Galvanic Corrosion Between Aluminum Alloys

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Galvanic series in seawater at 25oC (77oC) are indicated below ... Aluminum alloy 2117; Aluminum alloy 2017; Aluminum alloy 2024; Cadmium; Aluminum alloy ...

Really, I hope this isn't homework, cause we hate doing it, too.

I don't need to be scolded for not doing my homework. I need specific data and I have not found it yet. You have not found it for me either.

Perhaps I was not specific enough in my RFI - if I were you would not have been so eager to respond with a complaint. The links you provide are grossly general. I need specific data and I came to CR4 in case a knowledgeable subscriber has the information at their disposal.

Example (within my incomplete data set):

Alloy Potential (V)

7075 -0.99

5046 -0.87

5083 -0.87

5052 -0.85

3004 -0.84

3003 -0.83

1100 -0.83

...

This table is taken from: Atlas Steel Technical Note No. 7 "Galvanic Corrosion"

That is an interesting issue. I have known not to mix steel, aluminum, copper or other dissimilar metals in a corrosive environment. It is pretty obvious now but I didn't think about different aluminum alloys. I only searched for a couple of minutes but I couldn't find it either.

Thanks, Bruce.

Galvanic corrosion action is most agressive when the materials have a wide difference in galvanic potential (Cu and Al, for example). I'm wondering if the potentials of aluminum alloys are so close to each other that the galvanic action is minimal and no longer significant to even consider.

Might find it in here.....

http://www.keytometals.com/page.aspx?ID=CheckArticle&site=ktn&NM=187

Thanks, SolarEagle. While the article did not answer my qustion specifically, it was a good roll-up of various corrosive activities and even included the suseptibility of various alloys to each.

I'll explore th rest of that site.

Thanks.

Did you check out the book "corrosion of aluminum and aluminium alloys"?

http://books.google.com/books?id=iEeiQEeLOmYC&pg=PA31&lpg=PA31&dq=galvanic+corrosion+table+aluminum+alloys

It might hold a table for what you are looking for.

That's it! You are correct - the good ol US Army Missile Command comes through for us again. Thanks, IdeaSmith.

Gosh, has there really not been much work on this subject since 1970?

Even after more research I have not found much more really. Surprising that is after all we know Aluminium is far off from being dumped as construction material.

But then why do the same again when it has been done already!

"Truth-be-told", a *tremendous amount* of new data and knowledge is being gained all the time.

It simply is NOT being done by 'Online Open Universities', with everything displayed in blogs or posted in declassified ("so-old-as-to-be-outdated") materials.

Here's one book with a tremendous quantity of newer, broader-spectrum and more in depth research [ASM's Aluminum and Aluminum Alloys]

A couple statements from : (another VERY good reference )
"Corrosion of Aluminum and Aluminum Alloys" (edited by Joseph R. Davis)

"Anodized bare (Al) stock successfully resists corrosion when only occasional exposure to salt water is encountered."

"Chloride solutions did not cause exfoliation during *reasonable* periods of immersion; however, formulations of chloride-nitrate solutions were found that produced severe exfoliation of highly susceptible alloys of various types in only 1 or 2 days."

ASTM salt-spray / Fog tests (various versions) are DETAILED, with lots of photos...

Likewise , mechanical properties of various alloys are highly discussed therein...

Buy the print version HERE.

NACE is THE place to go (and get involved!) if you want the very LATEST & GREATEST info pertaining to corrosion of nearly *anything*. By joining, you have free access to SEARCH a plethora of papers, AND (of even better "value") download the latest Standards and Recommended Practices.

Don't fall into the trap of thinking 'google' can provide "everything-you-need-to-know" straight to your monitor/printer for you... or you'll be missing out on a LOT of the very BEST.

'cheers' ~

Aluminum is a super metal.

Better specific strength (strength-to-weight) than any metal other than titanium and much much easier to cast and form. Cheap. Lightweight - when strength is not paramount

What is the fluid, its temperature and its pressure?

Thanks for the information.

I am interested in the relative galvanic potentials between the various aluminum alloys so I can ensure that my tube is the cathode and the fin is the anode.

The environment, as you have mentioned, can vary somewhat with the region (near coast) but typically it is outside - rain, humid air, sun, thermal cycling... Also considered for HVAC condensors, but not necessarily a galvanic corrosion concern, are pesticides, herbicides, fertilizers (big one), dog pee, cleaning solutions (bleaches, deck cleaner, mildew cleaner), bird crap...

