Chromium Plating for Vehicles

1.     PRINCIPLES

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

Chrome plating is not difficult providing the part has been properly cleansed and the following requirements met:

·         Preparation of the chromic acid (CrO3) solution..(Do not acquire the hydrogenated [ H3CrO4 ] chromic acid crystals)

·         Temperature control of the bath (plating solution)

·         Preparation of lead anodes (peroxide)

·         Agitation method of the bath (bubbles)

·         Plating current density control and duration (controller)

·         Ventilation (for safety)

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!

2.     DECORATIVE OR HARD CHROME

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 mm to 1 mm. 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).

3.     CONTROL OF THICKNESS OF DEPOSIT

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 mm 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.

4.     PREPARATION OF THE CHROMIC ACID BATH

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

4.1.    BRIGHT CHROME

I use the basic formulation of 100:1 chromic acid/sulphuric acid proportions:

Chromic acid crystals = 33 oz. (936 grams)

Sulphuric acid fluid = .33 oz. (9.36 millilitre)

Distilled (or demineralized) water to make 1 gallon (3.79 litre).

Of course, you can vary these proportions in accordance with the quantity you wish to make up. So, to make up one pint for small work, simply divide everything by eight The dilution ratio of the sulphuric acid as purchased has to be taken into account and the amount used in the bath must be one of pure H2SO4 to 100 Cr03.

Be very accurate in this process; and:

ALWAYS ADD ACID SLOWLY TO WATER -- NEVER ADD WATER TO ACID –

If you have access to de-mineralized water from your home dehumidifier, it can be a good substitute for the recommended distilled water.

Also, I recommend the use of surgical rubber gloves when handling any of the chemicals called up in this article. Pharmacies (Chemists) carry them and they are much easier to replace than the skin of your hands.

The chromic acid crystals yield about 52% pure chrome metal. For reasons, which must remain unexplained at this stage, a freshly mixed solution will only deposit passably good chromium. The same solution, like a good wine, improves with age... So use it for experimentation when first mixed, before you undertake any serious plating - I keep mine in a sealed glass container and it is good for years. Filter as required between uses - plating current will be around 0.75 A/sq.in. For bright chrome and up to 1.4 A/sqin. for dull 'hard chrome'.

4.2.    BLACK CHROME

Black chrome can also be plated the same way and still have similar characteristics to the bright chrome. For aesthetic or anti-reflective applications, it may be preferable in some cases. I have not yet used it, but the formula is as follows:

Chromic acid 33 oz (936 g)

Acetic acid = 28.2 oz (800 g)

Barium acetate =1.0 oz (28 g)

Distilled (or demineralized) water to make 1 gal. (3.79 litre).

Operation of this bath will be at 90° to 115° F (32.2° C. to 46.1° C.) and at a current density of 0.25 to 0.63 AIsq.in. (More on how to set this up later).

4.3.    TEMPERATURE CONTROL

Temperature is critical for good (or any) results. This is best maintained automatically by using a thermostatically controlled electric heater right in the bath. A simple and cheap expedient for this requirement is to use a tropical fish-tank heater available at any pet store. And, while you're there, pick up a fish tank air pump, plastic piping to suit, and one air valve control, too.

The 115 V heater comes in a quartz tube with a temperature control knob on top. This acts on a bi-metal strip contact tension and can easily be cranked up to maintain the required 45° C. to 50° C. (1130F. to 122° F). A thermometer covering this range is also required.

It is important this temperature range be maintained throughout plating times.