Steel corrosion protection: can/should Fishoilene be sealed with any coating?

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Greendrake

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#296040 16-May-2022 19:24

I am considering options for protecting some welded steel objects from corrosion. These will be buried in soil (a mixture of silt, clay and gravel) and will at times experience some considerable forces both up and down which could possibly damage the protective layer(s). (For those curious, these are feet for timber piles for a solar panel sail which will be pulled up or pushed down depending on the wind direction).

Hot-dip galvanising would have been ideal but is not practicable (the feet are bulky, heavy and I don't know where, how much etc.)

So, my initial intention was to just coat them with CRC Zinc It.

Recently one guy suggested Fishoilene instead. This looks like a good alternative for a thing to be buried, but I am not sure whether it then can (or need to) be painted over.

Fishoilene "will gradually set to a pliable protective coating. Not recommended for overcoating" — as the tin that I've got says. But "pliable" coating doesn't sound like something that can withstand gravel pressing it: it would perhaps give in and expose the steel to the wild.

Then, "overcoating" here is a bit confusing. Does it refer to applying Fishoilene over an existing coating (as opposed to over bare metal/rust)? Or does it refer to applying some other coating over Fishoilene afterwards?

Can/should Fishoilene be top coated with something (e.g. Heavy Duty Primer)?

Interestingly, the Fishoilene spec does not mention any seal/finish options, whereas the spec for Cold Galvit (an alternative surface preparation stuff) does recommend some seal/finish products. From this I infer that Fishoilene is supposed to be the one-and-only coating, and then one needs to be gentle with the coated objects as the coating is "pliable".

Does that sound about right? Or can someone testify that Fishoilene can successfully be painted over and reinforced?

Thanks in advance!

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#2914717 16-May-2022 20:13

If you are burying in soil with gravel, won't the gravel remove any coating and thus provide entry for corrosion?

I would suggest a more robust material, e.g. concrete to coat the steel, namely, cast the steel in concrete in situ

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Rust

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#2914719 16-May-2022 20:21

Fishoilene is an oil product and so never cures like a paint coating. It thickens over time but never 'dries', so other coatings are not likely to be durable if applied over the top.

The only long-term solution for your project, if you must use steel, is hot-dip galvanizing. It may seem relatively expensive, but it will last a very long time.

Consumer grade coatings used on an underground structure are unlikely to provide any noticeable additional life over an un-coated item.

There are industrial coatings that chemically bond to steel, but they are designed for structures too large to hot-dip, and would cost as much.

Greendrake

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#2914720 16-May-2022 20:25

elpenguino:

If you are burying in soil with gravel, won't the gravel remove any coating and thus provide entry for corrosion?

I guess it will depend on how strong the coating is, how sharp the gravel is and whether it moves against the surface vs applies just static force.

The gravel will be rather occasional, not crushed (so not very sharp corners), and I don't anticipate that it will move/scratch the surface. At most, some stones will be just pressing it, mostly through a (however thin) layer of clay/silt. From this I guess that, provided that the coating is hard enough, it should be fine.

Rust:

Consumer grade coatings used on an underground structure are unlikely to provide any noticeable additional life over an un-coated item.

😮

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Ge0rge

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#2914721 16-May-2022 20:27

I would suggest you re-visit hot dip galv.

I'm not sure what your idea of bulky / heavy is, but the last item I got dipped was 4.7m long, 2.2m wide and weighed just over 500kg. The kettle at Webforge Palmerston North is 8.2m x 1.3m x 2.4m.

Galvanising HB serve the majority of the lower North Island, and will pick up / deliver free of charge if you're happy to wait for their scheduled truck runs.

Perry Metals have a kettle in Auckland, Hamilton, Tauranga and Wellington.

Valmont also have a kettle in Christchurch.

Have you considered building the brackets from pre-galved steel? You take off a small amount to weld, and then hit the weld with galv sticks while it's still hot to effectively hot-dip the exposed metal. Not as effective, and normally done on structures you can't get to a kettle, but might be am option.

Otherwise, using your current plan of burying steel - do it once and do it right with hot dip.

elpenguino

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#2914725 16-May-2022 20:36

Rust:

Fishoilene is an oil product and so never cures like a paint coating. It thickens over time but never 'dries', so other coatings are not likely to be durable if applied over the top.

