Metal Finishing Guide Book


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Page 313 of 903

ANODE REACTIONS It is estimated that between 5 to 10% of the reaction goes to generating trivalent chrome. How much trivalent chrome is formed depends on several factors. Introduction of impurities into the bath also causes Cr(3) to form. The formation of trivalent occurs within the reaction of plating and it is also reduced in the operation at the anode in the following reaction: 2PbO +O2 j 2PbO2 (lead peroxide) 2H2O j O2 + 4H+ (oxygen evolves) Cr+3 ��� 3e- j Cr+6 (trivalent chrome oxidized at the anode back to hexavalent state) While this anodic reaction seems at first very simple, the oxidation only takes place while current is applied. The lead (typically an alloy of Pb/Sn/Sb) is being oxidized and forming an oxide that is distinguished as a dark brown coating. When the anodes are inactive they form a yellow film of lead chromate (PbCrO4). This lead chromate film can grow until it develops a scale that blocks the flow of current. It is typical for plating operations, both functional and decorative, to activate the anodes by ���dummying.��� This allows the current to blow off the chromate film and reform the peroxide film. When the film gets too heavy, anodes should be cleaned. Caution: Remember that lead is highly toxic and all necessary protection should be worn to avoid contact or breathing the dust. The best practice is to soak the scaled anodes in a proprietary anode cleaner design for this purpose. The current applied to the anode is very important to generating and maintaining this dark brown film. Too low an anode current density will not cause the peroxide to form and trivalent will continue to build. The anode current density should be between 5 to 8 ASD or 0.35 to 0.5 asi. If the anode hook is hot then as a general rule the anode area is too small and more anodes or larger diameter anodes should be used. Pure lead anodes will quickly dissolve. They need to be alloyed to inhibit corrosion and also to add stiffness to the soft metal; 7% tin has been used successfully for this purpose. Antimony is also added in a smaller quantity and adds rigidity to the anode; 2-3% antimony is generally sufficient to keep the anodes stiff and straight. Long anodes will generally have a core to aid in getting current evenly distributed to the anode and to provide more rigidity. If good distribution is needed, i.e, for I.D. of gun barrels or oil field pipe, the anodes will generally have a copper core. When using copper the manufacturer should silver the copper. This prevents the copper from being attacked by the chrome solution should a hole develop in the lead. Sometimes a steel core is used but if used it should be tinned and/or silver plated to prevent attack. This is particularly important when using the mixed fluoride chemistry. Proper sizing of the anode relative to the work area is very important. Anodes sized too small will warp. This can also occur with high or low current. For example, burning the hook into the anode gives the best connection. These are areas that should be advised by the manufacturers of anode materials. To effectively convert trivalent chrome back to hex, the anode area should be sized to be twice the area of the work size. Any signs of build up in Cr (III) the anodes should be checked for scale and size. Since lead anodes are consumed they will become smaller over time and the effective area will be reduced, limiting their abil306

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