How to tell brass or bronze?

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bluedragon

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I've got lots of "bronze" type underwater fittings on my boat (1969 vintage)...prop shaft & propeller assembly, seacocks, skin fittings, rudder fittings and many of the bolts securing them. I've assumed until recently they are all bronze, but is there an easy way of confirming this vs. brass? I'm trying to get my mind around the galvanic corrosion risk and to what extent the anodic protection currently in place is actually needed? The only pitting I can see is on the prop blades which are pock marked but structurally sound.
 
By chemical analysis! Bronze is an alloy of copper and tin, brass is copper and zinc. However if you are able to do that you would not have had to ask the question.

They are a slightly different colour. I suggest you get a bit of brass and a bit of something you know to be bronze grind or file a bright patch on each then compare the colours with a similarly prepared patch on the various bits you want to identify. Also take into account the colours of the naturally oxidised areas.

If anything is dezincification resistant (DZR) brass you probably won't be able to distinguish it from "ordinary" brass other than by analysis for the minor constituents.

Anything the looks pink is not likely to be bronze or DZR brass for that matter and should be replaced.

[ QUOTE ]
to what extent the anodic protection currently in place

[/ QUOTE ] <u>Cathodic</u> protection you mean.

Bronze should not need protection although brass will dezincify without it, and maybe even with it.
 
Unfortunately analysis is the only way. I have been trying for some time to develop a simple test (I am a metallurgist) but everything has been unsuccessful. Unfortunately colour is not a very reliable guide either, as brasses in particular are changed considerably by their copper content. If your fittings are original and have been in place since 1969 I would not be worrying too much.
 
If you know a friendly scrap merchant (is there such a thing?) there are simple to use hand held spectrometers such as Niton. They will actually tell you the Ansi grade.
 
Checking again today and looking at the Blakes seacock (which is DZR brass I believe), I think many of the fittings may well be DZR. However, if that's generally as good as bronze, it doesn't matter which is which I guess. I've got no "pink" bits anywhere, but the prop corrosion is a bit mystifying. It was covered in what I can best describe as a hard grey mineral deposit, and underneath, the metal surface is quite pitted. No sign of pink...just the brass/bronze missing in numerous small craters over the surface. Any comments?
 
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