Galvanic corrosion of Brass Tsuba

[JohnTo]

Hi Guys,

I’m writing for advice on whether to try to remove galvanic corrosion from a brass tsuba, or to leave it alone.

For those without a background in electrochemistry it may be useful to describe what galvanic corrosion is.  If two different metals are in electrical contact and placed in an electrolyte (a liquid, such as salt water, which enables an electric current to flow) the most reactive metal will corrode much faster than if it was on its own.  An early example of this was in 1761 when sheets of copper were nailed to the hull of HMS Alarm in order to prevent marine fouling.  Although the copper did prevent weed and marine animals from attacking the wooden hull it was found that the iron nails attaching the copper plated completely corroded far quicker than other iron nails on the ship.  The iron nails acted as an anode and the copper as a cathode of a battery and the sea water provided an electrical conducting path between the two metals.  Of interest to us, as tsuba collectors, is that copper is often used for sekigane on iron tsuba, but I have yet to see any galvanic corrosion of the iron.  This is because of the dry environment around the nakago ana thus excluding the electrolyte component of the galvanic corrosive mix.

Iron is the most reactive of metals found in tsuba and thus most likely to corrode: gold, silver and copper the most unreactive and brass between the two extremes.  Probably the most common examples of galvanic corrosion of tsuba are to be found in early Heianjo tsuba in which the seal between the iron plate of the tsuba and the brass inlay was not sufficiently tight to prevent moisture seeping in between the brass and the iron.  The resulting galvanic cell caused to iron to rust and the increase in volume of the rust, compared to iron, forced the brass inlay out.  Later examples of Heianjo tsuba were often coated with a thin film of clear lacquer to prevent water ingress.

So now let me turn to my brass tsuba which depicts two gilt and shakudo takazogan horses and some background engraving on a flat base plate of brass.  Its mumei, wakizashi size (6.7 x 6.3 cm), a bit battered and not a particularly great piece, possibly Mito or Nara schools.  There appear to be three areas where the patina has been discoloured, namely:

1.      On the omote the gilt (copper?) and shakudo horses plus gilt flowers on the tree are surrounded by dark areas stretching out about 2mm from the inlays.  I assume that this is due to galvanic corrosion of the brass by the copper/gold inlay. (labelled 1 on pics)

2.      On the ura are two small pieces of copper imbedded into the brass ji and these two are surrounded by dark areas of stained brass.  I don’t know if these pieces of copper were picked up by accident in the brass mixture or if they were hammered into the brass blank to fill in flaws.  Whatever, these two pieces of copper retain a bright finish, as would be expected from the cathode of the galvanic pair of metals and it is the brass that has oxidised and become discoloured. (labelled 2 on pics)

3.      The lower right quadrant of the ura shows areas of red copper in the ji.  This, is probably not actually galvanic corrosion but due to the zinc in the surface of the brass being oxidised and rubbed off as it was formed by the clothing of whoever was wearing the sword, leaving behind a zinc depleted layer, i.e. copper.  (labelled 3 on pics)

My analysis of the history of this tsuba is that the dark staining was caused by storage in damp conditions, thus providing the third component of a galvanic cell, the electrolyte.  The copper-coloured areas appeared during normal wear near a warm moist (sweaty) part of the body and by rubbing against clothing.

My initial attempts to clean the tsuba were with ethanol on cotton wool buds to remove the general grime.  But I found that the dark stains around the horses were resistant to this.  I wondered if Renaissance Metal De-Corroder would remove this film.  I have used it on a badly rusted iron tsuba and it does work by softening up rust as an easily removed sludge leaving behind the underlying iron undamaged, but lacking any patina which would have been lost anyway beforehand during rusting.  Being cautious I tried this out on an old brass pipe coupling which had become similarly decoloured.  Rubbing on the De-Corroder with a cotton wool bud worked within a couple of minutes, but instead of revealing a nice brass coloured piece of metal, it was copper coloured.  The oxidised zinc had been removed to reveal a zinc depleted micro layer, i.e. copper, underneath.  I then treated the test piece with a paste of 100/200 mesh alumina in WD-40 using a cotton wool bud and sure enough the micro layer of copper was removed, revealing bright shining brass underneath.

So, here is my dilemma, do I leave the tsuba as is, or do I remove the dark stains with De-Corroder and then polish out the resulting micro layer of copper?  Areas of the ji would then be a mixture of bright and dull brass unless I polish all of the ji and wait bout 20 years for the brass to oxidise to a more aged patina.  It would be good to have the horses stand out from the ji, but perhaps surrounded by a bright, rather than dark, halo would not be an improvement.

Your help and comments will be appreciated.

With best regards,

John

Just a guy making observations, asking questions and trying to learn

[Matsunoki]

Simple……leave it.

[Exclus1ve]

I can’t help but agree with Colin.

You definitely shouldn't do anything, especially in such an “indelicate” way. Firstly, rust removers completely “eat up” the top layer of the tsuba metal, and patina no longer forms on it. Secondly, you should not use oils like WD-40, as they are hygroscopic and contain aggressive additives.
And if you remove the natural plaque around the inserts, the tsuba as a whole will look much less attractive…

Edited Thursday at 08:58 PM by Exclus1ve

[Jake6500]

As a general rule in this hobby, when in doubt do nothing... Sorry I have nothing new to add to the discussion but Colin said all that needs to be said!

[Spartancrest]