Electric potential - Aluminum Steel Interface

[bluepgt]

Hello All,

I have the following materials:
Material 1: Steel that is zinc flake coated
Material 2: Aluminum (Blank)
Material 3: Steel that is Z100 coated

I have a connection as such
Material 1 | Material 2 | Material 1 | Material 3


This is not exactly my area of expertise and that is why I'm writing here. I can't really explain the whole situation but the results from the corrosion test are not so good. My concern is that the electric potential between Material 2 and Material 3 is relatively high and that Material 2 gets eaten away. Could somebody maybe comment on this or guide me to some readings?

Thank you in advance

 

[MikeHalloran]

None of that matters if you keep the joint dry.
Obviously, you can't.

Some boats are built with a steel hull and aluminum structure.
The trick is that the metals are joined by a band of detonation-bonded aluminum/steel, where there is no crevice in which water can accumulate.



Mike Halloran
Pembroke Pines, FL, USA

 

[bluepgt]

Hello Mike,

Thank you for your answer.
My concern is that the spacing between Material 2 and Material 3 at 0,8 mm is vulnerable to accumulation of water. The Aluminum is untreated and I would think that the Aluminum will act as a anode and the zinc coated steel as a cathode and water being the electrolyte.

Do you think this idea could be valid or could I calculate the electrode potential now with data from a galvanic series that I find online?

 

[Tmoose]

Or, measure whatcha got with a voltmeter while in a salt water bath.

MIL STD 889

http://www.everyspec.com/MIL-STD/MIL-STD-0800-0899/MIL_STD_889B_955/

i was thinking it had some comments about materials closer than 0.XX volts being acceptable, but I did not find that just now.

Can you paint the aluminum with epoxy?

 

[WGJ]

Look up the Galvanic Series to see how the potentials vary between the metals that are actually in contact and not the steel that has been plated.
Try this link and look at page 24:

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=13&sqi=2&ved=0CIcBEBYwDA&url=http://

http://www.dtic.mil/cgi-bin/GetTRDo...=r6lybiRYRT4d7s-SV3ODrQ&bvm=bv.56988011,d.d2k

Page 12 - Zinc is has a potential of -0.79v with a Copper reference
Page 12 - Aluminium covers quite a range, depending on the alloy, but let's say about -0.75v, worst case.
Mild steel - about -0.65

http://www2.sandvik.com/sandvik/014.../HandbookWeb/349C27F72301CF3DC1256B490032613D

As far as I can see, Z100 is just another form of Zinc coating.
Zinc finishes are interesting as there are many ways of coating steel with zinc, from traditional hot-dip processes through to Dacromet, Sheradizing and electroplate.
Each has its own benefits and costs. Making parts out of pre-finished steel brings its own problems owing to the presence of 'raw' edges produced by punchin, shearng, bending or other processes.

So, Al in contact with ZN: -0.79v in contact with -0.75v = difference of 0.04 volts, very low, so neither should affect the other.
However, should a raw steel edge come into contact with Al, via a coating of wet mud, salty wet mud or some other contaminant containing either or both moisture and salt, then the potentila will be = Al - Steel = -0.79 in contact with -0.65 = 0.14 and the Al will corrode in preference to the steel.

Bill