Corrosion Retardation

Hello Bartfink

I'm trying to understand your nickel and aluminium corrosion situation.

You want to know which will corrode first, if isolated in two separate buckets with exactly the same liquid in each, and no electrical connection between buckets.

Is that what you intend?

If so, the answer is easy.

There are a number of liquids which attack aluminium furiously, yet do not bother nickel so quickly.

Caustic soda, N.O.S.(NITRIC ACID,PHOSPHORIC ACID), will etch or corrode aluminium quickly, but nickel will be much slower.

Is that what you need to know?

Reply here, with...

Kind Regards....

You understand the hypothetical experimental set up correctly, however I want to know if there are any circumstances in which the NICKEL will corraode faster than the aluminium?

Are there any exceptions to the rule with rergards to Nickel and Aluminium?

I really am looking for any circumstances in which this can occur.

For example..... a frequently stressed Nickel (active) foil in warm milk will corrode quicker than a passivated aluminium bar in a separate bucket of warm milk. (I'm not saying this is correct, it just a example).

I appreciate your input. Any ideas?

Bartfink.

Maybe I'm missing something here but if the aluminium and nickel are in different buckets does the galvanic series even come into consideration? If the two metals are not in electrical contact does it matter which is more nobel?

If the metals are in two seperate buckets then I would expect that there is a material that will oxidize nickel and not aluminium.

If the metals are in electrical contact in the same bath of electrolyte then I agree that it would take some type of thermally induced corrosion reversal to have the nickel oxidize more quickly than the aluminium. I am not sure that a corrosion reversal between Ni and Al exists.

Although the nobility of nickel vs. the activity of Al is well understood, in the original post, it was stated: "The Ni and Al do not necessarily have to be neighbouring in the same medium (galvanic corrosion) but should be considered in like mediums..."

This leaves ALL SORTS of endless possibilities, and thus, should not be addressed with a simple off-the-cuff answer based on "Standard-Fare" knowledge. An *inquisitive* student asked the question. That student is probably coming up with all kinds of "What-Ifs", which might just require a bit of thought or investigation.

The Nickel Institute has all sorts of information which might prove useful, here, including a Technical Assistance board on which to post questions-messages.

Likewise, the National Association of Corrosion Engineers International (www.nace.org) has ListServer networks for corrosion professionals to assist one-another with areas outside of our own expertise. I always defer such peculiar postulations to the group before assuming that I "know-it-all"... Regards to one and all ~

You raise a good question. Is the galvanic series a "red herring" in the separate bucket senario?

You state....

"If the metals are in two seperate buckets then I would expect that there is a material that will oxidize nickel and not aluminium."

Great, but any ideas as to which one or what type? If so it would be the answer I am looking for.

If a "thermally induced corrosion reversal" existed I would be interested as to how this worked!!

Anyone?

Thanks ca1ic0cat.

Thanks for the insight.

I have had a look into Pourbaix diagrams. I see that they (like the galvanic series) is medium specific. Please correct me if I am wrong. See http://www.corrosion-doctors.org/Thermo/ironE-pH.htm

"describes the potential-pH equilibrium diagram for the system iron-water at 25^oC"

As I am looking for any conditions might make Ni corrode faster, I would need to look at a Pourbaix diagram for every medium......a little time consuming.

Great case study in your earlier responce!

As you stated......

"Remember that the "contributor" for corrosion to occur (with MOST metals) in any electrolyte is the concentration of positive metal ions (the H⁺ ion in acids applies ... the stronger the acid, the greater the concentration of those ions ; hence, "pH" ~ potentz Hydrogen)."

Thank you. For the insight. This tells me that the higher the concentration of the H+ ions the quicker the corrosion will be.

P.S. El Paso experience 2008 reminds me of Bournemouth 2005 in which I needed plates and screws......not to my head.

To have galvanic action the materials must be connected.

Is the Ni active or passive.

Is it pure Al.

