Rust Stains on Exterior Galvanized Steel Framing 2

[Eng_Struct]

Hi Group,

Please refer to the image below. The photo is from an exterior stair framing from a low to a high roof. All framing members were originally galvanized. We have observed rust stains primarily at the joint locations and for the life of me, I am not able to figure out why galvanized steel will rust.

Does anyone have experience with this or can provide thoughts?

 

[phamENG]

Probably damaged the zinc coating and didn't patch it. Those joints should have been touched up per ASTM A780 using a zinc rich paint. Even so, they'd be more prone to corrosion than areas that have good and intact HDG coating.

 

[hokie66]

Are you sure those stubs are HDG? What does the detail look like? Is that a weld at the lower joint?

 

[dvd]

The lower support looks kind of like it was painted with zinc paint and not galvanized. Also, I wonder what material the flashing is, and whether this may be a galvanic corrosion of the zinc.

 

[bones206]

I suspect galvanic as well. Copper or lead flashing would be lower on the galvanic scale so the corrosion potential is there.

 

[milkshakelake]

Others have mentioned the cause for the rust, which I agree with. It's a quality control issue, because the actual G60/G90 stair framing steel hasn't corroded (unless it's much newer, which is a possibility).

For the solution, I'd clean the rust and paint it with something like Rustoleum. Repaint every 3-5 years.

 

[desertfox]

I agree with phamENG, Zinc is a sacrificial coating which means that a small steel surface open to the elements is protected initially from corroding by the zinc coating, however as the zinc is sacrificial it means the steel surface exposed gets larger and larger over time, it gets to a point where the steel surface is too large to be protected by the zinc and hence corrosion.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Eng_Struct]

Thank you, Everyone!

Some super useful feedback.