Chromating is a process used primarily on zinc plating, zinc die-casting (pot metal), and cadmium plating. ‘Golden Cad' is really a cadmium plate that has been ‘chromated’ with a yellow chromate.
Zinc and cadmium are electroplates or metals which are readily attacked by mild acids, and will deteriorate rapidly, even if daubed lightly with tomato ketchup! To 'harden' the plate it is placed in a mild solution of special acid (Yellow Chromate, Blue Chromate or Drab Olive Green). This slightly attacks the plate, oxidizing it. The oxide forms a tough corrosion resistant crust, which protects the softer zinc or cadmium metal.
Chromates will only work on zinc die-cast (pot metal), or parts previously plated with zinc, cadmium or Copy Cad. Prepare the part for chromating by zinc or 'Copy Cad' plating steel, or bead blasting pot metal. The plate needs to be in excellent condition, fresh and bright, otherwise defects will occur in the treatment. Do not handle the parts with your bare hands prior to dipping, as you will leave body oils in the form of fingerprints, which will show up on the finished item.
Old pot metal, (probably 25 years +) these parts need to have fresh metal exposed without any oxidation present. To achieve this on carburettors, blast clean with baking soda and then rinse off in fresh water before dipping. If the result is blotchy, then blast clean again, and zinc plate the part to provide an even, fresh layer of zinc over the part. NB. Baking Soda dissolves in water and will therefore not clog any important carburettor ports.
When using any of the following processes, it is important that the parts are rinsed thoroughly and they are allowed to dry overnight before handling.
1. Iridescent Yellow Chromate Process has an oily look to it, with ripples of yellows, blues and greens. It is commonly found on brackets, brake boosters and carburettors. The Yellow Chromate Crystals are acidic and marked as a carcinogen. Treat with great care and wear protective clothing, gloves and goggles or other eye protection! Make sure you read the labels and MSDS before use!
a. YELLOW IRIDESCENT FINISHES. Add 1 gal (4.55 litre) of DISTILLED WATER to a plastic tank Add 36 gms of CHROMATE CRYSTALS. Add 3 cc of Sulphuric acid (or 9cc of Battery Acid)
b. DEEP YELLOW - BRONZE FINISHES Add 1 gal (4.55 litre) of DISTILLED WATER to a plastic tank Add 72 gms of CHROMATE CRYSTALS. Add 3 cc of Sulphuric acid (or 9cc of Battery Acid)
Using the Iridescent Yellow Chromate
Raise the temperature of the solution to 27 deg C - using your immersion/aquarium heater. Hang the part from a copper wire and immerse in the solution for 30 seconds (or until the correct colour is achieved) swirling it to agitate. Remove and rinse in fresh water (Failure to do this will result in the part continuing to darken beyond the desired colour). Set aside and air-dry using a small fan.
Variations on the amount of chromate crystals will achieve different effects. Less chromate will produce a light brass colour and more will produce a dark bronze. Experiment!
When the process seems to no longer work effectively, add more crystals to the solution, or discard and make up a new batch. The latter is preferable if you are trying to achieve a constant result.
Store the unused liquid in a glass container, in a locked cupboard.