Galvanic or Electrolytic Pit Corrosion Propeller

[Goudurix]

Hi,

upon haulout of my sailing boat I observed a large number of small pits on the 3 propeller blades, mainly the backward facing blades.

It is a european boat (Jeanneau): all through hulls are unbonded. 12V ground is the engine block. This has 1 copper mesh connection to the bronze (underwater) propeller shaft strut.
Zinc anodes:
1 ball anode on propeller shaft close to the propeller
1 anode on the propeller nut
2 zinc disc anodes both sides of the bronze propeller shaft strut (that I added when connecting also the RF-ground of my SSB's antenna tuner to this strut => this connection has DC-blocking capacitors 1 microF and the SGC tuner is reported to already have DC-blocking caps in its RF ground lead).

All this anodes have well worked after 12 months season: the ball anode was well eaten away (time to replace) - 1 of the disc anodes well eaten away - the anode on the propeller nut has worked a bit but still in good shape (will replace anyhow).


It seems to me the anodes work well to prevent galvanic corrosion.

Hence I suspect electrolytic corrosion? Due to straw currents?

A guy doing the maintenance of charter sailing boats advised:
1) cut the ground lead of the shore power cable (WHICH I WOULD NEVER DO!!!!!!)
2) connect a heavy duty copper wire to the engine block negative, hang overboard with a heavy zinc anode conncted to it when in the dock stationary and on shore power (doubtfull? but maybe useful in case of straw current issue?)

European boat => the shore power ground is NOT connected to the ships 12V DC ground in any direct way.
I could only suspect 2 devices:
a)the shore power battery charger (it is a modern Mastervolt HF charger) - does it connect the shore power ground to the 12V DC ground?
b) the water boiler also has a 220V heating system (used very seldom so it is alwas physically plugged pout) - could it be faulty without tripping or connect the shore power ground to the engine block via the cooling fluid that runs through the boiler to heat water when running on engine?

Please your advices?

What could I measure?

(before maybe deciding to invest in an isolation transformer that maybe is unnesseceary in my case)

Jan

[FlyingCloud1937]

Pit corrosion, on the prop is indicative of cavitation corrosion.

Corrosion of underwater metal is a hard thing to diagnose over the internet.

If no other underwater metals are showing sign of corrosion.

Then suspect cavitation.....else cheap material in the prop. casting.

Lloyd

corrosion

Originally Posted by Goudurix

Hi,

upon haulout of my sailing boat I observed a large number of small pits on the 3 propeller blades, mainly the backward facing blades.

It is a european boat (Jeanneau): all through hulls are unbonded. 12V ground is the engine block. This has 1 copper mesh connection to the bronze (underwater) propeller shaft strut.
Zinc anodes:
1 ball anode on propeller shaft close to the propeller
1 anode on the propeller nut
2 zinc disc anodes both sides of the bronze propeller shaft strut (that I added when connecting also the RF-ground of my SSB's antenna tuner to this strut => this connection has DC-blocking capacitors 1 microF and the SGC tuner is reported to already have DC-blocking caps in its RF ground lead).

All this anodes have well worked after 12 months season: the ball anode was well eaten away (time to replace) - 1 of the disc anodes well eaten away - the anode on the propeller nut has worked a bit but still in good shape (will replace anyhow).


It seems to me the anodes work well to prevent galvanic corrosion.

Hence I suspect electrolytic corrosion? Due to straw currents?

A guy doing the maintenance of charter sailing boats advised:
1) cut the ground lead of the shore power cable (WHICH I WOULD NEVER DO!!!!!!)
2) connect a heavy duty copper wire to the engine block negative, hang overboard with a heavy zinc anode conncted to it when in the dock stationary and on shore power (doubtfull? but maybe useful in case of straw current issue?)

European boat => the shore power ground is NOT connected to the ships 12V DC ground in any direct way.
I could only suspect 2 devices:
a)the shore power battery charger (it is a modern Mastervolt HF charger) - does it connect the shore power ground to the 12V DC ground?
b) the water boiler also has a 220V heating system (used very seldom so it is alwas physically plugged pout) - could it be faulty without tripping or connect the shore power ground to the engine block via the cooling fluid that runs through the boiler to heat water when running on engine?

Please your advices?

What could I measure?

(before maybe deciding to invest in an isolation transformer that maybe is unnesseceary in my case)

Jan

[Hoofsmit]

Hi , sorry no expert , but I have experienced the same issue,
I think personally you can rule out cavitation as this tends to be be on higher rev props on planning craft.
As I understand it, galvanic corrosion is from dissimilar metals with the saltwater being the electrolyte ( battery acid)
Where as electrolytic corrosion is caused by stray currents,
So looking at it from a school boys view ( me singular brain cell) the cathode ( prop) is being oxidised so ... So my bet It's a stray current coming from you boat..... I'm sure I will be corrected..... I'm hoping to learn a bit here !

[westwinds]

Originally Posted by Hoofsmit

Hi , sorry no expert , but I have experienced the same issue,
I think personally you can rule out cavitation as this tends to be be on higher rev props on planning craft...

Electrolysis causes a shiny surface. Dave Gerr in his Propeller Handbook on page 43 states that cavitation causes pitting from vacuum bubbles imploding against the propeller metal. The force of this actually sucks the metal off. Cavitation also causes vibration very like an unbalanced propeller. His book also goes on for many pages about determining the proper size and pitch necessary to prevent cavitation on propellers used with displacement hull boats.

