[gbgreen59]

My boat is in fresh water, Lake Michigan. Can anyone tell me what the corrosion is as seen in the attached pictures? Does this look like galvanic corrosion. I had the boat yard put on Magnesium sacrificial anodes prior to launch last year. I can rum my finger across it and some comes off.

Should I remove this stuff with sandpaper or anything else like that? How can I prevent it from coming back or getting worse?

Thanks for the help.

 

[RichH]

Are there 'pits' into the metal underneath this accretion when you clean it?

If no, then the strongest possibility is simply bacterial/microorganism, etc. colonies and/or the 'excretions' of such colonies growing on the shafting. This happens a lot to bare metal underwater surfaces in fresh water, especially where 'slime', and other 'soft growth' etc. buildup is heavy.

IF you see that the actual surface is being attacked/etched by such growth, then consider to use a bottom paint on the shaft (one that is compatible with the metal shafting) to prevent 'whatever' is attaching and growing and is using the metal surface as a nutrient source.

 

[gbgreen59]

Are there 'pits' into the metal underneath this accretion when you clean it?

If no, then the strongest possibility is simply bacterial/microorganism, etc. colonies and/or the 'excretions' of such colonies growing on the shafting. This happens a lot to bare metal underwater surfaces in fresh water, especially where 'slime', and other 'soft growth' etc. buildup is heavy.

IF you see that the actual surface is being attacked/etched by such growth, then consider to use a bottom paint on the shaft (one that is compatible with the metal shafting) to prevent 'whatever' is attaching and growing and is using the metal surface as a nutrient source.

Thanks, this makes sense. There is quite a bit of slim where I am. There's no pitting; comes off pretty easily. I noticed, in the boat yard, that quite a few have this issue. And, some don't. Maybe I'll ask the boat yard if there is a product I could use.

 

[aloof]

Sandpaper. Monthly. 120 grit wet/dry. You are not going to hurt the shaft even with 20 years of sanding.