I'm intending to build 4 raised vege garden beds, and want them to last as long as possible. Galvanised steel (locally it's commonly called corrugated iron) is likely to be a reasonably economic option.

I'd rather not line them with plastic, so I'm interested to know how long it is likely to take for the steel to corrode to the point where they are either no longer functional or look terrible (they're in an area that needs to be kept reasonably tidy looking).

What's a typical lifetime for a galvanised steel raised garden bed, regularly watered?

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Galvanized corrugated steel longevity in contact with soil is dependent on four variables (chlorides, moisture content, pH, and resistivity) that have the most profound effect on the corrosion rates. Decay rates vary from 0.2 microns to 3 microns per year depending on the variables above. Hot dipped galvanized lasts longer than some of the "plated" types that also have a polymer coating on top of them.

Overall the bed's frame constructed of wood on the outside that is taken care of with regular maintenance should last 30 to 120 years. I'll bet the wood will rot away first. You can mitigate this by treating the wood with various finishes and maintaining them.

But let's focus on the steel and its four variables. Moisture means how wet the soil is and what do you use. Rainwater collected in a cistern would be best; tap water will contain chloride and increase the chloride level. Also, most tap water has some level of calcium added which can buffer the chlorides and salt in your water supply, but I have found that hard water contains lots of additional chemicals. PH in garden soils tends to be acidic but keeping it in the 6 to 7 PH range can minimize issues. Chemical fertilizers will contain many more salts, including chlorides, than organic fertilizers. Organics have lower salts level, and overuse can cause spikes in acidity but the natural bugs in the soil will even that out over time. Soils should be soft and friable which will wick moisture away from the metal more quickly. Loam made with lots of sand, minerals, and inorganic components play into the soil resistivity, if your soil is high in mineral content it will be more conductive and can wear on the metal quicker, but if you build a good loam soil this will reduce this effect.

I encourage you to use local soil if you can, so you can get the flavor of your region in your food; "terrior" perhaps. By the way clay in your soil is good for the metal since it wicks and holds moisture away from the metal.

Your main issue will be constructing a wood frame that is on the outside and not in contact with soil. I would use wood that is rot-resistant, we have lots of hemlock lumber, and finish it with an oil finish or paint it, and keep it off the ground. Use tar paper between it and the ground if sitting on stones or blocks. You could even use some of the newer pressure treated woods as long as they are not in contact with the soils. The old pressure-treated wood treated with toxic metal compounds could leach into food.

A couple more things, I'm not sure what the zinc leaching rate through soil is. I do know that pressure-treated wood chemicals leach around half an inch in the first year. Plants love to pick up ions and accumulate them in their flesh. If you ever have any doubts have the Cooperative Extension lab in your state test the bed's soil and vegetables.

Don't go haywire with the number of galvanized screws used each hole is a place where corrosion will begin. Use barn screws where you can to hold the corrugated panels.

Post a picture of the plans or the finished product if you like.

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