Al and steel in contact

[mstrlucky74]

Anyone have experience with aluminum and steel being in contact with each each. We have an aluminum light shroid that's attached to half steel strut and stainless steel bolts. The engineer is saying the aluminum and steel can't contact each other as it will cause a galvanic reaction and corrode over time. True?

 

[Coppersmith]

True. But time is relative.

https://en.wikipedia.org/wiki/Galvanic_corrosion

When plumbers run copper lines using steel strut they put rubber insulators between the dissimilar metals.

 

[mstrlucky74]

So the engineer is correct?

 

[retirede]

So the engineer is correct?

In theory, yes.

The difference between theory and practice is that, in theory, they are the same, but in practice, they’re different.

In other words, galvanic corrosion can occur. To what degree depends on a lot of factors such as moisture present. It’s unlikely to cause any real problems in most installations.

Aluminum rigid conduit is terminated in steel boxes all the time.

 

[paulengr]

I doubt it.

Aluminum is -.76 to -.99 V. Steel is -0.58 to -0.71 V.. Copper is -0.31 to -0.40 V. So there is about a 0.5 V difference between copper and aluminum but only about 0.1 V between steel and aluminum. We don’t generally see much corrosion of copper in contact with steel at around 0.25 V difference and there is even less between aluminum and steel. There is still a difference but not much to drive the reaction. And I rarely see much corrosion that is much different from “general environment” corrosion in practice. Bigger differences like aluminum and copper generally are a problem but under 0.25 V you won’t see much if anything because the potential is so low.

 

[curt swartz]

I guess this engineer doesn't allow standard Aluminum conduit bodies (LB's, C's) or hubs to be used with steel raceways. Or diecast aluminum fittings to be used with steel EMT?

 

[Carultch]

I doubt it.

Aluminum is -.76 to -.99 V. Steel is -0.58 to -0.71 V.. Copper is -0.31 to -0.40 V. So there is about a 0.5 V difference between copper and aluminum but only about 0.1 V between steel and aluminum. We don’t generally see much corrosion of copper in contact with steel at around 0.25 V difference and there is even less between aluminum and steel. There is still a difference but not much to drive the reaction. And I rarely see much corrosion that is much different from “general environment” corrosion in practice. Bigger differences like aluminum and copper generally are a problem but under 0.25 V you won’t see much if anything because the potential is so low.

There's a huge difference between aluminum and stainless steel on that galvanic scale, yet those metals are mixed together all the time, and we think nothing of it. How exactly does this work, according to the theory of avoiding mixing dissimilar metals?

 

[LarryFine]

Even aluminum to aluminum can be a problem. I once had to cut an aluminum LB to remove it from aluminum conduit.

I made a diagonal cut part way around the hub of the LB (4") and then twisted it open with a large flat-blade screwdriver.

The threads were coated with white oxidation, so I wire-brushed them, ans used no-ox to add a coupling to extend it.

 

[electrofelon]

There's a huge difference between aluminum and stainless steel on that galvanic scale, yet those metals are mixed together all the time, and we think nothing of it. How exactly does this work, according to the theory of avoiding mixing dissimilar metals?

Having an electrolyte between the two is a requirement for the galvanic reaction, so things in dry areas will be generally unaffected. I do tell people to avoid stainless and aluminum in wet locations, as that can get very bad. I had to take my ladder rack off last year to replace a broken rear window, and it is aluminum with stainless hardware. It has been on there for 5 years and it could barely get the bolts out there was so much corrosion.

 

[retirede]

Even aluminum to aluminum can be a problem. I once had to cut an aluminum LB to remove it from aluminum conduit.

I made a diagonal cut part way around the hub of the LB (4") and then twisted it open with a large flat-blade screwdriver.

The threads were coated with white oxidation, so I wire-brushed them, ans used no-ox to add a coupling to extend it.

Aluminum oxidation is not the same as galvanic corrosion.
But as you point out, it can still be a problem!

 

[paulengr]

There's a huge difference between aluminum and stainless steel on that galvanic scale, yet those metals are mixed together all the time, and we think nothing of it. How exactly does this work, according to the theory of avoiding mixing dissimilar metals?

Stainless is kind of strange. The zinc acts almost independently, like galvanized. It corrodes leaving the steel untarnished. Also the behavior of oxides is important. The oxides of stainless form a protective layer just as aluminum oxide is a protective layer for aluminum.

 

[gadfly56]

Stainless is kind of strange. The zinc acts almost independently, like galvanized. It corrodes leaving the steel untarnished. Also the behavior of oxides is important. The oxides of stainless form a protective layer just as aluminum oxide is a protective layer for aluminum.

Where are you getting zinc with stainless steel or aluminum? In either metal, any significant quantity would be considered a contaminant. Brass is an alloy of copper and zinc. Bronze is an alloy of copper and tin. Stainless steel is a low carbon alloy of iron, chromium, nickel, and occasional bits of molybdenum, sulfur (not too much!) and what not.

 

[paulengr]

Where are you getting zinc with stainless steel or aluminum? In either metal, any significant quantity would be considered a contaminant. Brass is an alloy of copper and zinc. Bronze is an alloy of copper and tin. Stainless steel is a low carbon alloy of iron, chromium, nickel, and occasional bits of molybdenum, sulfur (not too much!) and what not.

I was thinking of chrome. Nickel generally isn’t in large quantities until you get into high temperature alloys like 309, 310, or the Inconels. It is not necessarily low carbon either (think of knife blades).

 

[gadfly56]

I was thinking of chrome. Nickel generally isn’t in large quantities until you get into high temperature alloys like 309, 310, or the Inconels. It is not necessarily low carbon either (think of knife blades).

304 stainless is also known as "18/8" for its chromium/nickel content. It is probably the most common stainless steel and has a max carbon content of 0.08%. Add 2% moly and you get 316, the second most common. The 309 stainless has 14% Ni, so if you think 8% in 304 is low, OK. Cutlery steel is usually a 420 or 440C stainless and has about 1.1% carbon, much higher than 304.

 

[rnatalie]

If you're talking about casual aluminum touching, you are also protected by the rapidly forming aluminum oxide on the surface of the aluminum. It will inhibit further galvanic action. How if you were making a tighter connection, then you may have an issue.