Steel + Aluminium melding?

"A "lasagna cell" is accidentally produced when salty moist food such as lasagna is stored in a steel baking pan and is covered with aluminum foil. After a few hours the foil develops small holes where it touches the lasagna, and the food surface becomes covered with small spots composed of corroded aluminum."

Yeah. Corrosion. It's typically galvanic corrorion, and you aren't going to stop it permanently. Depending on the application, your best bet may be to bolt and unbolt them frequently. Stainless steel bolts will not help.

I use lead-based anti-seize (from a tin I've had for 40 years). It only helps a little. It''s probably no better than petroleum jelly, but it makes me feel better.

Steel and stainless steel fasteners corroding aluminum is such a common problem, especially in marine applications, that a variety of coatings and even electrical protection schemes have been developed. The electrical methods work but are only practical on things like boats.

Your best bet is to keep them dry with really good coatings, and try to avoid entrapped air. Use lots of petroleum jelly. Entrapped air leads to pitting corrosion, which is not as bad as having a completely wet galvanic cell, but it's something to avoid.

Googling this , I came up with "galling" or "friction welding" , but some sort of galvanic corrossion process going on looks likely, despite no obvious iron rust or white aluminium corrossion product.

Galling happens when the surface of the metal tears. It's more likely when you're fastening or unfastening bolts under a high load. You aren't likely to encounter friction welding without a lot of speed and force.

A white corrosion on the aluminum is oxidatation. Likewise, red iron rust. Galvanic corrosion can produce white compounds, or black ones on iron or steel, but it also can result in a fairly clean-looking surface.

On boats that are left in salt water, they use sacrificial anodes of zinc, and the zinc preferentially erodes by galvanic corrosion. These can be completely clean even when they're severely corroded away.

Boat riggers use lanolin or a mixture of tar and lanolin for such things.

But aluminum is always a problem.

Joe Gwinn

Sounds like a great heritage mix, like oil-cloth. Was that used only in the past, or does its use continue to this day?

So, how well do the triple anti-sieze compounds work for this? Copper/Aluminum/Graphite in a petroleum lubricant base. [I've used an old tube of Permatex aluminum AS (and dielectric grease) for tune-ups/spark plug installations for 40+ years, all without problems.] AS has always been good for steel-to-potmetal outdoor equipment, too.

I was about to purchase one of these for the solar installation early next year.

Comments?

Superstrut, Unistrut short-spring nuts, stainless bolts, galv lags into the roof. I'll flash the top of each rail and elastomeric-goop each thru bolt in the roof, never EVER wanting a water leak to pop up.

Why not use a rubber isolation mount? That way one side would stay attached to the steel and the other side would be in the aluminum. No direct connection between them. Or use a rubber bulkhead fitting. Self sealing and no connection between the two components.

Yeah, stainless screws in aluminum in marine service is the worst. Very common on outboards. I started using a very expensive Loctite antiseize with about 50% by volume zinc powder. It helps a lot.

Pete Keillor

With steel bolts in aluminum , you will get galvanic corrosion. If the bolts are plated, it might be less so you do not see the white corrosion products. If you can find aluminum bolts, they would be better.

Dan

It's still current practice. I got the recipe from page 211 of the following book, bought in 2004 from a West Marine store near Boston:

"The Complete Rigger's Apprentice - Tools and Techniques for Modern and Traditional Rigging", Brion Toss, International Marine, Camden, Maine,

1998 McGraw-Hill.

By rigging, they mean the cables that keep the mast upright on a sailboat, despite the wind forces that propel the boat.

I have not tried these, and so cannot say, but the boating community has no doubt tried these, so I'd ask on a boating newsgroup.

Stainless steel screws into aluminum masts famously become impossible to remove due to corrosion.

Someone else mentioned having some success with something that was 75% zinc dust in grease - sounds like the stuff electricians use when making connections with aluminum wiring.

If you can arrange it mechanically, electrically isolating the aluminum from the stainless steel with nylon washers and/or FR4 sheets (choose materials that weather well) will stop galvanic corrosion more or less completely. But make sure there are no sneak paths for electrical currents to flow.

If this cannot be arranged, another nautical dodge is a sacrificial electrode of zinc or magnesium.

Joe Gwinn

That substance is sometimes called Noalox. It stands for no aluminum oxidation. Its applied in between wherever copper and aluminum surfaces are touching to prevent fires, corrosion, loose connections, etc..

It's really the best solution for bare metals.

Lanolin is still used today - on some of the most exotic sailing rigs.

It really is the best solution to galvanic corrosion of bare metals in contact.

I use it myself and I'm thankful when time comes to remove bolts from aluminum.

It's currently 1998 where you live. Interesting. ;)

Ayup, BTDT, got wet.

And, when you break off the head trying, they're nearly impossible to drill out without a jig. Otherwise, the drill bit skitters off into the very soft surrounding aluminum and drills deep INSTANTLY. DAMHIKT

Ick! (not fond of AL wire)

I found out recently that Superstrut (unistrut) is zincked and then yellow chrome plated. Hmm, more to think about with chromium against aluminum PV framing + galv and stainless hardware.

Since PV panels are entirely electrically separated from their framing, I wonder how much galvanic corrosion happens, during the course of a year's weather, between the rack, the frames, and the holddown hardware. Surely enough to warrant anti-sieze, but does all the hardware need more? I'll have to ask the local PV guys.

I haven't seen anything used by the pros during pro installation on YouTube vids, but what I have seen is all aluminum racking against aluminum PV frames with stainless holddown bolts.

I've sailed into Camden harbor on a day when the forest of tall wooden masts made it seem like 1898.

-jsw

Zinc Chromate paint was the stand-by for treating aluminum in the airplane business for years and years.

Ayup, things travels slowly around these parts.

Asking around in the relevant communities is the best way to find the best approach. I'd ask around in boating circles - if it works in a salt air environment, it will work all the better inland.

Joe Gwinn

What about thread inserts or Helicoils?

What is actually happening is that the galvanic corrosion produces aluminum oxide which as it has a greater volume then the aluminum causes the bolts to "freeze" in place. It is not a galling or friction welding process, but rather the effect of a bolt in a much too small hole.

To prevent this the usual practice is to insulate the aluminum from the steel in some manner. There are special sealers on the market and in emergency I have used silicone sealant.

But for long term service, the usual practice in making a two, or more, piece aluminum mast for instance, is to use fasteners what are very close to aluminum in the galvanic scale and thus eliminate the cause.

Actually stainless increases the problem as it is even further down the galvanic scale then plain steel.