I recently started welding and steel fabricating after finishing up a class. I'd like to start making some projects out of steel now rather than wood where I can use steel's strength-to-weight properties and increase my skills in metal fabrication. With wood I know there are some big rules about ground contact because of rot and moisture, primarily using pressure treated wood or other preservatives. Is there an approach when leaving steel in ground contact situations? I know galvanized metal is pretty corrosion resistant, but for welding I end up grinding off the galvanization because of the toxic gasses it gives off and needing a clean weld surface.
Is it as simple as wire brushing and putting a coat of paint on? Ground contact tends to be pretty rough, so I'd imagine a simple rattle can of paint is going to just chip off.
- can you design your projects so welds are above ground and only galvanized pipe is below grade
- tamp the drill hole around the pipe with gravel/sand so it drains and dries quickly, reducing moisture exposure
- use heavy walled pipe, like oilfield drill stem, below grade; yes it will corrode but it will take two lifetimes for that to matter so who cares
- allow a sheen of rust to form on purpose, then use an acid to convert the rust to a stable form of iron, hopefully slowing corrosion (that is the theory; I haven't tested it)
It will probably depend a little bit on what you are trying to accomplish, but generally the thicker the steel, the longer it will last. A piece of bare sheet metal on the ground will rust apart into flakes in no time, but I have pulled metal t-posts out of the ground after 30 years that while quite corroded, were still structurally sound. Your conditions may be different, but I think you could easily figure a decade or two per 1/8th of an inch of thickness before you start to lose structural strength.
A better approach if you are needing ground-contact would be to use concrete below grade, and then embed some way to fasten a steel structure to the footing.
Galvanized steel is good for resisting corrosion, and EMT is a cheap and versatile building material. Since it is so thin, I prefer to braze it. As you note, you have to strip off the zinc - and it is still a good call to wear a proper respirator while you work on it because you will invariably burn some of the zinc! If you go back and spray all the joints with a coat of silver spraypaint, you end up with a durable and clean-looking result. I have also often used the set-screw EMT couplers in projects, as then you can make modular stuff that can be easily taken apart. I made a greenhouse frame that way, and it can be dismantled into a pile of pipes that fit in the back of a truck.
There is "cold-galvanizing" (zinc-rich) paint for touchups of welded galvanized stuff. There's a primer for galvanized metal/aluminum (ACE Rust Stop?)
You could maybe coat the below-grade parts with epoxy instead of paint.
With ANY COATING , thorough, proper preparation is critical for good adhesion.
I think that design to keep the welds/mild steel above ground is a good one. The more thick/massive/solid the mild steel can be, the longer it may endure.
Cold connections like screws/bolts/rivets/epoxy is another way to not weld galvanized...
One more option is stainless steel.
If you look at any major metal structure set into the ground, pylons, pipelines etc you will notice a cable sneaking into the ground.
Its connected to a sacrificial anode, a block of zinc that slowly corrodes away instead of the major structure.
Corten steel actually corrodes by forming a self protecting layer.
Its similar to the galvanising process where the zinc will migrate to a scratch and seal it, with steel it creates a dark brown finish that will precent ultimate destruction of thye matertial.
Nifty I reckon!
John Daley Bendigo, Australia
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Coat it in brass. Clean the area of scale and flux. Heat the area with a torch to about 800 F, then hit it with a pure brass wire wheel on a drill. Brass resists corrosion in most normal circumstances, and it's used on fences and outdoor ironwork. Brass contains zinc and copper. If it was hardware that was just going to deal with humidity and not being rained on or getting wet, then I'd use baked on pine tar that works like seasoning a skillet. That's what I put on my homemade nails and hinges and stuff. The brass coating process also normalizes the weld region and takes the stress out of the transition zone between the weld and the mill finished steel. When you heat the clean steel, it will change colors rather quickly, this isn't a big deal, the brass will still adhere.
John C Daley wrote:If you look at any major metal structure set into the ground, pylons, pipelines etc you will notice a cable sneaking into the ground.
Its connected to a sacrificial anode, a block of zinc that slowly corrodes away instead of the major structures.[/url]
True! Except every form of cathodic protection I have seen requires the application of a DC voltage to reverse the corrosion cell, moving corrosion from the protected metal to the sacrificial anode. Is there a way to do this in a passive system?
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