Standards for Salt Corrosion Protection for Exposed Steel

[allgoodnamestaken]

Hi all,

I've been given a relatively simple project involving the design of a platform on which a 30' tall 16' diameter silo is going to stand. Trucks will be driving through the platform legs and a pneumatic system will allow them to actuate the silo discharge door to fill up.

The problem is that the product in the silo is road salt. I'm making the assumption that spills will happen and the steel platform will be in a relatively aggressive environment. There is no water near the site but the area is subject to heavy snows and that is when the use of road salt is most frequent.

I'm looking for some standards or examples on specifications for specifying paint or galvanization for the steel platform and its connections with the concrete foundation below.

Can anyone point me in the right direction?

 

[SlideRuleEra]

For hot-dipped galvanizing, consider the information in this link at the American Galvanizers Association:

http://www.galvanizeit.org/educatio...cal-faq-dr-galv/lifetime-vs-coating-thickness

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[TehMightyEngineer]

I was part of a project that involved repair and corrosion protection for a road salt storage shed with thin, galvanized steel walls so I can share some insight.

We saw increased corrosion at the bases of the walls where the salt would pile up adjacent to the steel and then be exposed to moisture. Areas where water or salt could not sit against the steel showed very limited corrosion.

Embed the platform legs in concrete (if able) such that the painted/galv. steel starts well above the grade of the platform. and slope everything so that no standing water will be able to pond in contact with steel. Design an accpetable corrosion loss amount into the project and have it documented, otherwise when this rusts they'll just let it go without a good number that they can target and design a maintenance program around. Avoid creating areas were salt can get in places that are difficult to clean or observe.

You'll probably want a good coating system; something like an organic zinc primer, 6 mils of epoxy mastic, and then a finish coat of polyurethane. 12.5 mils DFT total. That's what my company typically specifies for heavy corrosion environments.

Maine EIT, Civil/Structural.

 

[allgoodnamestaken]

Thanks TehMightyEngineer,

I anticipated some of your suggestions (grade sloping, minimizing spaces where salt or moisture can be trapped against the structure). I like the idea of encasing the bottom of the columns in concrete to make sure nothing pools against them.

