Process-wise, a zinc coating is electrically deposited on a substrate. That’s a barebones description for a much more complicated process. Sure, scant details are handy when articles are skimming over trivial subjects, but that’s not the case here. Really, the above statement can only be used as a jumping-off point, as an introductory sentence. One more time, then, what happens during the zinc plating process?
Implementing a Pre-treatment Phase
If this operation is to create a fully corrosion-resistant finish, there can’t be anything between the electro-deposited coating and the underlying substrate. First things first, let’s scrub the substrate clean. Only, it’s not as if someone can use a wire scrubber on every tiny screw or corrosion-prone fitting. To batch process contaminated metals, they’re dipped in alkaline detergents and washed in acid treatment baths. The caustic nature of the screening solution is carefully managed, and then the components to be treated are dipped and bathed or sprayed by an array of specially configured nozzles. This is corrosive work, so the workers in charge of the operation use automated machine stations to wash away the dirt, oils, and films of grease.
The Substrate Activation Stage
Are we ready to electrically charge the metal parts yet? Not quite, the pre-treatment phase isn’t over yet. Amazingly, with the metal acid and alkaline scrubbed, there are still a few contaminating elements trapped inside the parts’ substrate. Hydrogen outgassing is a known plating issue, so the metal is subjected to a hydrogen-cleansing electrical charge. This “electro-cleaner” rids the alloy surface of microscopically trapped quantities of hydrogen. Whether bound there by the forging process or some incautiously run heat treatment procedure, the plating work can’t continue until the substrate residue is discharged.
The Thin-Coating Electro-deposition Process
It is a thin finish. Unlike a layer of paint, this finish cannot dimensionally alter the surface upon which it is layered. Remember, corrosion-resistant fasteners use this treatment to prevent rust. If the finish was too thick, the screw threads wouldn’t do their job. The thin-walled surface coating resides in a bath. It’s a special zinc-based electrolyte, which contains a few other bond-enhancing chemicals, too. The parts, batch-mounted in barrels or racks, are lowered into the electrolytic bath. This is an electrical circuit, one that uses two electrodes. Just like a battery, there’s a cathode and an anode, and they’re trading electrons. A current flows in the bath.
In this process, the parts to be coated function as the circuit cathode. The ionic zinc liquid is charged as the anode. On introducing a D.C (Direct Current) power supply, the electro-deposition process begins. A layer of impenetrable, corrosion-proof zinc coats the target substrate. Job finished, all that’s left to do is a water rinse, which removes the last of the liquid electrolyte.