The coating is required to have high hardness in the process of hexavalent hard chromium plating. Chromium metal has high hardness only when it is directly converted from Cr6+ to zero valence and deposited on the surface of the plated part. However, in the process of hexavalent chromium plating, a small amount of Cr6+ will be converted into Cr3+ first, and then Cr3+ will be converted into zero valence. The hardness of Cr3+ transformed coating is lower than that of Cr6+ deposited coating directly. Cr3+ is inevitable in the plating solution, but it is allowed to keep the concentration at a very low level. When Cr3+ and Cr6+ are transformed into a metal layer with zero valence upon gradual accumulation to a higher concentration, the overall hardness of the coating will be weakened.  

Therefore, the anode is required to have a high oxygen evolution overpotential for oxidization of Cr3+ to Cr6+ in the plating process, so as to keep the Cr3+ concentration at a low level. Moreover, high current density of anode output is required in this special plating system.

Problems of traditional lead/tin anode
Lead-tin anode is semi-insoluble anode, and the inter-electrode gap will change
Lead ion reacts with sulfuric acid in the plating solution to produce lead sulfate slime with very low solubility, which causes environmental pollution and toxicity
If the slime is not cleaned in time, the coating defects will be caused, which are fatal in some highly demanding plating processes, such as plating of engine piston rings
After the production line is shut down, the anode surface will generate non-conductive lead sulfate to be brushed frequently

Advantages of platinum titanium anode  
It is insoluble, which can keep the high stability of the process without pollution and toxicity to the environment
The oxygen evolution overpotential can reach 1.7V, which is similar to the lead-tin alloy anode
It can be processed into a hieroglyphic anode similar to the shape of the product to maintain a uniform current density spreading from the plated part to the anode