The only long-term solution for your project, if you must use steel, is hot-dip galvanizing. It may seem relatively expensive, but it will last a very long time.

Consumer grade coatings used on an underground structure are unlikely to provide any noticeable additional life over an un-coated item.

There are industrial coatings that chemically bond to steel, but they are designed for structures too large to hot-dip, and would cost as much.

Username checks out. 😀

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Greendrake

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#2914726 16-May-2022 20:36

Ge0rge: I would suggest you re-visit hot dip galv.
...
Valmont also have a kettle in Christchurch.

I am near Invercargill. The feet are much smaller and lighter than the last item you got dipped, so I'll see indeed, thanks.

It's just the thought that, whereas hot dip would provide ultimate protection, won't a proper coating suffice for this use-case? Maybe not, according to Rust above.

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Ge0rge

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#2914727 16-May-2022 20:41

Southern Industrial Coatings - you don't even need to leave town.

mdf

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#2914728 16-May-2022 20:44

The link to the solar sail mentions 17 tonnes of concrete footing. That's nearly ~7m3! Appreciate there is some discussion in the linked thread about calculations and yours might be a different size, but that is a fair amount of rust protection right there. Are you not concreting at all?

Greendrake

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#2914730 16-May-2022 20:50

mdf:

Are you not concreting at all?

Nope, that's why protecting from rust.

Apart from rust protection, the concrete would have been just for the hold-down weight. Soil density is roughly the same as concrete (2.4 t/m3), so I reckoned why would I pour concrete? Soil will hold the feet down just as strong.

The site is also difficult for a concrete truck to access, so unless they have some 20 m long pipes to pump concrete across the paddock it's not an option.

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Lias

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#2914743 16-May-2022 21:10

Greendrake:

The site is also difficult for a concrete truck to access, so unless they have some 20 m long pipes to pump concrete across the paddock it's not an option.

I've got no idea how much extra it would cost, but they most certainly do have concrete pump trucks that can pump concrete way further than 20m. I used to flat with a guy who drove such a pump truck :-)

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Ge0rge

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#2914744 16-May-2022 21:10

Greendrake:
Apart from rust protection, the concrete would have been just for the hold-down weight. Soil density is roughly the same as concrete (2.4 t/m3), so I reckoned why would I pour concrete? Soil will hold the feet down just as strong.

I think you're missing something there. While strictly speaking the densities are close enough, you haven't got a cubic metre of soil in contact with your posts - the soil will separate and you've only got what's immediately in contact with the post as opposed to a solid block of concrete.

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Greendrake

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#2914745 16-May-2022 21:17

Ge0rge:

you haven't got a cubic metre of soil in contact with your posts - the soil will separate and you've only got what's immediately in contact with the post as opposed to a solid block of concrete.

That's what the feet are for. They've been designed to provide horizontal surface which the soil above pushes down. Yes the piles won't be a monolith with the soil but that's fine so long as they are held down with sufficient weight — which they will be as their feet will be.

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elpenguino

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#2914755 16-May-2022 21:50

So without seeing a drawing ( you drew the rest of your structure for SO but not the feet) , you're talking about a large horizontal area, sufficiently deep underground.

If that is correct, then that's all surface area to be attacked underground.

What's your expected life of the structure?

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Greendrake

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#2914756 16-May-2022 21:55

elpenguino:

If that is correct, then that's all surface area to be attacked underground.

Yes, that's why I am aiming to protect it.

elpenguino:

What's your expected life of the structure?

I know it would be around 50-70 years in this soil if it was hot dip galv. If a coating could do say 30 years that would be an acceptable compromise.

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elpenguino

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#2914766 16-May-2022 22:05

How high is the water table? Any tendency for a particular PH of the soil in the area?

What's the cost difference between making the large feet vs concrete with embedded anchor?

You would be surprised where a concrete truck can deliver to, although you haven't specified why access is difficult.

Whenever I consider doing something unconventional I ask myself, why is it usually done another way?

Good to see you have your elevation optimised for winter sun.

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