Cathodic Protection for Aluminum Boat: Zinc & Non-Use of Silver/Copper

[RPN]

Hi there,
Can anyone tell me if this is right.
If I were to have an aluminium boat and I want to give the hull cathodic protection what metal would I use.
I said that I would use a more active less noble metal such as zinc. by placing the zinc it becomes the sacrifial anode and the hull is the cathode and we all know that the anodes corrode. is this the right idea or am i wrong.

As well two metals not to use would be silver and copper. is this right.
Thanks for the advice!

 

[Gokul43201]

Zinc is usually the metal of choice for cathodic protection of steel hulled vessels. However, zinc is not more active than aluminum, and hence would not work in your case.

Look at http://members.aol.com/logan20/red_tabl.html . The choice is obvious.

Silver and copper certainly would not work, but then, neither would most any other metal.

 

[redwraith94]

If you had an aluminum boat, you wouldn't need cathodic protection, unless you wanted the vessel for more than say a couple of hundred years or so?

 

[Gokul43201]

Red, I'm sure this is some kind of textbook question, where other aspects of reality are not allowed to interfere with the thought process . Else, why would the OP ask about cathodic protection for something that is often used to make the anodes !?

 

[redwraith94]

Hmmm...good point...magnesium then...or perhaps lithium? ;) IDK how long lithium will last in water, but it will be one heck of an anode before it spontaneously dissolved!

 

[RPN]

Cathodic protection correction

Thanks for the table you provided. I think I can explain it a little better. On teh aluminium hull I would attached zinc due to it having a lower reduction potential. The zinc would corrode faster than aluminium and therefore be the anode. The most negative is the reducer.

I would not use copper or silver due to them having a positive reduction potential.the one with the most positive reduction potential is the oxidizer. they both have higher reduction potential making them gain electrons and oxidize aluminum.

Does this sound correct?

 

[Gokul43201]

Zn does not have a lower reduction potential than Al. Look at the table again.

 

[RPN]

Okay
Then I would use Mg as the anode as it has a lower reduction potential?

 

[Gokul43201]

Yes. Magnesium alloys are among some of the commonly used metals for CP.

Lithium isn't really much of a choice. It doesn't alloy very well with anything (except perhaps Al and Mg), and it virtually dissolves in water (forms a soluble hydroxide).

 

[pete worthington]

RPN's initial post is partially correct. If you chose to protect an aluminum boat hull in sea water then you would use zinc anodes since zinc is "more active" or "anodic" than aluminum (except for a few odd ball aluminum alloys - which are not used on boat hulls). If you chose to protect an aluminum boat hull in fresh water you would chose magnesium since it is even more anodic than aluminum and this protection is needed with an electrolyte with very little conductivity.

Notice I use the word "chose" since aluminum hulls with no dissimilar hardware or components submerged in the water near the hull do not need protection. Well designed aluminum hulls limit the use of dissimilar metals, even dissimilar aluminums.

I don't know about Gokul's reference to a reduction potential but I do know that the "long honored" galvanic scale of metals is very clear about placing magnesium, zinc, galvinized steel, aluminum, cadmium, mild steel...platinum in order of most reactive to least reactive. Again, there are a few exceptions but generally speaking this is the order that all engineers comply to.

An area that I have extensive experience in is the protection of aluminum outdrive or sterndrives. These drives are protected with zinc anodes in salt water and magnesium anodes in fresh water. If you put a magnesium anode attached to aluminum in seawater the level of electrolysis would dissolve the magnesium in a matter of weeks, where as if you put a zinc anode on aluminum in a freshwater there would not be enough of a potential difference to protect the aluminum.

By the way, dissimilar metals can not be avoided where the outdrive, gear housing or prop shaft and strut come through a hull, therefore, there would be some protection in the area of the propulsion units.

 

[Gokul43201]

I don't know about Gokul's reference to a reduction potential but I do know that the "long honored" galvanic scale of metals is very clear about placing magnesium, zinc, galvinized steel, aluminum, cadmium, mild steel...platinum in order of most reactive to least reactive. Again, there are a few exceptions but generally speaking this is the order that all engineers comply to.

The difference between the Std Reduction Potentials (aka the Electromotive Series) and the Galvanic series, is that the former assumes the electrolyte only contains ions of the two metals, while the latter series is dependent on the electrolyte and the most commonly used Galvanic Series is made for the case of seawater being the electrolyte.

So, in some relatively mineral-free fresh-water (ideally distilled water, but find me a river flowing distilled water) the EMF series may come close, but in seawater the Galvanic Series is the only true reckoning of corrosion potentials.

Pete's answers are the correct real life solutions to the problem, but as stated before, this is likely some textbook chemistry problem, and I'm certain the OP will be expected to look up the EMF series. Nevertheless, an understanding of the conditions under which this series is applicable (and where it fails) is most important. Also, a knowledge of the existence of a "real world" alternative is a big bonus.

 

[pete worthington]

Thank you for that point of clarification, Gokul. Here-in lies the difference between an engineering education and one of pure science. This Forum is very helpful for me as I hone my skills as a "third year" H.S. physics teacher after 20 some odd years in the mechanics/engineering industry. This type of thread helps me to better engage with teachers that are from a pure science program.

 

[katanoate]

mg sacrificial anode

I have an experiment where magnesium is wrapped around a nail and placed in a petri dish filled with agar/saltwater and indicators to locate anode and cathode. The nail does not corrode beacuse its oxidation reaction has been stiffled. Only query is: a white substance has formed around the magnesium on the nail. Magnesium oxide possibly? anyways, what is it, and why is it there? how is this combatted in industrial Mg anodes?

 

[JGM_14]

aluminum forms the oxide coating that prevents any furthur oxidation. try gold or platinum, or is cost an issue
the electrodes are commonly Mg alloy that helps prevent corrosion like stainless steel http://en.wikipedia.org/wiki/Elektron_%28alloy%29"