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topic 43730

Electrolysis between stainless steel and steel


A discussion started in 2007 but continuing through 2018

2007

Q. Hi

I have been searching on the web for an answer to whether or not electrolysis will occur if I affix stainless steel security mesh to a steel framed security door. Eventually I stumbled onto this site and would be really grateful if someone could advise me.

Thank you very much

Les

Leslie Gevers
Householder - Perth, Western Australia


2007

A. 'Galvanic corrosion' is probably a better term than 'electrolysis', Les. To understand galvanic corrosion, picture a dry cell battery or alkaline battery. The two poles of the battery are made of two different metals and the space between them is filled with a conductive glop (electrolyte). If you connect the two poles across a small light bulb, the bulb will light.

What is happening in the circuit is that one of the two metals is more noble than the other and wants to get to the other side and plate out and cover that other pole so the surface of both poles will become the same metal (it isn't essential that you understand why it wants to do that). But solid metal can't just up and walk. To get to the other pole it must dissolve as positively charged metal ions and travel through the electrolyte from one pole to the other. As the positively charged ions move from one pole to the other, their electrons must travel through the wire and bulb. When the electrons rejoin the positively charged ions, they turn back to metal. Eventually both poles are covered with the single metal and the battery is dead.

Your "battery" uses the two metals steel and stainless steel for the poles. It uses rainwater and salts as the electrolyte. And you are bolting or welding the two together in lieu of connecting them with wire and a light bulb.

If you have insulation between the two metals you have a battery with no load, so no current and no galvanic corrosion. If the item is kept very clean and dry you have no electrolyte and almost no galvanic corrosion. But carbon steel electrically connected to stainless will galvanically corrode otherwise -- but probably only quite slowly.

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



May 23, 2018

Q. Great read, thank you, Mr. Mooney. This is a perfect analysis of Galvanic corrosion in general but the specific question remains in my mind whether or not there would be a current between steel and stainless steel. your statement leads me to believe that you could (based on your statement) electroplate a piece of steel with stainless. if this is true, I have never heard of it. Also confuses me why there is no Galvanic corrosion between steel and the galvanizing zinc that covers it??

Hamid Bahrami
fire protection services - Houston Texas


May 2018

A. Hello Hamid. Yes, there is galvanic current between steel and stainless steel: the stainless steel is more noble, so it does impel the steel toward corroding. However, stainless steel tends to form a chromic oxide skin due to its chromium content and such oxides are electrically insulating and slow down the tendency for galvanic currents so much so that steel sheets with stainless cladding are a commonplace despite the theoretical susceptibility to galvanic corrosion.

The reason you don't see stainless steel alloys electroplated onto plain steel, however, is that we can't do it. In general it is difficult to electrodeposit alloys, and we aren't able to electrodeposit stainless steel.

There definitely is galvanic corrosion between steel and the galvanizing zinc that covers it. Steel is more noble than zinc, so the zinc gradually dissolves away to protect the steel. That is why steel galvanized with zinc is quite corrosion resistant -- because the deliberate galvanic corrosion of the zinc protects the steel.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



June 30, 2018

Q. Sir,
I have an on-demand natural gas water heater. The flue and its base is stainless steel. Sealing and securing the base of the flue to the cabinet was a painted steel collar that had a fiberglass mat between the flue base and the cabinet. On occasion, the flue will sweat and the condensate is deposited on the painted steel collar. The flue gases are slightly acidic. The collar rusted away. I replaced it with a stainless steel collar that I fabricated, the original cabinet parts are no longer available. What is the chemistry now that the stainless is fastened directly to the steel cabinet as opposed to the "floating" collar. The new stainless collar is now gasketed with a high temp silicone material.

Michael Buckley
- Casselman, ON Canada


June 2018

A. Hi Michael. If the silicone gasket is glued and there is no metal-to-metal connection between steel and stainless steel, no galvanic current can flow. If the stainless is bolted or sheet metal screwed to the carbon steel, you have the potential for galvanic corrosion. But realistically, you've seen and suffered the alternative of a carbon steel color, and it sounds like you're better off with acid-resistant stainless components that with a non-acid resistant steel component. Potential galvanic corrosion doesn't trump actual chemical corrosion :-)

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


July 6, 2018

thumbs up sign Thank you for your evaluation and prognosis of my carbon/stainless steel and the acidic moisture. My heater is at it's rated half-life. By the time the cabinet starts to deteriorate, I will require a new heater with a newer flue venting system. Thank you again

Mike Buckley

Michael Buckley [returning]
- Casselman, ON Canada



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