Everyone has experienced the pain and suffering of trying to remove a rusted bolt or nut. Unfortunately, the hardware store definition of “corrosion-resistant” and the actual definition can vary greatly. It’s common knowledge that most non-exotic materials will corrode with everyday use, especially when they are in a caustic environment. That’s why electroplating was invented – by covering the base metal with a very thin layer of resistant material, rust is no longer a problem. But how do you know which material to use for which environment? And will that plating stand up to the test of time?
Corrosion Resistance Testing
Perhaps the easiest and fastest method to test corrosion resistance is to put a sample inside a cabinet and continuously spray it with a caustic solution. The tests are given names based on the solution used – the two main solutions are NSS (Neutral Salt Spray), and the more corrosive CASS (Copper Accelerated Acetic Acid Salt Spray). Because of the higher than normal concentration of salts in these solutions, testing times can be accelerated and used to predict the plating’s effectiveness. This test is described in the ASTM B117 standard.
Many manufacturers, especially those in the automotive industry, do not trust the salt spray test because it lacks the real world condition of cycling between wet and dry conditions. An alternate method is SCAB (Simulated Corrosion Atmospheric Breakdown), described in ISO 11474 and SAE J2334, which follows a cycle of spraying the test piece with a salt solution and then allowing a drying period before repeating. While slower than standard salt spraying, a much better correlation is achieved between testing and real world results.
Regardless of what type of corrosive test is used, there is always an error relating the test to the real world application where the material will be used (unless the test perfectly replicates the real world environment). So are all ratings worthless? Certainly not – they can be used to quickly and easily compare different types or grades of material. For example, if real environment results are known for a specific grade of metal, corrosive test results for that metal can be compared to other metal grades.
Interpreting the Results
Again, it is important to note that most corrosion resistance results should be taken qualitatively, not quantitatively – you should be looking to compare results. Some ratings will give the number of hours until failure, while others may give a standard number of test hours and then a pass/ fail rating. For example, in our surface finishing tutorial, various different metals are listed along with the type and amount of plating. The Application Environment gives a basic idea of where such metals could be used. The base metals are run through various spraying times (more times for more caustic environments) to see where there is negligible corrosion, and thus to define the application environments.
Based on this chart, if a part needs to be made of aluminum and is going to be outside in the elements (Environment B), a plating of Ni30b, Cr 0.1 can be utilized. If a different material is needed, you can see what plating to use by quickly comparing the metals that are compatible with Environment B.
Top Corrosion-Resistant Misumi Products:
- Corrosion-Resistant Linear Guides
- Corrosion-Resistant Linear Slide Rails
- Corrosion-Resistant Duct Hoses (Excellent in Heat/Cold)
- Corrosion-Resistant Miniature Lead Screws
- Corrosion-Resistant Aluminum Alloy Plates (Made of Heat-treatable Alloy, Excellent in Strength)
- Corrosion-Resistant Stainless Steel Plates (Excellent in Atmospheric Environments)
Corrosion failure has always been, and continues to be, one of the largest concerns for designers and engineers. A large amount of innovation and research time is dedicated to new methods of stopping or slowing the effects of corrosion. As new materials and processes become available, testing and comparisons to existing options will be very important. As long as the same standards are used for the test procedures, users can select the best materials for the project.
Still not sure how to select the correct plating for your project environment? Leave a comment below, or email us at email@example.com!