[mpn1cxe]

I have an aluminum boat with a block of plywood enclosed in the transom, to support the motor.  The plywood has become soaked with salt water and the chemical reaction of the plywood, salt water and aluminum is eating holes in the aluminum of the transom.  A white paste-like substance oozes out.  I have cleaned and patched the holes, but they continue to appear.  It is not practical to remove and replace the wood.  I want to find a chemical to neutralize the corrosion process.  It is not electrolysis.  The paste-like substance is neutral.  I would appreciate any help. I attached a picture.

[xiankai]

paint the aluminium metal. the paint will provide a protective covering.

im not sure why aluminium decays, tho. the Al2O3 layer should be preventing further corrosion.

[eugenedakin]

Hello mpn 1cxe,

Xiankai has a very good idea, paint, or epoxy a protective covering to prevent further corrosion.  The white paste-like substance is most-likely aluminium chloride.  The reaction is electrolysis at the molecular level.  If electrolysis did not occur, neither would the corrosion reaction.  The Al2O3 protective layer does slow down the corrosion reaction, but does not stop it completely.  Before painting or applying epoxy, ensure that all materials are completely dry and salt-free.  This will minimize corrosion from increasing its rate one the cover is applied.

Any mechanical motion will eventually remove the protective Al2O3 layer and will expose 'fresh' aluminium metal, which can form aluminium chloride.  The Al2O3 layer can be as thin as 1 molecule.  With a layer this thin, even metal stress from the rocking motion of the boat will expose Aluminium metal.

Best of luck with your boat,

Eugene Dakin Ph.D., P.Chem.

[mpn1cxe]

Dr. Dakin,
Thank you fro the response.  However, I don't have access to the aluminum where it meets the wood.  The Transom is a block of plywood about 2" thic, sandwiched between the aluminum.  I have tried plugging the holes created by tje a;uminum chloride, and drilling into the plywood and filling with Gitrot (a product made for rotting marine wood).  I was hoping for a chemical that would neutralize the aluminum chloride, so I could attempt to seal the wood and keep it dry.     Any help would be appreciated.  I attached a picture  Chuck Edwards  chuckedw@verizon.net

[eugenedakin]

Hello Mr. Edwards,

Unfortunately, once aluminium chloride has been formed, I know of no economical methods to convert it back to solid aluminium.  The easiest method to lower the current corrosion is to: clean and rinse the area where the aluminium and wood meet with distilled water (has low chloride levels), and allow it to dry and remain in contact with atmospheric air (to form a protective aluminium oxide layer).  Once clean, an epoxy or paint can be applied to signifacently lower corrosion rates.

Replacement of corroded aluminium can occur by aluminium welding (if you know someone who can weld aluminium back into the spaces where corrosion has taken place), but this is a time-consuming operation.  Electroplating can be performed, but will usually provide a small deposition of aluminium around the entire boat.

I hope this helps,

Eugene