Re: Sacrificial Anodes and the laws of physics

[Regan Dale Copple <copple@xxxxxxxxxxxxxxxxxxxxxx>]

Les Singh,

> Interesting theory: should be news to all the specialists who use
> sacrificial anodes of magnesium and other metals that this does not work.

	They don't paint sacrificial anodes, wiseass.  As someone who
worked on military aircraft in the Marine Corps doing sanding and painting
in my squadrons corrosion control shop I can indeed attest to the point he
is making.  When painted aluminum rivets corrode they do swell up because
of the aluminum oxide Al3O4.  Unpainted aluminum and zinc sacrificial
anodes are unpainted and as the oxide forms it is usually taken off in
some fashion to expose fresh metal to the surrounding area.
	Dissimilar metal contact is one of the causes of aluminum rivets
corroding when in contact with steel panels, one of the other causes is
what makes the steel panels themselves corrode.  It's caused by the fact
that the aluminum rivet in contact with the steel panel forms a little cul
de sac so to speak.  When some moisture gets in there, the stage is set
for the corrosion to begin.  All corrosion is caused by a difference in
electrical potential, since corrosion is basically a material trying to
get to an equilibrium condition.  In this case the variation in potential
is caused by the different amounts of oxygen present at the outside of the
rivet-panel surface and the inside of the rivet-panel surface.  When the
moisture gets under the rivet it starts the cycle since the corrosion
first needs an electrolyte.  The moisture further exacerbates the oxygen
gradient between the outside and the inside and the result is two types of
corrosion present, the aluminum because of its place in the
electrochemical series, and the steel because of the oxygen gradient.
	
	There Mister Smarty-Pants Kiwi, satisfied?  Any further questions?
Class is dismissed.

					Regards,
					Regan Copple
					B.S. EECS/MSE UC-Berkeley
					79 Spider
					73 Montreal

"Black cars look better in the shade." - Gino Vanelli

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