Rust Penetration per day

[skylite]

After all the wide flange and purlins were delivered to the site.. it rained before we put epoxy primer on the steel. Do you need to remove every bit of rust before putting epoxy primer? What do you usually do?

Usually how many micrometer of depth does the rust penetrate into the steel per day?

 

[hokie66]

What type building or structure? If the steel is fully enclosed inside a waterproof enclosure, there is really no reason for the steel to require a coating. But if the steel is exposed to the atmosphere, it likely does need protection. You don't measure normal atmospheric corrosion per day, as it is quite slow.

 

[skylite]

I'm talking of exposed wide flange delivered to site and it rained before it is primered either by red oxide or epoxy primer. Just one day of exposure can rust a steel.. so what do you usually do with the rusted exposed steel before applying primer.. do you clean it away with clothe or just directly paint it with the primer?

 

[bridgebuster]

What does the paint manufacturer recommend? Check the catalog cuts. Some primers can tolerate a bit of rust - not loose, others can't. Perhaps you need to do some solvent cleaning per SSPC-SP1C followed by hand tool or power tool cleaning per SSPC-SP2/SP3.

 

[Ron]

skylite...first, let's first ask why the steel is being delivered to the jobsite without any coating. I suppose that might be a local practice, though in my opinion, not a good one.

Given that, you now have to clean the steel and apply a primer at least for further protection from exposure, and you have to do this under "less than ideal" field conditions.

Ideally, you would sandblast the steel to remove rust and loose mill scale. Then you would apply a sufficient thickness of a good quality primer that would be compatible with a variety of finish coats, if a finish coat is required. Otherwise, the primer will suffice for most applications except for exterior exposure.

Epoxy is not a common coating for building steel. That is usually reserved for specialty applications, bridges and high exposure conditions. For that, the steel will have to be sandblasted to a "near white" condition.

 

[BAretired]

Is the steel going to be exposed to the weather or will it be covered? If covered, you don't need primer at all.

https://www.aisc.org/DynamicQuestion.aspx?Grpid=6&QueId=1920&ste=_Preview&id=2100&type=0

BA

 

[Ron]

BAretired.....I agree with you in many instances that primer is not needed for completely covered steel; however, down here in the humid southeastern US, we have enough inward vapor drive and condensation to create corrosion issues. While it can take a while for this process to see any appreciable section loss, it does create a nuisance with staining due to condensate drips. For that reason and for protection at the jobsite, structural steel is customarily primed at the shop before shipping to the jobsite in this area.

 

[BAretired]

Ron.....yes, we usually see primer on structural steel here in arid Alberta too; I guess it is primarily for aesthetics during construction, but occasionally we see non-primed steel. I can't recall ever seeing primer applied in the field though other than field touch-up.

BA

 

[JStephen]

We see a lot of steel that is uncoated for weeks or months and then primed. However, that priming is always preceded by abrasive blasting, so you're either removing a light coat of rust or mill scale.