Aluminum/SS Galvanic Corrosion

[Viroos]

Is it OK to use passivated stainless steel hardware (retaining rings, screws, parallel keys etc) on Aluminum 7075-T6 parts? The product will be stored in a sealed package, but for tens of years. Or it will be better to use some carbon steel stuff with less electric potential difference between it and the aluminum body, but in this case I'm worried about the rusting of the hardware itself...
Also, will simple alodine coating help to protect the aluminum?
Thanks,
Max

 

[SwinnyGG]

You answered your own question when you said sealed package.

For galvanic corrosion you need metals of dissimilar potential, and an electrolyte.

If the package is sealed, and no electrolyte is present, there will be no galvanic corrosion.

In addition to that, Your assembly has been designed such that the relative sizes of the components are in your favor regarding galvanic corrosion. To avoid a galvanic couple with high current potential, the more noble metal should have less surface area than the more active one. An aluminum part with stainless fasteners is ok, a stainless part with aluminum fasteners would be bad.

 

[Viroos]

So in dry environment there is nothing to worry about?
And if to chose the hardware for aluminum parts, which kind would you prefer-carbon or stainless steel?

 

[SwinnyGG]

Galvanic corrosion cannot initiate without an electrolyte.

If the parts remain clean and dry they cannot corrode, period.

Fastener material choice is a product of many factors. I can't answer that one for you.

 

[moltenmetal]

The OP forgot to mention that they're filling the sealed container with seawater!

Presumably the fasteners won't stay in the package forever. At some point, they will be put into some service, where the choice between carbon and stainless steel fasteners might matter. If we knew that service we might be able to advise you better. If the ultimate service too is always dry, there is no need to go with stainless in an attempt to mitigate galvanic corrosion, if that is what you're asking.

 

[WKTaylor]

Viroos... "... for tens of years... " is a very rigorous requirement... and a bit odd.

CAUTION.
7075-T6 aluminum and the CRES alloys do not play well together with any electrolyte present... even moisture residue in the packaging or bare-hands contact while inserting the items in packaging can emerge as corrosion damage many years later.

Be aware that 'sealed packages' come in various quality levels. Seams may leak, membranes will be permeable and packaging may be punctured.

IF stored in a very dry climate controlled environment, untouched, good-quality packaging may truly provide indefinite protection.

IF stored in a humid/warm climate with exposure to moisture and/or occasional random handling, even superior quality packaging will not last indefinitely... 10-to-20-years at best.

There are many specifications for packaging out there that You should probably review simply because You are relying on it so much to protect these parts. Suggest You consider aerospace/military quality packaging for 'best chance' at long term protection.

NAS850 GENERAL PACKAGING STANDARD
NAS851 GENERAL PACKAGING STANDARD INDEXES
NAS3406 BAG, PACKAGING OF MULTIPLE ITEMS
NAS3434 PRECISION MACHINED ITEMS, PACKAGING OF
NAS3453 Fasteners, Aerospace, Packaging of
MIL-STD-2073 STANDARD PRACTICE FOR MILITARY PACKAGING

NOTES.
Packaging for extreme long-term storage should be done in an extremely low humidity environment with parts dry-purged and sterile 'cotton gloves' handling.
MIL-PRF-131 BARRIER MATERIALS, WATER-VAPOR-PROOF, GREASEPROOF, FLEXIBLE, HEAT-SEALABLE [or equivalent] should be considered for extreme long-term storage.
Layers of packaging may be required for severe environmental challenges and/or puncture-tear protection.
May consider purging the packaging with inert gas; and/or adding a corrosion inhibitor into the package, such as a VCI [ref NAS3400 VOLATILE CORROSION INHIBITOR]; and/or coating the parts with a CPC [corrosion protective compound] before inserting them into the packaging.

NOTE. A USAF small jet engine was intended for one mission only: yet had to last 10+ years in storage [on/off-wing during the interval]. At anytime during this interval, the engine had to start/run with 100% reliability, any temperature or climate condition, and operate for 3-hours. This problem was solved by a variety of means/methods/materials... and NOTHING was taken for granted.


Regards, Wil Taylor

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[drawoh]

Viroos,

We build aluminium structures assembled with stainless steel fasteners and we fly them over salt water. I have seen no signs of galvanic or any other corrosion. We use anodize and chemical film coatings.

On the other hand, we use aluminium 6061-T6, which is more corrosion resistant. Does it have to be 7075-T6? You could always use a little more of the 6061!

I have been reading up lately on salt water corrosion. Water vapour has a lower molecular weight than air. It is very much harder to seal out than air.

--
JHG