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topic 2404

Electrolysis between Aluminum & Stainless Steel


(1998)

Q. I have a client contemplating thermal sprayed aluminium to his traveling screens at a power station. Would there be a galvanic or sacrificial action where aluminum is in contact with stainless steel in a wet environment?

Greg N [last name deleted for privacy by Editor]
thermal spray - Victoria, Australia


(1998)

A. Aluminum and stainless steel are far apart enough in EMF to cause significant corrosion in a moist salt water environment.

An alternate that might work is nickel aluminide. I really like the metallographic results from a small system from TAFA. Beautiful laminar coating. virtually no porosity. It is a dual wire system. Miller has a big one but is too big for most work unless you are doing aircraft carrier anti-skid on the decks.

I would try one and see how it worked. If it worked okay for several weeks, do them all.

James Watts
- Navarre, Florida


(1998)

Q. I was wondering if you could describe in 50-100 words galvanic action in metals, in general terms. I'm currently studying and cannot find any info describing what takes place in this process.

Much appreciated, Thank you.

Christopher C [last name deleted for privacy by Editor]
- Gold Coast, Q,ld, Australia



A. Sure, Christopher.

Picture a dry cell battery. The battery consists of two different metals: the shell, which is the negative pole, is made of one metal (for example, zinc); the rod is made of a different more noble metal (for example, carbon) and is attached to the positive pole. In between the rod and the shell is a conductive gel or glop. For the battery to provide electricity (for you to use it), you have to connect wires that provide a metallic path from negative to positive that electrons can follow.

What happens is electrons travel from the negative pole, through the wire to the positive pole. This leaves a deficit of electrons at the negative pole. Some of the atoms of the negative pole (zinc shell) have thus been converted from metallic (neutrally charged) state to positively charged ions which dissolve into the solution and migrate through the glop and reach the carbon rod. The positively charged ions meet the electrons, which reduce the ions back to metallic state, i.e., the zinc plates out onto the carbon rod. At some point all the metal from the shell dissolves, or the rod gets completely coated with metal from the shell, so there is now only one metal exposed instead of two, and the battery is "dead".

Dissimilar metals in a device comprise such a battery regardless of whether we want them to or not. If you have two different metals, and they are sitting in salt water or another conducting liquid, you've got a battery. If you now provide a metallic path between the two metals (either a piece of wire or just bolting them together), the battery is "running", i.e., the more active of the two metals is corroding.

Luck and regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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