Wednesday, December 3, 2014

Crevice Corrosion Can Ruin Your Watermaker!

You may have heard of Crevice Corrosion. Perhaps you  have confused it with galvanic corrosion (electrolysis), but the chemical process is quite different.

Without explaining all the chemistry (as some of it is beyond my Chemistry 201 studies!), I will explain what happens. Essentially, Crevice Corrosion occurs to Stainless Steels when in the presence of seawater that has been depleted of oxygen. How does this happen?

Seawater that is trapped against stainless steel equipment loses its oxygen over time by causing oxidation of the materials it is in contact with. Crevice corrosion does not require dissimilar metals to occur. In fact, it can even occur where a plastic part is clamped to a stainless steel part (or even painted stainless steel!), if seawater is allowed to become trapped between the parts.

The result looks a lot like galvanic corrosion, but is not caused by the same problems.

Common boating equipment that frequently suffers from crevice corrosion is trim tabs. I have seen trim tabs that have been properly maintained, with zincs replaced regularly, fall prey to crevice corrosion. This usually occurs between the hydraulic ram base and the tab itself. You should inspect yours, and always make sure that the components of your trim tabs are well bedded in a waterproof compound (3M 4200 or 5200 or similar adhesive). Don't trust silicone for this critical task.

Another piece of equipment that we see suffering from crevice corrosion is your watermaker high-pressure pump and high-pressure fittings. This is due to inadequate fresh water flushing of your desalinator. Would you believe that some watermaker manufacturers still build systems with no easy method for fresh water flushing?

The potential for crevice corrosion is why I personally prefer Titanium Alloy or Nickel Aluminum Bronze (NAB) for high pressure watermaker pumps.

To learn more about crevice corrosion, please see this Wikipedia article on crevice corrosion.

Happy cruising!
Tim

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