Understanding Galvanic Corrosion - help

[skyflyer]

Our P bracket is bronze and is bolted to a pocket in the underside of the hull which is then faired in. In other words it is electrically isolated from the remainder of the boat.

On lift out this year we noticed where the anti foul had flaked off that it had gone a pink colour but following an earlier posting on here we gave it a quick rub over with some emery cloth and the yellow bronze was very rapidly restored.

Is it possible that the de-zincification has extended below the surface where it cannot be seen or can it ONLY occur where the metal is in physical contact with the electrolyte - in other words does corrosion have to take place from outside to inside or can it occur the other way.

How can I test the integrity of the P bracket without removing it?

 

[Billjratt]

Not sure about your 'integrity' question, however, when you say it's isolated - are you sure there isn't a method of connecting a ground wire to the inside of the fixing bolts?
The fact that you could polish off the depleted bronze indicates to me that it hasn't gone too deep-yet.. From it's mounted position, can it "see" an anode? Does the anode erode healthily?

 

[Tranona]

If it is isolated there is no chance of galvanic action with any other piece of metal on the boat. You do not say what the bolts are made of, no whether they are exposed to water, so am assuming they are the same material or completely encapsulated in the fairing. The dezincification will simply be the loss of the zinc content of the alloy leaving just copper - hence the pink tinge. Pretty sure this occurs from the outside in - as you have found the copper is only on the surface. Difficult to determine the extent of the dezincification by non destructive means. The usual signs of failure are bits breaking off (for example the thin ends of prop blades) or sounding "dead" - bronze type alloys ring if hit with a hammer, copper gives a dull thud. Unfortunately neither of these tests are easy or reliable on a casting like a P bracket because the casting itself is bulky so does not crumble at the edges and being solidly fixed the ringing sound gets absorbed.

 

[JumbleDuck]

If it is isolated there is no chance of galvanic action with any other piece of metal on the boat.

Brass woodscrews can and do dezincify on a boat, even though they are not connected to any other metal. I think it may be small scale electrolysis between grains of different composition which drives it, but I am not an electrochemist.

 

[VicS]

Brass woodscrews can and do dezincify on a boat, even though they are not connected to any other metal. I think it may be small scale electrolysis between grains of different composition which drives it, but I am not an electrochemist.

Yes pretty much what happens to brass. Depends largely on the alloy ITYWF. Low zinc content alloys are a homogeneous single phase while higher zinc content alloys consist of two phases. I believe the latter are more susceptible to dezincification due to interaction between the phases.

A peek at the copper zinc phase diagram will explain.

Note true bronzes are alloys with tin ,not zinc, so do not dezincify.

 

[JumbleDuck]

Yes pretty much what happens to brass. Depends largely on the alloy ITYWF. Low zinc content alloys are a homogeneous single phase while higher zinc content alloys consist of two phases. I believe the latter are more susceptible to dezincification due to interaction between the phases.

A peek at the copper zinc phase diagram will explain.

Note true bronzes are alloys with tin ,not zinc, so do not dezincify.

Many thanks. I hoped the tame electrochemist would come along! A shufti at Vyv Cox's web site suggest that there ain't much true bronze around these days, alas.

 

[jwilson]

Our P bracket is bronze and is bolted to a pocket in the underside of the hull which is then faired in. In other words it is electrically isolated from the remainder of the boat.

It cannot be electrically isolated as when in water (particularly seawater) there is a conductive path to other bits of metal, typically a S/S prop shaft, bronze prop, and any anodes nearby - usually at least one either on the propshaft or on the prop nut.

Whilst not technical, my experience is that brasses and some 'bronzes' dezincify from the outwards in - ie if a light sanding takes off the pink the rest will still be sound. Used to be normal to have wires inside bonding all metal hull fttings together - this has now changed and current advice is usually not to bond thus.

 

[macd]

we gave it a quick rub over with some emery cloth and the yellow bronze was very rapidly restored.

True bronze (cf Vic's post above) is not yellow, or even yellowish. Sounds like you have something else.

 

[JumbleDuck]

It cannot be electrically isolated as when in water (particularly seawater) there is a conductive path to other bits of metal, typically a S/S prop shaft, bronze prop, and any anodes nearby - usually at least one either on the propshaft or on the prop nut.

One path is not enough. Stick a copper electrode and a zinc one into a lemon - nothing interesting happens until you link them with a wire as well.

 

[VicS]

It cannot be electrically isolated as when in water (particularly seawater) there is a conductive path to other bits of metal, typically a S/S prop shaft, bronze prop, and any anodes nearby - usually at least one either on the propshaft or on the prop nut.

Whilst not technical, my experience is that brasses and some 'bronzes' dezincify from the outwards in - ie if a light sanding takes off the pink the rest will still be sound. Used to be normal to have wires inside bonding all metal hull fttings together - this has now changed and current advice is usually not to bond thus.

For galvanic corrosion to occur there must be a complete circuit. Through the water to another item is only half of a circuit. Ther emust be an electrical connection between the two to complete the circuit .