By the way, the best way to prevent electrolysis is with an isolation transformer.

[Andina Marie]

But if you can't stand the cost, weight and space of an isolation transformer a $99 galvanic isolator is the first line of defense.

[donradcliffe]

Originally Posted by FlyingCloud1937

Pit corrosion, on the prop is indicative of cavitation corrosion.

Corrosion of underwater metal is a hard thing to diagnose over the internet.

If no other underwater metals are showing sign of corrosion.

Then suspect cavitation.....else cheap material in the prop. casting.

Lloyd

corrosion

Cavitation occurs on the low pressure side of the blade, which would be the forward facing surface.

[westwinds]

Originally Posted by FlyingCloud1937

...Corrosion of underwater metal is a hard thing to diagnose over the internet...

Goudurix, can you get a picture of the propeller, several different views?

[boatpoker]

First of all be cautious of advice from anyone using the term "electrolysis" as it suggests an incomplete understanding of the terminology and therefore the issues at hand.

Cavitation corrosion is virtually unheard of at the speed and depth your propeller is turning at.

Your propeller is most likely manganese bronze which is actually a brass with up to 40% zinc content and these can suffer from dezincification caused by galvanism. Cheaper (Chinese) propellers have at best ... uncertain metallurgy and have been know to corrode themselves due to impurities in the metal. Likewise cheap offshore anodes have the same problem.

Your problem is almost certainly not caused by stray current as that type of corrosion occurs at a much faster and dramatic rate than galvanic corrosion.

I'd suggest you buy "brand" name anodes made anywhere but the far east and install a galvanic isolator (the safe way of cutting your shore ground connection).

[senormechanico]

Originally Posted by westwinds

By the way, the best way to prevent electrolysis is with an isolation transformer.

Or solar panels...

[Cheechako]

I think cavitation will show on the blade edges a lot. If no missing material on the edges I suspect another source. How about the SSB ground plane... could it be getting to the prop somehow? How bad is the pitting? maybe the boat sat a lot and corroded from barnacles etc and was subsequently cleaned?

[CharlieJ]

Some marine corrosion factoids:
1. Galvanic corrosion is a very slow process.
2. Stray current corrosion is a very, very fast process dependent only on the forcing voltage. A propeller can be completely eaten away by stray current at B+ potential in a few days time.
3. Corrosion problems are difficult to analyze when on the vessel experiencing the problem and virtually impossible to analyze by long distance.
4. Electrolysis is the dissociation of water into hydrogen and oxygen when a strong electrical current is passed through the water. It has nothing to do with the loss of underwater metal bits.

Post a picture of your propellers and other parts of the running gear and we may be able to help.

[FlyingCloud1937]

Cavitation: Cavitation sometimes is considered a special case of erosion-corrosion and is caused by the formation and collapse of vapor bubbles in a liquid near a metal surface. Cavitation removes protective surface scales by the implosion of gas bubbles in a fluid. Calculations have shown that the implosions produce shock waves with pressures approaching 415 MPa. The subsequent corrosion attack is the result of hydro-mechanical effects from liquids in regions of low pressure where flow velocity changes, disruptions, or alterations in flow direction have occurred. Cavitation damage often appears as a collection of closely spaced, sharp-edged pits or craters on the surface.

Electrolysis(aka;Electrolytic Corrosion) and Galvanic Corrosion are frequently confused. Galvanic corrosion is caused by two dissimilar metals being in contact with each other, in the presence of an electrolyte, such as seawater. Electrolysis is caused when an external current, called a stray current, finds a path between two metals in the presence of an electrolyte. The two metals may be of exactly the same type or different types.



How to use the galvanic scale

The potential difference (volts) can easily be calculated by reference to the scale. Let's take a fairly extreme example – lead and magnesium. The first is rated at about -0.2V and the second at about -1.4V . Take one from the other and we get the potential difference between them, namely: 1.2V which signals quite a lot of galvanic activity.




As I originally posted it's hard to due over the internet. Find a local Marine Electrician versed in corrosion survey.

Lloyd

[FlyingCloud1937]

Originally Posted by CharlieJ

Some marine corrosion factoids:

4. Electrolysis is the dissociation of water into hydrogen and oxygen when a strong electrical current is passed through the water. It has nothing to do with the loss of underwater metal bits.

Post a picture of your propellers and other parts of the running gear and we may be able to help.

That's one process.

It can and is also used to create hi purity metals by separating the different constituents, by control of the voltage....intentional corrosion. Or be used to electroplate.

Electrolysis is the electrochemical process intentionally or by acidental/stray dc current.

Lloyd

Electrolysis is commercially highly important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell.

Electrolysis is the passage of a direct electric current through an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes and separation of materials.
The main components required to achieve electrolysis are :

• An electrolyte : a substance containing free ions which are the carriers of electric current in the electrolyte. If the ions are not mobile, as in a solid salt then electrolysis cannot occur.
• A direct current (DC) supply : provides the energy necessary to create or discharge the ions in the electrolyte. Electric current is carried by electrons in the external circuit.
• Two electrodes : an electrical conductor which provides the physical interface between the electrical circuit providing the energy and the electrolyte

Electrodes of metal, graphite and semiconductor material are widely used. Choice of suitable electrode depends on chemical reactivity between the electrode and electrolyte and the cost of manufacture.

[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]

Just on a note
Aluminium props lasted 3 seasons, stainless 7 with no affect at all.