Stress-Corrosion Cracking and It's Effect on Karat Gold Alloys

By Stuller Technical Resources | July 21, 2011

You are behind yours sales counter when one of your repeat customers enters your store.  OnE look at her immediately warns you that something is wrong.  After a short greeting, the customer produces a piece of jewelry that has missing prong tips, and, even worse, the diamond or gemstone is also missing.  After a few questions and a quick visual examination, it becomes obvious that this is not the typical fracture, as seen when a prong has been torn off.  Closer inspection of the fracture surface reveals a grainy appearance that is very porous.  The odd appearance of the fracture surface and the infrequency of this type of failure lead you to believe that the problem is "bad metal" or "bad casting".  Was the problem really due to defective metal or casting or is there another explanation and solution?  The correct answer may surprise even the most skilled and experienced craftsman.

 

It is not surprising to initially blame the material as the cause of fracture but with the use of high powered magnification and literature that is not readily available to the craftsman the real cause can be found.  The photos below illustrate several products with a "mysterious' failure that you may have encountered during your career.  The prong tips broke off with little or no indication of onset of failure.  As a matter of fact, they all appear to have fractured in a brittle manner showing very little ductility.

  

All of the examples in the above photos illustrate that none of the failed prongs exhibit evidence of ductility around the broken prong tip or on the fracture surface.  Features like "orange peel" texture of a dimpled fracture surface are always present when a ductile alloy has been broken by bending, twisting or pulling too many times.  Most karat gold alloys typically used to produce jewelry with are usually quite ductile.  You are able to bend each of the prongs to correctly seat the stone with no obvious complications. 

 

This lack of ductility becomes very clear when the fracture surface of the broken prong tips are viewed under high magnifications using a scanning electron microscope (SEM).  The black and white photos seen above are SEM images taken at lower magnifications, typically 20x to 100x. 

 

The "rock candy" appearances of grains in these images are merely facets of grain boundaries.  Each grain has separated from its neighbors along the grain boundareis it once shared with them.  This is known as intergranular fracture, which is typically associated with the brittle mode of failure.  The brittle fracture surface often appears specular or shiny (when it is not covered with debris) to the unaided eye.  At low magnification, the intergranular fracture surface may appear porous.  At higher magnification, it becomes apparent the the pockets are created by clusters of crystals being removed at multiple levels.  This intergranular fracture is clear evidence of how the initially ductile gold alloys used in jewelry applications have become brittle at the notched areas of the prong tip.  So why did the prong fail in a brittle manner?

 

Brittle mode failure of a ductile gold alloy is not typical.  The three most common causes of brittle mode failure are stress corrosion, hot tearing and contamination of the alloy.

 

Hot tearing typically results in intergranular failure and occurs when stress is applied to the material at elevated temperatures near its melting point.  These fractures usually exhibit oxidation on the fracture surface and a "waxy" appearance on the crystal facets.  Examples of hot tearing would be fractures occurring during soldering operations and fractures found on castings due to fast quenching of investment molds.  If these types fractures are present in a prong, they would usually become obvious when the prong was manipulated during stone setting.

 

Gross contamination of gold alloys can also lead to intergranular fracture.  Lead is a well-documented contaminant in gold alloys, and levels as low as 0.01% can result in brittle failure.  Alloys with gross contamination would almost never make it through stressful operations such as rolling, drawing or stone setting without suffering severe cracking.

 

Stress corrosion cracking (SCC) is a brittle mode failure that is well documented and has been extensively studied in all families of different engineering alloys.  For example, certain types of stainless steel are known to be susceptible to SCC when exposed to chlorine or chlorine containing compounds.  Copper alloys such as brass or bronze are susceptible to SCC when exposed to ammonia.

 

Elements known to promote SCC in gold jewelry alloys can be found in the halogen group.  These corrosive elements include iodine, chlorine, fluorine and bromine.  For a karat gold jewelry piece to suffer SCC, two factors must be present and act simultaneously - residual stress (prong tension, etc..) and exposure to a corrosive environment.  The jewelry will not fail due to SCC if either one of the components is absent.

 

 

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