Colorado Model Engineering Society


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ELECTROCHEMICAL DERUSTING

If the part is greasy/oily, degrease it. A soak in the parts washer
followed by some drying time will work.

Find a PLASTIC container large enough to hold the rusty part. For a
hood hinge, a 5-gallon pail might work. Or an office type waste paper
can, or a tall kitchen trash can.

Go to the supermarket and look for "Washing Soda" in the laundry
detergent aisle. Its usually next to the "20-Mule Team Borax". The
chemical is sodium carbonate. Its also used in swimming pools and spas
as a pH increaser, available from any pool supply store. Check the
ingredients label to verify 100% sodium carbonate. This is also the
same chemical that is used as the neutralizer in the Prestone Heavy-Duty
Radiator Flush (the yellow cardboard can). You can substitute baking
soda (sodium hydrogen carbonate), but it will not work as well.

Fill the container with a solution of sodium carbonate. Use about one
tablespoon of sodium carbonate in the container for each gallon of
water, but the amount is not critical.

Submerge your part and attach the NEGATIVE clip of your battery charger
to the part. Make sure you have contact with the metal (scrape the
paint if necessary).

Insert a piece of scrap steel into the container, making sure that it
does not touch the rusty part. Old concrete reinforcing bar, black iron
pipe with any coating removed, hot rolled structural steel, cold rolled
steel, old axle shafts...anything steel will work. Make sure the scrap
steel is long enough to protrude above the surface of the water. You
can rig up something to suspend the steel if it is too short. The steel
is your "sacrificial anode". See below for why you should not use
stainless
steel for the anode.

Connect the POSITIVE clip of the battery charger to the sacrificial
anode. IMPORTANT: Make sure that the clip remains completely out of
the water at all times, or it will *literally* disappear!

Turn on the battery charger. For a new sacrificial anode, the 6-volt
setting might be enough. If the ammeter goes off scale, raise the
sacrificial anode so less is submerged and/or move it further away from
the rusty part and/or reduce the voltage. The amount of current is not
critical. A low current will require more time to de-rust the part. A
high current will unnecessarily heat the water and make a lot of foam.
Given a choice, be patient and use the lower current.

Bubbling will occur. Hydrogen will be liberated at the rusty part
(where the red rust is converted into black rust), and oxygen at the
sacrificial anode (where new red rust is formed). As the rust is
removed from the part, microscopic rust particles will rise with the
oxygen and form a brown scum on the surface. Keep the positive clip out
of the foam! The sacrificial anode will form a rust layer. You can
interrupt the process to wire-brush the rust off the anode from time to
time, so it keeps working efficiently.

The process works best line-of-sight, so the rusty part may need to be
turned occasionally to expose all sides to the anode.

You can let it run indefinitely without any damage to the de-rusted
part. Any heavy rust layer will be loose, or will fall off on its own.
The time to completely de-rust the part will vary from minutes to
days, depending on the volume of rust and the current.

When the part has been de-rusted, rinse it with hot water and dry it
quickly. A hair dryer or heat gun works well for this.

The dry part will have a thin layer of loosely adhered black rust. If
the geometry of the part is simple, it can be brushed off using a fine
bristle wire brush or wire wheel on an electric drill. The part should
then be submerged in dilute phosphoric acid for a while, agitating it
occasionally. Wear nitrile gloves and splash goggles when working with
acid. When the black rust disappears, treat the part with a conversion
coating (follow manufacturer's instructions. Dry it quickly, again with
hot air.

35% phosphoric acid is available at Home Depot as "Behr Concrete Cleaner
and Rust Remover". Dilute it with 4-5 parts water. The de-rusted part
*must* remain completely submerged in the acid, or an etched line will
form at the air/liquid interface. This is due to atmospheric oxygen
dissolving into the liquid, making it extremely corrosive at the
air/liquid interface.

The de-rusted part is now ready for painting with the primer of your
choice.

You can do a Google search for electrolytic rust removal if you want
more info. There are some very recent scientific papers on the exactly
how the chemical process works. Museums use this process to de-rust
artifacts. Only the rust is removed; the base metal is unaffected.

You can scale the size of the bath up or down to suit the size of the
part. Some plywood attached to stakes in the ground can be used to form
a large shallow pool; with a heavy plastic liner it can be used to
de-rust an entire car frame. A large plastic storage tub can be used to
de-rust an engine block. If the part is too big for the bath, you can
de-rust it in stages. The pH of the bath is >7, so it is not corrosive,
even at the air/liquid interface. I use a tall kitchen trash can to
de-rust drive shafts. When one half is finished, I just turn it over
and finish the other end. 32-gallon Rubbermaid trash cans work well for
many large items.

Rusty parts that are "frozen" will be loosened by this process. Wait
until after de-rusting before trying to break them loose! In
particular, do not apply any penetrating oil before de-rusting.

Paint will be loosened by the de-rusting process because the hydrogen
bubble formation acts as a wedge to pry it from the surface. If you
want to use a chemical paint remover, its best to do so before
de-rusting.

The de-rusting solution never goes bad. Just replenish the water as
needed.

The de-rusting solution is only mildly alkaline, and can be used to
water your garden. The rust accumulated at the bottom can be used to
feed plants that want a lot of iron in the soil e.g. raspberries and
blueberries. Or just dump the liquid down the drain and put the rust in
the trash.

Remember: KEEP THE POSITIVE CLIP DRY AT ALL TIMES, and do not allow the
rusty part to touch the anode!

SAFETY WARNING: Always unplug the battery charger when reaching into
the de-rusting bath. If you don't, you will become part of the circuit
and could
be electrocuted! Long-gauntlet nitrile gloves are also recommended.
Ken

WHY TO NOT USE STAINLESS STEEL ANODES

Many people using the electrolysis method for rust reduction swear by
stainless steel, stating (incorrectly) that it's not consumed, stays
clean and seems safe.
Stainless steel is indeed consumed when used in the electrolysis
process, although slowly. The main problem with using it is the
hazardous waste it produces. Stainless steel contains chromium. The
electrodes, and thus the chromium is consumed, and you end up with
poisonous chromates in your electrolyte. Dumping these on the ground or
down the drain is illegal. The compounds can cause severe skin problems
and ultimately, cancer. Hexavalent chromate is poisonous. These
compounds are not excused from hazardous waste regulations where
household wastes are. These compounds are bad enough that government
regulations mandate "elimination of hexavalent chromate by 2007 for
corrosion protection."
Does your electrolyte turn yellow? That's a sign of chromates. If you
have been using stainless steel for the anodes (positive electrodes),
wear rubber gloves when working with or near the liquids. If you need to
dispose of it, allow it to evaporate into powders and dispose of the
powders in sealed containers during your local "hazardous waste clean-up
days".

Best bet - don't use stainless steel no matter how tempting it is.

 

Bill in Boulder, "Engineering as an Art Form!"



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