We’ve all seen it. That crispy, flaky, nodular, usually dark deposit on the high current density extremes (edges, etc.). Usually, turning down the current is the reaction done immediately to try and remedy the problem. But exactly why does burning occur? An understanding of the mechanisms involved with the formation of a burn can better help a plater to avoid this nasty problem.
Going back to high school chemistry (if you can remember that far back), you may have seen demonstrated the two electrodes from a battery, generator, or rectifier placed in a beaker of water. You will see gassing at both electrodes. If you collect the bubbles, separate for the anode (positive) and cathode (negative), you will note that twice as much volume of gas comes off the cathode. This is hydrogen gas. The gas collected from the anode is oxygen. These are, of course, the constituents of water is H2O.
Well that little experiment illustrates what I’d like to call as the ground state of electrolysis in an aqueous solution. This is what such a solution “wants” to do. Now in a plating bath, you’re wanting to overcome this tendency by supplying the metal ions that you which to plate from that bath. We will use nickel (Ni) as an example. At the cathode (the part being plated), reduction is occurring. Hydrogen ions (H+) are reduced to hydrogen gas by the incoming electrons from the cathode. You want enough nickel ions (Ni+2) to “step in” to the cathode film to take the place of the hydrogen ions to gas generation and therefore be reduced on to the part as nickel metal.
If there is not enough supply of nickel ions to replace the nickel that has already been reduced in the cathode film, the electrolytic “pressure” will start reducing the hydrogen ions to hydrogen gas. Due to the rapid depletion of hydrogen ions in the cathode film, the pH will rise sharply in that film. Remember, pH is a measure of the hydrogen ion concentration. You will also have an increase of hydroxide ions (OH) in the immediate vicinity. The pH will rise to the point, in the vicinity, that precipitation will occur of nickel hydroxide (and some iron if in the area) and this is what a burn basically is. So, obviously, the better you can facilitate the entry of the Ni ions into the cathode film, the less chance of burning. These include:
- Better solution agitation, especially near the plated work.
- Higher temperature.
- Higher nickel ion concentration.
Also, proper buffering of the bath with the boric acid helps in that it supplies extra hydrogen ions into the cathode film to keep pH there from going up too high to where the nickel starts precipitating in the cathode film causing the burn.