Difference between sacrificial anode and passivating metal (1 Viewer)

[tonyzhen]

Hi Everyone
I know that this may seem obvious to many of you, but what is really the difference between a sacrificial anode and passivating a metal as a means of protecting a ship from corrosion? In Excel HSC Chemistry, it is said that galvanising iron (that is, coating with a layer of zinc), is method involving a sacrificial anode. The zinc corrodes in preference to the iron, forming an impervious layer of ZnCO3.Zn(OH)2. It is this impervious layer that protects the ship.

However, isn't this similar to the technique of passivating a metal? Aluminium, which is an example of a passivating metal also corrodes, forming an impervious layer of aluminium oxide. Again this oxide prevents further corrosion. Whilst it is more reactive than iron, aluminium, however is not really classified as a sacrificial electrode.

What is really the difference between them?

Any suggestions?

Thank you everyone

 

[YoloStudent]

Passivating metal refers to the metal. Aluminium and Chromium for example. Those whole metals are passivating.

Sacrificial anode is any metal that is more reactive than the protected metal to cathodically protect it.

What that example is doing is combining both by coating a protected metal with a metal that is more reactive than the protected metal AND using a metal that is passivating.

Passivating and sacrificial anode combo: Passivating metal corrodes preferentially - forms a non-porous layer - prevents further corrosion.
Requires a passivating metal that is more reactive.
Passivating: Any metal that forms the protective layer. The difference here is that it can be ANY metal that forms an non-porous layer.
Sacrificial anode: Any metal that is more reactive than the protected metal. The difference here is that it is ANY metal that is more reactive and thus corrodes preferentially.

Hopefully it's clear enough. Ask again if you need more help.