Metal finishing Q&As since 1989

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How fast can you run Salt Tests

Hello, my name is Emily and I am doing a science fair project on the corrosive effects of Salt water on Metal. I am doing this science fair project because of the recent bridge collapse. I would like to know how long it would take to run a salt test and if I can just use a humidifier? I would really appreaciate it.

Thank You.

Emily L
student - Byers, CO, US
February 7, 2008

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This is a general reply, Emily, it is not based on personal investigation of that bridge failure, but: bridges suffer corrosion collapses because of lack of an adequate inspection program, not because people did not correctly predict the life of the bridge. The public employees charged with bridge inspection (or maybe the officials charged with funding the program) did not meet their responsibility.

There is no accelerated corrosion test which does a really good job of predicting corrosion because the principles are different. Corrosion resisting tarnishes and skins that build in real life, due to the reaction of metal with carbon dioxide in the air, for example, don't have a chance to build in accelerated tests. It is important that specifiers as well as students understand this because materials that are fabulous in the real world, like galvanizing, cannot match the performance of greatly inferior coatings in accelerated tests.

Although there are a number of different accelerated corrosion tests, the most common, the salt fog test, takes from about 100 to about 1000 hours. A good test for a student or school would probably be the CAMRI test described in www.zinc-diecasting.info/zdc-PDF/SurfaceFinishes.pdf. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 11, 2008

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The last that I read, there were 2 major factors in the Minneapolis bridge collapse. The major one was the gusset plate was not properly sized by a significant factor. The next is bird poop. It has chemicals in it that promote rusting at an unknown and erratic rate.

The gusset problem is terribly close to a new design bridge collapse in Australia and 1 or 2 other places. It is an hour long show that was on the history channel. Your school may be able to get a loan of the DVD for you.

James Watts
- Navarre, Florida
February 13, 2008

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Thanks, Jim. I am not familiar with that particular brdge collapse, so I could only talk about Emily's corrosion project, and general corrosion failures. Certainly, if the bridge failed because it was misdesigned, that's a whole different issue

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 13, 2008