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Anode Cleaning |
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December 10, 2008
Nickel metal is highly reactive in an Oxygen atmosphere, with the exposed surface quickly forming an oxide layer that is very stable and inert, sealing the underlying Nickel. This is why Nickel is used in applications where corrosion is a problem. Instead of continuing to rust like Iron, Nickel rapidly forms an oxide layer that insulates it, preventing further corrosion. Nickel anodes having this oxidation layer are said to be passivated.The insulating properties of the oxide layer affect the electrolytic and dissolution properties of the Nickel. In normal Sulfamate Nickel electroforming, Nickel ions are picked up by electrical current from the surface of the anode and carried to the electrolyte to replace the same number of Nickel ions that is deposited at the cathode. When Nickel anodes become passivated, as they quickly will when exposed to air, they become so highly insulated that the current is unable to remove metal ions from the surface of the anodes. When this happens, Hydrogen ions are formed at the anode, but not Nickel ions to replace the ones deposited at the cathode. In other words, Nickel is plated from the solution, draining it from its Nickel content. A consequence of this is that the Sulfamate portion of the Nickel Sulfamate is left behind where it further breaks down into various Sulfur compounds and Ammonium ions, poisoning the electrolyte. In Sulfamate Nickel electroforming, the electroform solution serves only as the electrolyte, and is not designed to provide the metal ions for deposition. If the passivated areas are unevenly distributed over the surface of the anode, the current field between the anode and the cathode will be affected, causing uneven distribution of the Nickel deposit of the cathode, i.e. increased TTV. With the introduction of DVD’s, and the tightening of the TTV profiles, this process became increasingly important. A sure sign of passivated anodes in Sulfamate Nickel electroforming is a rapidly falling pH; Hydrogen ions continues to be generated at the cathode and dissolves in solution. Hydrogen ion dissolved in an aqueous solution is...anyone...you, in the back...yes, that's correct; ACID - hence the reduced pH. To prevent break down of the Sulfamate, reduced Nickel concentration, reduced pH and increased TTV, the Nickel pellets must be activated before use, to remove the oxide layer from the anode surface. Leaching the pellets over night in an acid solution carries out the activation (see below). Once in electroform solution, Sulfur-depolarized anodes will remain active. A Sulfur inclusion in this type of specialized anode material prevents the Nickel from Oxidizing during plating, as the current more or less continuously removes Nickel from the anode surface, maintaining them in an active state. Note that in addition to Sulfur depolarized Nickel anodes, S-Nickel, there also exist Sulfur-free variants called electrolytic Nickel anodes, labeled P-Nickel or E-Nickel. Without the Sulfur, electrolytic Nickel anodes rapidly become passivated even during plating, and should not be used in the Sulfamate Nickel electroform process.
A well-approved procedure to activate the S-Nickel pellets is described below:
Put one gallon (4 liters) of DI water in a large plastic bucket. Add 100 grams of Sulfamic acid, and 40 ml of wetting agent to the DI water. Mix until completely dissolved. Add the amount of S-Nickel pellets required for one weeks production, or up to 5 pcs 10 kg bags (=1-box). Allow the pellets to soak for a minimum of 24-hours before use. Rinse the anodes before refilling them to the anode baskets. - Replace the solution on the 7th day, as Ammonia will form and can potentially be carried into the electroform solution, if not rinsed properly.
|  Sulfamic Acid
is corrosive.Read the MSDS
and use protectivegoggles & gloves ! |
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