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topic 59429

Calcium compound as corrosion product of copper alloy

October 16, 2013

Q. Yes, I know this sounds daft, but I have found a layer of a calcium compound along all of the fracture and most of the cracks of a Nickel Aluminium Bronze fastener which has failed by Stress Corrosion Cracking.
By SEM, I'm only getting Calcium and Oxygen as major elements, and at the root of many of the cracks there is none at all with only normal corrosion products and Chlorine etc.
There is only the calcium layer on the fracture, with none at all on the thread flanks or shank, so I'm sure I haven't added anything odd during preparation.
The sample is mounted for examination, but the resin contains only very little Calcium, so I'm fairly sure it's not come from this.
Microscopic examination shows the layer to be hard and well adhered to the fracture surface like an oxide or similar.
I'm completely at a loss.
Any thought chaps?

Martin Rich
Company Metallurgist - Plymouth, UK


October 22, 2013

A. Hi Martin,

How much can you share about the conditions of service? At the moment we don't know what environment the part has seen, so it is a little difficult to give you any ideas.

Remember that a lot of areas around the south of the UK are chalk hills so there is a plentiful supply of calcium.

Brian Terry
Aerospace - Yeovil, Somerset, UK


October 23, 2013

A. This can happen as a result of water contact.

blake kneedler
Blake Kneedler
Feather Hollow Eng. - Stockton, California


October 25, 2013

Q. Brian.
Thanks for that, I think you could guess at my company business.
The environment is occasional sea water.
The coating is extremely hard, so I'm guessing it's the Oxide or Hydroxide and probably not Carbonate.
Could a hard Calcium compound result from a cool application?
I'm still floundering here - any help appreciated.
Martin

Martin Rich
Ship Repair - Plymouth, UK


October 29, 2013

A. Hi Martin,

Calcium carbonate can be pretty darned hard and a nuisance to get rid of. Calcium oxide is also possible. Easy way to check is to try and break a small piece off and put it in some hydrochloric acid, if it effervesces it is carbonate, if not possibly the oxide.

As for a source of calcium...mmm...that isn't so easy. Although there is calcium in sea water it is much more dilute than the water that runs off the chalk hills so I would expect to see a build up. Saying that a lot of shellfish have hard, tenacious shells that are essentially calcium carbonate.

How quickly is this deposit forming? Are we looks at days, weeks, or even years for formation?

Brian Terry
Aerospace - Yeovil, Somerset, UK


November 11, 2013

A. It is a pity there is no information about the service conditions that have resulted in the SCC, as we are all shooting in the dark. However, I reckon the deposit could be calcium hydroxide and come from a fresh water source. My thinking is based on the fact that saline water will contain a lot of chloride - which will be easily detected throughout the corrosion products by EDAX (on the SEM). I would be most surprised to find calcium oxide as a deposit, as it is very reactive and is hydrolysed to calcium hydroxide by water, whilst at the same time producing a fair amount of heat. Unfortunately EDAX is unable to detect hydrogen and in fact struggles with any element with an atomic number lower than boron. Perhaps more information and a photograph may help.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

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