CORROSION TESTS AND GOLD JEWELRY
Recently we decided that we must do some testing to verify how well our jewelry “holds up” in a corrosive environment (humid environments, perspiration contact, etc.). Do you have any suggestions for us?
Q. Our company designs jewelry and then outsources the manufacturing. Our jewelry typically is made of copper or brass and gold plated. In the past we have relied on our vendors to test the jewelry for corrosion as well as the presence of lead and other possible impurities. Recently we decided that we must do some testing to verify how well our jewelry “holds up” in a corrosive environment (humid environments, perspiration contact, etc.). Do you have any suggestions for us? C. J.
A. The old standby is the salt-spray test, which is described in ASTM B 117-07a, available from ASTM International (www.astm.org). The electroplating industry has relied on this standardized test for many years. The main concern about this test is that the results quite often do not correlate well with the real world.
Another test which to my knowledge does not have “official recognition” is one that was developed by the Swiss watch industry and was reported in Metal Finishing a number of years ago. This test may actually give you better correlated results with the real world. The test procedure follows.
- Mix together the following:
Glacial acetic acid—25% by weight
Sodium acetate trihydrate—50% by weight
- Place the mixture into a sealable glass container. The container should be designed in such a way that the sample is exposed on all sides to the atmosphere inside the container.
- The sides of the receptacle should be lined with white blotting paper which dips into the liquid.
- The sample is positioned above the mixture using a glass hook. The height above the mixture should be at least 30 mm.
- Maintain the temperature of the container at 23oC ±2oC for 24 hr.
- After 24 hr, inspect the sample for green deposits (copper) or white deposits (zinc). Lack of deposits indicate no corrosion.
A variation of this test uses artificial perspiration in a similar set-up. One formulation of artificial perspiration is as follows:
The recommended temperature for this test is 40°C.
|Sodium chloride||20 g/L|
|Ammonium chloride||17.5 g/L|
|Glacial acetic acid||2.5 g/L|
|Lactic acid||25 g/L|
|Sodium hydroxide to adjust 4.7 pH||As needed|
A primer on this inexpensive and highly efficient process.
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