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Symbol: Cr
Atomic number: 24
Atomic weight: 51.996
Classification: Metal
Crystal structure: Body Centered Cubic (bcc)
Characteristic: Hard, brittle, silvery gray metal.
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Decorative chromium is a thin chromium deposit applied to a metallic or plastic substrate in order to impart particular surface properties:
Decorative chromium is thiner than hard chromium. Additionally decorative chromium depositions have micro-discontinuity (micro-porous, micro-crack) structure in contrast to hard chromium, which is normally crack-free.
Typical applications of decorative chromium Electroplating:
The best appearance and corrosion resistance is achieved when decorative chromium is deposited on a a surface undercoated by nickel or copper and nickel layers.
Copper undercoating is deposited by copper strike followed by copper electroplating. Copper covers defects on the substrate surface improving its decorative appearance. Copper also protects the substrate metal against corrosion.
Thickness of copper layer is 0.2-0.5 μinch (5-13 μm).
Nickel undercoating is deposited by a Nickel electroplating process in one (bright nickel) or two (semi-bright nickel and bright nickel) layers. Nickel provides corrosion resistance and levelling of the substrate surface. Additionally nickel activates the surface preparing it for chromium plating.
Thickness of semi-bright nickel is 0.5-1.2 μinch (13-30 μm). Thickness of bright nickel is 0.2-0.8 μinch (5-20 μm).
Decorative chromium is deposited directly on bright underlying nickel.
Thickness of decorative chromium depositions may vary in the range 0.01-0.03 μinch (0.25-0.75 μm).
Decorative chromium coating possesses numerous invisible defects: micro-cracks and micro-pores. These micro-defects play important role providing Cathodic protection of the nickel undercoating (chromium serves as cathode and nickel as cathode). Since the micro-cracks or micro-pores are uniformly spread over the chromium surface, corrosion reaction is not localized and therefore it proceeds slowly.
Decorative chromium may be electroplated in either hexavalent or trivalent chromium bath.
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The main component of hexavalent chromium plating solutions is chromium trioxide (CrO3) referred also as chromic acid. The second component is a catalyst, which is either sulfate (SO42-) or fluoride.
By-product of the electroplating process in hexavalent chromium solutions is trivalent chromium (Cr3+). Ions of trivalent chromium continuously reoxidize to the hexavalent state (Cr6+) at the anode. Normal level of the trivalent chromium is about 1-2% of the chromic acid concentration. Higher contents of trivalent chromium may cause reduction of throwing power and plating rate, pitting and treeing of the deposit. If the trivalent chromium is too high (more than 2%) reoxidation operation should be carried out at high anode area/cathode area ratio (30) at cathode current density 20 A/ft² (2 A/dm²).
Cathode current efficiency of chromium electroplating is low: about 10-20%.
80-90% of the electric current passing between the anode and the cathode is used for gaseous Hydrogen formation.
In this process chromic acid is catalyzed by sulfate ions (SO42-).
Chromic acid/sulfate ratio is one of the most important process parameters. It varies within the range 80 - 200.
Low ratio solutions are characterized by high plating rate but low throwing power. High chromic acid/sulfate ratio may cause gray or even “burnt” deposition on high current density areas. Normally plating solutions with chromic acid/sulfate ratio 100 are used.
Bath formulation:
Chromic acid (CrO3): 20-35 oz/gal (150-263 g/l);
Sulfate (SO42-): 0.13-0.23 oz/gal (1-1.73 g/l). Source of sulfate ions is sulfuric acid.
In this process chromic acid is catalyzed by a mixture of sulfate and fluoride ions.
Fluoride bath have higher than sulfate baths current efficiency. Additionally fluoride baths may operate at higher current density not causing burning and treeing. As a result plating rate in fluoride baths may be 50% higher than in conventional sulfate catalyzed baths. Fluoride ions are chemically active and may attack the unplated surfaces. In order to prevent etching of the areas, which are not to be plated, they should be masked.
Bath formulation:
Chromic acid (CrO3): 20-35 oz/gal (150-263 g/l);
Chromic acid/sulfate ratio (CrO3/SO42-): 180-190;
Fluoride: 0.2-0.3535 oz/gal (1.5-2.63 g/l).
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High temperatures are used for deposition of crack-free chromium coating. Increase of the bath temperature causes reduction of the plating rate. Excessive bath temperature may result in formation of soft dull deposit.
Too low current density decreases economical effectiveness of the process. Too high current density may cause “burning”. Optimal current density is determined by the bath temperature: higher temperatures require higher current densities.
Properly operating anodes are coated with dark brown lead peroxide. If an anode has a lighter color (yellow-orange) it should be cleaned. Cleaning operation is immediately followed by immersion of the anode connected to the operating power supply into the bath. This operation results in formation of conductive lead peroxide coating on the anode surface.
High ripple exceeding 5% and current interruptions may cause dull or even laminated deposit.
Agitation helps to homogenize the bath temperature.
Problems and troubleshooting of the electroplating process are similar to those of hard chromium.
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Trivalent chromium (Cr3+) electroplating process has been recently developed as an environmentally friendly replacement of decorative process with toxic hexavalent chromium.
The cathodic reaction in the trivalent chromium electroplating process is as follows:
Cr3+ + 3e- = Cr
By-product of the electroplating process in trivalent chromium solutions is hexavalent chromium (Cr6+) forming under certain conditions at the anode:
Cr3+ = Cr6+ + 3e-
In order to prevent formation of hexavalent chromium ions conventional 93Pb-7Sn anode is modified. According to the the anode modifications two trivalent chromium plating processes are available in the market:
Single process uses inert Graphite anode, which is in direct contact with electroplating solution. The electrolyte of the single-cell process contains chlorides at high concentration.
Anodes of the double-cell process are made of 93Pb-7Sn alloy (like the anodes in hexavalent chromium plating process) however they are separated from the trivalent chromium solution. The anode is placed in a box containing diluted sulfuric acid. The box walls made of an ion-selective membrane are not permeable for trivalent chromium ions. The electrolyte of the double-cell process contains no chlorides.
Advantages of trivalent chromium over hexavalent chromium decorative plating process:
Hexavalent chromium solutions are carcinogenic, oxidizing and more acidic (more corrosive) than trivalent chromium.
Chromium content in trivalent solutions is about 1/5 of that in hexavalent solutions.
Trivalent chromium deposits are micro-porous. Hexavalent chromium should be pre- or post-treated to form micro-crack structure.
Trivalent chromium solutions have lower viscosity therefore the parts transfers less electrolyte to the rinse baths.
Disadvantages of trivalent chromium over hexavalent chromium decorative plating process:
More frequent bath analysis is required.
The colors of trivalent chromium are metallic white, stainless steel or pewter. Blue-white color of hexavalent chromium is preferable. Maximum thickness of chromium deposited in trivalent solution is 1 μinch (25 μm). This limitation does not allow to use trivalent chromium for plating hard (functional) chromium.
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