Corrosion, now I know what I was missing...

[Lon nan Gruagach]

Corrosion is the oxidisation of metal, in steel and iron this is rust, others dont have specific names.
Steel/iron is a significant problem since oxides of most other metals provide a protective surface, but iron does not and the corrosion continues until nothing is left.

Corrosion is an electro chemical process which can be prevented by several means including cathodic protection.

Cathodic protection works by preventing or very quickly reversing the occurrence of anodic areas on the structure. Achieved by providing a source of electrons either from a voltage source or a sacrificial anode.

When 2 dissimilar metals are immersed in an electrolyte and are electrically bonded they will most likely form an anode, cathode pair depending on their relative reactivity to the electrolyte. Despite any protective oxidisation that might occur the anode will corrode faster than normal while the cathode will corrode slower or not at all (until the anode is all gone that is).

So, why and when to fit anodes:
When the metal will corrode due to simple oxidisation or being part of a bonded anode cathode pair.
What anode to fit:
One that is more reactive to the electrolyte than the most reactive metal that you wish to protect.


What was I missing?
A bad case of old wives tales built up in world that thinks it knows it all.
Why post?
didnt want to continue a thread drifting spat.

Top tip, when you insult someone by saying read what you just linked to, have a read of it yourself.

 

[RichardS]

What was/is the old wives tale?

Richard

 

[l'escargot]

What was/is the old wives tale?

Richard

The OP maintains on a thread on the mobo forum that we fit anodes to our boats primarily to prevent them rusting not to prevent galvanic corrosion:

Am I missing something, all this talk of galvanic corrosion and dissimilar metals, what ever happened to plain old rust? Isnt that why anodes are fitted in the first place, to prevent corrosion of the most reactive metal underwater even if it is the only metal underwater.

And as for stainless not bothering plain steel, when was the last time you left a steel knife on a wet stainless draining board, and cleaned the rust stain of afterwards...

Two of us have tried to explain that he is missing something and it would seem he still is, so he has started another thread about it...

 

[Lon nan Gruagach]

The OP maintains on a thread on the mobo forum that we fit anodes to our boats primarily to prevent them rusting not to prevent them rusting not to prevent galvanic corrosion:



Two of us have tried to explain that he is missing something and it would seem he still is, so he has started another thread about it...

Indeed, I recall you told me to read https://en.wikipedia.org/wiki/Galvanic_anode, even though it was me that posted it, that it would show that anodes only prevented galvanic corrosion. I refer you to the first section:

In brief, corrosion is a chemical reaction occurring by an electrochemical mechanism.[1] During corrosion there are two reactions, oxidation (equation 1), where electrons leave the metal (and results in the actual loss of metal) and reduction, where the electrons are used to convert water or oxygen to hydroxides (equations 2 and 3).[2]

Fe → Fe2+ + 2e−
O2 + 2H2O + 4e− → 4OH−
2H2O + 2e− → H2 + 2OH−
In most environments, the hydroxide ions and ferrous ions combine to form ferrous hydroxide, which eventually becomes the familiar brown rust:[3]

Fe2+ + 2OH− → Fe(OH)2
As corrosion takes place, oxidation and reduction reactions occur and electrochemical cells are formed on the surface of the metal so that some areas will become anodic (oxidation) and some cathodic (reduction). Electric current will flow from the anodic areas into the electrolyte as the metal corrodes. Conversely, as the electric current flows from the electrolyte to the cathodic areas the rate of corrosion is reduced.[4] (In this example, 'electric current' is referring to conventional current flow, rather than the flow of electrons).

As the metal continues to corrode, the local potentials on the surface of the metal will change and the anodic and cathodic areas will change and move. As a result, in ferrous metals, a general covering of rust is formed over the whole surface, which will eventually consume all the metal. This is rather a simplified view of the corrosion process, because it can occur in several different forms.[5]

CP works by introducing another metal (the galvanic anode) with a much more anodic surface, so that all the current will flow from the introduced anode and the metal to be protected becomes cathodic in comparison to the anode. This effectively stops the oxidation reactions on the metal surface by transferring them to the galvanic anode, which will be sacrificed in favour of the structure under protection.[6]

Hey, whaddya know anodes will prevent rust, as I said

Am I missing something, all this talk of galvanic corrosion and dissimilar metals, what ever happened to plain old rust? Isnt that why anodes are fitted in the first place, to prevent corrosion of the most reactive metal underwater even if it is the only metal underwater.

And as for stainless not bothering plain steel, when was the last time you left a steel knife on a wet stainless draining board, and cleaned the rust stain of afterwards...
Read more at http://www.ybw.com/forums/showthrea...v-negative-terminal/page2#gKUPKbaHyHRvETLE.99

 

[l'escargot]

...Hey, whaddya know anodes will prevent rust, as I said

You're still missing something. You asked: "...what ever happened to plain old rust? Isnt that why anodes are fitted in the first place..." No it's not.