An analogy is one of your DC circuits**. Connecting a bulb to the positive terminal of the battery will not cause the bulb to light. The circuit must be completed by a connection to the battery negative.

This is why anodes have to be bonded to the items they are fitted to protect. No bonding = no circuit = no protection


** or Dumble Jacks lemon

 

[bluerm166]

Is this activity perhaps related to the use of a copper based antifouling which Jefa Rudders ( http://www.jefa.com) in their useful technical sections point out as providing electrical continuity over the whole hull,and onto metal items onto which it is lapped ? One often sees antifoul carried over in this way (unless there's an extremely unlikely colour match of a specialist AF ).

 

[skyflyer]

So the real question is:-do I need a new p bracket? How can I tell? (Whatever material this one is made from it is original and it went pink and a light sanding has made it a brass like colour

There is an anode not too far away on the hull (I assume bonded) as it is about 25% depleted No anodes on prop shaft though

 

[Bosun Higgs]

can it ONLY occur where the metal is in physical contact with the electrolyte - in other words does corrosion have to take place from outside to inside or can it occur the other way.

Its like a battery. The current flows through the electrolyte from the surface of one metal to the surface of the other metal. So dont worry.

 

[jwilson]

For galvanic corrosion to occur there must be a complete circuit. Through the water to another item is only half of a circuit. Ther emust be an electrical connection between the two to complete the circuit .

So why did I change brass skin fittings and seacocks that were definitely starting to dezincify? Standard modern GRP boat, skin fittings on plastic hull each individually connected to plastic pipes, no inter-fitting bonding. There must be current flows through the water, though maybe less than if items were electrically linked by wires.

 

[JumbleDuck]

So why did I change brass skin fittings and seacocks that were definitely starting to dezincify? Standard modern GRP boat, skin fittings on plastic hull each individually connected to plastic pipes, no inter-fitting bonding. There must be current flows through the water, though maybe less than if items were electrically linked by wires.

Yes, there must be current flows through the water, but there must also be current flows not through the water. VicS has already explained how a single unconnected object can undergo electrolytic corrosion: basically it's because it's made of crystals of two different materials which are linked by physical contact and by the water.

 

[vyv_cox]

So the real question is:-do I need a new p bracket? How can I tell? (Whatever material this one is made from it is original and it went pink and a light sanding has made it a brass like colour

There is an anode not too far away on the hull (I assume bonded) as it is about 25% depleted No anodes on prop shaft though

If your P-bracket is like thousands of others it is made from manganese bronze, which despite its name is a 60/40 brass with minor additions. It is subject to dezincification, which is a form of galvanic corrosion between phases in the alloy, as VicS has explained. When dezincification becomes severe the remaining metal is red in colour, has almost no strength and crumbles at thinner sections. It sounds as though yours is not very severe in the exposed part but I understand your concern over the base that is embedded in the hull. Other than cutting some grp away to inspect it i can only suggest that you subject the P-bracket to some serious loading to ensure that it is sound. I would strap a long timber lever to it and heave it sideways in both directions.

 

[VicS]

So why did I change brass skin fittings and seacocks that were definitely starting to dezincify? Standard modern GRP boat, skin fittings on plastic hull each individually connected to plastic pipes, no inter-fitting bonding. There must be current flows through the water, though maybe less than if items were electrically linked by wires.

Brass on its own can dezincify as explained earlier in the thread.

For what we normally recognise as galvanic corrosion a complete electrical circuit is required. The water forms only one half of the circuit.

If two metals with different electrode potentials (copper and iron) for example are immersed in salt water and connected together electrically ( or are in contact with each other) current will flow from one through the water to the other and then back to the first via the electrical connection. The more anodic of the two (iron in this example) will corrode.
But break the electrical connection and no current will flow.

 

[alahol2]

Brass on its own can dezincify as explained earlier in the thread.

For what we normally recognise as galvanic corrosion a complete electrical circuit is required. The water forms only one half of the circuit.

But break the electrical connection and no current will flow.

Surely, then, the simplest way to protect a P bracket would be to epoxy paint it under the antifoul and not bond it internally?

 

[vyv_cox]

Surely, then, the simplest way to protect a P bracket would be to epoxy paint it under the antifoul and not bond it internally?

Almost exactly what I do. Bonding paint to manganese bronze seems to be quite difficult and my first two attempts were unsuccessful. Then i found that Hammerite special metals primer sticks to it very well, with prop antifoul over that.

 

[MYStargazer]

Almost exactly what I do. Bonding paint to manganese bronze seems to be quite difficult and my first two attempts were unsuccessful. Then i found that Hammerite special metals primer sticks to it very well, with prop antifoul over that.

I was wondering about protecting bronze rudders by coating in epoxy and anti fouling over the top. But I was told this would concentrate galvanic action on the tiny gap on the shaft where the rudder enters the hull, as this must remain uncoated if the rudder is still to move easily. Is this true?

If coating works to help prevent galvanic action, should the covered parts still be connected to a separate anode in the normal way - just to be on the safe side?