Corrosion of brass scaleplates..etc.

[The Philpott]

..Into the muddy world of copper corrosion- and the use of the word 'verdigris'.

Verdigris tends to get used as a general term for greenish-blue corrosion(s) of copper, but i researched the term 'Bronze disease' yesterday, which refers specifically to chlorides of copper attacking copper or it's alloys. Now this really is a nasty one- when Copper (I) Chloride is formed it readily reacts with any water present to form Copper (II) Chloride and Hydrochloric Acid. A destructive cycle is generated! The Chloride/s can even form under more stable layers of Carbonates or Oxides, expanding and spalling out through to the surface.

The chief identifier for Chlorides is the corrosion is a fuzzy and flaky consistency, whereas Carbonates are more solid.

This is a confusing subject, but it seems to be the case that many of these green-blue copper salts are hygroscopic- which leads me to this particular painted scaleplate which has suffered recently. I made the mistake of leaving a donor (parts) meter too close to the outer wall of the shed/workshop, which has generated (or more likely, reactivated) some salts beneath the paint. On close inspection each mouldy looking pustule in the picture had a tiny drop of moisture at it's centre, rubbed away with a cotton bud without further drama.

This corrosion probably initiated through pinholes or micro cracks in the paint, caused by light degradation or strong daylight super-heating the surface in the past. I will now use this as a test piece to see if the deterioration can be halted.
Not sure where to start with that though, i'm not convinced 3 in 1 will properly penetrate through all the green stuff. Suggestions welcome. I could even try different treatments on different areas.

Dave

[Superscope]

How about trying WD40 ?

The all in one instant repair for everything oil ?

The only real problem I have found with the stuff is, that it can sometimes stain
White surfaces, especially Plastic.


Ian

[The Philpott]

I believe that i wiped it with wd40 when i got it..wouldn't do any harm to re-apply.

[Tractionist]

Hmmm ..... another term for surface oxidation of copper is "patination". Copper covered roofs are some of the most durable [and expensive] around ..... and if not treated with 'patination oil' upon completion will turn to [what many regard as a pleasant shade of] green. This 'patina' prevents further corrosion and ergo the material exhibits a very long life.

The key word here is 'oxidation' i.e. a reaction involving exposure to oxygen [i.e. a 21% component of air]. Sooo ..... it would seem that your damaged surface coating [paint or sometimes enamel] has permitted oxidation of the copper sub-straite.

Doubtless a wipe over with WD40, or silicone polish etc. will do much to re-seal the surface and thereby prevent further oxidation ...... but obviously this should be applied [initially] judiciously in a small and inconspicuous area.

[Bazz4CQJ]

One problem here is that the moisture is already in there; putting anything like WD40 on top may well help keep it there.

I have no prior experience with anything like this, but I'm inclined to agree with your analysis of how it happened and suggest that the water needs to be slowly driven out - a vacuum oven (starting at 30'C) would be useful, possibly raising to no more that 40'C over a day or two. I can offer to do that for you if you you don't happen to have access to a vac oven.

I'm not very sure about what to do next; putting it back in the case together with a pack of desiccant is one possibilty. The long-term effects of applying some chemical to the painted scale are uncertain? WD40 certainly is not a long-term sealant; maybe something like a light car wax?

B

[emeritus]

I am no metallurgist, but the workshop course that Plessey sent me on circa 1970 incuded a lecture on corrosion. I don't recall anthing about brass, but do recall that stainless steel has to be left unpainted to ensure its corrosion resistance. If painted with perfect coverage, ok, but in damp conditions a pinhole will result in a self-perpetuating corrosion cell (pitting corrosion) becoming established. Possibly a similar situation exists with some types of brass.

[The Philpott]

Further to post No.4, i am advised the brown oxide on a well handled 50 year old 2p coin or the dark green coat of carbonate you find when you dig up a 100 year old penny, are indeed protective.

Thanks Bazz for the offer of a vac oven. I will remove the scaleplate and see how things develop- the meter movement is not a usable one so i can take my time seeing if the water can be driven out before applying anything.

I certainly need to keep an eye on another avometer that a previous owner decided to put in a dishwasher- i got it working due to it's superior environmental seal but now we're a few years on and anything could be happening inside it.

Dave