Corrosion on prop

[morgandlm]

I beached the boat to scrub the hull and clean the prop last Saturday - an annual job. This year the prop was not just a bit weedy but had a number of things that looked like scabs which on scraping away left pits in the bronze. The prop is a standard fixed two blade bronze one and is the original from the late 80's. The shaft anode is relatively new and is eroding away "normally". I have cleaned and polished the prop as usual and will inspect again on lift - out end October or November. The "scabs" looked like some sort of corrosion and I'm sure were not animal. Non scabby surfaces of the prop were black - this black came off with polishing. Sadly, I had no camera with me.

I've never found anything like this before in previous similar operations - any ideas?

Grateful for any advice / comments

David Morgan

 

[Tranona]

Usually corrosion (dezincification) affects the tips of the blades first, going pink in colour and brittle. When the boat is out, tap the prop with a small hammer and if it rings it is OK, if it sounds dull then it is suspect. Take the prop off and put it in brick cleaner which will remove all the nasties growing on it so you can better assess its condition.

 

[Halo]

Could the scabs be pitting caused by cavitation ? Have you been motoring hard or do you have an uprated engine?
I suggest painting the scabs with Hamererite special metals primer. On one of my props this seemed to resist futher pitting very effectively.
The only other thing that comes to mind is earth leakage causing corrosion- do you have a charger running ?

 

[ex-Gladys]

Sounds like the after effects of barnacles, where they glue themselves on

 

[coreng]

Hard to say without zoomed pics. What do you mean by eroding "normally" ? Because from the corrosionist point of view, normally is nil.

 

[VicS]

Hard to say without zoomed pics. What do you mean by eroding "normally" ? Because from the corrosionist point of view, normally is nil.

Not so .
A zinc anode bonded electrically to a nearby big chunk if bronze, or more likely manganese bronze, and immersed in the same pool of seawater is going to be gradually dissolved / eroded/ corroded, or what ever word you prefer to use

That is the basic electrochemistry at work and in this case there is emf of about 0.7 volt driving the process which we call "galvanic corrosion"

 

[coreng]

My mistake : I missed the word "anode" !!!