Removing corrosion from prop and repainting

[desmobob]

I take care of a small aluminum boat for a couple of female friends.  The boat (14' Duranautic w/15HP Suzuki four-cycle) doesn't get used much.  Last season, it sat in the water under a covered boathouse with the engine tilted up part-way so that the sacrificial zinc block on the motor was out of the water, but the prop and bottom of the lower unit were in the water.

 

The prop and skeg ended up with white powdery corrosion of some sort on the edges and bubbling up through the paint in some spots. I'd like to remove it and repaint the aluminum prop.  Should I use something acidic or basic to remove the corrosion?  Or just the wire wheel on my grinder?  Is there a specific paint I should use to repaint the prop?  Is a special primer needed?

 

If it was my boat, I'd just wire brush it off and hit it with a rattle can.  These two gals are perfectionists/neat freaks, so I want to try and do the best job possible.  ?  Thanks for any advice.  

 

Bob

[WRB]

The white corrosion products are aluminum oxides from the aluminum. 

Tom

[desmobob]

20 minutes ago, WRB said:

The white corrosion products are aluminum oxides from the aluminum. 

Tom

Thanks.  I did some searching... Aluminum oxides have some interesting properties, one of which is that it can act as an acid or a base.  I found that aluminum oxides are soluble in weak citric acid (which I have a bunch of).  I'll try that for dissolving any remaining residue after some wire brushing.

[WRB]

The question is why did corrosion start in fresh water? The reasons are; electrical power in the water from dock lights or the boat battery being on using the ground as the engine or acid from the exhaust fuel residue. It's also possible weed abatement using acid to kill aquatic weeds may have contributed. Shouldn't have severe galvanic corrosion leaving the engine in the fresh water even with the zincs dry.

Removing the paint to bare aluminum, rinsing with distilled water and primer coat with brush Alodine then painting should resolve the corrosion issues depending on the severity of the corrosion.

Good luck,

Tom

 

[desmobob]

There is no battery in the boat and no power at the dock.  The lake is AAA quality clear, clean water with no chemical treatment in that area of the lake (Lake George NY).  I've seen the same issue with a boat I used to keep docked in that lake.

 

Thanks for your help,

Bob

[Jig Man]

You might be seeing calcium deposits.  Try some hydrochloric acid (toilet bowl cleaner will work) in a small area and see if it helps.  Around here we get lots of it.  I use it on my boat and motor regularly to keep it off.

[desmobob]

21 hours ago, Jig Man said:

You might be seeing calcium deposits.  Try some hydrochloric acid (toilet bowl cleaner will work) in a small area and see if it helps.  Around here we get lots of it.  I use it on my boat and motor regularly to keep it off.

I'm thinking that' probably not it.  I'm in the Adirondacks in northeastern NY.  We have very little limestone in our area and that's why our mountain lakes and ponds have been harmed so badly by acid rain in the past.  Areas with water running through limestone have lots of calcium to help stabilize the pH.

 

That said, I must report that the highest level of chemistry I studied was in high school, and I passed by the skin of my teeth.   ?    You might be absolutely correct.

[slonezp]

Oxidation is metal trying to return to its natural state. Water and oxygen are a metals worst enemy.

Clean of the prop with a wire brush. Use a self etching primer and enamel to refinish.

[WRB]

21 hours ago, desmobob said:

There is no battery in the boat and no power at the dock.  The lake is AAA quality clear, clean water with no chemical treatment in that area of the lake (Lake George NY).  I've seen the same issue with a boat I used to keep docked in that lake.

 

Thanks for your help,

Bob

Aluminum doesn't corrode in fresh water without some type of electrolyte creating galvanic dissimilar metal reaction. The boat is aluminum? I am sure the hull isn't corroding. Aluminum oxides with good air circulation protect the metal from corrosion. 

The white aluminum corrosion products indicate the metal is eroding and if not mitigated will continue to erode.

You have eliminated every cause of galvanic reaction except exhaust acids, very unusual to have excessive HNO2 build up of exhaust products. I would think after cleaning and painting the problem will be resolved. A very good corrosion sealant for aluminum is ethylene glycol or anti freeze.

Good luck,

Tom