a really simplified diagram of what I'm intending looks like this:


~~~/--~~|-----| silo
~~|-----|
~~|-----|
~====== steel platform
~~|-----| columns
~~|-----|
~~[]---[] bottom of columns encased in concrete (3" curb? 6"? no idea what's appropriate)
=========== reinforced concrete slab foundation with thickened edges

As far as coating this is what I had thought appropriate based on the Canadian Institute of Steel Construction recommendations (I'm in Canada), but looking at it now it's probably just a minimum. I got it from here:

http://www.cisc-icca.ca/solutions-c...es/coatings/tips-on-painting-structural-steel

I had chosen Surface Preparation SP-10 and Environment Zone 2b which gives me:

1) Near-White blast cleaning of steel
2) 3 mils zinc primer
3) 4 mils high build 2-component epoxy mastic

For a total dry film thickness of only 7 mils compared to your 12.5. I'm not sure how much cost difference there will be in the thicker application but I will inquire. I didn't have a polyurethane layer which sounds like a good idea.

Can anyone briefly describe why I would want to pick an organic vs inorganic zinc primer? Googling led me to even more confusion - zinc silicates vs zinc epoxy vs zinc siloxane...

 

[TehMightyEngineer]

I've attached the detail we typically use for repairs to corroded column bases so you can probably modify as you need.

As for organic vs inorganic zinc primer I too have not found a good answer on that other than my boss telling me to use organic zinc in our specs. I'd love to hear a good answer from someone here.

Maine EIT, Civil/Structural.

 

[hokie66]

The terminology 'organic' and 'inorganic' is confusing. An inorganic zinc primer essentially just leaves a coating of zinc, while an organic primer means it is epoxy based, with some zinc filler. Inorganic zinc is usually only used in new work, where near perfect preparation can be achieved. In repair work in the field, or for touchup, some type of epoxy based product is normally better.

 

[allgoodnamestaken]

Thanks for the help!

 

[TomDOT]

Hokie, an inorganic zinc (IOZ) does NOT leave just a coating of zinc. IOZ is zinc dust in a silicate binder matrix. Since there are no organics in the cured binder, it's called inorganic. Organic zinc is zinc dust in an organic binder matrix, most commonly epoxy.

Uncoated, high ratio IOZ performs very well in a variety of environments, much better than organic zinc. See the NASA KSC beach studies. It's a little less clear on the performance when you are using it in a multicoat system.

IOZ is less forgiving of application errors than OZ. We only use it on new build.

 

[hokie66]

Fair enough, but for those of us who are not chemists or coating specialists, it is confusing. I was just trying to differentiate between organic and inorganic. What does the word "organic" mean in this context? I associate it with "epoxy", but don't know why.

 

[ajk1]

My experience: don't rely on epoxy for corrosion protection. May be ok to protect the zinc rich paint from damage, but it does not provide very good corrosion protection especially at sharp corners where any paint including epoxy paint, runs away from the corners and you won't get the specified thickness (used to be, maybe still is, a good discussion of this in AISC Handbook). Zinc-rich paint will provide much better corrosion protection than epoxy. Specify the dft thickness and site measure it to verify you are getting it. However I would very much prefer hot-dip galvanizing in your case than zinc rich paint. The cost difference is generally small.

For the protection of reabr in the concrete, if you are in Canada consider specifying CSA exposure class C-XL concrete (yes is a lot more expensive than C-1 concrete) and good concrete cover. Use Life 365 to compare the life of different protection systems and concrete cover. Use waterproofing membranes wherever they are not subject to damage from equipment blades used to push the salt around or to lift it. Refer to CSA S413 for protection systems. Epoxy coated bars add a little to the life, so you can also include them if you wish, but the downside is wider crack widths. MTO no longer uses them in their bridges, and they are not accepted by CSA S413 in Table 1, but they may add a bit of life.

You have a challenging assignement. We did a similar facility about 5 years ago.

What design life are you aiming for?

 

[TomDOT]

"Organic" in this instance means that the binder (the glue that holds the paint together and adheres to the substrate) has carbon atoms. This is based on the classic "Organic Chemistry" definition, well predating the use of the term by the modern "green" movement.

Epoxy is the most common binder used for organic zinc paints, but there are others such as moisture cured polyurethanes*

"Inorganic" in this instance means that the binder has no carbon atoms. I only know of one family of inorganic binders used in zinc paint - silicates. Typically it's ethyl silicate, which reacts with water in the air to release ethanol and crosslink into the cured film.

*Okay, I believe they are technically polyureas, but are commonly called polyurethanes due to the widespread use of polyurethanes in the coatings industry, and concerns that customers might not like "urea" in their paint. If you need to go deep enough into the coating chemistry to where it matters, you should already know that "moisture cured polyurethane" really means polyurea.

 

[TehMightyEngineer]

