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-cell process

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.

• Double-cell process

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:

• Lower toxicity.

Hexavalent chromium solutions are carcinogenic, oxidizing and more acidic (more corrosive) than trivalent chromium.

• Lower chromium concentration.

Chromium content in trivalent solutions is about 1/5 of that in hexavalent solutions.

• Higher productivity.
• Simpler process control.
• Less sensitive to the rectifier ripple.
• No treatments are required for formation micro-porous/micro-crack structure.

Trivalent chromium deposits are micro-porous. Hexavalent chromium should be pre- or post-treated to form micro-crack structure.

• Lower drag-out.

Trivalent chromium solutions have lower viscosity therefore the parts transfers less electrolyte to the rinse baths.

• Higher throwing power.
• Less expensive waste treatment and ventilation systems.

Disadvantages of trivalent chromium over hexavalent chromium decorative plating process:

• Trivalent plating solutions are more sensitive to contaminations.

More frequent bath analysis is required.

• Trivalent solution components are more expensive.
• Limited deposit thickness.
• Deposit color does not meet some requirements.

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.