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Friday, July 8, 2011

Chromium Electroplating


Chromium can’t be deposited from solution only chromic acid (CrO3) and water. There must also be present in bath one or more acid radicals which act catalyst to bring about or aid in the cathodic deposition of chromium. The purity of chromic acid used is often not specified or established and yet the nature

All that remains is the requirement of time - so don't let the apparent complexity of the task discourages you because the results are very worthwhile, indeed.
I have studied the industrial processes involved, reduced them to pint-size applications for model engineering, and experimented enough to be able to tell you what works. We have a lot to learn and the process has been laid out for you in ten easy steps. So, here we go!


All chromium is about the same hardness; 800 to 1000 VHN - very hard! The main difference lies in the thickness of the deposit.
For decorative purposes, chrome sits best on nickel which itself adheres very well to copper - this combination also offers the best corrosion protection resistance. Decorative chrome thickness will vary from a few hundredths of a mil to 1 mil. The mirror finish will only be as good as the finish you put on the surface before you put on the chrome.
For functional purposes, to take advantage of the extremely low chrome coefficient of friction, or for wear build-up (bearing surfaces or pistons, as examples), hard chrome is plated in thickness as required from 1 to 50 mills.

When used as a bearing surface. Chrome must be micro-finished (more on this later) and will then provide a coefficient of friction lower than any other metal when used against steel, iron, brass, bronze, babbitt, or aluminium alloys. Do not use chrome against chrome. Because chrome is also much harder than casehardened steel, we then have a perfect set-up for longwearing working surfaces. Chrome will resist mostly all organic and in organic compounds and acids, except hydrochloric acid (muriatic).


Given fixed parameters for temperature, plating solutions, anodes, set-up, and current density, thickness is a function of time. Expect around .75 to 1.2 mil per hour of plating time.
I have plated up to 20 mills successfully at home - admittedly this was by accident because I was aiming for 3 mills deposit to refinish a piston! It had previously taken six hours using a particular chromic acid solution to deposit 3 mills of excellent chrome. I thought to shorten plating time I would increase the current density from 600 mA to 800 mA and the temperature of the solution was tweaked from 450 C. to 500 C. (1 13oF to 122o F). I then plated, with agitation, for five hours and wound up with an hour-glass shaped piston, due to a 13 mill chrome deposit measured at mid-skirt level and 21 mils on the edges (formed by the bottom of the skirt and the piston crown).

Let that be a lesson to all of us: Never change more than one parameter at a time.
Subsequently, grinding of the same piston was successfully carried out; which attests to the excellent adhesion of the chrome to the base metal (steel) as prepared earlier.
Of course, the piston was then lapped to a perfect fit in the re-lapped bore (no rings involved in that .020 cu.in. engine). We'll come to the grinding and lapping notes later. Chrome will lap to a superb finish, to a degree of precision obtainable by no other method and limited only by the machinist's patience and skills.


NOTE: The chemical formulations given in this article are in avoirdupois ounces per gallon of solution .To convert these to metric measure, simply multiply the oz/gal number by the conversion factor of 7.5 to obtain grams per litre.

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