Brass Corrosion

[Trophies]

Good morning,
I am hoping to get some expert information in regard to corrosion which has affected some brass plate that I have supplied to a customer (I did the engraving).
The material is 1.2 mm brass and is presented as being suitable for exterior applications. The material is CuZn39Pb2 or C37700 and should be Cu 59%, Pb 1.8% and the balance Zn. After engraving and paint-fill the surface was abraded and then coated with a clear coating.
The plates (20 in all) were then attached to a limestone wall using brass screws. The position gets full weather and wind from the ocean.
Unfortunately the corrosion/pitting shown has occurred in less than 3 months. It is most severe on the plain brass surface but has also permeated under the clear coat.
Would this be an expected result? Can you suggest what may have affected the brass? Is it likely that the material was not to spec? Is there any way to prevent a recurrence?
We are in the process of replacing the brass with 316 Stainless. Is this likely to be more successful?
Thank you for your time.

 

[EdStainless]

Are these out where rain can wash them off?
That is important, if they are somewhat protected then corrosion is much worse.

It also looks like your fasteners are brass plated steel. That is no good. You need actual brass or bronze fasteners.

I would not used 316 SS. There are plenty of cases of 316 pitting in coastal environments.
You could go to higher alloy grades (2205, AL-6XN, 254SMO) and use them uncoated, but they are expensive and difficult to work with.

If you want the look of brass go for a real marine grade, such as AlBronze, NiAlBronze, or Admiralty brass. There are CuNi alloys (90/10 or 85/15) that are very corrosion resistant in seawater environments, but again they are more difficult to work with.

I would also suggest that you not clear coat them. Any small defect in or damage in coating (such as from the screws) will result in crevice corrosion under it (as salt collects and concentrates). This may mean that you will need to use a more matte finish, but it will be much more durable.

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Plymouth Tube

 

[TomDOT]

Ed, I think you covered everything.