Galvanic Corrosion

[DustinMechEng]

Two metals that form a galvanic couple (brass and 304 stainless steel for example) are immersed in salt water and near eachother (approx 1 foot apart), but not touching.

Will the brass corrode? Is the salt water enough of an electrical conduction path to create galvanic corrosion?

Thanks for the help!

 

[Heckler]

Yes, the salt water is an electrical conduction path to create galvanic corrosion.....The extent of corrosion would be a function of environment, material potentials, and distance. I would have to suggest some physical testing to be totally sure of the extent.

Heckler
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[Compositepro]

There will be no galvanic corrosion because there is no current loop. The two metals in an electrolyte form a galvanic cell (or "battery") but there has to be conductive path, not ionic path, for electron flow. What you have is a battery with no load attached.

Concentration gradients in the electrolyte could lead to some corrosion in some cases.

 

[deanc]

http://www.ocean.udel.edu/seagrant/publications/corrosion.html

See if this helps,

 

[mshimko]

Two opposing views are provided above; I've taught the subject and agree with Compositepro - there will be no galvanic corrosion because there is no "external electrical contact.

These are the requirements for galvanic corrosion:
- anode and cathode
- thermodynamic tendency for a reaction to occur ("delta G < 0)
- contact with a common electrolyte
- external electrical connection (I've seen this last requirement described many ways; I find it useful to think of a "wire or wire equivalent" connecting the anode and cathode)

Take away any of these, and there will be no galvanic corrosion.

 

[rb1957]

the two metal components in the brass alloy won't undergo galvanic corrosion, but brass can still corrode in salt water (which i recognise is a different question).

I'd express it as there is no flow of electrons between the atoms of the constitutent metals when the alloy is sitting on the bench. The salt water doesn't really factor into the problem (the alloyed elements are sufficiently close contact to allow electrons to flow if they wanted to). But the salt water, being corrosive, will probably corrode the brass.

 

[kenvlach]

1) Brass (straight Cu-Zn) shouldn't be used in seawater. It will corrode, sometimes by de-zincification. There are alloying combinations to prevent this, but better to use a naval bronze or something else more suitable.

2) If brass & SS are in a closed system, the SS may move from a passive to an active state of corrosion due to the presence of dissolved Cu & Zn ions along with the Cl of seawater.

 

[0707]

Considering the two metals separately in salty water they will both corrode because they are not pure homogeneous materials each material by itself will have cathode and anodic areas so corrosion will occur corroding anodic areas.

As both metals are close together but not touching each other, chloride ions of salty water will be more prone for SS than for brass. I would forecast severe pitting end chloride SCC for SS304 and dezincification for brass.

luis