Why are some old brass lenses so corroded, and others are not?

[Pere Casals]

DZR brass has nothing to do with old brass lenses. It is a relatively modern alloy (started after 1950' and 60').

You should know that "DZR" is more than a kind of brass, it is also property of any brass, and any ancient brass is more or less DZR, this is more or less resistant to selective leaching, for example an addition (or impurity!) of some 0.02% of Arsenic to alpha phase brasses produces high resistance to dezincification, this is more DZR. Brasses of alpha or beta phases are determined by Zinc content, being less than 45% in alpha phase kind.

Sometimes you seem to be extremely well informed...

I'm somewhat informed because some decades ago I studied metallurgy in my degree, and I remember basic things like why gold shines: because electronegativity level, the highest of any metal: 2.54.

Just pointing that corrosion resistance of a metal or alloy can come from chemical or from physical barrier origin.



PD: edited to add "or Impurity !". In ancient times metallurgy had tricks that today are well explained.

[Pfsor]

You should know that "DZR" is more than a kind of brass, it is also property of any brass, and any ancient brass is more or less DZR, this is more or less resistant to selective leaching, for example an addition of some 0.02% of Arsenic to alpha phase brasses produces high resistance to dezincification, this is more DZR. Brasses of alpha or beta phases are determined by Zinc content, being less than 45% in alpha phase kind.
I'm somewhat informed because some decades ago I studied metallurgy in my degree,

A pity you didn't learn that DZR brass alloys were not used for old brass lenses. And no brass was called DZR in that time regardless of your definition.
Shouldn't you complete your studies a little bit?

[Pfsor]

Just pointing that corrosion resistance of a metal or alloy can come from chemical or from physical barrier origin.

No kidding? Who would say that!

[Pfsor]

... for example an addition (or impurity!) of some 0.02% of Arsenic to alpha phase brasses produces high resistance to dezincification, this is more DZR.

In ancient times metallurgy had tricks that today are well explained.

When they started to produce DZR brass (and call it so!) it was in the 1950' and 60'. The dezincification itself was recognised as such in about 1920. Not so ancient times... unless every time, passed by, is for you more or less ancient one, of course...

[Richard Wasserman]

I used to have an antiques business and routinely dealt with brass that was as old or older than the lenses being discussed. My understanding, based purely on empirical evidence, is that the degradation you see on the brass lenses would have more to do with how they were stored and cared for than the exact alloy used. If brass is kept clean, dry, and away from harmful acids and salts, it holds up well.

[Pfsor]

I used to have an antiques business and routinely dealt with brass that was as old or older than the lenses being discussed. My understanding, based purely on empirical evidence, is that the degradation you see on the brass lenses would have more to do with how they were stored and cared for than the exact alloy used. If brass is kept clean, dry, and away from harmful acids and salts, it holds up well.

That's correct - that's why I said "Add to it different air quality (humidity) and you have differently corroded brass lenses." But the opposite is true too - different alloys have different reactions to the same air quality, so it 18 and 20 minus 2...

[Pere Casals]

A pity you didn't learn that DZR brass alloys were not used for old brass lenses. And no brass was called DZR in that time regardless of your definition.
Shouldn't you complete your studies a little bit?

Any kind or brass has more or less "DeZincification Resistance", in the XIX or today. A particular highly DeZincification Resistant brass alloy is called DZR. Those XIX lens barrels that are corroed probably had low DeZincification Resistance. Those that arrived OK possibly are lower than 45% in Zinc and contain more than 0,02% As impurity or addition. Don't you think?

No kidding? Who would say that!

It is what I was teached. To give you an example you can easily understand consider COR-TEN Steel, https://en.wikipedia.org/wiki/Weathering_steel

All metals having an electronegativity that allows their oxidation but are corrosion resistant this is because their superficial oxid layer is protective. Example is AISI 304 Stainless Steel, and Copper, and Tin, and Silver... if you make an scratch then you see the metal, as some time passes a very thin oxid layer covers it, becoming darker, that darkening comes from surface oxidation, and this layer becomes protective as it makes a physical barrier that does not allow O2 to reach the metal under.

If you polish that layer then air O2 oxidizes again the newly exposed surface, making a new protective layer, like if it was paint.

[Pfsor]

... if you make an scratch then you see the metal, as some time passes a very thin oxid layer covers it, becoming darker, that darkening comes from surface oxidation, and this layer becomes protective as it makes a physical barrier that does not allow O2 to reach the metal under.
If you polish that layer then air O2 oxidizes again the newly exposed surface, making a new protective layer, like if it was paint.

I think I have had my dose... Thank you.

[Pere Casals]

Richard, storage conditions and lacquer would be important, as you say, but also consider that some kinds of brass, like "naval brass", can withstand large amounts of abuse without degradation, while some lower priced brass (lower copper for example) can be easily ruined.

Regards

[Myriophyllum]

Hi,

I think one problem with old brass is that some people find it a good idea to polish all lacquer (zapon or similar) off. They don't apply any new lacque. The blank brass now looks shiny for some time, then gets corroded and looks terrible. Depending on brass type and storage, of course.

Best
Jens