Paint for Exterior Steel Application and Corrosion Protection

[MidwestSE]

Hi everybody,

I'm used to working in the industrial realm where galvanized steel is regular. I've been asked often about paint systems that can be used in lieu of galvanizing for low-risk applications for non-industrial applications - e.g. a steel beam used for a residential deck.

Is anyone aware of any documents, either by an industry source (SSPC/NACE) or supplier (3M, etc) that has recommendations of what type of paint to use? There must be some sort of generic paint system/etc that is rated to provide standard levels of corrosion protection - but I can't seem to find it anywhere.

What information do you provide when specifying paint for exterior steel corrosion protection?

Thanks!

 

[dik]

It largely depends on the degree of protection, and the environment and amount of money you want to spend. If a container type of structure it also depends on the content. I have about 40 different coating systems that I've used depending on the work. If the unit cost for the least costly is 1 then the unit cost for the most expensive coating system could be 40 or 60. A good relatively inexpensive system I've used on numerous sawmills is (I don't know how this will format):

SYSTEM 08: QUALITY PRIMER WITH TOPCOAT (EXTERIOR)
BASE METAL -STEEL
CLEANING -TO SSPC-SP 1 AND SSPC-SP 6
PRIMER -DEVOE 'DEVGUARD 4160 - MULTIPURPOSE TANK AND STRUCTURAL PRIMER'
TOPCOAT -DEVOE 'DTM URETHANE-ALKYD GLOSS ENAMEL'
ALT: DEVOE 'DEVTHANE 379H - ALIPHATIC URETHANE GLOSS ENAMEL'

There is likely an American equivalent. This is a realtively fast drying system; some coating systems are slow drying and many have very limited times during which you can re-apply the next coating. It can get very tricky and you need to consult a good paint tekkie. There are some really good coating system people on this site. Many clients have their own paint systems that they have found to work over the years. There are also specialised paint systems for particular tasks.

In Canada there are two common coating systems noted in our steel handbook. They are the CISC/CPMA 1/73A and 1/75. I often see engineers spec these, not really knowing what they are. One consultant specs a double coating of 1/73A, not realising the spec is performance only and only has to 'last' six months... does't matter how many coats. There's no requirement that it will take a top coat. Lots of little things you have to look into.



Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[JedClampett]

You might want to drill down and see if coating is really any cheaper than galvanizing. I'm assuming that's the motivation. If you coat, that's another trade, that's notoriously hard to work with (paint fumes?), that has to be brought in. The steel needs to have its surface prepped (sand blasting) before the first coat is applied. There's a wait between coats, overspray, monitoring, etc.
I've had GC's tell me galvanizing is preferable. You put the steel up, touch up the galvanizing and you're done.
But if you choose to coat, the Cadillac system is two coats of epoxy and a polyurethane top coat. It's likely more coating than you need.

 

[canwesteng]

A lot of paint companies release ISO 12944 coatings guides. Although I don't think you can get away without epoxy or thick urethane coatings for an ISO compliant system. I'd guess galvanizing, or weathering steel, will still be the cheapest.

 

[STrctPono]

Hot dip galvanizing is certainly a very good corrosion protection. If you are looking for an alternative you would be looking at a 3 coat system.

1st coat either an organic or inorganic zinc rich primer. This is basically intended to act like the galvanizing coating as there is a large percentage of zinc suspended in the paint. Research the difference between them since inorganic primers need to allow time to weather prior to applying the midcoat. We typically always specify organic (epoxy based) primers.
2nd coat epoxy midcoat. Prior to applying the midcoat, they should stripe coat all the edges of steel to ensure you get proper DFT
3rd coat polyurethane topcoat for color retention and to prevent chalking.

There are other technologies that are promising such as fluoropolymers but currently these 3 coat systems are the industry standard go to for corrosion protection.

There is a consortium called NEPCOAT (out of Northeastern US) that we take guidance from when painting steel bridges. Many times our girders are too big for the galvanizing kettles around the US and we are stuck with having to rely on the paint. These are outdoor structures that take a beating from the environment and these proposed approved coating systems are the best systems from each of the individual manufacturers. I think you will find this helpful. When we write our specs (which is a rather in depth specification) we allow the Contractor to choose any of the systems from the Qualified Product List B.

Link

 

[GGedge]

You can follow the approach given in the ISO 12944 which requires:

1) define the exposure class - more specifically in ISO speak the "corrosivity category"
2) define the durability requirement - the expected life to first maintenance
3) select an appropriate specification from the examples given

As noted above many paint suppliers publish suggestions on 3) for each category in 1 and 2.

Not sure from your post what exposure condition is for the beams in question, are they inside a building with controlled environment or external?

Cheers

GG

 

[MidwestSE]

Thanks everybody for the replies.

For the case which prompted the question, the only exposed steel is a total of (3) 1/2" thick steel plates that are going to be used in flitch beams for an exterior deck with a roof over it.

The primary motivation is not so much cost as it is the project schedule. The lead time for getting the steel galvanized in the area of the project is a little lengthy and might wreck the project schedule. The contractor and I are looking for other ways to solve this type of problem - it has happened to both of us enough in the last year to warrant a little digging.

Thanks!

 

[GGedge]

A alternative, admittedly a bit unusual for buildings, is metallization/thermal sprayed metal with either zinc or aluminium. 100 to 125 microns (4 to 5 mil) would give similar performance to galvanizing. Might be worth trying finding a local applicator to see if they can do it quicker than a galvanizer.

Cheers

GG

 

[BAretired]

I recall specifying Galvacon, a cold galvanizing compound by Lanco for applications like this.

BA

 

[XR250]

Honestly, if it has a roof and it will only get wet occasionally, I would leave it as bare steel. I run into plenty of these that are 30 years old with little to no rust.

 

[dik]

that and Galvafroid... both good zinc rich primers.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik