If some stainless steel and some aluminium are left in contact then am I imagining it or is there an electric current induced. Not much of one of course, but if they are left for long enough then does some metal transfer occur?
Anna
~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|
No, corrosion mainly.
Can you explain a bit more? Corrosion as in rusting ? - so only if there is water present - and aluminium doesn't rust just gets a coating of protective aluminium oxide doesn't it?
I'm very hazy on all this for which I blame my Dad. He was a chemist by profession so there was NO WAY I was going to study chemistry at school with the result that I have no idea what I am talking about
Anna
~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|
Galvanic corrosion. In a suitable electrolyte the aluminium end of an aluminium/stainless steel pair will corrode, the steel end will not.
and you can stop galvanc corrosion by applying electricity. Or if you apply with wrong polarity, it causes it. Caused many problems with gas line corrosion where tram rails went in days gone.
NT
Not quite. Galvanic corrosion is the effect of two dissimilar metals in an electrolyte forming a cell.
If you apply electricity you have electrolytic corrosion - similar in its effect but not its cause.
Excuse me if my chemistry is a little out it's been a few years but...
When you join any to dissimilar metals together you are essentially creating a battery. The rate of corrosion will be based on a number of factors probably the most important of which are:
1) The ion transfer potential of the electrolyte (e.g. what ever is between your two bits of metal). If you get say salt water between them the rate of corrosion will be much greater than if you get deionized water between them 2) How good the electrical connection is between the two materials 3) The difference in the electronegativity of the two materials. This gives you an idea of how readily they will transfer charge in an ideal situation.The ways to stop galvanic corrosion are therefore to interrupt one of the three things listed above that cause corrosion. Probably the easiest if you really want to have Al and Fe in the same piece is to separate them with rubber washers.
Graham
Anna Kettle wrote:
Electrolytic corrosion, it occurs when dissimilar metals are left in contact in the atmosphere. In essence, you are creating a small cell or battery, and with acid rain, small currents flow.
It was - and I suspect still is - a major problem in the electricity supply industry where both copper and aluminium are used for overhead conductors. Joints between the two metals are almost invariably where the faults occur. unless special bimetallic joints, specially made for use in the particular situation, are used. Before these were made, the remedy was physical protection of the joint with grease and denso tape. Bimetallic joints have now been in use for some 20 to 25 years at a guess, and I suspect these will be about ready to start playing up, BICBW.
I somehow doubt that a change of porential of a few milliamps is likely to be of great concern with overhead cables.
I heard sometime ago that highly polished faces of silver and gold blocks can -if placed one on the other, weld together.
The first arc lamp produced by Priestly or someone from that era, was powered from a bank or battery of copper and silver coins with sulphuric acid electrolyte.
Zinc oxidises in air giving off some sort of electronic pulse when struck by photons (or whatever light is.)
Torch batteries were originally zinc containers with a carbon core separated by a paste made with some sort of acid. This developed through platinum reduction of ethanol in satellites, into the modern watch battery.
And a piece of aluminium rubbing up against a piece of iron will eventually become inert due to layers of salts and oxides inhibiting contact. Either that or they will rust apart.
Milliamps of potential, eh? No, the problem is that a few hundred micro-ohms of contact resistance in a joint that's carrying a few hundred amps will do a lot of damage just due to the heat generated. And (I suspect) that aluminium alloys chosen for good electrical conductivity are not ones which have good corrosion resistance.
Had any experience in the Electricity Supply Industry, have you?
Try this:
Obviously not if he thinks potential is measured in Amps.
Air, or nothing
Thats OK then I don't think I need to get excited about corrosion in this case. Thanks everyone
Anna
~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|
Of its outside, you do.
Al rivetted to steel frames goes quickly in some sorts of trailers e.g...
Err, if it's outside and subject to the elements, they'll take their toll.....
Its in a well ventilated and well maintained attic space so water will not be a problem
Anna
~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|
Mmm. You should be OK then for 15-20 years..
Now you've worried me again. I was thinking more like 150-200 years
Anna ~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|
Move to the Mojave desrt then. Theres a reaosn why its got the biggest 'hibernation' center of mothballed airplanes in the world...
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