When designing anything it is advisable not to have more than 0,2v between dissimilar metals that are connected in an electrolyte otherwise galvanic action will occur. The two materials can be protected by an anodic or cathodic protection systems.

Correct. I see OP's question as not being limited to "galvanic" corrosion but to corrosion in general. He (OP) seems to understand that glavanic corrosion puts the two metals in the same solution, but also wants to know about "two metals in two isolated solutions" problem.

Each commentor should top post as to which of these two questions he is commenting on. This will help to sort the thread into (exactly two) buckets (galvanic and general).

To quash any confusion....

I understand that the ISOLATED BUCKETS and SAME BUCKET may offer different reactions however, I am looking for any scenarios in which Ni will corrode faster than Al.

Therefore you can have any arrangement possible just as long as you can offer a solution in which Ni will corrode faster than Al. I just want the mediums to stay the same.

Thanks for highlighting this.

Thanks MOBI,

I have not stipulated active or passive. So you can choose. Whichever leads to a plausible scenario.

Alloys are not being considered by myself at this time.

You state....

"The two materials can be protected by an anodic or cathodic protection systems."

IN THE SAME BUCKET,
Can the aluminium be protected enough so that Ni will corrode faster (gold plating would be too easy)? Allowing the aluminium oxide to form etc.

Thanks.

So three metals are now involved. In bucket 1 is the nickel. In bucket 2 we find aluminum and the cathode metal, usually zinc. Now, the nickel (bucket 1) corrodes faster because in bucket 2, the aluminum is protected using the "passive cathodic protection system". What does "passive cathodic protection mean"? Adding zinc anode to protect the aluminum cathode. Bucket 1 having no "cathodic" protection is allowed to corrode at a faster rate. Problem solved.

Clearly, where Post 14 says: "In bucket 2 we find aluminum and the cathode metal, usually zinc..." it actually means to say that the Zinc is anode metal ... anodic with respect to the aluminum (with which it must be in metallic-contact).

... as suggested within Post#5 ... this IS one of those peculiar instances wherein the seemingly impossible situation can, in fact, exist. [That is, so long as the zinc remains sufficiently active to provide continued protection to the Aluminum. Zinc, which is amphoteric like alum, can be passivated by CO₂, carbonates, etc.] ... (T.M.D.)

Yes, sorry. Zinc is the sacrificial anode in this "passive cathodic protection system" in bucket 2. Yes they would be electrically bonded Typical aplications are water heaters and propeller shafts on boats (not aluminum).

My experiences with corrosion have virtually been with the same electrolyte, seawater and the various materials involved in ship building and all their machinery and systems. Along with this various methods of corrosion protection.

I think in answer to your question, my advice would be to consult a metallurgist or a corrosion engineer. Surely we have a member that would fit the bill.

I was always taught, and have taught others, that if they have a severe corrosion problem to consult one of these well qualified persons because if you don't your problem can get right out of hand..............and it's not particularly funny when a stoker is cleaning rust from the bilge and comes up and tells you he has put a chipping hammer through the hull well below the waterline and you are out in the middle of the Coral Sea........didn't present too much of a problem, all that was needed was a few softwood wedges.

Keep in mind that many metals appear twice in the galvanic series, being either more anodic or more cathodic depending on environment (PH etc.). One also need take into account passivating layers of oxide on a given metal and if the oxide is resistant to or dissolves in the electolyte. Another item to watch is ratio of anode to cathode (area exposed to electrolyte).

Regarding the statement: "Keep in mind that many metals appear twice in the galvanic series, being either more anodic or more cathodic depending on environment (PH etc.). One also need take into account passivating layers of oxide on a given metal and if the oxide is resistant to or dissolves in the electolyte" ...

The second part of the statement is correct, which is what accounts for "double-entries" of a material (such as "Stainless 304 Active" & "Stainless 304 Passive") on a given Galvanic Series.

Since any Galvanic Series is, by nature, pH specific (pH does not change, within any given Series), the first sentence is not correct... what you meant was stated correctly by the second sentence itself. [[Not just semantics, here...]]

...~~... Best Regards ---