Date: Sun, 17 Jan 1999 15:37:40 -0500
Reply-To: "Carroll A. Smith" <cas@DIGITAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "Carroll A. Smith" <cas@DIGITAL.NET>
Subject: Re: corrosion & anodes (long)
Content-Type: text/plain; charset=iso-8859-1
YauMan Chan wrote:
> >>> Austin <austins@IX.NETCOM.COM> 01/16 10:36 PM >>>
> Thus the question for all the electron cognizenti out there - does an a=
node
> have to be electrically attached to a vehicle ground[...]
>
This is a case where a little electrochemistry may not be enough .......
> Here's a 2 minute lesson in electrochemistry:
> A battery is created whenever you have two "dis-similar" metals in cont=
act with each other or through a third medium call the electrolyte. Thus=
, you can stick a copper pin and a zinc pin into a lemon or potato and li=
ght up a light bulb - you have created a battery. We can look up the diff=
erent metals "Potential volts" in any chemistry handbook under "Electroch=
emical Series." Thus under Copper is listed .0158v and under Zinc is -0.7=
63v, and so the Cu/Zn lemon battery gets 0.158-(-0.763) or 0.92 Volts. W=
hile it is acting as a battery, the metal from the Zn side will migrate t=
o the Cu side as ions - thus the Zn side will be "eroded" while the Cu si=
de will get coated with Cu+lemon juice compounds.
Not always. Usually water is involved in its ionized state, ie H+ and OH-=
, or if you prefer HOH+ for the cation, in secondary reactions at both =
the cathode and anode. This is why hydrogen and sometimes oxygen is liber=
ated at the surfaces of the reacting electrodes.
> The kicker here is that the higher the potential difference between the=
metals, the more reactive this process is.. I.e. faster it will happen.
True if there is no inhibiting surface reactions which in the case of alu=
minum there almost always is. So for aluminum, generally not true.
> The key, however, is that there has to be an electrolyte for this to ha=
ppen. Acids (lemon juice) make good electrolyte and most base/alkalies d=
o
> not. Salt solutions also make good electrolytes.
No problem here!
>
>
> This is what is suppose to happen when the coolant acts as an electroly=
te between the engine and the radiator. The radiator is copper Cu
Many radiators are aluminum, I do not know what your VW has.
> and the engine is something else - what? If the engine is steel, with =
is mostly Iron (Fe) or cast ion, the potential differences is only (Cu=3D=
0.158 and Fe=3D-0.036 ) 0.194volts). If the engine is Aluminum (Al) the=
potential difference is (Cu=3D0.158 and Al=3D-1.706) 1.864 volts. As yo=
u can see, the reaction rate is substantially higher in the case of an al=
uminum engine.
Not necessarily.
>
>
> Between the engine and the radiator, for most cars, there only common c=
urrent path is the coolant which acts as electrolyte. Thus we have create=
d a gigantic battery and when the reaction is taking place, the engine ge=
ts "eroded" while the copper radiator gets clotted up (Al migrates and de=
posits to the Cu.) From the above paragraph, you can see that the cast i=
ron engine is a lot less "reactive" than the Aluminum engine, ie you can =
make a much battery with Al and Cu than with Fe and Cu. The engine is th=
e anode and the copper radiator is the cathode.
>
> To stop, this nonsense is easy. Do not immerse these metals in an elect=
rolyte.
Yup! Go aircooled.
> Make the coolant alkyl and not acidic. Make sure the is no "salt" in th=
e coolant. "Salt" used in the chemical sense is not just table salt. Pho=
sphate compound is a good salt. This is what all the fuss is about the b=
lue phosphate-free coolants.
>
> Now, in the case of the boat it is not so easy. The hull, if metal, or =
any metal parts in contact with water is the anode, sea water is the elec=
trolyte and mother earth is the cathode.
> So you have a HUGE battery and the reaction is very fast and furious. =
Sea water is a very very good electrolyte, in fact as good as sulfuric ac=
id used in batteries!
Wrong! If this were so aluminum hulls would disappear into the ocean. The=
y do not. In fact one of the longest lived trash in the ocean is the comm=
on aluminum beer can. Aluminum hulled boats are among the longest survivi=
ng hulls known.
> Here's a case where a sacrificial anode is effective. A metal, more ac=
tive on the electrochemical series than the metal of the boat is immersed=
in water, but in electrical contact with the boat, "moves" the electroly=
sis action from the boat to the sacrificial anode.
>
> So, while a sacrificial anode works wonders in marine applications wher=
e the anode is Fe compound like steel, it is not very useful in the Vanag=
on engine. For it to be effective, first, it must be metal more negative=
than Al on the electrochemical series and there are very few, Al being o=
ne of the most negative of common metals. You are left the likes of Magn=
esium (-2.3v) or Potassium (-2.94v) or some Al compound made for that pur=
pose.
Sacrificial anodes are alloys that are "doped" to bring their open circui=
t potentials high enough and current capacity high enough to be good sacr=
ificial anodes. Mercury used to be used, magnesium is most likely a major=
component, maybe lithium or potassium.
> Second, it must be immersed in the cooling jacket of the engine itself =
and be in electrical contact with the aluminum and immersed in the coolan=
t.
This is exactly correct and the major reason that dangling an anode into =
the reservoir of your engine's coolant tank will do no good worthy of men=
tion.
>
>
> Yau-Man Chan
> 87 GL
No disrespect for Yau-Man is intended here. I earned my living as a marin=
e corrosion scientist for many years and the practical aspects of corrosi=
on control are not as simple as electrochemical texts might suggest. A g=
ood standard reference is "Corrosion Engineering" by Fontana & Green, McG=
raw-Hill. My issue is dated 1967; there are probably later editions and o=
ther texts as well.
Aluminum is a most remarkable metal. Finely divided and mixed with an oxi=
dizer such as potassium nitrate it is a powerful explosive. Conversely in=
some chemical environment it forms tight corrosion resistant films that =
resist corrosion to a remarkable degree. For aluminum engines I would use=
ONLY what the factory recommends and avoid chlorides (brackish water for=
example) at all costs. Distilled water would even be good insurance if y=
ou have water supplies from coastal sources. There are some parts in the =
coolant path made of steel (head studs for example) that do corrode prefe=
rentially but the aluminum will corrode in crevices where the inhibitive =
effects of the coolant do not reach freely. Free copper ion can be devast=
ating to aluminum as some saltwater sailors found when they painted their=
aluminum propeller gear casing with copper antifouling paint. Copper rea=
cts withthe aluminum, sets up a local corrosion cell, and perforates the =
aluminum in sea water in short order. So don't put
any copper pipes inline with your coolant to an aluminum engine. (This is=
why I doubt that your radiator is copper.) Another problem with aluminum=
boats is that some owners like to hook up battery chargers and other ele=
ctrical devices to their boats while in dock. If there is a fault in the =
isolation of the power, current will accelerate the corrosion process. Mo=
st marine operators are aware of this and go to some lengths to avoid the=
problem.
Makes you wonder why you bought this thing, doesn't it|?
cas
