Steel vs. Stainless Steel Bleed Nipples and Tefgel

Info on parts and accessories for the Mégane II, what to use and where to buy them.

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AlexB
Driving Legend
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Joined: Tue Mar 22, 2005 8:17 pm
Currently Drives:: Renaultsport R.S.250 Cup

Steel vs. Stainless Steel Bleed Nipples and Tefgel

Postby AlexB » Fri Oct 02, 2015 8:45 pm

I needed a new set of bleed screws for RS250 -- there are original Brembos, stainless steel and zinc-plated soft steel nipples available. So, I looked into the table of Electrochemical Potentials...

Now I understand why Brembo use soft steel nipples in the aluminium calipers. The electrochemical potential between these metals is small. So, Brembo coat their screws with some sort of a sealant. A stainless steel screw would attack the aluminium in the reaction, thus damaging the caliper! A zinc-plated screw is OK, just zinc will dissolve quite quickly.

Now I also see why the rear steel calipers use the steel zinc-plated screws. Zinc acts as a sacrificial anode in the reaction. Once it's gone, the remaining steel to steel pair is neutral and will only corrode in presence of air. Plain steel screws are also OK, neutral, but the exposed part needs to be painted or passivated somehow.

These are a few values from the table:
Aluminium A5 -- Steel: +0.09V (steel protected, aluminium corrodes very mildly)
Aluminium A5 -- Stainless Steel: +0.84V !!!bad for aluminium -- very quick corrosion!!!
Aluminium A5 -- Zinc: -0.31V
Steel -- Steel: 0V
Steel -- Zinc: -0.40V
Steel -- Stainless Steel: +0.75V !!!bad for steel -- very quick corrosion!!!

All the above is only true in presence of electrolyte, e.g. salt water. So, I found a special marine anti-seize thread sealant Tefgel (a relative to Krytox due to use of a fluoroether as a base oil). It is specifically designed to protect the fasteners made with dissimilar metals and exposed to sea water. So, plain steel or zinc-plated bleed screws and Tefgel in the thread are good for any calipers, Brembo or regular.

Stay away from the fancy stainless steel bleed screws!
AlexB
(no, a different AlexB)

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