Earth current flow and corrosion

[Pelico]

Since the corrosion thread has been raised again, it prompts me to ask a related question. Having studied the FAQ and read just about all of the corrosion posts in the archive, I noticed one comment about the location and route of earthing currents that has had me wondering ...

 

Was it a throwaway or do you need to be careful about the position of earths as current flow between the chassis and panels will add to the corrosion problem ?

[Sheds Moderator]

Yes, in theory, though EL corrosion happens in small areas with smaller currents in practice. However I wouldn't be at all surprised to see a steel washer on an ally panel and used as say a lighting earth demonstrate spectacular corrosion, more so than if there were no current flow. In point of fact I have seen this on an old tintop. Whether this was just coincidental I don't know. Interesting.

[Chelspeed]

Corrosion is a result of a local electric cell set up where two dissimilar metals at different points in the galvanic scale (some sort of chemical thing which makes some metals more positive and some negative) are in close contact separated by a conducting medium (eg salty water). This produces a current flow which lead to one metal, the more positive I think, being eaten away, the "bits" being eaten away carried by the electric current to the other.

 

You can suppress this corrosion by imposing an external voltage on the bit you're trying to protect that's higher than the galvanic corrosion voltage, so that net current flow is the opposite way so causing the second to corrode preferentially. This is impressed current cathodic protection and is used to protect steel things that are buried.

 

But I don't think this will have anything to do with using the chassis as an earth where the current flow is within the metal itself rather than from one metal surface through a conducting medium and everything in the chassis is at the same voltage, ie 0V or "earth". But I'm a mechanical engineer not chemical or electrical so the last bit moght all be bollox.

[Sheds Moderator]

I was thinking in terms of the actual attachment point - eg a steel washer on an ally panel. The steel washer is the earth return for an elec system so the (conventional) current flows from the steel to the ally with the electrons actually going the other way. Under normal circumstances ally corrodes in preference to steel where they are in contact. Hang on I'm going away for a think, the system described has to be affected by current flow based on what you say about induced cathodic protection.

[Sheds Moderator]

OK, sorted it. Oxidation is loss of electrons, ie they will flow from the ally to the steel assuming ally is more reactive (ISTR this is the case). Given that a steel earthing strip on a neg earth car will have electrons going in the same direction this will increase the rate of corrosion, all other things being equal.

 

In that case I'd ensure that you only earth on the (steel) chassis and cover them up with paint or waxoyl afterwards.

 

Could all be theoretical, with no real difference in practice since all 7s are corroding fairly quickly unless cared for in dry garages kneedeep in waxoyl. However I have seen engines in scrappers with an earth strap attachment point so corroded that the thing has come away.

 

What do they do on things like oil rigs?

[R2D2]

The standard electrode potential for aluminium is around -1.66 volts.

The standard electrode potential for steel is around -0.44 volts.

 

When these two metals are connected by a soution of weak acid such as rain water they will form a voltaic cell with an emf of around 1.22 volts.

 

I think that the aluminium will lose electrons (lower electrode potential) and be oxidised and the steel will be reduced.

 

There will also be some corrosion effects from differential aeration of the electrolyte so the steel can still corrode across its surface.

 

I think that negative earth battery connection is a bit of a red herring, I agree that electron loss can be considered as oxidation but electron flow doesn't imply loss of electrons as they should be replaced by the battery. Painting the connection is still a good idea as it prevents the formation of the cell.

 

Cathodic protection of cars is something that has been discussed for years. It is common to cathodially protect ships, platforms, pipelines and underground fuel tanks.

 

The common factor is that these structures are all immersed in a wet environment and so it is quite easy to apply the technique.

 

The problem with cars is that they get wet in localised areas and the CP protection system just isn't effective for this type of application.

 

There are a couple of "snake oil" salesmen in the US that claim to have an Automotive CP system but the basic science is very weak

 

 

 

 

Edited by - Chris Flavell on 30 Oct 2003 09:49:07

[P Brews]

An interesting point is that carbon fibre has a galvanic potential of +0.2 to +0.3 volts. This means that if you have any carbon fibre parts in contact with aluminium, you are likely to see severe corrosion and need to pay careful attention to insulating the parts from each other.