Detecting and Dealing with Stainless-steel Corrosion
Posted by Darrell Nicholson at 11:34AM - Comments: (2)
October 26, 2011
Whether you are laying up your boat for the winter, getting ready to head south, or preparing for the winter sailing season closer to the equator, now is as good a time as any to give your stainless-steel hardware a close inspection.
Practical Sailor has an interesting article on titanium hardware in the upcoming December issue, and this brought to mind several previous reports we’ve done on the problems with stainless steel. Although high-quality stainless can provide years of reliable service, sailors need to be aware of its limitations. Owners of used boats with hardware of an unknown age should be particularly scrupulous when carrying out routine inspection of stainless-steel rigging and hardware.
Small, load-bearing hardware such as shackles are particularly susceptible to failure after years of service. Shackles are among the most common products submitted to our hall of infamy, the Gear Graveyard, for reporting and/or analysis. Another common failure point is deck-level, swage-end fittings. The attached photo of an approximately 10-year-old Nicro snap shackle gives an example of what can happen when corrosion and cyclical loading take their toll on this common piece of hardware.
Crevice corrosion is probably one of the most virulent forms of corrosion that attack these small components. Any crack, seam, or flaw in the surface of the metal can become a fertile place for this type of corrosion to develop. All it takes is a small amount of oxygen-deficient salt water to become trapped in a tiny crevice, and the crevice becomes anodic, "sacrificing" electrons to the much larger surrounding cathodic surface area. Seemingly harmless rust stains are typical indicators of crevice corrosion, but the stains are not always obvious, sometimes hidden from view. Along with shackles and deck-level swage fittings, chainplates, turnbuckles, and any welds are also common sites for this and similar types of corrosion to develop.
Should you find signs of crevice corrosion, your course of action will depend on what is corroded and how serious the corrosion is. Regular, freshwater rinses, and keeping stainless steel micro-polished and clean can help prevent crevice corrosion, but some stainless-steel failures are often impossible to detect until it is too late. When in doubt, seek out the opinion of a local rigger. If you have recently purchased a used or new boat, having a rigger go over the rig, top-to-bottom can be worth the expense. It is a lot easier (and cheaper) to replace a suspect chainplate than deal with a broken mast at sea.
For more on stainless-steel corrosion, check out Marine Metal Warning, and Used Stainless Steel Hardware. You can also link to an interesting “Gear Graveyard” report on a different type of snap-shackle failure, available only as a PDF. That report is located in the “Also with this article” box in our December 2009 Mailport.
In the meantime, stay tuned for our report on titanium in the December issue.