Metal finishing Q&As since 1989

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Real World Salt Spray Performance?

I'm seeking information on how the real life corrosion resistance relates to salt spray test time.

The products I am dealing with are mechanical fasteners (i.e. bolts). For example, if a zinc chromate plated bolt will not exhibit red rust in a salt spray cabinet for 72 hours, how long can it be expected to last in real life service in different environments? I understand that there will be great differences in performance for areas such as California vs. Indiana vs. Georgia, but are there any published data giving average plating life performance for any of these areas?

Ron Tomallo
Lake Erie Screw Corporation - Cleveland, Ohio, USA
2001

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Salt spray does not correlate to real life. Neither does any other corrosion test. In fact, corrosion tests do not correlate to each other. It's not a pleasant fact, but that's how it is. People have studied this and tried to make it work, but as of yet, there is no exact correlation. The standard procedure is to set a number of hours your part should resist salt spray exposure before white and/or red rust, and use this as a pass/fail criteria. The test then can do an ok job in weeding out bad parts.

You will need to draw upon past experience to know how long your parts will last in real life service. If you want better real life corrosion resistance, then you can use a stronger finish and/or raise your salt spray hours, but there is no direct correlation. The problem with corrosion testing is that one is trying to replicate a chemical reaction that takes years. A rusting process that takes a month instead of years is not the same reaction. You can get good data, but it will never exactly get the same result as you will in the real world. Don't you love chemistry?

Tim Neveau
Rochester Hills, Michigan
2001