Zinc and steel experiment

[hpeer]

Calling chemists on this one. It would he a neat experiment to do but I dont have the situation to allow it.

Here is the experiment.

Take 2 each 5 gallon plastic containers, filk each with 4!gallons of sea water.
In Bucket A place a 1 pound hunk of clean mild steel.
In bucket B place a 1 pound hunk of clean mild steel AND a 1 pound chunk of zinc anode for a boat.

Cover both and wait a year.

At the end of a year what will be the condition of the steel in each bucket? The zinc?

[grantmc]

I don't doubt that thousands of such experiments have been done. There are literally thousands of books about corrosion and sea water.

Although these days such experiments would use aluminium rather than zinc I would expect.
Zinc has been mostly superseded, as it's toxic and doesn't perform so well.
Even most engine manufacturers prefer aluminium anodes. Mercury, BRP Evinrude, Honda, Suzuki, Yamaha, Volvo Penta; all of them dropped zinc and now use aluminium.

Also beyond the steel hull, fixtures like props tend to go first.

Here's a book I found very helpful (although more technical than I wanted): The Corrosion Handbook for Boats. But I bought it in a package with some Nigel Calder books and provided some fascinating insights.

OP what was your point in making the suggestion?

[fstbttms]

Originally Posted by hpeer

At the end of a year what will be the condition of the steel in each bucket? The zinc?

I suspect that very little would happen to either. There would be no electrolytic corrosion as there is no source of of electricity and galvanic corrosion only occurs when two dissimilar metals are in contact with each other in an electrolyte, which is not the case here.

What were you anticipating would happen?

[hpeer]

No anticipation of what will happen, which is why I asked the question the way I did.

In a more practical note.

We have a steel boat and sometimes have water in the bilge. Kind of hard to not have water in the bilge. Wo I was wondering if there was any benefit to putting a zinc down there, just sitting on the bottom. Would it do any good? Any harm?

[FabioC]

I think you are asking two very different questions.
1. What would happen in the specific experiment. I agree with who have said nothing would happen. This is because the experiment is not a good approximation of reality, since there is no source to establish current and related galvanic corrosion (if you drop some source of leakage current in the bucket, then it is another story).

2. Would an anode in the bilge of a steel boat help? I would think in general not, since the hull of a steel boat should be grounded by definition, so you should not have corrosion in the bilge.

If for some reason it is not and you are seeing sign of corrosion, or if some parts in the hull are isolated in some way (e.g., thru-hulls) the placement of the anodes needs to be thought through for the specific boat. The reality is that galvanic corrosion is unavoidable, it will always look for the weaker points. The anodes are sacrificial and protect a certain part. The other side of that is that anodes may "perturb" galvanic corrosion as well, meaning that placing an anode somewhere may increase exposure somewhere else. In general, if the thru-hulls do not show sign of corrosion. or if the current anodes are working well the way they are, it is advisable "not to mess around" with anodes, since that could perturb things. Generally speaking, "randomly dropping an anode somewhere" is not a good thing, at best it is going to be irrelevant, at worst it may cause unexpected chain reaction.

[Dave Lochner]

Originally Posted by hpeer

No anticipation of what will happen, which is why I asked the question the way I did.

In a more practical note.

We have a steel boat and sometimes have water in the bilge. Kind of hard to not have water in the bilge. Wo I was wondering if there was any benefit to putting a zinc down there, just sitting on the bottom. Would it do any good? Any harm?

Stagnant water in the bilge provides the conditions for microbiologically induced corrosion (MIC). Ever present bacteria feasts on the sulfur in the water and forms small encapsulated colonies on the steel. Their by product is acidic and it causes the steel to corrode.

The same happens in fuel tanks with water in the bottom of the tank. Its not the water so, its microbe digestive by products.

An anode won't prevent this kind of corrosion, the prevention is a dry bilge and/or frequently replaced water.

