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Aluminum in Concrete - no good

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Re: Aluminum in Concrete - no good

Postby Arno » Fri Sep 08, 2017 1:41 am

MarkL wrote:If this occurs, how can aluminum be used in marine applications where part of the aluminum is submerged and part is above water?


In this application theres loads of dissolved oxygen in the (sea)water, so apart from the normal corrosion issues there's no potential being generated from the big difference in oxygen concentration like in crevice corrosion situations. The water in this case also gets constantly renewed/replenished so the various (by)products and parts of the reaction get washed away and can't keep the cycle up.

Key point here is that for crevice corrosion like this to start the trapped water needs to get into an stagnant and oxygen depleted state to start the reaction as (combined with naturally dissolved salts) it's what generates the galvanic cell. The lack of oxygen also prevents the alu from (re)forming it's oxide skin so the reactive base metal remains exposed all the time to keep on reacting.

The water itself is not the issue and as long as its free to 'breathe' so the alu can oxidise and skin-over then it's also fine.

You will see the same effect on boats or other setups where for instance 2 alu parts are bolted together and the mating surface is not sealing well so a film of water can get between them. This stagnant water film will start the same reaction and when the parts are taken apart the surface between the parts and around it will have siginificant chunks eaten away.

Bolted pipe flanges can show this effect as a progressing corrosion 'wedge' between the mating sufaces that starts on the outer edge and then begins working it's way into the join.

In many cases it's often not an issue but (also on many other metals) when there are spots in a design where water can pool/collect and get trapped in confined spaces where it has little to to exposure to the oxygen in the air anymore, trouble may be ahead..

Water seems so harmless, but it REALLY wants to be friends with almost everyone and steal their ions if you leave it alone for a while :lol:

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Re: Aluminum in Concrete - no good

Postby Arno » Fri Sep 08, 2017 1:47 am

Poland308 wrote: It's worse in salt water than in fresh.


Yup.. Seawater is a great electrolyte! And then people start adding stuff like carbon-fiber into the mix (eg. racing ocean going sailing vessels) and that makes it even worse! :shock:

And somehow these guys are not too thrilled of sticking a few tonnes of sacrificial anode metal blocks on their ships to help reduce it :lol:

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Re: Aluminum in Concrete - no good

Postby MarkL » Fri Sep 08, 2017 9:07 am

Arno wrote:Key point here is that for crevice corrosion like this to start the trapped water needs to get into an stagnant and oxygen depleted state to start the reaction as (combined with naturally dissolved salts) it's what generates the galvanic cell. The lack of oxygen also prevents the alu from (re)forming it's oxide skin so the reactive base metal remains exposed all the time to keep on reacting.

I went off and read about crevice corrosion, which I had never heard of, very interesting. I understand the mechanism but don't understand the reference to a galvanic cell. I don't see any mechanism for galvanic action, it appears to be driven only by lack of oxygen to maintain the oxide layer on aluminum (or stainless). Were you speaking literally or figuratively about a galvanic cell?
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Re: Aluminum in Concrete - no good

Postby Poland308 » Fri Sep 08, 2017 4:43 pm

If you look at the galvanic tables you will see that any two metals will corrode through galvanic corrosion. The farther apart they are on the chart means they will corrode faster. Humidity in the air above 50% is enough to get the process started. Rain and snow melt contribute even more. Especially if your roads are treated with salt, or worse yet calcium chloride that's sprayed so often in a brine solution in northern climates. Even natural ocuring salts in the ground and ground moisture is enough to cause real problems with buried pipe.
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Re: Aluminum in Concrete - no good

Postby MarkL » Fri Sep 08, 2017 5:37 pm

Poland308 wrote:If you look at the galvanic tables you will see that any two metals will corrode through galvanic corrosion. The farther apart they are on the chart means they will corrode faster. Humidity in the air above 50% is enough to get the process started. Rain and snow melt contribute even more. Especially if your roads are treated with salt, or worse yet calcium chloride that's sprayed so often in a brine solution in northern climates. Even natural ocuring salts in the ground and ground moisture is enough to cause real problems with buried pipe.


I understand galvanic action, but I don't see how it's involved in the explanation Arno provided because there is only one metal involved (aluminum). Galvanic corrosion requires two different metals. Crevice corrosion only requires the oxygen at the surface to be depleted, there is no need (that I can see) for galvanic action to occur.
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Re: Aluminum in Concrete - no good

Postby Poland308 » Fri Sep 08, 2017 7:02 pm

Cement contains enough other metals to start the process. If you add in the plastic bag. It becomes a place the condensation forms and collects. This becomes the focal point of the corrosion.
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Re: Aluminum in Concrete - no good

Postby Arno » Mon Sep 11, 2017 1:19 am

MarkL wrote:I understand galvanic action, but I don't see how it's involved in the explanation Arno provided because there is only one metal involved (aluminum). Galvanic corrosion requires two different metals. Crevice corrosion only requires the oxygen at the surface to be depleted, there is no need (that I can see) for galvanic action to occur.


Yes. You are correct.

It's just that the underlying principle that kickstarts crevice corrosion also bases itself partly on the flow of electrons and ions a bit like galvanic corrosion does when an electrolyte (water with some salts) is present.

Although in this case it's the lack of oxygen that can start to break down protective oxide layers ('steal' the O2 from it) and resulting slighty different O2 concentrations allows a kind of 'conveyer belt' of electrons and ions to start up and slowly (although scan be still fairly fast..) eat away at the base metal. (fun fact.. exhaust oxygen sensors use a difference in oxygen concentrations in- and outside the exhaust between a membrane to create a tiny little 'battery'...)

The specific circumstance here is that the lack of oxygen means it's an effect that also happens and continues to metals (like alu) that normally 'skin over' and protect themselves so it can be a pretty sneaky effect. Can usually be designed out as long as the designer makes sure there's no spots were water collects in joints but can drain or run off.

I was perhaps muddy-ing the water by bringing up galvanic corrosion in this context and not getting the reference clear. Apologies for that.

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Re: Aluminum in Concrete - no good

Postby MarkL » Mon Sep 11, 2017 8:54 am

Arno wrote:The specific circumstance here is that the lack of oxygen means it's an effect that also happens and continues to metals (like alu) that normally 'skin over' and protect themselves so it can be a pretty sneaky effect. Can usually be designed out as long as the designer makes sure there's no spots were water collects in joints but can drain or run off.

Over the years I've noticed places on docks and boats where two aluminum surfaces were pressed together and were corroded. I always just shrugged and thought it was crud growing in there, like algae or something. Now at least I know what's going on.
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