The reference eletrode, then, would be the other aluminum alloy, right? I'm really just concerned with the interaction between the aluminum tube and aluminum fin.

You seem to have some experience here - what data am I looking for if not the relative galvanic potential between aluminum alloys?

Thanks again - I'm interested to hear more.

Yes , your specific concern is the electrochemical relationship between the tubes and the fin material. Some of the materials from which you might choose the tubing are identified by ASTM B210 (Standard) ... scroll to about 1/3 down the page.

Regarding the "cleaning solutions (bleach)"... "NOT!" Make SURE that the instructions for any units using Al for the tubes/fins WARNS against spraying-down with hypochlorite for cleaning(!)

The easy resolution for you is to determine the tubing to be used (based on the necessary mechanical properties and compatability with the system), and then work with the fin-material provider to select fin-stock that is sufficiently close to the tubing.

That is what (the original) "Potentiometers" were made for (like my L&N unit here):

. . . measuring Corrosion Potentials.

I am not exactly (not even remotely!) familiar with the effects from bending/folding punching etc upon aluminum (e.g., fin-stock)... (where is Milo?)... but you might look into this. It might CHANGE (i.e., "increase-the-spread") between the materials that you initially select.

The SHIPBUILDING industry learned a serious lesson regarding the "corrosion" effects of "cold-working" plating for web-frames many decades ago.

By "punching" those "corrugations" ("pleats / folds") into plating in order to hopefully preclude the need for welding-on L-angle stiffeners, the plating developed SUCH WIDE disparities (electrochemically-speaking) between the "flats" and the "bends", that corrosion across-the-surfaces was *horribly exacerbated*...!!!

They quickly went BACK to welding-on the necessary stiffeners.

Wishing y'all success...!

Great info, Tom.

Yes, in corrosion testing we have to consider all the crud homeowners, apartment complexes, landscapers, ... get on their condensor coils. Bleach is a big one and so are some fertilizers. Actually, inside the home (the evaporator "A-coil") has a much different environment and corrosive elements (these coils are wet and impacted by perfumes, adhesives, drywall, cooking oils, detergents, cat litter boxes, you name it). A larger problem inddors is formicary corrosion.

Here's a decent article on the issue of heat exchanger corrosion. Now go out and clean your coils!

http://www.epatest.com/EdArticles/PDFs/Detect-Elimin%20CoilCor.pdf

Very well written / succinct article...(!)

Near the bottom of page 2 the author says:
"alkaline cleaners also need to be rinsed thoroughly just like acid-based cleaners because alkaline residues can also corrode aluminum and other materials."

He is referring to the fact that aluminum (like zinc, lead, tantalum, and a few other metals, as I posted previously HERE) are "amphoteric" ... (Note: CS is *not* amphoteric, as posted HERE) .
This means they are 'happy' at neutral pH, but will corrode much faster when placed into EITHER an acidic OR an alkaline environment.

Thus , alkaline cleaners will initiate and / or exacerbate corrosion of aluminum....and, this is ALSO why galvanizing (zinc; as depicted in THIS post) is NOT a suitable means for protecting conveyors, tanks, etc for handling cementitious (alkaline) materials.

A "PS" / afterthought...

Buy yourself a NACE Corrosion Engineer's Reference Book and you will see (similar to the Table that lyn posted, but reverse-order) that all of the various aluminum alloys fall into a fairly narrow margin of potential difference.

... And, displays *thusly* on an "Atmospheric Corrosion" table:

Little-known (or acknowledged) by many people is the fact that (just as seawater differs in its concentrations of different constituents around the globe), AIR quality differs between the "big-city" and rural counties... and plays a part in longevity of both men AND 'their machines'...

There's a gooble-de-gook plethora of data in said handbook ... well worth the investment...!

Tom,

I would like your thoughts on this follow-on question -

The potential difference between copper and aluminum is large, resulting on galvanic corrosion of the aluminum in contact with the copper. As you have noted, all the aluminum alloys fall into a very narrow margin of potential difference.

What am I really asking? - in a Cu-Al heat exchanger coil, galvanic corrosion consideratons are significant. What about an Al-Al coil - is the galvanic potential so small that other forms of corrosion will eat up the coil long before any small galvanic corrosion causes a failure?

Are we done caring about galvanic corrosion?

