From: nimkp@imp.imp.uib.no (knut pedersn)
Subject: Chemical removal of rusty spark plug
Date: 29 Feb 2000
Newsgroups: rec.crafts.metalworking

Chemical removal of rusty spark plug.
Thanks for all suggestion on how to remove a rusty spark plug from an
aluminium top. After a week soaking with penetrating oils, I started to test
chemical methods. First I put a iron screw and a piece of aluminium in a
saturated solution of aluminium sulphate, temp. 40 C degree after one week the
screw is still in perfect order.
Then I started test with nitric acid. Concentrated  acid is 65% sp. weight
1,4.
In the test glass vials with 10 ml mix acid and water were used. In each vial
I put one 1/8" screw ½" long of iron and an aluminium rivet of same size. The
weight of screw and rivet were determinated with an analytical balance.

One part acid and one part water, typical exothermic reaction temperature
raise to 60 C degree. After 10 min. the screw had almost dissolved.
                start weight            end weight
screw           0,6489 g                0,0031 g
rivet              0,3294 g                0,3282 g

One part acid and four parts of water, temperature did not raise. After one
hour the screw were reduced to a tiny needle.
                start weight            end weight
screw           0,6563 g                0,1657 g
rivet              0,3448 g                0,3432 g

One part acid and nine parts of water, temperature did not raise. After four
hour the screw were reduced to the half.
                start weight            end weight
screw           0,6582 g                0,3384 g
rivet             0,3447 g                0,3430 g

My chemical book says that nitric acid is an oxydative acid, in reaction with
aluminium the natural oxid layer will increase and so protect the aluminium.
Then to the honda engine. I took off the top removed some of the insulation
and started to add the one to one mix into the hole in the plug. It was a
heavy reaction with gassing, hydrogen and some nitrogendioxid. After a few
minutes reaction stopped, the old brown mix were removed and new mix added.
Same process again and again every five minutes. I used a syringe to handle
the acid. Be careful it is a nasty stuff. After four hour the wall thickness
was reduced to 0,5 mm. I tried a wrench and the plug went in two pieces. One
hour later with acid treatment, a tap was easily run through the hole and the
job was finished. No damage to the aluminium at all.

I beg you pardon for using so many words on a rusty spark plug. Hoping it may
bee to help for someone who want to remove iron from aluminium.
Knut
Bergen Norway