Copper brushing and galavanic corrosion?

[billyO]

Hello all.  On another knife forum, someone posted a question about using copper foil instead of gold for Keum Boo (a Korean technique for applying gold foil to steel).  And the other day, while looking for my small brass brush to apply the brass finish to a small piece I came across a small copper brush which got me wondering if I could use this for a copper finish instead of a brass finish.  But then I immediately thought of the problem of galvanic corrosion and it got me wondering....
I see an earlier thread where it seems as if the technique is possible to get a copper finish but no mention of galvanic corrosion. 

Won't this be an issue in the long run? (I'm reminded of something one of the recently deceased founding members of the NWBA apparently said to a student years ago when asked if their forging was good enough:  Ike replied, "Remember, when making something in steel, if it's ugly, it'll be ugly for a long time.")
Thanks.

 

[lary]

Copper sulfate is what worked for me. I'm still not convinced there actually is a copper brush available to get a similar effect like brass brushing. I have seen corrosion when the copper sulfate gets accidentally spilled on steel and left for a while.

[Frosty]

You can't get galvanic corrosion unless electrolyte closes the circuit between the poles.

Yes, copper will provide a "plated" layer like brass but it requires higher temps. On the up side you don't need a brush, use the copper wire made up of bundles of thin filaments and lightly fluxing is NOT cheating. I just dissolve borax to saturation in clean fresh water. Warm makes it easier to dissolve it doesn't matter if there's undissolved in the bottom.

I've done copper highlights with nothing more special than a good wire brushing and low red heat.

You can get neat effects by sprinkling brass or copper filings on highlight hot steel but this likes a bit of flux steel oxidizes quickly.

Frosty The Lucky.

[billyO]

Thanks for the more detailed info on how to go about this, Frosty.

5 hours ago, Frosty said:

You can't get galvanic corrosion unless electrolyte closes the circuit between the poles.

Sure, but unless it's in a climate-controlled environment, won't humidity eventually cause this over the long term?

Have a good week, all.

[Frosty]

From my understanding exposing the outside of the poles would require complete contact so I suppose galvanic corrosion could nibble at the edges, Maybe. You could be right we need someone who actually knows this stuff, my memory isn't good enough anymore and I don't want to do a bunch of searching and reading. Right now anyway.

Frosty The Lucky.

[billyO]

Thanks for the response, and no worries.  
In my non-exhaustive searching here and other places, there's no clear answer to this.  It seems like this might be an issue only in the long run, unless there is an electrolyte in the mix, which most don't seem concerned about.  It seems that I tend to be an outlier by thinking this way, but like I said in the first post, "when making things in steel . . ."

Have a good week.
 

[Buzzkill]

 

To the best of my understanding Frosty has it right.  If the two metals are bonded properly, as in a solder or brazing situation, there is no opportunity for any electrolyte to get between them to cause corrosion.  However, at the edges or any other place where there is even the slightest opportunity for something to get between the two metals it is likely to occur over time. 

Generally speaking though, if the piece is kept dry, or is lightly oiled, you should not see any galvanic activity between the two metals in the scenario you presented.

[Frosty]

To clarify what I meant by galvanic action at the edges I'll burn a few more lines of post space. I was envisioning a piece of steel, say 1/4" x 1" x 6". Brass brushed 2" on one end and Copper brushed 2" on the far end. No matter how well bonded there are 3 dissimilar metals in contact. If an electrolyte comes in contact them I can imagine galvanic reactions happening at the boundaries between steel and the other two metals. Then there's the potential for reaction between brass and copper. BUT it would all happen at the boundary between the metals, the edges.

This is why galvanized steel, say roofing tin doesn't corrode UNTIL it's scratched or you drive a nail / screw through it. Even a galvanized screw breaks the layer of zinc protecting the less reactive steel. 

When ever you look at an old piece of galvanized roofing you'll notice the rust stains begin at penetrations of the zinc plating. 

The above is different than welding zincs to steel boat hulls to control corrosion but it's a different thing. Sort of.

Frosty The Lucky.

[Mike BR]

They say galvanizing self heals, but I think what they really mean is that scratches cause a couple that protects the steel at the expense of the zinc. (Which seems like the same mechanism as a zinc anode on a boat or structure.)  You do get surface rust on a scratch, but I rarely see galvanized rust to failure until significant areas of the zinc are gone. 

[gewoon ik]

I wouldn't worry about the galvanic corrosion. You will not life long enough to see the problem.

And if it is humid and/or salty enough, the mild steel will rust anyway, maybe a tid faster because of the copper.

Btw bronze is also galvanic higher than steel. Same with stainless and carbon steel.

The blades you made with a welded in piece could have the same problem. But again, to have a real effect or very long time or something that is very bad for the knife anyway.

Lomg story short. Don't worry, let us see your results

[Frosty]

Gewoon, when you say, bronze is galvanic higher than steel. Do you mean bronze corrodes more slowly than steel if they're in contact with an electrolyte? 

Zincs and zinc galvanizing themselves are what we refer to as "sacrificial" metals meaning the zinc corrodes instead of the steel in galvanic conditions just like the sacrificial plate/pole in a battery.

Frosty The Lucky.

[gewoon ik]

Yes, the steel tries to protect the bronze by sacrificing itself.

Like zink is lower than steel and is sacrificing itself to protect the steel.

Hence the zink and/or aluminium blocks on ships. And more around the propellor and seachests (especially if boxcoolers are installed).