How to Prevent Stress Corrosion Cracking
Stress corrosion cracking, which results from the combined effects of stress and corrosion, occurs as a result of combining three controlling influences--a susceptible metal, tensile stresses, and a corrosive environment. Minimizing one or several of these influences is the key to preventing stress corrosion cracking.
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Instructions
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Controlling Stress Corrosion Cracking
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Select a material for your product that is resistant to stress corrosion cracking. Reduce the effect of the damaging elements of the service environment by choosing metals that will stand up to the corrosive agents your product will be exposed to. Thoughtful metal selection makes the end product less vulnerable to this type of cracking.
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Stress-relief anneal the product by heating parts to a specific temperature (which varies by material) in an annealing oven and then cooling slowly; this process relieves the residual stress in the metal. This strategy is most practical for small parts and is most commonly used for carbon steel. Stress-relief annealing minimizes the stress component of stress corrosion cracking by changing the metal's microstructure and, consequently, some of its physical properties, including strength and hardness.
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Even out the stresses by shot-peening or grit-blasting specific areas of weakness, such as welds; this strategy often is used for larger structures that can't be annealed. Shot-peening and grit-blasting both create a layer of compressive stress just below the surface of the metal; this is achieved by bombarding the surface with shot or grit, hard enough to create dimpling. The dimples are areas of compressive stress; the goal is to get all the dimples to overlap, thereby creating a surface that is entirely stressed by compression. Compressive stress increases resistance to stress corrosion cracking and controls the stress component mechanically.
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Apply an appropriate coating that will isolate the material from the environment. Depending on the exact circumstances of the system, including the product material and the corrosive agent in the environment, your choice of coating will vary, but the concept is basic. Coatings provide a layer of protection that minimizes the metal's exposure to corrosion, thereby limiting the effect of the corrosive agent.
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Paint the exposed surfaces. Painting protects the metal through high electrical resistance. This means that the paint retards the current flow from anode to cathode that weakens the metal. Note that paint is effective in resisting stress corrosion cracking because of its electrochemical properties; it is not an effective mechanical barrier to the environment.
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Tips & Warnings
Plan your strategy to prevent stress corrosion cracking during the design stage of your product; preventing this problem is easier than trying to fix it.
When designing components for the food service industry, avoid using coatings that are toxic, such as those containing cadmium.
References
- Photo Credit metal and wood image by Steve Lovegrove from Fotolia.com