[GordMay]

Originally Posted by FabioC

... 1. What would happen in the specific experiment. I agree with who have said nothing would happen. This is because the experiment is not a good approximation of reality, since there is no source to establish current and related galvanic corrosion (if you drop some source of leakage current in the bucket, then it is another story)...

An external source of current is not required, in a bi-metalic couple.

For galvanic corrosion to occur, the metals involved must be connected in two ways.
First, they must be in electrical contact, either through direct physical contact, or through another conducting material (wire, bolt, etc), to allow an electric current to flow from one metal to the other.
Second, they must also be in ionic contact, so an ion current can flow between them. This requires an electrolyte, namely, a solution containing ions such as from a dissolved salt, an acid or a base. For ionic contact, the metals might be completely immersed in the electrolyte or coated with a continuous film of electrolyte that wets both metals, as can happen when the humidity is high.
The contacting metals form a bimetallic couple, because of their different affinities (or attraction) for electrons. These different affinities create an electrical potential between the two metals, allowing current to flow.
The metal higher in the galvanic series of metals, the anode (active, or less noble), provides protection for the metal lower in the series, the cathode (more noble).

The potential difference (voltage), between two dissimilar metals, is the driving force for the destructive attack on the active metal (anode). Current flows through the electrolyte to the more noble metal (cathode) and the less noble (anode) metal will corrode. The conductivity of electrolyte, temperature, and dissolved O2 content will also affect the degree of attack. The cathode to anode area ratio is directly proportional to the acceleration factor.

If zinc & steel are in direct solid contact, with each other:
In our example, zinc, (or as suggested aluminum or magnesium), which all lie below iron and steel in the galvanic series, are used as sacrificial anodes, to protect the steel in the seawater. The more active anodic metal, zinc, should corrode more rapidly than it would alone, whereas the more noble cathodic metal, steel, should corrode more slowly.
If they are not in direct solid contact, just dumped in (as per OP), there will be no galvanic corrosion.

[hpeer]

Thanks guys. Reading with interest.
And learning.

[GordMay]

The corrosion, FabioC referred to, resulting from an externally supplied source* is referred to as Stray Current corrosion, sometimes incorrectly called electrolytic corrosion.

In the electrolysis process, the external current (stray current) alone drives metal atoms into electrolyte as water-soluble ions. The environmental factors such as oxygen concentration, chloride, and pH, that are so critical to natural corrosion processes, are no longer relevant. The extent of damage, or loss, of metal is directly proportional to the magnitude of stray current leaving the structure, at the point of discharge.

In stray current corrosion, the driving force for corrosion is an external electrical current, that is virtually unlimited in its magnitude, depending on its source; while in galvanic corrosion, there is no external electrical current involved, the driving force for galvanic corrosion is the potential difference (voltage) between the two dissimilar metals. This means that the potential damage, by stray current corrosion, can be many times greater, than that by galvanic corrosion.

* Usually, the current flowing through paths other than the intended circuit. ie: A ground fault, for instance.
There are also Impressed Current (counter-intuitively) protection systems, beyond the scope of this discussion.

[billgewater]

Read up on Luigi Galvani. He discovered in 1780 that current flows between dissimilar metals in an electrolyte (he used frogs legs to detect the current).
GordMay has got it right. The more active metal goes into solution thus protecting the less active metal. The zinc atoms lose electrons and form + zinc ions in solution. This will make the zinc negative relative to the iron atoms. The circuit must be completed by having both metals making good electrical connection. Ultimately the zinc block "disappears".
However, the zinc does not totally save the other metal as the effect may be very local and is influenced by stray currents in the steel due to bad wiring practices, motion of the electrolyte etc. The electrolyte used is also highly relevant. Clean pH 7 water is a really poor electrolyte but many other liquids are even better e.g. many oils, air. Read up on the issue.
Some primer paints (e.g. zinc epoxies) contain the zinc and also keep the electrolyte off the steel. It's a complex area but there is lots of info. available.