Rogershaw's post probably explains it better than me:

Yes you are missing something.

Rust is when when Iron in the steel combines with Oxygen to form iron oxide and there are chemically 2 types Fe2O3 and Fe3O4.

The Fe is the iron and the O is of cause Oxygen.

Most metals also Oxidise to same extent by combining the metal and Oxygen but most produce a stable and therefore protective layer on the metal. Aluminium oxide and Chromium oxide of 2 examples.

Rust or Iron oxide is not stable and is porous so flakes off and allows oxygen to again attack the base metal continuously.

Galvanic corrosion is in effect the transfer of metal from a anodic metal to a cathodic metal by and electrolytic process. Any form of metal plating is an example of this. Zinc, Nickel, Chromium and even Gold and Silver can all be electroplated in this way. One job I had was designing a plant to refine gold by using the electroplating process for a local gold refinery

The problem is that on a steel boat with yellow (copper based alloy) metal fittings if there is an electrical contact between the steel and the yellow metal the material process is for the yellow metal the be plated with the iron in the steel thus destroying the structure thus strength of the steel. We therefore use zinc anodes the will electroplate the yellow in preference to the iron.

The general way to protect steel/iron from rusting is by preventing the Oxygen getting to the surface of the steel/iron by painting the steel of even coating it with a more stable metal like zinc. nickel, cadmium or several more stable metals.

Stainless steel is interesting because the protection of the iron in the stainless steel from rusting is with a layer of chromium oxide on the layer that forms in the presence of oxygen. But that is not the whole story and too complicated to discuss here.

The position on the galvanic scale (this determines how electrically reactive metals are to each in simple terms) of stainless steel and mild steel is quite close so the effect is there but small enough to ignore. There are even special welding rods the allow stainless steel to be welded to mild steel reducing this effect at that joint.

I expect you will just rest on selective quoting in a different context from Wikipedia though. But hey ho everyone can rest peacefully that their anodes will stop their GRP hulls, yellow metal skin fittings, props and stainless steel shafts from rusting - and you know what, the fact there won't be a bit of rust on any of those things probably proves you right...

 

[superheat6k]

The basic issue here is the statement that corrosion is oxidisation. Yes oxidisation is one form of (electrolytic) corrosion, but oxygen has nothing to do at all with galvanic nor most forms of electrolytic corrosion, and these two are also quite different.

 

[RichardS]

If the old wives' tale is that "we fit anodes to prevent rusting" that's not an old wives' tale that I've ever heard.

Richard

 

[superheat6k]

If the old wives' tale is that "we fit anodes to prevent rusting" that's not an old wives' tale that I've ever heard.

Richard

Indeed - I have never seen brass rust.

 

[Lon nan Gruagach]

Now I see what l'esgargot and Rogershaw were trying to say and failing rather spectacularly. Is that materials are selected or treated to avoid single material corrosion. In the case of mixed metals this is not feasible so cathodic protection is used.

Could have said that with out being all snarky couldnt ya?
Oh, and hardy har har, yes, I know its only ferrous metals that rust duh!

 

[l'escargot]

Now I see what l'esgargot and Rogershaw were trying to say and failing rather spectacularly. Is that materials are selected or treated to avoid single material corrosion. In the case of mixed metals this is not feasible so cathodic protection is used.

Could have said that with out being all snarky couldnt ya?
Oh, and hardy har har, yes, I know its only ferrous metals that rust duh!

Now you're getting it - I think... :encouragement:

 

[vyv_cox]

A couple of points of order ?

Rust is the hydrated form of iron oxide, Fe(OH), not iron oxide FeO, ignoring the correct chemical valencies.

Anodes are placed to preferentially corrode, protecting any metal that is more noble than themselves in the galvanic series. As has been pointed out, aluminium or zinc anodes are used to protect steel structures and vessels but also to protect combinations of metals that might corrode galvanically between themselves, e.g. a manganese bronze propeller on a stainless steel shaft. In this case both metals are more noble than the aluminium or zinc and are protected.

 

[rogerthebodger]

I tend to separate "corrosion" into in to 2 forms due to the potential confusion that occurs in the use of 1 term for 2 dissimilar processed.

1) is oxidation that is rust in steel but most metals do oxidise to a greater or lesser extent. (maybe except the platinum group metals gold, silver and platinum)

2) galvanic corrosion where as said 2 dissimilar metals in an electrolyte, electrically connected together.

One other point about galvanic corrosion of steel is that when it does take place the surface of the steel tends to become very shiny and pitted with no rust until the shiny surface comes into contact with oxygen then it start to rust.


Thks Vyv for correcting in rust formula as not a chemist of metalagist just a plain old mechanical/electrical design engineer.