I assume by the term "Irridite" ["alodine", chem-film, etc] You mean that are going to apply a chemical conversion coating to the Aluminum, such as per MIL-C-5541 Cl 1A or 3.
Bare CRES & bare aluminum by nature are galvanically very dissimilar. Throw in ANY electrolyte and aluminum will be severely attacked by the CRES.
To "covert the aluminum surface, chemical conversion coatings [various formulations] are all acidic electrolytes; and will flow naturally into every pin-hole and crevasse by capillary action. Once in a confined [microscopic] space, the acidic materials are "hell" to flush-out!
Mix CRES and aluminum and a thin layer of trapped acid and You have a guaranteed corrosion failure.
Strongly suggest You apply conversion coating ["Irridite"] to the finish-machined aluminum part W/O any inserts. Insure that all blind and threaded holes/ports are heavily flushed-out [with deionized or pure water] to remove all traces of the trapped "wet" conversion coating. Then install Your CRES inserts.
CAUTION: if possible, the CRES inserts should have a cad, zinc or zinc-nickel plating [etc], with a post-plating chromate or phosphate treatment... or if plating is impractical, apply a thin resin [epoxy] coating to the insert exterior [aluminum contact surfaces]. And if practical [for additional dissimilar material isolation], install the insert "wet" with sealant or a corrosion-protective epoxy primer.
Do I sound a little paranoid about this situation??? I hope I do, cause I've seen numerous material failures [big-time corrosion] around steel or CRES inserts in aluminum. Even inserts in areas flooded with lubricants will eventually be drained and overhauled by cleaning processes which can in-turn entrap caustic or acidic residues by this same chemical processes noted above!!!
Wil, as a metallurgist, I would agree with you from a theoretical standpoint.But, the immersion time with Irridite is so short that none of these bad things are likely to happen. I have coated aluminum-steel friction welds in a phosphate bath (also a conversion coating) as a powder paint pre-treatment and nothing bad happened. The painted parts then passed 400 hour salt spray testing.Now, if we were talking anodizing with steel inserts in aluminum, that would be a catastrophe. The steel inserts will be rapidly eaten away by the sulfuric acid bath, combined with the driving force of the anodizing current. I've done lots of ASTM B117 salt spray testing on aluminum components with steel inserts , which most people would agree is a bad galvanic couple in an agressive electrolyte.It takes 96 hours or so of ASTM B117 testing for anything even to begin to happen.The attack you mentioned will occurr, but it takes a while.All metal eventually goes away due to corrosion.
You seem to have missed Wil's points entirely. The problem isn't immersion time, it is residence time in crevices (for example, weeks/months/years). Also, he is speaking obviously from experience and not theory. There are other threads here at Eng-Tips discussing stainless steel-aluminium couples and the passivity of them. It is bad engineering to dismiss potential complications for this system.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
No,Cory, I fully understood Wil's comments. Here is my take on this, as the question was posed by Newtonville:
Process A--Irridite/rinse the aluminum part and then put the ss inserts in.
Process B--Put the ss inserts into the aluminum and then Irridite/rinse the assembly.
Now, setting aside for a moment the issue with potentially trapped Irridite , consider that an Irridited aluminum part will be in galvanic contact with the stainless insert in either case, as Irridite in not an insulator.In this context, I can't see any difference between Process A and Process B. The original question, as I interpreted it, was a process question not a question about the corrosion implications of putting ss inserts in contact with aluminum.
Ultimately, Newtonville needs to tell us what kind of part this is and in what environment it is to be used if the corrosion aspects are to be addressed. And we can certainly do that!
Thank you for the comments. I want to make my question a little bit more clear.
My question was about the process as well as the corrosion implications. Based on what I have read so far, process is not a problem. But there are two camps on corrosion implications.
Process B in swall's comment is the case. The assembly is used in a coastal environment.
My original worry was that some of the aluminum surface would be masked by the SS insert and not getting iridited.
Coastal environment, eh? I'll be the first to admit that there is a genuine corrosion concern here. Not just galvanic, but general corrosion of the aluminum as well. Irridite is not a sufficient surface treatment for aluminum as a stand alone coating in this sort of envirnment. You need at least a top coat (paint layer) over the Irridite. Anodizing as the base treatment would be even better.
In general, I believe that anodizing would be ineffective in blind threaded holes## [without special cathodes* inserted in the holes]... whereas chemical conversion coatings would have a very high penetration/wetting/coating capability on threads and blind recesses [make sure You "rise-like hell" to insure complete removal of the material]!
## Ref to MIL-A-8625 for practical anodizing geometry limitations. NOTE: anodizing also requires significant deburring and radiusing of edges to insure contintuity of the coating to eliminate potential corner brittleness and flaking].
Note, also, that I strongly suggested inserts be coated with chromate or phosphate [post-plating] treatment or coated with phosphate; AND then plating/coating should be resin-sealed; AND then the insert should be installed "wet" with an added organic isolation finish [primer, sealant, etc] to seal the thread-area from moisture penetration.
The process sequence for irridite (alodine or chem-film), is best done without the inserts installed. This will reduce the need to mask off the CRES inserts, additional rinsing and flushing. As I understand his original question, the concern was "would the CRES inserts mask any of the aluminum during the irridite process?"
My preference is to irridite (chem film) or anodize without any CRES inserts installed. If left unmasked, the CRES inserts will be corroded away from the process solutions used. (From a personal experience I mistakenly concluded that inserts weren't installed, when in fact the insert had corroded away).
However, that said, if proper masking is done, to prevent the leaking of any process solutions, then there wouldn't be any problems with the CRES inserts. The areas that were masked, though, would not have any irridite and would have to be touched up by hand. (Again, this from personal experience. Done by Manufacturing processes and not by Engineering Design.)
