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
|____| www.kettlenet.co.uk 01359 230642

Anna Kettle wrote:

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?


No, corrosion mainly.

Anna

~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repairs
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

On Fri, 25 Feb 2005 18:01:00 +0000, The Natural Philosopher
wrote:

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
|____| www.kettlenet.co.uk 01359 230642

On Fri, 25 Feb 2005 16:43:32 GMT, (Anna Kettle)
wrote:

If some stainless steel and some aluminium are left in contact then am
I imagining it or is there an electric current induced.

Galvanic corrosion. In a suitable electrolyte the aluminium end of
an aluminium/stainless steel pair will corrode, the steel end will
not.

http://www.ocean.udel.edu/mas/masnotes/corrosion.html


--
Peter Parry.
http://www.wpp.ltd.uk/

Peter Parry wrote:
On Fri, 25 Feb 2005 16:43:32 GMT, (Anna Kettle)
wrote:

If some stainless steel and some aluminium are left in contact then
am
I imagining it or is there an electric current induced.

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

On 25 Feb 2005 10:48:59 -0800, wrote:


and you can stop galvanc corrosion by applying electricity. Or if you
apply with wrong polarity, it causes it.

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.

--
Peter Parry.
http://www.wpp.ltd.uk/

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 a

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. http://en.wikipedia.org/wiki/Electronegativity

Assuming that you have the two metals bolted to one another you have
essentially a perfect electrical connection. If you put two metal faces
together you will have some sort of electrolyte between then no matter
how much you clean them (even if it looks clean to you there is probably
a mono-layer of water on the metal surface). Although after thorough
cleaning whatever is left will be a very poor electrolyte. Aluminium
has an electronegativity of 1.5 and Iron 1.8. So the difference, 0.3,
isn't that great (you couldn't realistically use it as a battery)
meaning that corrosion will probably be quite slow.

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:
On Fri, 25 Feb 2005 18:01:00 +0000, The Natural Philosopher
wrote:


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
|____| www.kettlenet.co.uk 01359 230642

On Fri, 25 Feb 2005 16:43:32 GMT, Anna Kettle wrote:

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?

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.


--

the dot wanderer at tesco dot net

"The Wanderer" wrote in message


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.



--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG

Michael Mcneil wrote:

I somehow doubt that a change of porential of a few milliamps is likely
to be of great concern with overhead cables.

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.

--
Andy

On Fri, 25 Feb 2005 23:26:57 +0000 (UTC), Michael Mcneil wrote:

"The Wanderer" wrote in message


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.

Had any experience in the Electricity Supply Industry, have you?

--

the dot wanderer at tesco dot net

Peter Parry wrote:
On 25 Feb 2005 10:48:59 -0800, wrote:

and you can stop galvanc corrosion by applying electricity. Or if
you
apply with wrong polarity, it causes it.

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.


Try this:
http://www.corrosion-doctors.org/CP/Introduction.htm

NT

In article , "The
Wanderer" says...
On Fri, 25 Feb 2005 23:26:57 +0000 (UTC), Michael Mcneil wrote:

"The Wanderer" wrote in message


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.

Had any experience in the Electricity Supply Industry, have you?


Obviously not if he thinks potential is measured in Amps.

On Fri, 25 Feb 2005 21:52:27 +0000, doozer
wrote:

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

Air, or nothing

2) How good the electrical connection is between the two materials

Good in parts

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. http://en.wikipedia.org/wiki/Electronegativity
Aluminium
has an electronegativity of 1.5 and Iron 1.8. So the difference, 0.3,
isn't that great (you couldn't realistically use it as a battery)
meaning that corrosion will probably be quite slow.

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
|____| www.kettlenet.co.uk 01359 230642

Anna Kettle wrote:

On Fri, 25 Feb 2005 21:52:27 +0000, doozer
wrote:


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


Air, or nothing


2) How good the electrical connection is between the two materials


Good in parts


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. http://en.wikipedia.org/wiki/Electronegativity
Aluminium
has an electronegativity of 1.5 and Iron 1.8. So the difference, 0.3,
isn't that great (you couldn't realistically use it as a battery)
meaning that corrosion will probably be quite slow.


Thats OK then I don't think I need to get excited about corrosion in
this case. Thanks everyone


Of its outside, you do.

Al rivetted to steel frames goes quickly in some sorts of trailers e.g...

