to 60% faster than conventional, and up to 20% faster than the fluoride process. Due to catalyst stability, the etch-free process can be operated at higher current densities and temperatures than the fluoride process.

Figure 6. Deposition rate for etch-free, fluoride, and conventional baths. Fluoride data was from a customers sample plated in the laboratory.

Energy cost
At an electricity cost of 8 cents/kWhr the power cost is about $6/kg of CrO3 for an etch-free deposit. The power cost of a conventional deposit would be $11/kg and for the fluoride process would be $8/kg. The etch-free process has lower power costs due its higher efficiency and lower voltage (compared to the fluoride process). The fluoride process will have a higher average voltage due to the substrate etching. The conventional and fluoride have 80% and 30% higher power cost respectively, compared to the etch-free process. Data and conditions are shown in Figure 7.

Anode Cost
Anodes last about three times longer in a conventional or etch-free process than anodes in the fluoride bath. A large tank using the fluoride process can have an annual anode cost of $60,000. The savings with a conventional or etch-free process would be about $40,000/year. Additional savings can occur by switching from the more expensive lead tin alloy required in fluoride processes to the less expensive lead antimony anodes.

Figure 7. Power cost for etch-free, fluoride, and conventional baths. Rectifier efficiency is 85%. Fluoride process has higher voltage due to impurities from LCD etching.

Low-Current-Density (LCD) Etch
The LCD etch occurs at lower cathodic current densities than where plating occurs. Etching is minimal unless fluorides or chlorides are present. These halides attack the oxide that protects metals and allow the hydrogen to etch the substrate. The LDC etch rates of iron samples are shown in Figure 8 for the three chemistries. The etch-free and conventional chemistries etch 1,000 times less than the fluoride process.

Difficult to Plate Substrates
Nickel, high nickel alloys and tool steels can most easily be plated in the fluoride process. Fluoride provides very good activation for many substrates with an immersion or anodic etch. With the use of an activating solution, anodic etch and or a hydrogen wash; nickel, high nickel alloys, and tool steels can be plated in conventional and etch-free processes with the same adhesion strength as with a fluoride process. When the fluoride process is used the LCD areas must be masked to prevent etching of the substrate.

Additional Processes

Higher-Speed Etch-Free Processes
The etch-free catalyst is stable and non volatile at higher temperatures, allows the use of this type of chemistry at higher current densities. Current densities of up to 200 A/dm² can be used to obtain efficiencies of 30% and deposition rates of 4.6 microns/minute. These deposition rates are about 10 times faster than plating speeds obtained from conventional chemistries. Higher plating speeds allow smaller or fewer plating tanks and allows inline plating with other production processes.

Self-Regulated Processes
Numerous self-regulated conventional and high-speed fluoride processes are available. These chemistries contain sparingly soluble compounds to control the sulfate concentration and or the fluoride concentrations. The chemistries are partially or completely self-regulated and in some cases platers need only to maintain the chromic acid concentration to maintain the plating process.

Crack-Free Processes
Original deposits from crack-free processes contain no microcracks. These coatings have niche uses. Time, mechanical, or thermal stresses can cause these coatings to form macrocracks. Crack-free deposits are softer than microcracked deposits.

Supplemental Information

Anodes
Insoluble anodes are used for hexavalent chromium plating. Lead alloys with 7% tin or 6% antimony or tin and antimony are the most common anodes used. Lead/tin alloy anodes are more corrosion resistant and last longer than lead/antimony alloy anodes. Lead/tin alloy anodes should be used in fluoride baths. Lead/antimony alloy anodes are more rigid than the lead/tin alloy and must be used in deep tanks. Lead/tin/antimony alloys give some of the advantages of both alloying metals.

Different types of insoluble anodes are used for trivalent chromium plating depending upon the formulation. One process uses special graphite anodes that will last “indefinitely” if not mechanically damaged. Another process uses titanium anodes coated with metal oxide that needs to be recoated every several years. Both anodes eliminate the creation of hazardous lead salts in the plating solution.

Rectifiers
Direct current rectifiers should typically have a voltage range of between 9 and 15 v and sufficient amperage for the tanks. In hexavalent chromium plating it is critical that the ripple (alternating current), over the full range of amperage that will be used, is less than 5%. High ripple can cause soft and dull chromium and poor cohesion.

Trivalent chromium processes do not require low-ripple rectifiers. Nickel plating rectifiers with a voltage range of 6–12 v can be used. Fifteen-volt rectifiers are sometimes used to optimize productivity by increasing rack density and amperage without burning.

Tanks and Auxiliary Equipment
Tank linings may be a flexible polyvinyl chloride material of an approved type, either sheet or sprayed. Lead lined tanks are not recommended for fluoride or etch-free baths. Bricks are often used to line tanks. These must be compatible with the solution. Glass-containing bricks should not be used with fluoride solutions.

Metallic temperature controlling coils or heat exchangers must be electrically isolated from the steam and water lines. Titanium cannot be used with fluoride baths, while tantalum or niobium can be used. Kynar or PTFE coils can be used in all baths. Pumps, pipes, and ventilation equipment should be made from the above materials or from other chromic acid or fluoride/chromic acid resistant materials.

Figure 8. LCD (2 A/dm2) etch rates of iron for etch-free, fluoride, and conventional baths.

Trivalent chromium processes can use plastic, PVC, ABS, polyethylene or rubber tank liners. Copper anode rails should be nickel-chromium plated or plastic coated to avoid possible copper contamination of the plating solution. Under some plating conditions, heating and even cooling might be necessary. Titanium coils with thermostat-controlled water can be used. All equipment used to store or transfer the solution must be plastic lined to avoided metallic contamination. Ventilation is frequently not used due to a lack of misting but plastic air ducts with airflow similar to what is employed on nickel electroplating tanks may be used.

Purification
Some trivalent chromium processes utilize ion exchange resins to remove nickel, copper, iron, and zinc contamination from the plating solution. The operating solution may be passed directly through the resin even while plating. The regeneration solutions for the resin goes directly to a normal nickel and chromium waste treatment system along with the rinse water containing trivalent chromium solution. Plastic lined filter pumps containing carbon can be used to remove organic contamination if necessary. Continuous solution filtration is not required. Since contaminants can be easily removed, atmospheric evaporators are used to evaporate water from the plating solution so that dragged out plating solution can be returned to the plating tank.

Filtration
Filtration is used in plating solutions that are used for plating chromium on optically flat rolls. Filtration is occasionally used in other applications. The proper filters must be used and must be precleaned and replaced on a regular schedule. If they break down they can cause pitting in the chromium.

Decorative hexavalent chromium solutions are very seldom filtered because particulate matter in the solution typically does not cause a plating problem. Filters are many times placed on trivalent chromium plating lines so that carbon polishing can be conducted to remove organic contamination when required.

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