Aluminum and copper -- Galvanic corrosion

[Curiousphy]

If a copper water pipe is connected to a piece of aluminum through a copper wire, where will the galvanic corrosion take place? On the wire to aluminum joint eating away at the aluminum, or will the whole water pipe suffer? (does copper corrode aluminum, or vice versa, or each other?) thanks

 

[PhanthomJay]

If a copper water pipe is connected to a piece of aluminum through a copper wire, where will the galvanic corrosion take place? On the wire to aluminum joint eating away at the aluminum, or will the whole water pipe suffer? (does copper corrode aluminum, or vice versa, or each other?) thanks

Well I am not a materials expert by any means, but my years of experience tell me that
1. Copper to aluminum connections should be avoided;
2. It is the copper that corrodes the aluminum, and in the process, both the clamp and wires corrode at the connection;
3. When copper to aluminum wire connections are unavoidable, use as large a mass as possible of an aluminum bolted clamp; and
4. Install the aluminum above the copper, to prevent the copper ‘salts’ from ‘dripping’ onto the aluminum.

 

[Curiousphy]

If I treat the aluminum piece and the copper wire as consumable and easily replaceable, is the copper pipe itself totally safe from corrosion through this connection? actually, would this give the copper pipe some protection and effectively make the aluminum piece a sacrificial anode?

What if the copper wire was connected to a metal that was less reactive than copper, would it corrode the wire at the connection or corrode the pipe as well? would the copper wire provide a corrosion buffer for the pipe?

 

[tech99]

If I treat the aluminum piece and the copper wire as consumable and easily replaceable, is the copper pipe itself totally safe from corrosion through this connection? actually, would this give the copper pipe some protection and effectively make the aluminum piece a sacrificial anode?

What if the copper wire was connected to a metal that was less reactive than copper, would it corrode the wire at the connection or corrode the pipe as well? would the copper wire provide a corrosion buffer for the pipe?

Why do you not keep everything copper? Once you introduce aluminium it will create problems however you do it.

 

[Nik_2213]

Bulk aluminium is only stable due to the superficial oxide layer. If this is breached, the metal can corrode rapidly.

Similarly, I've known a copper hot-water tank fail because, the plumber discovered, it was resting directly on a flush floorboard nail...

Examples of rapid aluminium corrosion:
IIRC, Mercury thermometers etc are not allowed on aircraft lest break or spill attacks the structure.

'Waterglass' for sealing eggs is seriously alkaline. It ruined an aluminium pan used in error, tainted that batch of eggs. Although the 'waterglass' was labelled correctly, our complaints department sent the victim a very nice set of steel pans as 'goodwill'.

Our lab had a few aluminium weighing scoops as their 'tare' was significantly less than s/steel. I happened to find one that had sprouted a 'feather' of oxide, like a 'crystal garden', but in air. It continued growing as I watched, several mm/min. Ooh, pretty ! Sadly, I never learned what had set it off, and my few experiments failed to reproduce the effect. But we did replace those few Al scoops...

 

[PhanthomJay]

If I treat the aluminum piece and the copper wire as consumable and easily replaceable, is the copper pipe itself totally safe from corrosion through this connection? actually, would this give the copper pipe some protection and effectively make the aluminum piece a sacrificial anode?

What if the copper wire was connected to a metal that was less reactive than copper, would it corrode the wire at the connection or corrode the pipe as well? would the copper wire provide a corrosion buffer for the pipe?

The aluminum becomes the anode, but in the failed corroded copper-aluminum cable connections I have seen, the corrosive failure has been limited to the immediate area at the connection. The rest of the aluminum wire and copper wire beyond the joint has undergone just the normal weathering process in forming the copper (green) and aluminum (darkened gray) oxides.
Best to use copper to copper connections, although copper to steel or copper to bronze connections are fine.

 

[Curiousphy]

The aluminum becomes the anode, but in the failed corroded copper-aluminum cable connections I have seen, the corrosive failure has been limited to the immediate area at the connection.

Thank you, that's good to know. I know you can connect two dissimilar metals via non-contact connectors, but it's also good to know that you can use wires as corrosion buffer.

 

[Curiousphy]

Why do you not keep everything copper? Once you introduce aluminium it will create problems however you do it.

I just wanted to understand the corrosion process better. I already knew what would happen if everything was copper :)

 

[Curiousphy]

@Nik_2213 Interesting story :) reminds me of youtube

 

[Tom.G]

although copper to steel or copper to bronze connections are fine.

