Question about Grumman aluminum canoe corrosion

Hello, long-time lurker on these boards, but registered since I was planning to buy a 15 foot Grumman aluminum canoe for $200 (my first canoe), and wanted to get some repair and maintenance advice.

The canoe was used and stored upside down on a saltwater dock for the past 3 years (prior to that, freshwater only). The seller points out that there is some surface corrosion on the inside aluminum (see pic of corrosion around serial number plate (as newbie, I think I’m only allowed to post a max of 1 picture…other pics show similar surface corrosion on inside only)). Otherwise, the canoe appears in decent condition. The seller says there is no corrosion/pitting on the outside aluminum, and that the canoe does not leak.

So my question is once I purchase it, how should I clean it to remove the inside corrosion and resultant pitting, and then protect it, so that the corrosion/pitting does not progress through the inside aluminum to the outside hull of the canoe (aluminum thickness is 0.050 based on Grumman’s metal plate). I’m fine with aesthetic imperfections, but just don’t want the pitting to progress to leaking holes. Thanks in advance for any suggestions and advice.

I work with aluminum for a living, you can use aluminum brightener to remove most of the oxidation, but you’ll want to do small areas at a time. Other option is to use to 180 grit sandpaper to take off the high spots, spray it with an etching primer, then paint it. Oxidation is a pain to remove with sandpaper, being as aluminum oxide is the primary abrasive in most sandpaper. You might want to just pressure wash it and call it good.

Thanks auronotcs for your knowledge and suggestions. From reading other postings, my understanding is the smooth gray oxidation layer is protective, so if that is the case, I don’t mind that. What worries me is the whitish powdery corrosion, since I’m thinking that can’t be protective, and beneath it, are pits which probably still have some salt residue embedded (from the seller having exposed it to salt water), and which may further deepen the pits if left alone untreated.

I was wanting avoid painting it, since many postings mention that paint adherence isn’t great with aluminum which I figure is why you suggested using an etching primer. But then the painting (vs leaving bare aluminum), would require future maintenance :frowning:. But I like your pressure washing idea and then calling it good. However, would just pressure washing get down into the pitting to remove any residual salt water residue that might cause future ongoing corrosion? Would it be common for this type of pitting to actually progress to the point where it would breech the hull and cause leaks, if properly cleaned and future salt water exposure is avoided?

Your pressure washing suggestion makes me think that maybe I should pressure wash it, and then use some 180 grit sandpaper to remove the high spots, and then re-pressure washing it again to try to get into the pitting crevices might be the best way to arrest any further pitting damage. I just didn’t want to buy the canoe, and then a few years later, start having problems with pinhole leaks from the inside pitting progressing to the outside of the hull…maybe I’m worrying too much. :grinning:

I’m not exactly sure if that would arrest the corrosion or not, but I would bet that it would. Most aluminum won’t corrode to failure, and its used extensively in salt heavy environments. The prior owners must have let salt crystals form on the aluminum itself. The only aluminum I’ve seen fail was a masterlock that had stacked aluminum sheets located inside of an automatic carwash… It was exposed to a sodium hydroxide based soap day in and day out, literally turned to powder when I touched it.

For aluminum to rust it has to be in contact with another metal, which causes galvanic action. If you remove the lumps of corrosion give it a good mild acid wipe down to react with any errant sodium, rinse with freshwater and it should be good to go.

There I go again nerding out about aluminum, but that’s my two cents.

Thanks auronotcs for your very helpful insight. I’m appreciating your “nerding out” about aluminum, as it is very helpful, since I like to understand why things work.

Based on your extensive experience working with aluminum, I’m reassured that the soon-to-be my Grumman aluminum canoe should last many years.

I agree that the prior owner probably had salt crystals form on the aluminum, which I think contributed to the formation of the white corrosion deposits…I’m guessing this is what is called “poultice corrosion”. I don’t really know much about poultice corrosion, but had read something about it, where poultice corrosion occurs when moisture affects the unprotected metal alloy, and results in the whitish powdery poultice material (which is said to be aluminum hydroxide). From my reading, the poultice further traps moisture, resulting in a vicious cycle of more poultice formation. The mechanism of the corrosion is said to be from (1) the poultice limiting oxygen to provide the protective gray oxidation; (2) formation of a more ACIDIC pH due to increased concentration of negatively charged Chloride ions (accelerated by the saltwater, NaCl) with the retained moisture, in combination with the positively-charged aluminum (and water), creating an acidic environment which further increases the corrosion/pitting and more poultice formation. The negatively-charged Chloride ion supposedly gets deep into the aluminum crevices/pits since it is attracted to the positively-charged aluminum.

Now this all sounds good, although I don’t remember my college chemistry well enough to really know if it is correct.

But power washing and cleaning off the poultice makes alot of sense, since it will increase the oxygen exposure to the aluminum for the protective gray oxidation. With the acidic environment causing poultice formation, would a mildly basic solution be better than a mild acid wipe?..again, my college chemistry was too long ago. :frowning_face: Drying well seems most important.

I did read about people treating their aluminum boats with something called ACF-50 (“penetrates corrosion deposits, where it chemically emulsifies and displaces the electrolyte”) or Corrosion Block, to protect after cleaning and drying. Have you had any experience with either of these? Thanks very much for your “nerding”…I like it! :+1:

My understanding of it is that the corrosion has an alkaline ph. It forms aluminum hydroxide. Acid can damage aluminum, but the anodizing process actually involves dunking the aluminum in acid and passing an electrical current through it causing the surface molecules to align forming a thin layer of aluminum oxide which is incredibly resilient.

All of the shining/cleaning/corrosion removing chemicals I’ve seen for aluminum are acidic. I’m fairly certain that acid is the way to go on this.

Sounds great. I really appreciate all your time and patience helping me. And nothing beats knowledge and experience, which you seem to have oodles of from working with aluminum. So many thanks!..especially for a $200 canoe! But at $200, it’s still my “new” (used) canoe, and so even used, I’m going to have to baby it for at least a few seasons until it becomes my “used” used canoe, and then it may become like everyone else’s used aluminum canoe which sits on support stands behind the house, with weeds growing around it (or maybe not). :grin: Thanks so much again auronotcs!

When that happens you can get a few bucks for scrap Aluminum.

Lol, @ string…I’m hoping I enjoy many many years of the Grumman, and then when I’m really old, I’ll pass it down to my nephew, since everyone says that the Grumman canoes are built to last, although I can see him selling it for scrap. I guess by that time, any corrosion and poultice holes won’t matter. Of course, I could be the old eccentric neighbor that has this ugly monstrosity out in the very front of his yard, using it as a planter…after all, with the corrosion holes, it now has a place for the water to drain out if used as a planter! :wink:

I have the same problem, but more advanced. The corrosion on my 18-footer has gone right through the hull, which manifests as darkened patches of bumpy aluminum. I can poke a finger through in a couple of spots.
Very occasionally I paddle the boat in salt water, but I always rinse it thoroughly. Otherwise it is exclusively used in freshwater and stored in the garden, a long way from saltwater. Do I need a zinc puck to prevent this? NewbieCanoebie have you made any progress with your issue? I also have the white powdery corrosion.

Try white vinegar. Autobody shops are having to train people to deal with aluminum with the new cars I read. Once corrosion starts it’s hard to paint over properly.

I use Alumabrite on my drift boat.