ajk1: Good point on the epoxy at the corners. We typically specify epoxy paint for the damage resistant qualities you mentioned. We often find that industrial sites suffer more from damaged coatings than from coating application issues. However, with a salt-rich environment you're probably right that coating quality is just as important as damage resistance of the coating.

I mentioned galvanizing but you're right to point out that hot-dipping the steel will probably do better than a paint system for this harsh of a service environment and will definitely be easier to ensure a well installed corrosion protection system. If the OP has a lot of welded joints that can be an issue but it's not impossible of course. Overall the cost would probably be a little more initially but the maintenance down the road will likely be cheaper.

I'm personally not a fan of epoxy coated rebar. For their cost I don't think they're worth it and they are so prone to coating damage during installation that I don't think people get the service life they think they're getting. FRP bars would make much more sense to me than epoxy coated steel rebar.

Maine EIT, Civil/Structural.

 

[allgoodnamestaken]

Thanks everyone for all the great advice and suggestions.

Currently I am specifying at minimum:

Exposed Structural Steel:
[ul]
[li]Near White Blast Cleaning (SP-10)[/li]
[li]Inorganic Zinc Primer 2-3 mils DFT[/li]
[li]High Build 2 component Epoxy Mastic 4-6 mils DFT[/li]
[li]Polyurethane 2-3 mils DFT[/li]
[/ul]
Total DFT of 8-12 mils.

Interior of Silo:
[ul]
[li]Near White Blast Cleaning (SP-10)[/li]
[li]100% Solids Elastomeric Polyurethane 25-40 mils DFT for silo and silo top[/li]
[li]100% Solids Elastomeric Polyurethane 40-50 mils DFT for lower silo cone[/li]
[/ul]

I am worried about the steel rusting in between blast cleaning and the application of the polyurethane, but every product guide seems to say not to prime.

I am also asking for prices to be given for hot dipped galvanizing of the platform steel.

Regarding concrete:

1) I will be requesting minimum class C-1 from CSA A123.1
2) I will be specifying large clear cover to rebar (75mm) to increase lifespan.

Most people I talk to seem to dislike epoxy coated rebar so I'm avoiding it.

Does it sound like I'm on the right track?

 

[allgoodnamestaken]

regarding clear cover:

I am thinking of using 75mm for sides and bottom, but reduce clear cover for the exposed surface. I am worried about wide cracks. Does anyone have any experience with a good solution to this catch-22 (high cover, wide cracks)?

 

[hokie66]

I agree about the epoxy coated rebar not being reliable, but have you considered galvanized rebar? I would use galvanized rebar, and 50mm cover, if this is the interior of the salt tank.

Back to terminology, when I hear "zinc rich" coatings discussed, I think epoxy, or "organic", with zinc. And as discussed, inorganic zinc gives much better protection, if applied to clean surfaces. But hot dipped galvanizing is far superior to any applied coating system.

Flash rusting in the silo? I thought that was concrete, supported on structural steel.

 

[Agent666]

Wider cracks don't impact on the durability as long as the crack width at the required cover is less than the limit. In other words it's just a visual thing provided if you worked out the crack width at the minimum cover irrespective of the actual cover. British standards allow this approach I believe.

This also means with a higher cover you aren't actually getting as good a benefit to the durability as you have assumed unless you aim for smaller crack widths at the surface.

 

[dik]

SYSTEM 24: TWO PART EPOXY COATING WITH ZINC RICH PRIMER AND TOPCOAT
BASE METAL - STEEL
CLEANING - TO SSPC-SP 1 AND SSPC-SP 10
PRIMER - DEVOE 'CATHACOAT 302HB - REINF INORGANIC ZINC PRIMER
SUBCOAT - DEVOE 'BARRUST 235 - MULTIPURPOSE EPOXY COATING'
TOPCOAT - DEVOE 'DEVTHANE 379H - ALIPHATIC URETHANE GLOSS ENAMEL'

The above is one I use often... the Cathacoat product can be used with slip critical connections. As TomDOT noted the inorganic material is used only for new applications where shop quality application is available else the IOZ doesn't work so well. The epoxy coatings tend to dull or craze over time and the Urethane gloss enamel is used to give a long term, 'attractive' coating that is visible.

Dik

 

[TomDOT]

Yep, that's a nice benefit of IOZ you mention, dik - most IOZ meet both Class A and Class B slip, unlike the organic zincs. No need to mask off the faying surface in the shop, and likely a benefit to not doing so, as you get some additional corrosion protection at the faying surface.

 

[dik]

Thanks... that's why I use it... I've used that coating spec on numerous projects...

Dik

 

[allgoodnamestaken]

Thanks again everyone for the advice. I went away for the long weekend (Canada!) and forgot to check this thread.

@hokie66 - The platform and the silo above it are both steel. The foundation underneath the platform is concrete.

The silo interior will be lined with a very high build 100% solids polyurethane that is used for a lot of high durability and high salt environment situations. It's also inert, non poisonous (actually certified for potable water), non flammable, etc, which will be good if someone ever needs to go into the silo. But as I said before, the period in between surface preparation and application of the polyurethane is where I'm worried about flash rusting. I'll just mention something about blasting off any rust before application.