[sru_tx]

While selling furniture in my booth, I am often asked about placing some of my steel/wood pieces outside. Typically I say that I don't recommend it. at all. ever. But seeing how often I get the question I have started working on a new idea for a table that can be placed outdoors.

My thought process went something like this:
What metal? Aluminum? doesn't rust (check) already have some (check)
What to make the top out of? Cast concrete? Weather tolerant (check) Artsy (check)

Marriage made in heaven... or so I thought.

So I put together a prototype and take the unfinished piece to this weekend's market just to gauge response. Interestingly a vendor friend dropped by and over the course of conversation I find out that aluminum and concrete don't mix. The alkali nature of concrete attacks the aluminum. The resulting corrosion doesn't stop once the concrete has cured. It just keeps going, corrosion expanding, until the concrete breaks. Doing research online I now find that it's commonly known issue (electrical conduit in slabs, fence posts in concrete) and just isn't done unless it's coated with a bitumen (asphalt) material or possibly some other coatings.

Question: has anyone here (fence/gate builders?) successfully placed aluminum in concrete? recommended coating material?

The simple solution is to switch from aluminum to steel or stainless but I like the look and have a frame ready to be cast.

thanks.

steve

here's a pic of the rough prototype. no cleanup no polishing.

concrete_proto.jpg (83.42 KiB) Viewed 4187 times

[cj737]

You can certainly treat the concrete with a sealer or stain/paint to separate it from the aluminum. You can also treat the aluminum with paint, powder coat, or epoxy. But raw-raw won't last (as you discovered).

[sru_tx]

Sealing the concrete would only help if the curved support was mounted on the concrete pads. In this case an aluminum frame is embedded inside the concrete on each end to support the piece and the concrete. I'm testing a roof patch material but it's such a mess to work with. I'll check into an epoxy paint. thanks

[Mattwho777]

While selling furniture in my booth, I am often asked about placing some of my steel/wood pieces outside. Typically I say that I don't recommend it. at all. ever. But seeing how often I get the question I have started working on a new idea for a table that can be placed outdoors.

My thought process went something like this:
What metal? Aluminum? doesn't rust (check) already have some (check)
What to make the top out of? Cast concrete? Weather tolerant (check) Artsy (check)

Marriage made in heaven... or so I thought.

So I put together a prototype and take the unfinished piece to this weekend's market just to gauge response. Interestingly a vendor friend dropped by and over the course of conversation I find out that aluminum and concrete don't mix. The alkali nature of concrete attacks the aluminum. The resulting corrosion doesn't stop once the concrete has cured. It just keeps going, corrosion expanding, until the concrete breaks. Doing research online I now find that it's commonly known issue (electrical conduit in slabs, fence posts in concrete) and just isn't done unless it's coated with a bitumen (asphalt) material or possibly some other coatings.

Question: has anyone here (fence/gate builders?) successfully placed aluminum in concrete? recommended coating material?

The simple solution is to switch from aluminum to steel or stainless but I like the look and have a frame ready to be cast.

thanks.

steve

here's a pic of the rough prototype. no cleanup no polishing.

concrete_proto.jpg

Maybe try powder coating

[Poland308]

Cast the concrete with galvanized or ss bolt threads sticking out. Then just bolt the top on with some plastic shims between the concrete and aluminum.

[MarkL]

Put a heavy garbage bag over the aluminum before you pour the concrete. As long as the bag doesn't rip during the pour, the aluminum will never be exposed to the chemistry of the concrete. The point where the aluminum enters the concrete will be a problem because if you just cut the bag flush with the concrete, there will be a small bit of contact between the two. Maybe wrap the point where that interface occurs with electrical tape which will extend beyond the concrete.

[Arno]

Put a heavy garbage bag over the aluminum before you pour the concrete. As long as the bag doesn't rip during the pour, the aluminum will never be exposed to the chemistry of the concrete.

True, but a secondary problem can occur where water (condensation, seepage) gets between the plastic and the aluminium and gets trapped shutting sections off from oxygen. The outer portions still remain exposed to oxygen and this sets up a kind of galvanic internal 'cell'.

This then sets off a crevice corrosion process on the aluminium which basically 'eats away' all the aluminium and likely will crack the concrete with the resulting aluminium salts that are deposited and take up more space.

Coating the part of the aluminium and probably up to an inch or so above the level of the concrete with a tough, chip-resistant paint or surface treatment (anodizing is no use. it gets eaten too) that fully seals the surface from any later water ingress is the key.

In this case with a ornamental piece something like a silver coloured epoxy or powdercoat on the area that gets cast into the concrete will probably be a good option and should not be obvious once it's all set.

For dry/indoor constructions you can actually have 'bare' aluminium cast into concrete as long as the concrete cures and dries out completely. This then basically removes the caustic environment as there's no more water to facilitate the ion transfer and it won't continue eating away the alu. Once water gets into the equation it starts to go again though..

Bye, Arno.

[sru_tx]

Thanks for the responses.
Powder coating is a good idea but I'd like to find something I can do in-house to keep the costs low. I realize that I will need to change the internal foot design to be much cleaner and easier to ensure that all surfaces are coated. My prototype has so many nooks and crannies it will be impossible to fully coat.

Arno, I didn't realize the chemistry required moisture. I found lots of info regarding aluminum electrical conduit buried in concrete slabs. From all of my reading I didn't get the impression that moisture and curing had any effect. Ultimately I need to clean up the design and coat coat coat.

I will clean up the design and then try a thick coating of epoxy paint or possibly a spray on plastic coating like a tool-dip product.

thanks

[MarkL]

True, but a secondary problem can occur where water (condensation, seepage) gets between the plastic and the aluminium and gets trapped shutting sections off from oxygen. The outer portions still remain exposed to oxygen and this sets up a kind of galvanic internal 'cell'.

This then sets off a crevice corrosion process on the aluminium which basically 'eats away' all the aluminium and likely will crack the concrete with the resulting aluminium salts that are deposited and take up more space.

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

[Poland308]

Galvanic corrosion is a big issue with boats. But aluminum is lighter than ss so it's often used despite this fact. It's worse in salt water than in fresh.

[Arno]

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

Bye, Arno.

[Arno]

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!

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

Bye, Arno.

[MarkL]

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?

[Poland308]

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.

[MarkL]

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.

[Poland308]

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.

[Arno]

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.

Bye, Arno.

[MarkL]

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.

[Wood Welder]

Rebar is sand blasted then epoxy coated for use in beach-front construction.

They say four coats to be completely waterproof in some applications like boat bottoms, but that may be overkill for your project.

[Harry72]

Just use steel and put a good coating system on it