Minute and almost undetectable, crevice corrosion can have major consequences. Nothing demonstrates that more than the historic case involving Aloha Airlines flight 243, which suffered extensive fuselage damage and was required to make an emergency landing. The cause of the damage? Metal fatigue originating from a crack site caused by crevice corrosion. Hopefully, we now have your attention!
How does Crevice Corrosion Occur?
Crevice corrosion causes havoc on metal surfaces, stainless steel notwithstanding. Finding the usual culprits responsible for crevice corrosion is not so difficult seeing that they’re usually found in really tiny crevices. Lap joints, gasket surfaces, threaded connections, tube-to-tube sheet joints, washers to base plates and even surface deposits, anywhere that holes may appear are potential crevice corrosion points. Due to location, this type of corrosion is sometimes called gasket or deposit corrosion. Understandably, this is more likely in seawater applications.
Is Crevice Corrosion Preventable?
The thing is, we’re looking at what is called the anodic reaction, which is better supported by chromium metal ion than by nickel or iron alloys. Austenitic stainless steels seem better able to resist crevice corrosion than other grades of steel.
Molybdenum (Mo) and nitrogen (N) noticeably affect resistance. Mo appears to arrest the rate of attack. As a general rule of thumb, stainless steels, such as the 6% molybdenum austenitic and super-duplex grades to give the best crevice corrosion attack resistance.
Design may still be the best offence. Using insulating gaskets is not enough prevention. The prevention strategy lies in good fit-up and adhesion of non-metal joints and ensuring crevices occurring between two material is averted.
In summary, the old adage “prevention is better than cure” applies in full to avoid an insidious attack of crevice corrosion.
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