Anna



~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repairs
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

On Sun, 27 Feb 2005 11:31:02 GMT, Anna Kettle wrote:

snip

Thats OK then I don't think I need to get excited about corrosion in
this case. Thanks everyone

Err, if it's outside and subject to the elements, they'll take their
toll.....

--

the dot wanderer at tesco dot net

On Sun, 27 Feb 2005 12:26:09 +0000, The Natural Philosopher
wrote:

Anna Kettle wrote:

Thats OK then I don't think I need to get excited about corrosion in
this case. Thanks everyone


Of its outside, you do.

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
|____| www.kettlenet.co.uk 01359 230642

Anna Kettle wrote:

On Sun, 27 Feb 2005 12:26:09 +0000, The Natural Philosopher
wrote:


Anna Kettle wrote:


Thats OK then I don't think I need to get excited about corrosion in
this case. Thanks everyone


Of its outside, you do.


Its in a well ventilated and well maintained attic space so water will
not be a problem

Mmm. You should be OK then for 15-20 years..
Anna

~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repairs
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

On Sun, 27 Feb 2005 23:10:37 +0000, The Natural Philosopher
wrote:

Its in a well ventilated and well maintained attic space so water will
not be a problem

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
|____| www.kettlenet.co.uk 01359 230642

Anna Kettle wrote:

On Sun, 27 Feb 2005 23:10:37 +0000, The Natural Philosopher
wrote:


Its in a well ventilated and well maintained attic space so water will
not be a problem


Mmm. You should be OK then for 15-20 years..


Now you've worried me again. I was thinking more like 150-200 years

Move to the Mojave desrt then. Theres a reaosn why its got the biggest
'hibernation' center of mothballed airplanes in the world...
Anna
~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repairs
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

I am curious. What could you possible want to install for 150 to 200 years?

I would imagine given that amount of time dissimilar metals will corrode
significantly. Probably to the point of failure. Is it vital that they
are dissimilar metals and that they are irreplaceable for 150 years?

Aluminium is an interesting case because it is actually a highly
reactive metal* but in the presence of oxygen it forms a tough oxide
(rust) coating that stops further reaction. Perhaps as long as you don't
continually damage that coating (e.g. they joint is completely
immobile) it might last quite well.

*You would be amazed at the speed of the reaction. I have seen aluminium
foil nearly catch alight after it has had the oxide coating removed by
chemical treatment.

Graham

Anna Kettle wrote:
On Sun, 27 Feb 2005 23:10:37 +0000, The Natural Philosopher
wrote:


Its in a well ventilated and well maintained attic space so water will
not be a problem


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
|____| www.kettlenet.co.uk 01359 230642

doozer wrote:
I am curious. What could you possible want to install for 150 to 200
years?

a house?

NT

wrote:
doozer wrote:

I am curious. What could you possible want to install for 150 to 200

years?

a house?

NT


LOL. Yes maybe, but this isn't the whole house. It is a little
(presumably) item in the loft made from iron and aluminium. I can't
think of anything that fits the given description (and needs to last
for 150+ years) in any house that I know.

Graham

On Tue, 01 Mar 2005 11:09:02 +0000, doozer
wrote:

I am curious. What could you possible want to install for 150 to 200 years?

I would imagine given that amount of time dissimilar metals will corrode
significantly. Probably to the point of failure. Is it vital that they
are dissimilar metals and that they are irreplaceable for 150 years?

Its not actually me that is using it but a fellow building
conservator, working in the attic of an important building - the
ceiling is trying to fall down and we are trying to stop it doing
that. Standard practice is to do everything in stainless (threaded
rod, screws, washers, wire etc) so I was surprised when they started
using aluminium wire

Aluminium is an interesting case because it is actually a highly
reactive metal* but in the presence of oxygen it forms a tough oxide
(rust) coating that stops further reaction. Perhaps as long as you don't
continually damage that coating (e.g. they joint is completely
immobile) it might last quite well.

It should be immobile so lets hope so

Anna
~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repairs
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

doozer wrote:

I am curious. What could you possible want to install for 150 to 200 years?

I would imagine given that amount of time dissimilar metals will corrode
significantly. Probably to the point of failure. Is it vital that they
are dissimilar metals and that they are irreplaceable for 150 years?

