Aluminium and screws

[simmitc]

If one wishes to secure a piece of aluminium to some wood (who mentioned Travellers?) then which screw material will create least corrosion - steel, stainless steel, zinc plated, brass, etc. I have not seen aluminium wood screws, and would suspect them of being too soft anyway. I'm thinking periodic table, dissimilar metals and so forth. Thanks for any advice.

[drivewasher]

Stainless?

[mbo145]

I would use chrome plated brass, but they are expensive and need a pilot hole! I used a combination of those and zinc plated steel screws on the traveller as I figured they lasted 50 years the first time round so I will be over 90 when I have to do it again!

[mbo145]

Oh and my dad had an old Royal Blue coach from the 1950s, where the aluminium polished strips were fixed on with chromed brass screws there was no corrosion but the replaced steel screws had lots of powdery corrosion on the aluminium! It took a long time to polish it all out, and was still quite pitted.

[les]

Brass would be good from the timber point of view. In an ideal world it is thought that certain metals react, as you imply. I'm not sure what the term is, but say for example each metal has a number, then the less corrosion is achieved if two metals touch whose numbers are closer together, not a very scientific explanation but I think it's something along those lines. However sorry not to be able to answer your question directly. Of course in the real world I doubt if you would have too much trouble whatever you chose

[Neil MG]

Quick search:

"Corrosion risks with aluminium and stainless steel in contact

Aluminium and stainless steel together also appears to be a bi-metallic corrosion risk, from the 'nobility' table.
With this combination the affect of relative surface area on corrosion is important.

A large area of 'cathode' relative to 'anode' will accelerate the anodic corrosion. Although aluminium is anodic to stainless steel, large relative surface areas of aluminium to stainless steel can be acceptable, dependant on local conditions.
Stainless steel fasteners in aluminium plates or sheets are normally considered safe, whereas aluminium rivets or bolts holding stainless steel parts together is an unwise combination, as there is a practical risk of corrosion.

In a marine environment, severe localised pitting corrosion to the aluminium treads has been observed where un-insulated stainless steel bolts were used to secure the treads in place.
On the same ladder however, bolts with sound insulating washers did not show any pitting on the surrounding aluminium.

This illustrates the beneficial effect of breaking the corrosion cell by isolating the two 'dissimilar' metals in marginal cases."

In short if you paint the aluminium first and the screw doesn't break the paint you would be ok with stainless.

[mike.perry]

[quote="mbo145"]Oh and my dad had an old Royal Blue coach from the 1950s,

Any photos? Was it a Bristol ECW with the luggage rack on the roof?

[RogerRust]

Ok time for some school boy physics

If you look at the table below
Aluminium is at -1.670 and brass is at -0.350 which gives you a difference of 1.3 volts
Steel however is at -0.440 which gives you 1.23 volts

so steel is very slightly less likely to corrode the aluminium

BUT
zinc is at -0.762 so a good galvanised steel screw only gives 0.908 - better still

We have all seen this in action you know that cotton woolly dusty corrosion where there is damage to a piece of galvanised steel that's down to the galvanic corrosion.

I once saw a lorry door fall off where they had used brass bolts on the aluminium door.

For what is worth I would suggest
1 paint the panel first
2 use plastic washers

By the way at the other extreme gold plated screws would corrode the aluminium in next to no time!! with 3 volts difference


Anodic end (this is where the corrosion occurs)
Standard Electrode Potential (Volts)

Lithium-3.045
Potassium-2.920
Sodium-2.712
Magnesium-2.340
Beryllium-1.700
Aluminum-1.670
Manganese-1.050
Zinc-0.762
Chromium-0.744
Iron; Mild Steel -0.440
Cadmium-0.402
Yellow Brass -0.350
50-50 Tin-Lead Solder -0.325
Cobalt-0.277
Nickel-0.250
Tin-0.136
Lead-0.126
Hydrogen reference electrode 0.000
Titanium+0.055
Copper+0.340
Mercury+0.789
Silver+0.799
Carbon+0.810
Platinum+1.200
Gold+1.420
Graphite+2.250
Cathodic end, passive - (no corrosion here)

[simmitc]

Thanks to all for the information. The short answer seems to be not to try to fix the panel at all I shall try paint, plastic washer, and zinc plated screws. If I'm still around in another 40 years, I'll see how it worked