Faraday's Law: The One Process You Can Calculate Exactly

**The part is the cathode**, connected to the negative terminal. Metal ions in solution are positive, so they are attracted to it and are reduced to metal there. **The anode is usually the same metal being plated**, and it dissolves at the same rate as the cathode gains, keeping the bath concentration constant. That is why a nickel bath has nickel anodes and a copper bath has copper. **An inert anode** — lead, platinised titanium, graphite — is used where a soluble one would not work, chromium being the main case. Then the bath is depleted as plating proceeds and must be topped up. **Measure the current, do not assume it.** A bench supply's display is a guide; a meter in series is the measurement. Every number in this rung depends on the current actually being what it says. **Measure the area properly**, including both faces and the edges of a flat part. A part plated at "1 amp" with its area guessed at half its real value gets half the thickness intended. **Agitate.** Solution next to the cathode is depleted of ions as it plates, and without movement the deposit goes rough and burnt. A magnetic stirrer, air sparging, or moving the work — every real plating bath moves.

The calculation predicts; the balance confirms. Doing both is how the current efficiency of a particular bath is discovered, and after that the calculation can be trusted. **Weigh before and after**, dry, on a scale reading to a milligram for a small part. The gain is the metal deposited. **Compare with the prediction.** The ratio is the current efficiency of that bath at that current density and temperature, and it is worth writing on the tank. **An efficiency above 100% means something is wrong** — the part was not dry, or it gained weight some other way, or the current was higher than recorded. It is the same impossible-number rule as a chemical yield. **Measure the thickness directly** where it matters: a micrometer across a flat part before and after, which works down to a few microns; or a magnetic or eddy current gauge, which is what industry uses; or a cross-section under a microscope, which is destructive and definitive. **Expect the thickness to vary across the part**, and that is not an error in the calculation — Faraday's law gives the TOTAL metal deposited, and the distribution is the next rung's subject.

A plating bath is a chemical system with a working window, and drifting outside it degrades the deposit long before the bath stops working. **Temperature.** Higher temperature allows higher current density and gives a softer, less stressed deposit; too cold and the deposit is brittle and burns at modest current. A Watts nickel bath runs at 45 to 60 C. **pH.** Nickel wants about 4.0 to 4.5. Too low and the efficiency falls as more current goes to hydrogen; too high and basic salts precipitate into the deposit as roughness. **Concentration.** Falls as the bath is used if the anodes cannot keep up, and low metal concentration shows up first as burning at the edges. **Additives.** Brighteners, levellers and wetting agents. They are consumed, they break down into products that accumulate, and they are the usual reason a bath that worked last month does not now. **Log every run:** date, part, area, current, time, temperature, pH, and the measured weight gain. Six lines, and they turn "the plating has gone dull" into a curve with a cause. **A Hull cell** is the standard diagnostic: a small tank shaped so one test panel sees a whole range of current densities at once. One panel shows the bath's working range, where it burns and where it is too thin — and it uses a few hundred millilitres of solution.