corrosion in a steel+aluminium construction

[Walterke]

Hi

I'm currently designing a workplatform. I'd like it to be as light as possible so I am considering the use of aluminium for the floor plates (another project is running with a plastic floor, but this has currently been put on hold), where the base structure will be built up out of steel tubes.

How much of a problem will galvanic corrosion be in such a construction? Most connections will probably be made with rivets, so there would be "naked" steel-aluminium contact.

NX 7.5
Teamcenter 8

 

[kingnero]

Will this be outside? environment? (seaside, wet/dry, temperature, ...).

aluminum, steel or stainless rivets?
you say "most connections", what will the other be?

provide some more info, as this isn't that hard to work out.
Google will also be helpful.

 

[MiketheEngineer]

Dry environment - usually no problem for at leat 20 years or more.

Wet - a few months

Do all steel or all AL

 

[Cockroach]

The good news is that aluminum oxidizes fairly readily in air to produce one of the most stabilist compounds known to man, AlO. It is very difficult to break down, thermally stable and extremely tough. This layer of passification typically is a few atoms thick and does not conduct an electrical charge.

So what I'm saying is that once you cut a piece of aluminum, it basically oxidizes right away to create an insulation layer protecting the aluminum below. This layer gives shiny aluminum that dull appearance.

Placing aluminum in contact with steel would initially promote galvanic corrosion in the presence of an electrolytic medium, but then the steel would rust (iron passification) and aluminum oxide (aluminum passification). I would expect the two materials not to be extremely valent to eachother as a result.

Damage to either passification layer would only promote oxidation of the freshly exposed metal, hence limited. I think you would be okay.

What I love to do is degrease the steel components, then case harden by liquid nitration, approximately 1/128 inches deep. As an absorption process, the steel maintains the original geometric tolerances, yet increases hardness from HRc 18-22 to HRc 55-65. Nitrated steel does not easily rust and maintains a very deep, black opeg appearance.

So I would recommend doing nothing to the aluminum, case harden by liquid nitration of steel components in contact with the aluminum and building your construction.

Good luck with it.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Walterke]

kingnero, I say most connections because the design is still in it's basic form and I'm not quite 100% sure yet how I'll be doing this. I've considered steel-alu welding, but still have to do some research on this.

As for google: there's a lot of conflicting information on which of the 2 will start oxidizing, the Al, or the steel. If, like cockroach said, the aluminum will rush and not the steel, there is no problem(because of the aforementioned AlO) however if the steel rusts, it might go very fast my work platform probably won't last for a year.

I'm fairly fresh out of school, so I only know these things in theory and was just wondering how people take care of this practically.

What if I would galvanize&paint the steel parts and connect the aluminum with stainless steel rivets?

Building the entire platform out of Al is not possible as I am assigned to use certain stock parts for the construction.

NX 7.5
Teamcenter 8

 

[kingnero]

Environment? dry, inside? or not? this is the first thing you need to design for.

When you say rivets, I assume it's only the alu plates you're going to use, in a steel structure? if so, you can bond them in (see attached sketch) using brackets to avoid lateral movement.
This will also provide isolation between the dissimilar metals.

 

[Walterke]

work platform will be used in construction so outside environment. I assume they will cease working when it's pouring rain, but it will probably get wet from time to time (even of only to clean it)
I'd like to use a 2,5/4mm alu tread-plate (not sure if this is the right english term) for the floor.
Are you suggesting gluing the alu to the steel? or am I missing something?
I don't have any experience with gluing metals to be honest.

NX 7.5
Teamcenter 8

 

[kingnero]

traanplaat = tread plate.

Yes I am suggesting using glue (in a silicone-like substance) (brandname Tec7; Max superbond; anything MS Polymere based).
This way you avoid all direct metallic contact. Using the brackets you secure the plate position (structural function).
Glue only needs to hold the plate in position (no structural function).

Whenever there's water involved you'll get galvanic corrosion, especially when using bolts or rivets as you'll create galvanic cells.
With glue you'll avoid this totally.

I've done this before, 10 years outside (winter and summer) and still a good bond (not a work platform but a passageway for periodic maintenance). Advantage is that it does not completely dry out, so it handles temperature differences (+ differential expansion) without problems.

 

[MiketheEngineer]

Never thought about gluing - but NOT a bad idea!! You could also use plastic ferrules (tubes) to isolate Steel from AL - done all the time.

 

[Walterke]

traanplaat = tread plate.

How did u know I speak dutch?

Either way, I'll think about the gluing, though I'm rather skeptical about it holding under the working conditions. There's a difference between a walkway and a work platform swinging at the end of a telehandler.

NX 7.5
Teamcenter 8

 

[kingnero]

@ Mike:
Those ferrules only work (satisfactory) when you have access to both sides of the connection, like a bolt and nut assembly.
In this case, I'd expect self-drilling and self-threading screws to be used (through the Al. plate and into the steel members). Gasketing will be difficult and costly anyway in this kind of connection.

Maybe there exists something like rivets with isolating liners, however I haven't seen those (yet) (or somewone will prove me wrong).

 

[flash3780]

Have you considered using zinc galvanized steel and galvanized steel bolted connections?

http://www.galvanizeit.org/aga/abou...ng-does-hdg-last/in-contact-with-other-metals

//signed//
Christopher K. Hubley
Mechanical Engineer
Sunpower Incorporated
Athens, Ohio
--

http://engineeringliberty.wordpress.com

 

[Walterke]

The tubes we use are galvanizes, but, because of the welding process (for assembly) some of the galvanization will be gone. We've considered hot-dip galvanizing the entire structure, but we don't find anyone with a bath that's big enough.

Apparently there's also a process called metalizing, which "sprays" a zinc-alu layer onto the steel. I'm assuming something like this would suffice to protect the steel?

NX 7.5
Teamcenter 8

 

[LostHippie]

there are many ways of dealing with this:

you could choose an aluminum alloy series that has a similar galvanic potential to the mild steel, you can find galvanic potential tables all over the internet.

you could install sacrificial anodes of some sort electrically connected to various portions of the structure (ex. zinc blocks). That way the zinc will corrode instead of the steel (the anode is the one that actually destructively corrodes...the aluminum will be cathodically protected.

You could also use a DC power supply dialed to approximately the same voltage generated by the galvanic cell, and apply this voltage with the poles reversed to the galvanic cell to counteract the flow of electrons (if you shoot for something slightly less than the voltage generated by the structure, it will significantly slow the rate of corrosion).

I have seen the sacrificial anode technique used on bridges and water heaters to significantly lower the rate of corrosion...but care must be taken to replace these anodes as they are used up.

Anyway, hope this helps. Peace,

LostHippie

 

[Walterke]

Treadplates are standard ALMG3 5754 H114. So I can't change anything about that.

I've considered sacrificial anodes but was hoping they would not be necessary.

DC power seems a bit far-fetched for our application. But thanks for the idea.

I was really hoping the metalizing would solve my problem. Does anyone have any experience with this?

NX 7.5
Teamcenter 8

 

[swall]

With galvanic coupling situations, you want to minimize the area of the cathode relative to anode area. I think your design inherently does this, given that steel framing (cathode)is contemplated with aluminum tread plates. Isolating the steel to aluminum contacts is also beneficial, as has been mentioned. Powder coating of the steel structure after all welding and fabrication would probably be effective in preventing coupling.You would need to chose a powder coating that will not chip when you rivet the aluminum plates to the steel. Framing made from galvanized steel would mitigate galvanic concerns as zinc and aluminum are usually satisfactory when galvanically coupled in out door environments. There are zinc rich paints that can be used to touch up the galvanize after welding.