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Galvanic corrosion: Is it a concern in our projects?

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Posted on August 25, 2023 at 18:54:09
jorisdallaire
Audiophile

Posts: 352
Location: Quebec city
Joined: July 26, 2012
 I have been wondering about this after reading about the HUGE problems the US Navy has with its aluminium bodied ships and their stainless steel propellers. Reading about glavanic corrosion and the anodic index of metals on Wikipedia, it turns out stainless steel and 6000-series aluminium I use is one of the worst combinations one can do; aluminium and gold or brass are even worse, and these are metals we regularly find in our projects. We also have zinc-plated and nickel-plated hardware, silver-plated parts maybe...

Of course we don't operate our gear in sea water (I certainly hope so ;), but there are currents and magnetic fields present and it seems the usual home environment conditions are sufficient to enable reaction. I don't live in a swamp but humidity can reach 65% in my home sometimes.

I just started using aluminium chassis and so don't have long-term experience, having used mainly steel. Anyways most of my projects don't reach the "chassis stage" :-) Maybe members have experienced some problems? If so, what are our options to mitigate the effects if at all possible? In the 70's people used aluminium in home wiring and if you want to join these with regular copper or even with a wire nut I remember there is a special paste to apply to the wiring beforehand. In the special case of aluminium chassis, the metal develops a surface oxyde that mostly prevents corrosion, but I'm not sure it would be effective when in contact with a screw made of incompatible material, and the screw/washer would probably breach that surface layer anyways?

As always, any insight highly appreciated.
-Joris

 

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RE: Galvanic corrosion: Is it a concern in our projects?, posted on August 25, 2023 at 19:58:33
Triode_Kingdom
Audiophile

Posts: 9896
Location: Central Texas
Joined: September 24, 2006
 This topic comes up frequently on forums visited by those of us who own aluminum boats. The degree of corrosive activity depends not only on the specific metals involved, but also which of the metals comprises the larger surface area. In the case of aluminum and stainless, it turns out that this is pretty much a non-issue, at least in fresh water, when it involves stainless bolts (small surface area) in an aluminum hull (large surface area). Saltwater of course creates greater activity, but even so, corrosion as a result of this combination is generally not significant unless also accompanied by fairly large electrical current through the hull. For this reason, most installations of trolling motors, stereos, lights, etc. use a floating ground.

A bigger worry for our projects (or mine, at least) involves the installation of stainless machine screws in aluminum threads. I frequently use panels 1/4" to 1/2" thick to hold various bits of the build together, and it's commonly known in machining circles that stainless screws can gall and freeze in threads cut in aluminum. This sometimes occurs for no apparent reason and even following many insertions and removals that would seem to relax the fit. I haven't experienced this issue in a chassis, but it happened a few times over the years when I was building aluminum accessories for my hot rods or motorcycles. This is one reason (in addition to durability) that most aluminum chassis threading in commercial work is followed by insertion of steel Heli-Coils.


 

RE: Galvanic corrosion: Is it a concern in our projects?, posted on August 26, 2023 at 09:36:56
jorisdallaire
Audiophile

Posts: 352
Location: Quebec city
Joined: July 26, 2012
 Thank you TK for validating what I suspected, that screws can seize in the material due to rust buildup in the threads and that it is not a rare occurence. I have threads in my aluminium structure so to be safe I checked which screw materials would be within the recommended range of anodic index between the two metals, which is 0.25 V difference. That is a quite narrow range and for the 6000-series aluminium the only compatible metal was plain carbon steel, with zinc plating possible at 0.3V difference.

Steel is slightly more "noble" than aluminium so in this combination it is the aluminium that will rust. The contrary happens with zinc-plated hardware; I wonder which is best, having the screw rust or the main material.

I chose "black oxyde" screws, and that's fine for the innards but they look a bit too industrial for the visible parts. I would like to use stainless for these but in the absence of a corrosion mitigating process I'll have to settle for zinc-plated screws. They don't look that bad anyways. I really have to keep that in mind because I have a lot of small M3 screws in the design and even some M2.5. I think even a small amount of corrosion can seize these screws and then you can't use much torque to unscrew them if they do freeze in the aluminium.

 

RE: Galvanic corrosion: Is it a concern in our projects?, posted on August 27, 2023 at 23:40:21
pictureguy
Audiophile

Posts: 22509
Location: SoCal
Joined: October 19, 2008
 Not only that....but a MINUTE (really tiny) current is also generated between dissimilar metals and proportional to temp.

That's why I'd be careful on the most sensitive stage....the phono stage....where 1mv of dc on the circuit could be bad news.....