Aluminium is an interesting case because it is actually a highly
reactive metal* but in the presence of oxygen it forms a tough oxide
(rust) coating that stops further reaction. Perhaps as long as you don't
continually damage that coating (e.g. they joint is completely
immobile) it might last quite well.


Trouble is al oxide is fine to protect against pure water, but any
acidity and ist gone. Salt likewise.


Its very interesting to visit old museums and see the occasioanl iron
age sword, as a rotted sump, but almost perfect bronze swords...iron
doesn't last AT ALL well.

Most of the early cast and wrought iron stuff is on the edge of
destruction and needs painting everyt year.

*You would be amazed at the speed of the reaction. I have seen aluminium
foil nearly catch alight after it has had the oxide coating removed by
chemical treatment.

OK so electrolytic rection occurs but does it matter?

A battery is set up which has the effect of moving aluminium atoms
into the stainless steel and vv. Which is welding. Which is no problem
in this situation

Have I got this right?

Anna

~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repairs
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

Anna Kettle wrote:
OK so electrolytic rection occurs but does it matter?

A battery is set up which has the effect of moving aluminium atoms
into the stainless steel and vv. Which is welding. Which is no problem
in this situation

Have I got this right?

Not really. What will probably happen is your aluminium will slowly turn
to dust (aluminium oxide) while you iron stays relatively shiny an new
looking. I wrote and article detailing how a battery works last year.
Hopefully it's easy enough to follow and doesn't have to many glaring
mistakes )

http://www.crazysquirrel.com/other/batteries.pdf




Anna

~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repairs
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

In article , "Anna Kettle"
says...
On Tue, 01 Mar 2005 11:09:02 +0000, doozer
wrote:

I am curious. What could you possible want to install for 150 to 200 years?

I would imagine given that amount of time dissimilar metals will corrode
significantly. Probably to the point of failure. Is it vital that they
are dissimilar metals and that they are irreplaceable for 150 years?

Its not actually me that is using it but a fellow building
conservator, working in the attic of an important building - the
ceiling is trying to fall down and we are trying to stop it doing
that. Standard practice is to do everything in stainless (threaded
rod, screws, washers, wire etc) so I was surprised when they started
using aluminium wire

Maybe something to do with thermal expansion? Can they not prevent
the two metals from contacting by using some sort of barrier, like
teflon tape?

On Wed, 02 Mar 2005 09:13:25 +0000, doozer
wrote:

Not really. What will probably happen is your aluminium will slowly turn
to dust (aluminium oxide) while you iron stays relatively shiny an new
looking. I wrote and article detailing how a battery works last year.
Hopefully it's easy enough to follow and doesn't have to many glaring
mistakes )

http://www.crazysquirrel.com/other/batteries.pdf

Thanks I had a look at your battery article and read the chemistry
part with difficulty [1]. Aluminium wire doesn't seem a good idea so I
think I had better have a word with my friend

[1] The font doesn't show up at all well in my browser (Firefox). Its
very blurry

Anna



~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repairs
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

(Anna Kettle) wrote in
:

[1] The font doesn't show up at all well in my browser (Firefox). Its
very blurry

Looks fine to me - what version of Adobe (Acrobat) Reader have you got
installed? I suggest upgrading to 7. If using a TFT screen, check out the
Cooltype settings.

One reason that this occurs is colour management - the text might be set to
100% black but this is translated into some lower value for display. I
think that earlier versions of the reader allow you to choose to diplay
using the colour settings in the document or RGB - if so, choose RGB.

--
Rod

www.annalaurie.co.uk

I wrote it in Latex and exported it as PDF this is a known problem with
this set up, usually people say that it looks like the paper has got
slightly wet and the ink has run. When I get the time I will export it
again with the newer PDF libraries which I believe fix the problem. In
the mean time the latest acrobat (if you are using windows) seems to
display it correctly.

Graham

Rod Hewitt wrote:
(Anna Kettle) wrote in
:


[1] The font doesn't show up at all well in my browser (Firefox). Its
very blurry


Looks fine to me - what version of Adobe (Acrobat) Reader have you got
installed? I suggest upgrading to 7. If using a TFT screen, check out the
Cooltype settings.

One reason that this occurs is colour management - the text might be set to
100% black but this is translated into some lower value for display. I
think that earlier versions of the reader allow you to choose to diplay
using the colour settings in the document or RGB - if so, choose RGB.