cor-ten steel protection and combination with A36 1

[sennafan]

We are repairing a steel structure originally constructed using 50 ksi cor-ten structural steel. Corrosion in various members differs from nil to completely missing webs and/or lower flanges in places. Framing members will be replaced selectively, depending on the extent of corrosion.

We have advised replacing heavily corroded members with new cor-ten steel (to avoid possible galvanic corrosion between the existing A588 steel and new steel such as A572). We are also advising that the new steel be painted to reduce corrosion. The owner has decided not to clean and paint the marginally corroded original members that are to remain.

How concerned do we need to be about dis-similar metal issues combining A572 steel with A588?

For shop painting of new members, we are considering an epoxy primer for the new cor-ten members. Any suggestions about paint type? Would a 2nd coat of paint be worthwhile?

The structure is a 35 year old parking deck over a fresh water lake that was framed with cor-ten beams and stringers left bare. The framing is right at water level - the lower flanges of the deepest beams are under water for several months of the year.

Any advice would be appreciated.

 

[strider6]

Sennafan,

I don't think that galvanic corrosion will be an issue. Regarding the structure you said that a part of it" the lower flanges of the deepest beams are under water for several months of the year".
Have you ever considered to apply a cathodic protection system for this parts that are below water level for several months?

regards

Strider

http://www.corrosionist.com

 

[EdStainless]

Cor-ten will stand up to weathering, but not being submerged. The submerged material will need to treated like a pier. coatings and CP are going to needed. You need to look into marine structures to see how it is done.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[kenvlach]

Dissimilar materials isn't an issue between A572 & A588* steels.
The weathering steels need exposure to atmosphere and wet-dry cycling to develop a denser surface oxide (mostly). They have no advantage over ordinary steel such as A36 if submerged, as noted by EdStainless. For coatings, blast to white metal & use appropriate marine grade epoxy primer and polyurethane topcoat.

*A more appropriate steel, although less available, is per ASTM A690/A690M-07, 'Standard Specification for High-Strength Low-Alloy Nickel, Copper, Phosphorus Steel H-Piles and Sheet Piling with Atmospheric Corrosion Resistance for Use in Marine Environments.'
However, note its disclaimer:
'The atmospheric corrosion resistance of this steel is substantially better than that of ordinary carbon steels with or without copper addition (see Note 1). The steel has also shown to have substantially greater resistance to seawater "Splash Zone" corrosion than ordinary carbon steel (Specifications A 36/A 36M and A 328/A 328M) where exposed to the washing action of rain and the drying action of the wind or sun, or both. Where the steel is not boldly exposed, the usual provisions for the protection of ordinary carbon steel should be considered.'
[my emphasis]

http://www.astm.org/cgi-bin/SoftCar...EDLINE_PAGES/A690A690M.htm?L+mystore+voyw2563

Some further info:
'Maintenance Coating of Weathering Steel: Field Evaluation and Guidelines, Final Report, FHWA RD-92-055, March 1992'
abstract is at

http://www.emetalizing.com/section.cfm?Cat=Weather

'PERFORMANCE OF WEATHERING STEEL IN HIGHWAY BRIDGES
A Third Phase Report'

http://www.steel.org/Content/ContentGroups/Construction2/Bridges/AISI_Construction_Pe12.htm

For a scientific basis of the ~protective coating that forms on weathering steels, see
'Structure of Protective Rust Layers Formed on Weathering Steels by Long-term Exposure in the Industrial Atmospheres of Japan and North America,'

http://db1.wdc-jp.com/isij/abst/199803/is380285.html

 

[HgTX]

Excellent info from kenvlach.

As others have said, there's not much of a dissimilar metals problem, so there's really no point in using A 588 if you're just going to paint it. Painted A 588 doesn't function any better than painted A 572. A pinhole in the paint will rust just as badly in both.

You might want to revisit the question of whether or not to paint the remaining A 588. A small investment in adequate corrosion protection now could save them another expensive rehab in the future.

Look into thermal-sprayed aluminum or zinc. TSA seems to be popular for offshore structures around Texas.

If you really can't do the thermal spray for some reason, look into an immersion-grade zinc-rich primer. Look for a high zinc loading--the Texas requirement for zinc-rich primer on bridges is 80% in the dry film. And watch out for percentages printed on cans--often that refers to the purity of the zinc, not the percentage of zinc in the coating.

Also if you're welding the new material onto the old material, even if you stick with the current decision not to
paint the old material, you'll still need to paint the welded areas. Zinc-rich coatings shouldn't be welded through (hazardous fumes, not to mention that "weldable" primers aren't very much so), so the paint will need to be held back, and there will be a few inches of surface to paint. This area will need adequate surface preparation including removal of chloride contamination.

Have fun!

Hg

Eng-Tips policies: faq731-376

 

[sennafan]

Thanks all! Much appreciate the information. We will discuss the options with the client and probably at least get a quote for cleaning and painting the steel to remain. Environmental concerns, cost, and schedule were all part of the initial decision to avoid painting the parts to remain. I'll also look into cathodic protection.

sennafan