Comments and Guidance
by George Munch W5VPQ

Comments and Discussion

A process of removing rust (oxidation of ferrous metals) while doing least damage to the remainder of a piece of equipment was discussed on one of the antique radio forums a while back, and it proved quite useful. Anyting that will survive immersion in a mild solution of baking soda and water can be treated, and little or no damage is done to other metals in the process.

Porous insulators tend to absorb enough water and baking soda to become lightly conductive, so they must be protected or removed but, otherwise, the process is so simple one wponders why it is not better known. It is simply a matter of passing a current through a solution of baking soda and water, producing hydrogen and oxygen is tiny quantities, and lifting rust away from the underlying steel.

Equipment required to perform this consistes of a 12 volt power supply of a few amps capability (battery charger), water, baking soda and a stailess steel rod. A 12V lamp added to the circuit precludes damage from shorts, although use oif a current limited supply would make it unnecessary.

Guidance:

1. Use a plastic container large enough to hold the item to be derusted AND contain sufficient solution to immerse the rusted area.

2. Mix a teaspoon or so of baking soda per quart of water; the amount is not critical - just enough baking soda is needed to make the solution conductive.

3. Use a stainless steel rod for the anode (positive terminal); the object to be derusted is the cathode (negative terminal).

4. Connect 12 VDC and watch for bubbles at both electrodes. Bubbles indicate the process is working as expected.

NOTE:  Polarity is critical, of course. The anode is slowly consumed, so don't use anything of value. Stainless steel is suggested because it adds the least objectionable material to the solution during the process.

5. Use a lamp (a headlight bulb is ideal) in series with the circuit, as a short tends to fry any and all clipleads in use! The lamp is also an instant indication of a short, and the voltage drop across it is negligible under normal conditions.

The process will proceed most quickly on the surfaces of the derusting cathode at the location(s) closest to the anode; almost no action, if any, will occur inside closed spaces where current flow is low or nil. Consider a "U"-shaped article; for such an object, an auxiliary anode should be added and placed between the legs of the U to insure current flow inside where it is needed.

This derusting process is agonizingly slow on large or heavily rusted objects; if such an article is being processed, set up in an out-of-the-way location and check it once or twice a day. Lightly rusted items will clean faster and should be examined every few hours. Add baking soda and water only as the solution is consumed. After some time, a lot of the diplaced rust will end up floating on and suspended in the conductive solution, making it a real mess. Once it is so nasty you can't stand it, discard the solution and replace it with a fresh batch.

While most of the rust floats free from the derusting cathode, some will remain in the affected area and require removal by mechanical means. Also, the derusted area tends to look darker than the surrounding metal; it can be cleaned up with a "soft" wire brush or "Scotchbrite" or a similar product.

For more discussion and details on this and other deruting processes do an internet search on "Electrolytic Derusting" or "rust removal". One particularly good article appears at http://www.stovebolt.com/techtips/rust/electrolytic_derusting.htm.