Re: "is the galvanic potential so small that other forms of corrosion will eat up the coil long before any small galvanic corrosion causes a failure?"

I should HOPE that if you intend to (properly) *address* corrosion issues, here, that they would ALL be addressed properly such that electrical/mechanical wear-tear-burnout will be the overwhelmingly-predominant failure modes...

Most certainly, galvanic corrosion between your Al tubing and Al fins will be minimal... but, do not neglect to insure appropriate electrical isolation wherever the Al is joined to steel, brass or 'whatever-else'... AND (as I was attempting to emphasize in Post#24) you might wish to investigate the "activity-change" imparted upon the fin material that results from the processes to which IT is subjected ("learn from the mistakes that OTHERS have already learned from!").

IF you don't have the technical abilities "in-house"... then, as I ended Post#16 above: "(seek a corrosion engineer specific to the aluminum industry for advice and you will do well)"

Remember that in the end, your "preference" will be to have the FIN material "ever-so-slightly" more anodic (more active) than the tubing material, rather than vise-versa.

Wishing you the greatest success in this endeavor...!

"wOw" ...

gone only a "few minutes", for a trip out to the

"Vampire-Mobile"

. . . . and *5* other posts roll-in ...(!). SINCE you didn't apply either a grin () or a wink (), it is difficult to say for CERTAIN whether you are simply being facetious or not...

Just in case , "Visit": NACE International (checking my link just now, their website seems to be exhibiting difficulties, but... y'all know the Organization to which I refer).

Wow, now that's a GA answer.

There is something I have wondered about over the years that is only a small expansion of the OPs question. Since all alloys are a mixture of different metals is there any "corrode thy self" action between the atoms of an alloy? This is kind of a weird question since the surface of most metals would be subject to surface oxidation anyway. The corrosion would probably be limited to the surface.

A science show that I saw a few weeks ago had electron microscope photographs of the bronze of bells that rang with a thud and bells that rang out for several seconds. There was a difference both in the amount of alloying elements and the uniformity of the mixture. It would seem like a uniform mixture would corrode differently from one that is less uniform.

Hi Bruce - yes and good question. The atoms of various alloying elements either take substituion locations within the base metal lattice or are concentrated in the boundaries between the crystalline grains.

The following quote is from the link provided by SolarEagle, post #7:

"Intergranular (intercrystalline) corrosion is selective attack of grain boundaries or closely adjacent regions without appreciable attack of the grains themselves. Intergranular corrosion is a generic term that includes several variations associated with different metallic structures and thermomechanical treatments. Intergranular corrosion is caused by potential differences between the grain-boundary region and the adjacent grain bodies.

The location of the anodic path varies with the different alloy systems. In 2xxx series alloys, it is a narrow band on either side of the boundary that is depleted in copper; in 5xxx series alloys, it is the anodic constituent Mg₂AI₃ when that constituent forms a continuous path along a grain boundary; in copper-free 7xxx series alloys, it is generally considered to be the anodic zinc- and magnesium-bearing constituents on the grain boundary. The 6xxx series alloys generally resist this type of corrosion, although slight intergranular attack has been observed in aggressive environments."

Another link I have found informative, you may also: http://www.alueurope.eu/talat/lectures/1252.pdf

I appreciate your interest in the subject.

The traditional mix is 4:1 ratio of copper to tin (78% copper, 22% tin)...

From the wiki.....

"This metal combination produces a tough, long-wearing material that is resistant to oxidation and subject only to an initial surface weathering. Verdigris forms a protective patina on the surface of the bell which coats it against further oxidation.^[2] Bell metal of these ratios has been used for more than 3,000 years, and is known for its resonance and "attractive sound."^[1] This allows for a better bell resonance and causes the bell to "vibrate like a spring when struck", a necessary quality as the clapper may strike the bell at speeds of up to 600 miles per hour. The forces holding the tin and copper together cause vibrations rather than cracks when the bell is struck which creates a resonant tone.^[1]

In Russia, church bells are commonly cast with a unique mixture of copper and tin, often with silver added, to produce their unique sonority and resonance, mastered early in Russian Christian history.^[3]"

^{http://en.wikipedia.org/wiki/Bell_metal}

Re: "Since all alloys are a mixture of different metals is there any "corrode thy self" action between the atoms of an alloy?"

TRICKY_WORDING , there !!!

