White Rust Deposit 1

[Guastavino]

Got an existing 1968 building (Mid-atlantic USA) with some white rust with red deposits (see below photos. First is before I scraped it. After is after I scraped it). The red deposits come right off and deck seems to be in pretty good shape underneath. Problem is they have coolers above this slab and condensation has been forming. That will be addressed, but I'm more curious about what those deposits are?

I'm thinking best way to clean is to power tool wire brush clean. But should they treat it with a chemical too? Indoor space, so once moisture issue is taken care of, it shouldn't rust further. So I'm thinking that would be an unnecessary expense.

Thanks!

 

[TehMightyEngineer]

The white rust is corrosion of the zinc galvanizing. The red deposits are a mix of zinc corrosion and steel corrosion products. It's highly localized corrosion of the steel because areas where the zinc is still galvanically bonded to the steel will corrode the zinc first.

I'd wire brush it until you've removed all corrosion products (down to either uncorroded zinc or bare metal). Then I'd spray the affected areas with a "cold-galvanizing" zinc rich spray paint. Solve your moisture problems first, of course.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Ron]

What TME said!