galvanized, weathering, painted steel

[delagina]

almost all steel i work on uses galvanized steel.
am interested when weathering, painted, etc.. are used.

can someone link me to this.

my online search hasnt really helped me.

thanks,

 

[JedClampett]

Weathering steel was used a lot on bridges in the 60's and 70's. It was marketed as a magic material that answered all your wants and needs. It turned out the weathering layer didn't adhere as well as expected in wind and rain, so the sections started to reduce in thickness. This eliminated the advantage (no coating ever), so its use has greatly gone down.
As far as your ohter question, I'm not sure what you're driving at. Painted steel (or at least primed) should be used everywhere. Unless steel is not exposed to oxygen or water, it will corrode.

 

[delagina]

thanks,

i only have experience designing galvanized steel. i have not designed any steel that is painted.

 

[kelowna]

The question is very vague Delagina, what kind of designing do you do? Bridges/buildings/mechanical/industrial?

In general the decision of using weathering, painted or galvanized steel will come from analyzing a number of factors, such as location of the structure, use, environmental conditions, expected use and maintenance, design life...
The three systems provide different levels of protection at different costs.

It is very difficult to summarize it but in a couple of lines, paint is cheap, OK for most applications but requires maintenance. Galvanized gives the best protection with less maintenance required at a price. Weathering steel is supposed to be maintenance free but requires dry/wet cycles to develop the proper patina and has other issues such as staining.

 

[STATICPH]

Hi D,

See you into Structural design....

I do not know where in the world you come from but,

I work in South Africa in the cheaper residential structures and galvanizing are often prompted to be expensive and "the ordering of it" time consuming, therefore we paint most of our structural steel work,
that is often not the case with bigger projects thou,

however, the Galvanizing society had a huge marketing thing going on and they have compared the two systems and therefore I think I understand the differences,

Painted methods need more maintenance and application is important.

Galvanizing need lessor maintenance IF DONE AND INSTALLED CORRECTLY and therefore is cheaper over time,

Galvanizing is a chemical method while painting is a mechanical method – the paint get “sucked” to the steel,

Preparation for paint is done by sandblasting to remove slag - that is never done over here anyway, and the slag is burned automatically when galvanised

The design calcs is the same as strength are not influanced (I do not know if this is true with all grades of steel)

Tx for giving my 2 cents

 

[connectegr]

The clearest line in the US between galvanized and painted steel projects, is industrial vs. commercial projects. Nearly all of our industrial projects are galvanized. Most are exposed structures or have specific corrosion concerns which require the additional protection of galvanizing. On the other hand nearly all of our commercial projects painted or provide no coating at all. For highrise construction or exposed steel structures (stadiums and arenas), painted coats are used to provide a Class B slip coefficient. Class B can be achieved without paint, in projects that do not have exposed steel or where fire proofing is applied directly to the steel.

http://www.FerrellEngineering.com

 

[delagina]

thanks. i work in industrial projects, refinery plant.

 

[merganser]

Not sure exactly what you're looking for, but I've found the following AISC publication helpful when specifying painted protection for exposed structural steel:

http://www.aisc.org/WorkArea/showcontent.aspx?id=17506

There is a good discussion of the relative costs of different levels of painted protection and various surface prep methods..

 

[dik]

The attached is from my General Notes.

 

[Ron]

delagina...this group used to be known as the Steel Structures Painting Council...now they are just SSPC. Anyway, here's a link to some good info.

http://www.sspc.org/

Also, check NACE International...

http://nace.org/content.cfm?parentid=1001&currentID=1001&CFID=2540786&CFTOKEN=32621722

 

[vthomidis]

......just to add that combination of galvanizing and paint ( primer and top coat ) is very common in thin gauge steel sheets used in :
- electric/domestics appliances industry
- car manufacturing
- insulating panels used as roof and wall claddings
(mainy in preengineered steel buildings )

 

[HgTX]

@JedClampett: weathering steel for bridges is quite alive and well.

Hg

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[JedClampett]

Do they coat it?

 

[Ron]

JC...weathering steel is typically not coated.

 

[HgTX]

Some designers or owners call for coating the ends because if the joint leaks deicing-salted water, that would be really hard on any steel, coated or not. (I've seen plenty of painted girders with their ends rusting away under a leaky joint.)

But "wind and rain" are not an issue for weathering steel. (And I'd be interested in seeing your source for that claim.) It's not good for splash zones or low-water crossings with frequent immersion, because with frequent rewetting the protective patina doesn't get a chance to form, but otherwise, it does well with ordinary weathering exposure ("wind and rain").

Hg

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[frv]

I'm with HgTX on this..

The whole idea of weathering steel is that the protection is, for lack of a better term, "built in".. In other words, it is a material property of the steel itself that a protective coat forms on the steel to prevent further corrosion, much like aluminum.. It is not a "coating" in the traditional sense of the word.

 

[JedClampett]

This is stuff I heard a long time ago. But I dug around and found this:
"The FHWA TA recommends caution in employing weathering steels in areas where the material could remain wet for extended periods of time due to high levels of rainfall, humidity or fog. The FHWA TA recommends evaluating these conditions using ASTM Test G 84 "Time of Wetness Determination (On Surfaces Exposed to Cyclic Atmospheric Conditions)." If the average time of wetness exceeds 60 percent, use of weathering steel is not recommended. No measurements of the time of wetness were taken at any of the bridge sites visited, so an assessment of the adequacy of the FJWA TA recommendation cannot be made. It has been reported by others that weathering steel in bridges located in the Northwest portion of the United States, west of the Cascade Mountain range, and southeastern Alaska, has not performed satisfactorily and required painting...."

However, contrary to what I said before, most of the experiences have been satisfactory. Read the whole report.

http://www.steel.org/Content/ContentGroups/Construction2/Bridges/AISI_Construction_Pe6.htm

 

[vthomidis]

...agree with frv ,same idea as stainless steel in which chromium is the main element forming a protective coat on steel surface ( i.e. the reaction of chromium / oxygen )...

On the other hand, in my country weathering steel ( CORTEN ) is mainly used in Electric Power Authority's projects and in train manufacturing.

 

[HgTX]

@Jed: yeah, WS not recommended for what would essentially be immersion service, and one needs to design the structure to drain well because it doesn't like ponding either.

But for applications where it will work (which is most bridge applications in my opinion), I'm a fan.

Hg

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