House plumbing is a victim of this. My 'plastic' pipes were part of a recall.....and installed when the house was build in about '85. When I got around to replacing with PEX the brass or copper barbs on the HOT WATER side were just about gone. I was maybe 6 months from a series of essentially Unfixable leaks which would have caused additional and costly damage.....

And don't forget that copper oxidizes readilly. turns green, in some cases. And at that point is also a semiconductor......At one point Selenium or Copper Oxide was used as rectifier materials....
Too much is never enough

 

RE: Galvanic corrosion: Is it a concern in our projects?, posted on August 28, 2023 at 16:27:43
6bq5
Audiophile

Posts: 4209
Location: SF Bay
Joined: August 16, 2001
 Galvanic corrosion is one aspect of the trend towards entropy - however, as alluded to - there are other issues that may or may not be more significant.

Galvanic corrosion mostly affects items in salt water, and to a lesser extent fresh water. there are galvanic tables which list out metals in an order from Anodic to Cathodic - the anodic is what disappears - or is sacrificial. The presence of salt water is particularly important as it will conduct electricity quite well, and salts have metals that can be displaced or can displace the anodic material. One last not to Galvanic corrosion, adding something like Selenium to a Stainless alloy (like 303, making it 303SE) makes an otherwise cathodic material QUITE anodic. so read the fine print. Also, if you live near the sea, and have lots of fog, you probably have a higher risk of some type of galvanism. ...

There are also issues with metals that gall, or have physical characteristics that do not play well together. Aluminum is particularly difficult, it can gall with many materials, and comes in many different alloys (it is mostly the 5### series that are used for marine applications, like hulls. Aluminum also has a relatively high coefficient of expansion - which can create problems if it were to get warm...
It will also oxidize fairly quickly even inland, creating a white cancer, or rust that is rather hard, and difficult to remove. This is also known as Aluminum Oxide, an abrasive (Moh hardness of 8, scale 0-10), Al2O3, a common material for sharpening / grinding stones.
Happy Listening

 

Good point about the phono stage (nt), posted on August 28, 2023 at 17:04:15
jorisdallaire
Audiophile

Posts: 352
Location: Quebec city
Joined: July 26, 2012

 

Very informative post, thank you (nt), posted on August 28, 2023 at 17:07:43
jorisdallaire
Audiophile

Posts: 352
Location: Quebec city
Joined: July 26, 2012

 

RE: Galvanic corrosion: Is it a concern in our projects?, posted on September 2, 2023 at 23:10:04
pictureguy
Audiophile

Posts: 22509
Location: SoCal
Joined: October 19, 2008
 Anti Seize? Does that help?

Also? NKG, the sparkplug people plate the threads with 3 layers of metal......Plugs are designed to NOT seize in an Aluminum head and Anti-Seize is NOT recommended.

Fake NGK plugs will, when removed, frequently take a chunk of HEAD with. I think at that point, even Helicoils are not possible.....

Home water systems are prone to galvanic problems. Our house when I was a kid had iron pipes and incredibly hard water. My dad replaced everything he could get at with copper AND installed several nylon 'bushings' to isolate sections.
Any boat may also have this problem....fresh or salt. ESPECIALLY if plugged into shore power.

And I've experimented with Stainless bolts in salt water and could etch one thru given a lot of time.

the ONLY alloy I know to be very stable in Salt Water would be H1 or H2, alloys used by Spyderco in its 'Salt Water' line of specialty knives.....
Too much is never enough

 

RE: Galvanic corrosion: Is it a concern in our projects?, posted on September 3, 2023 at 10:15:57
6bq5
Audiophile

Posts: 4209
Location: SF Bay
Joined: August 16, 2001
 Stainless steel is a huge basket of materials that are not monolithic in their characteristics or performance.
and, to the point in your article about H1/H2 steels, an Austenitic steel can become martensitic with cold work - this includes milling, lathe work bending etc.

Passivation can relieve these stresses- and for some Stainless steel, it MUST be passivated for the final fabrication to be stainless....

Stainless steels are grouped together because of their alloys -
most are high in Chromium, with additives such as nickel, Vanadium, and Molybdenum

Wrought materials have different chemistries and properties from Cast materials, and magnetism can be 'tuned'

There are also Inconel steels, a very special version of the "electric alloys" made by Crucible steel (I think closed) that were very resistant to corrosion and wear, but hard to machine...

Then there is Beryllium Copper - very strong, non magnetic, non-sparking(when dropped or struck)....
Happy Listening

 

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