Galvanic or Electrolytic Pit Corrosion Propeller

[FlyingCloud1937]

Aluminum anodes for brackish water, or for a boat that spend part time in salt and part time in fresh, or brackish water.

Anode FAQs

Lloyd

Originally Posted by Hoofsmit

Technically I'm sure you are correct, but we are trying to figure out the loss of material on a prop, not make a battery ! Or trying to electro plate our through hulls!
So do you think it is being caused by a stray current or poor quality prop material or ' bad' anodes?
I known magnesium anode mix is used in fresh water for anti galvanic corrosion protection and zinc rich in salt water.
I moored in a estuary where the salinity varied a lot dependant of rain full and tidal state and had both fitted for a season, the zincs sacrificed more than the mags.

[Hoofsmit]

Great link, explains a lot about the negative current impact on anodes and metals that need protecting

Thanks

[westwinds]

Goudurix, have you changed engines or propellers on your boat?

[JPA Cate]

Goudurix,

If you have a bronze prop, and there is pink metal surrounding the pits, then you have had de-zincification on your prop, and this is caused by stray current. Could be poor wiring in the marina, or could be from your own boat.

I'm not an expert, but when I read how many zincs you're using, I wondered whether you are over-zinc-ed, which can cause problems, as well.

Perhaps the suggestion to find a qualified marine electrician to survey your boat for stray current is not a bad idea. You, yourself can check the electrical potential in various berths at your marina. This can be educational.

Good luck with it. i hope your prop's okay.

Ann

[Goudurix]

Thanks fot all your replies so far!
I am aware of the difference between galvanic corrosion/currents and stray current corrosion.

Unfortunately too late to send any pics of the prop; I put the boat in the water Saturday.

I heard about "over-zincing"; I have never measured the cathodic protection voltage of my system for which I would need a silver electrode.
The reason why I added 2 zinc discs to both sides of the propeller shaft strut, is that some years ago I observed purple-ish coloring of the strut, after I had connected the SSB auto-tuners RF ground to this bronze strut. There was already a copper mesh connected to it from the engine ground, thus serving also as the boat's (12V system) ground. Fearing problems due to the SSB (questionable) I did add the 2 bronze discs and put in high voltage capacitors (1 microF in total) between the ATU and the bronze strut to block any DC and ground loop voltage differences.

The zincs are branded and expensive.

As for the prop: there was no purplish coloring around the small pits. There was however, as every year, a difficult to remove calcified crust on the blades, indeed it made me think about barnacles - but could they cause the pitting?

As for the dock power system ground, I checked any leaks between this ground and the battery chargers output and 12V negative: infinite resistance. Tested for any faults in the 220V circuit of the water boiler - it seems OK.

There is definitely no connection between the dock power ground and ships ground or the engine (...I know...European rules not as stringent...) so I suspect adding in a galvanic isolator or isolation transformer would be unnecessary.

Jan

[Celestialsailor]

Originally Posted by Cheechako

How about the SSB ground plane... could it be getting to the prop somehow?

I did notice hauling out this time that where I placed my copper plate outside the boat, had a lot of bottom paint corrosion around it. Even though it was connected to a zinc. I am eliminating the copper plate for a KISS system to cure this.

[senormechanico]

I installed a thin sheet of vinyl (to act as an insulator) beneath the zinc anode for all the bonding system and another one between the hull and keel cooler for the fridge. No paint burns.

[Celestialsailor]

The paint burns I have go 12"s all around the 4"X10" copper plate.