What Causes Stress Corrosion?
Frequently our customers ask questions regarding the loss of stones due to prong failure, the effects of chlorine, and other chemicals on jewelry. This excellent article by Randy Welch will help you provide concise information and technical support to consumers. Please save this document to e-mail to those customers with e-mail, and keep a hard copy to hand out.
Stress Corrosion
by Randy Welch
It's summertime. Hot, lazy days hanging out by the pool or spa with something cool to drink. And of course, making a fashion statement with as much jewelry as possible shown off against a beautiful tan.
Unfortunately, if that's a description of your customers, then they are looking at some possible massive jewelry repairs and lost stones. The reason - stress corrosion.
Alloy Factors
Stress corrosion is very common in low karat alloys, characterized as 10K or less. In alloys with gold atomic content of 40 percent or greater it is almost non-existent. Alloy systems have a significant impact as well. For example, it is more common in white gold, which is affected by the nickel content. Nickel contents below 7 percent and above 10 percent are very susceptible. As a rule, 8 to 10 percent are standard nickel levels in a 14K white system.
Another factor is whether the metallic system is homogeneous. Cast structures are not usually considered homogenous due to micro and macro segregation. Cold work followed by proper annealing can help reduce this; the more times repeated, the more homogenous the system becomes.
Stress Factors
Technically, stress can be defined as fluctuating strains in a metallic system, which work against each other to create a nucleation for possible failure.
Mechanical deformation is a primary source of stress: bending, stamping, drawing, rolling, and hammering. The quantity of deformation has an impact, as does the dispersion of isolation of the stress to one area. It seems that isolated stress increases the potential for failure under the proper conditions.
Heating is often overlooked as a source of stress, not to mention the segregation of the metallic system. Overheating, length of heating, and repeated heating encourages grain growth and segregation within the crystalline structure. Isolated heating of strained metal could also be cause of varying stress levels. Careful consideration should be given to cooling rates of heated areas. The source of heat can vary from bench torch soldering to grinding.
Notching has been studied at length in application to structural assemblies, and should be applied to jewelry fabrication. Notches and cuts, which come to a sharp angle, are undesirable due to the concentration of stress at the points of intersection. A small radius helps to distribute the stress over a wider area.
Compression is a factor rarely considered. Stretching and compression combined, along with quantity of compression, can cause failure.
Corrosion is the reduction of metals to a metallic salt. In gold jewelry, aqueous or gaseous environments can cause it. Stressed areas are possible sites of fracture, acting as entry routes for corrosives.
Mercury is a great example of wet metal corrosion, as is the exposure of copper-based alloys to liquid bismuth. The liquid metal migrates through the metallic structure, propagating a fast fracture.
Chlorine is extremely detrimental to karat metals and should be avoided. Special care should be taken with detergents, cleaners, and other industrial or household formulas. Consider that levels of chlorine and other chemicals can be increased by evaporation of fluids, leaving concentrated deposits. In particular, this makes spas and swimming pools undesirable for gold jewelry.
Additionally, concern is warranted about fumes from pickle baths and cleaners found in your shop. These, as well as acid fumes, are sources of corrosion.
Precautionary measures
- If stress has been applied or loaded into a product, consider a 35-minute soak at 650°F; allow to air cool. This will relieve the stress with no adverse affects to solder joints or tarnishing.
- How much stress can you handle? When reducing sheet or wire, avoid overworking. This can be indicated by edge notching, which is undesirable. Reductions of 50 percent are standard between anneals. Thirty-seven percent reduction is considered full hard.
- Avoid sharp notching; use a slight radius if applicable, especially if stretching and compression must be applied to the notched areas. Cut seats with a round burr or file as smooth as possible, no less than 30 percent, or no more than 50 percent. Consider using Stuller solitaire heads with pre-formed seats. (Series 309N, 401N, 140/309M, 140/401N).
- Use 18K or platinum instead of low karat nickel whites. These metals have a much higher tolerance to corrosives.
- Avoid too much or excessive time exposures to heat, use heat sinks and consider using a lower melt solder to avoid using a hot torch. Allow pieces to cool slowly. Repetitious heating to red zones is not recommended unless sufficient cold work is applied between heats. Avoid dull cutting tools and use a lubricant to avoid generating heat.
- Avoid contact with corrosive liquids, solids, or gasses. Clean notches well, as these can trap liquids, which evaporate leaving concentrated deposits. The seacoast and semitropical areas produce chlorides, which combine with the salts produced through perspiration to form a corrosive element that discolors the skin. Smog fumes filled with particles of sulfur dioxide and phosphate gradually attack jewelry.
- Of particular concern is prong work. Care should be taken not to stretch the prongs out of parallel. This increases the angle required to move the prong tip over the stone, increasing stress and compression. Stress corrosion can be controlled with proper precautions being applied from the manufacturer down to the bench jeweler and education of the consumer. It's our responsibility to control and decrease stress--make it a point to work "stress free".
Editors Note: thanks to John C. Herbbst of Palm Bay Jewelers, Victor Gemma of Grand Jewelry Company, Lowell Shortes of Brooks Jewelers, Ralph Pierro of Pierro's Jewelers, Victor Settings, and W.R. Cobb Company, who also contributed directly or indirectly to this article.
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| Article Created on 8/7/2003 |
Last Modified on 8/7/2003 |
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