Between any two (adjacent) "identical atoms" (irrespective as to whether they are part of a "chunk/piece" of an element, or part of an alloy), there is no electrochemical potential difference... and thus, no "Corrosion Potential".

BUT , between dissimilar atoms , "most certainly" there can be a potential difference.

More-to-the-point , tho (speaking "Real_World" observations), you are dead-on insofar as defining what is known as "Local Cell" corrosion (as opposed to Galvanic corrosion).

A plate of steel is NOT truly "homogenous" (especially when referring to the electrochemical attributes).

There are trillions of DIFFERENT potentials scattered all-throughout the plate. That IS why a plate of steel left in the rain will quickly rust all-over itself. The 'local' anodic sites give-up electrons TO the local cathodic sites ... and, it is THERE (at those cathodic sites) where the "Reduction of oxygen" takes place. Thus, having exhibited the "anodic" reaction ("giving-up" an electron) AND the "reduction" reaction (acquiring an additional electron), REDOX reactions have taken place ... 'SIMULTANEOUSLY'... thus fulfilling the defined requirements for corrosion of a metal (REDOX reactions; reduction & oxidation occuring simultaneously).

NOTICE that "oxidation" ITSELF has NOTHING WHATSOEVER to do with "oxygen".

"Corrosion of metals" CAN and DOES occur quite frequently in the total absence of oxygen. People EASILY accept the "lie" that "oxidation takes place whenever oxygen eats the metal" (or something similarly stupid) simply because most corrosion of steels takes place IN THE PRESENCE of oxygen (either in the air or dissolved in seawater or other water source), and thus, the reduction of O₂ is the predominantly-observed reduction reaction.

ADDITIONALLY , specific to your query: "any "corrode thy self" action (in alloys)?

YES , in the DESIGN and MANUFACTURE of anodes for protecting submerged pipelines, offshore platforms, ships etc ... PLAIN / PURE Zinc cannot be used, even as electronegative as it is, because it will "passivate" VERY readily in seawater due to the dissolved CO₂, carbonates, etc.

THUS , zinc is alloyed with some aluminum AND (in years gone by, mercury, but nowadays) Indium, or in some cases magnesium, to INSURE that the anode REMAINS "active" throughout its life.

Me thinks that covers it .....

At last. Some decent, useful information.

Also check which alloys are commonly used for your shapes. If both the tubes and the fins turn out to be 6061/6063, you should be fine. Then check further with salt-spray tests on sample assemblies.

Actually, we make the tubes and sell to the OEM's that build the coils. We consider many things in the selection the tube alloy - strength (tensile and yield are both important), formability, brazability, corrosion resistance - and must also consider that it is compatible with the typical fin stock used in the industry.

We are looking at clad alloys also that come with their own anodic layer, so part of the tube is protectd from corrosion at the expense of the other part.

As a person once involved in chemical water treatment of heating and cooling systems, I trust you are very aware of the importance of the correct chemicals to be added to your recirculating water. Scale and corrosion inhibitors plus a continual bleed-off are essential to the longevity of cooling systems. Beware of chloride ions from salt spray in coastal environments - I have seen systems destroyed by this because the engineer in charge didn't like the idea of "buying all those chemicals." Needless to say, the replacement cost for the equipment was far higher.

...uh... no disrespect whatsoever intended here, but ...

if you read (past-tense) the OP, as well as his subsequent 'additions', you would have posted something entirely more ON-topic...

I felt that this was a discussion........ similar to brain storming......... EC is looking at it from another angle, and the value of his input is to be determined.

I tagged this discussion because it interested me and I was watching it and In my opinion, I'm glad he gave his input. and gave him a GA for his effort because I saw value in his post even though it had no data.

Keep it up energyconsersion, I for one appreciate your post......

Thanks and I might add to "buying all those chemicals" with "and seeing them all go down the drain." Unfortunately, that is the way of things - you do have to maintain chemical concentrations within prescribed limits and you have to continuously or intermittently bleed the system to avoid the build up of undesirable levels of Ca⁺⁺, Mg⁺⁺, CO3^--, SO4^--, Cl^- from the feedwater and the environment in the recirculating solution.

Hate to see this go off on a tangent,

but what you say is very important......... just did a cooling tower expansion (that required adding another cooling cell) at one of our plants. The first cooling cell had a galvanized cold basin, while the rest have stainless. And I talked to our chemical guy prior to putting in the new cell. that galvanized basin really rises heck to balancing the chemical.........