Design of Lead Anodes :
Lead anodes are used in Chrome Plating Baths.
Why Lead is used as an anode ?
Lead has high corrosion resistance,but in fact the lead anodes that are used are never made of pure lead. Chromic acid in the bath eats away at pure lead,when there is no current. Pure lead is also quite soft, does not hold its shape very well and sags under its own weight. This is why lead is alloyed with other metals in order to give it specific properties. Various amounts of antimony and tin are added to the lead depending upon the application.
Tin: Provides improved corrosion resistance and conductivity, reduces surface scaling and improves trivalent control. Used primarily in high fluoride baths.
Antimony: Provides hardness, rigidity and resistance to curling or sagging and is used whenever strength is required. High antimony contents, however, tend to produce excessive surface scale and a less than optimum trivalent control.
Types of alloys used :
7% tin – 93% lead: Used in all type baths including high fluoride solutions.This alloy has an improved peroxide surface film for better trivalent control and reduced scaling.This alloy is softer and may sag if too heavy or too large. This alloy is recommended for anode weighing upto 25 kgs.
7% Tin – 2 % Antimony – 91 % Lead – This alloy gives benefit of both Tin & antimony. It has improved conductivity and provides rigidity to anode. Anodes of this alloy shows minimal sagging and reduced scaling of surface.
10% tin – 90% lead: This alloy has an maximum improved peroxide surface film for better trivalent control and reduced scaling. Due to more Tin content cost of anode also increases.
6% antimony – 94% lead: This is a very common alloy that was used in the past . The antimony provides both hardness and rigidity and is particularly well suited for large or heavy anodes. The surface film scaling is heavier in this alloy.
Inserts Used in Anodes :
One of the most common problem in Long Anodes is current transfer from the bottom of anode .
MS rod insert is generally used in round anodes having length more than 2000 mm and Diameter 50 mm. Its helps in effective transfer of current in the bottom anode and also provides rigidity to the anode.
Shape of Anodes :
Shape of anodes is also one determining factor in plating. Anode shape be decided on the type of job to be plated and life of anode desired.
Increase in anode surface area would in anode to cathode ratio , thus improving cathode efficiency. Surface area of anodes are increased using corrugations and waviness.
Life Cycle of Anodes :
Life of anodes depends on volume of anode i.e weight of anode.
Anodes are prone to decomposition over time, and/or corrosion with exposure to certain ions or reducing acids in the plating bath. Corrosion starts from the weakest surface areas, like scratches, fissures and pores.
Bus Bar Connection Hook :
Only Copper hooks are used for connecting anode to bus bar /feeder line coming from rectifier. Copper Hook is TIN plated and perfectly integrated to lead alloy.Tin coating helps in perfect bonding of copper hook with lead alloy.
Copper hook is fabricated to fit on bus bar perfectly. Maximum surface area of copper hook should be in contact with Bus Bar , which helps in reducing heating problem of anode.
Copper hook can be tin coated to improve efficiency of current transfer.
PVC / Plastisol coating is done on the Copper hook to prevent corrosion from various gases and drip leaking of chromic acid from Jigs,Jobs while handling them.