Copper pipe connected to Galvanized pipe results in removal of the Zinc galvanize and continued corrosion of the steel pipe. Connecting them with a wire does not help; if anything it would make things worse. The Copper and Zinc, or Steel, act as a battery with the water being the electrolyte. When the are in electrical contact the battery is short circuited and starts dissolving the electrode. Same as what happens when a Carbon-Zinc flashlight battery is used, the Zinc case dissolves and it starts leaking

I lived with that for a while where the water heater was connected to the cold water pipe with Copper flex tubing (it should have been Brass or maybe Bronze rather than Copper). The result was we could not drink the tap water and the flow rate was reduced. The Iron content was so high that we got stomach aches from it! Just like if you take Iron supplement pills on an empty stomach. Ouch! (Tasted bad too.)

 

[PhanthomJay]

Copper pipe connected to Galvanized pipe results in removal of the Zinc galvanize and continued corrosion of the steel pipe. Connecting them with a wire does not help; if anything it would make things worse. The Copper and Zinc, or Steel, act as a battery with the water being the electrolyte. When the are in electrical contact the battery is short circuited and starts dissolving the electrode. Same as what happens when a Carbon-Zinc flashlight battery is used, the Zinc case dissolves and it starts leaking

I lived with that for a while where the water heater was connected to the cold water pipe with Copper flex tubing (it should have been Brass or maybe Bronze rather than Copper). The result was we could not drink the tap water and the flow rate was reduced. The Iron content was so high that we got stomach aches from it! Just like if you take Iron supplement pills on an empty stomach. Ouch! (Tasted bad too.)

That is interesting. The connections I am familiar with involve stranded copper about one-half inch diameter (1.5 cm) cable in contact with a galvanized steel clamp. The clamp is much more massive than the wire, and I am unaware of any problems. I've seen several such connections about 100 years old. The clamps are heavily corroded from decades of exposure to weather, and not from the copper wire within. I still specify bronze or steel clamps when in contact with aluminum (can't afford copper clamps!).

 

[Tom.G]

That is interesting. The connections I am familiar with involve stranded copper about one-half inch diameter (1.5 cm) cable in contact with a galvanized steel clamp. The clamp is much more massive than the wire, and I am unaware of any problems. I've seen several such connections about 100 years old. The clamps are heavily corroded from decades of exposure to weather, and not from the copper wire within. I still specify bronze or steel clamps when in contact with aluminum (can't afford copper clamps!).

Different situation. I was referring to connections that were all bare metal immersed in an electrolyte such as water. I agree, exposure to weather is not usually a problem.

The OP was a bit unclear about his exact situation, so I responded to one possibile condition... and as a point of information to others.

Cheers,
Tom

 

[Greg London]

Dumb question: if a copper wire bolted to aluminum wire causes corrosion, why doesn't the 90copper/10aluminum version of bronze do the same thing and corrode?

I just started looking into casting bronze (ive done some aluminum before) and it sounded like copper/aluminum is stonger/better than copper/tin. No one mentioned corrosion.

 

[PhanthomJay]

Good question. I didn’t say that correctly. When connecting copper to copper , use a copper or steel or bronze clamp. When connecting aluminum to aluminum, use an aluminum or steel clamp. Now when connecting copper to aluminum, this is bad and should be avoided, but if unavoidable, use a steel or aluminum clamp where if aluminum clamp, it must be of large mass with respect to the size of the copper, and the copper should be on the bottom side so that it’s salts do not ‘drip’ onto the aluminum. This is all with air as the surrounding medium.

 

[essenmein]

I'd inadvertently created this Al to Cu corrosion engine, Cu ground strap connected to Al heatsink, after few weeks the ground connection would go higher resistance and subsequently melty melty.

Tinning the copper with solder before assy prevented the effect, or at least slowed it to the point it was no longer noticeable, it was a prototype, that joint was changed for for production intent lol.

 

[SHD]

A number of years ago, I worked for a pulp mill. After the wood chips were cooked and pulped, the pulp, at 3% consistency, had to be separated from impurities.

This was accomplished by pumping the stock (under pressure) through cyclonic centi-cleaners attached to a stainless steel header with nylon hoses. The hoses connected to the header with standard garden hose fittings. Since SS fittings were expensive, brass connectors were often subbed in.

The punchline, after all this preamble: The process engineers decided the pulp had to be acidulated, so sulfuric acid was added upstream of the cleaners. It took about 2 days before the brass corroded enough to fail and they all blew out, spraying pulp all over. It was 3 feet deep on the floor and the mill had to shut down for 3 days for repairs.

All because nobody knew that two dissimilar metals in an acid bath would corrode.

 

[Nik_2213]

@Nik_2213 Interesting story :) reminds me of youtube

Yes, serendipitously, I came across that video just this week...


Difference, my experience lacked liquid mercury. Was it a mercury salt which deliquesced ? Or a copper salt, which had reduced to copper at a so-slightly damp scratch ? Having used 'activated zinc' in Uni organic syntheses, 'tis an enduring puzzle...