The authoritative public forum
  for Metal Finishing since 1989
-----

Aluminum corrosion on boat lift beams sitting on pressure treated pilings

Help -- our aluminum Decco boat lift beams are sitting on pressure treated pilings in a salt water canal off of the Intracoastal waterway in Jacksonville Florida. Something that looks like white puss or soap is eating away at the metal. There is a half moon about 1/2" in the back beam. Does anyone know what is causing this and what we should do?

Dan Stephens
hobbyist - Jacksonville, Florida
2007

---

2007

There are various treatment chemicals used in wood preservation, but "tanalith" is one that's common in some countries, and its acidic when wet.
Aluminium is resistant to corrosion mostly because it forms an oxide layer that's tightly adhered to the metal, and this protects the metal underneath.
But apply some acid or alkali and in wet conditions, and in air, and you'll get corrosion.
Corrosion conditions will produce alumina paste, a white sludgy, pasty looking substance, and this sounds like what you've got. Alkalis will dissolve the aluminium, acids will etch it. Both mean converting metal to oxide.
Solution: remove the acid/alkali, or insulate the aluminium from it. Is it possible to move the beams elsewhere? If not use some untreated wood to separate the metal from treated wood.
If you cannot separate them, then either wash them very often to rid accumulations of chemical, or completely dry the point of contact, and keep it very dry.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

This is a very common problem when aluminum is in direct contact with treated wood. The solution to the problem is to put isolation material between the aluminum and treated wood. This isolation material is typically thick plastic sheeting. Material sold as piling wrap works well when cut to fit. Anything will work as long as it is electrically nonconductive and able to withstand the rigors of "dock life".

Erik Nelson
- St. Petersburg, Florida
July 16, 2008

---

You can place any rubber between the lift and piling.
The piling is 2.5 CCA (google) and the lift is aluminum then you have salt water (creates a battery) However it sounds like you have a electrical problem which could end up hurting someone. I would have a electrician check it out.

Mike Rice
aluminum boat lifts - Pensacola, Florida
November 19, 2009