cold-galvanized cable trays

[AusLee]

Hello,

I have a project that specifies Hot-Dip galvanized cable trays, with 325 grams per square meter of prtoection material (i guess this is equivalent to the time in the cuve or the thickness).

The contractor is proposing cold-galvanized cable trays saying that it is acceptable for indoors and we will save money.

I am not einstein in galvanization but if i am going to protect a piece of metal at cold then i would paint with with zinc rich material - is there something called cold-galvanization? is it ok to use it indoors, above the false ceiling? I said i will agree if the savings are considerable, but will i be saving greately over Quality as well?

Thanks.

 

[BJC]

I am pretty sure he means "Pre-Galvanized" which means the tray is made from galvanized steel sheeting. The final product is galvanized except where is was cut or welded.
Check anybodys tray catalog and the difference is explained.
Depending on your application pregalve tray may work fine and it is a lot cheaper. Cold Galv refers to a zinc rich paint that is usually applied to bare metals when pregalved (or other galvanized metal) is cut or welded.

http://www.libertpaints.be/LibertPaints.nsf/LPFPO/Galvanox/$File/ENG.GALVANOX.pdf#search="galvanox"

Being indoors or out may not be the deciding criteria for determing what tray to use. Some water treatment areas have a chlorine concentration that is very small but can corode even galvanized tray in a remarkably short time. There are fiberglass ssytems for atmospheres like that.

 

[waross]

The alternative to hot dip galvanizing is electroplating. The zinc thickness is much less than with the hot dip method. I have heard some electroplate galvanizing described as cosmetic rather than protective.
If you have the same environment above the false ceiling as you have below it, electroplate will probably serve. If the area above the false ceiling is unheated and subject to high humidity at times, you may experience corrosion.
respectfully

 

[peebee]

"Cold galvanized" is synonymous with "electroplated" so far as I know.

"Pregalvanized" does not refer to the method, but rather when the galvanization takes place (ie, pre or post tray fabrication). With pregalvanization, the steel is exposed with no zinc covering wherever it is cut or welded. It is still afforded some cathodic protection from the zinc, but there is no zinc covering.

Electroplating is generally acceptable for normal (non-corrisive) indoor environments. BUT, electroplating should be avoided in data centers due to the formation of "zinc whiskers". Hot dip galvanized generally does not have the whisker problem. Google "zinc whiskers" for more information on that issue.

Anyway, in a normal (non-corrosive) office environment, electroplated tray should be fine.

 

[jraef]

No, BJC is correct. Cold Galvanized means they used galvanized material for fabrication, then treated the welded and drilled areas with a zinc rich coating. It is NOT the same as HDG, and his point about corrosion is dead on. IMHO, unless you know the absolute reason why HDG was specified, you cannot ASS-U-ME that just because it is indoors, cold galv is acceptable.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[peebee]

I never said cold galvanized was the same as HDG, I said it was generally used to refer to electroplating. And I stand by my statement:

From

http://www.tateaccessfloors.com/pdf...tin.pdf#search="cold galvanized electroplate"

"The electroplating method is referred to as “cold” process galvanization because of its low working
temperature"

From

http://www.answers.com/topic/galvanizing

"electrolytic process (also called cold galvanizing or electrogalvanizing)"

If you look around, though, you can also find the term "cold galvanized" to refer to just about any process occurs at a lower temperature than HDG, such as mechanically applied zinc powder or zinc painting systems in addition to electroplating.

I don't believe ASTM or any other standards organization uses the term "cold galvanized", so this term is very much open to interpretation and in my opinion should therefore be avoided. If you're concerned about the type of process, but don't need hot dip, you need to be specific regarding the type of "cold" process you want and should preferably refer to the appropriate ASTM standard.

 

[AusLee]

Hello,

Thank you for your links, i understand a little bit more the subject now, but as i was reading, i found two contradictory statements. The first one is:

"Note that by leaving the steel dipped for too long in the galvanization bath, the layers of protection will not get more and more thick by the time. The amount of Zinc that comes to deposit depends chemically on the nature of the steel. "

and the other statement is a table in the OBO-Bettermann catalog, in which they show various levels of protection (in micrometer).

So is it true that leavong the element longer in the galvanization cubicle will result in the deposition of a larger quantity of zonc, or not?

 

[peebee]

You could find answers to most of your questions with just a little help from Mr. Google. Here's what he pointed me to:

From

http://www.wittgalvanizing.com/galvanizing_process.htm

:

"Factors influencing the thickness and appearance of the galvanized coating include:

+ Chemical composition of the steel
+ Steel surface condition
+ Cold working of steel prior to galvanizing
+ Bath immersion time
+ Bath withdrawal rate
+ Steel cooling rate....

"As the galvanizing reaction is a diffusion process, higher zinc bath temperatures and longer immersion times will generally produce somewhat heavier alloy layers. Like all diffusion processes, however, the reaction proceeds rapidly at first and then slows as layers grow and become thicker. Continued immersion beyond a certain time will have little incremental effect....

"The thickness of the outer pure zinc layer is largely dependent upon the rate of withdrawal from the zinc bath. A rapid rate of withdrawal causes an article to carry out more zinc and generally results in a thicker coating."

 

[stephenw22]

Ask whoever specified using hot-dip galvanized tray for the reason why they did.

If there's no particular reason, then you'd be fine.

Will using pre-galvanized tray save you that much money? Depending on the installation, the cost of the tray could be a lot less than the labour to install it.

Hot-dip galvanized trays have much better corrosion resistance. Unless I was installing tray inside an environmentally-controlled office/commercial building, I'd probably just use hot-dip tray over pre-galv.

If corrosion is a concern, there's always fiberglass, stainless, aluminum, epoxy-coated, etc. Just give your tray supplier a call.

 

[mc5w]

If I were installing a metal cable tray outdoors or in a damp, wet, or corrosive environment, I would assemble it, test the section to section ground continuity using a buzz box welder rigged to act as a high current ohmmeter, and then varnish the heck out of its with 2 coats of urethane varnish.

There are 2 advantages to using urethane varnish as a protective coating:

1. You can visualize any corrosion that sneaks in through varnish cracks and so forth.

2. Urethane varnish is chemically related to Crazy Glue(R) which means that once urethane varnish cures it becomes as impervious as a man made membrane can get.

Sure, urethane varnish is not UL listed but I have worked in 2 environemts that knew how to corrode stainless steel. One of them was a food plant and when you place stainless steel in an anaerobic environment such as in a swamp or a blind threaded hole, the environment reduces the chromium and nickel oxide layers and then electrolytic corrosion occurs between the different metals. The other environment was a place that reprocesses foundry wastes. The combination of air moisture and ammonia gas rots galvanized steel. The combination of ammonia, water, oxygen, and fluxing chlorides dissolves or severely tarnishes all useful metals except indium and maybe also bismuth and remotely also copper free aluminum that is indium plated. Since aluminum requires a small amount of magnesium for structural strength, corrosives will tend to pockmark aluminum unless it is coated with urethane varnish.

The U.L. listing for corrosion resistance is valid for sodium chloride a seawater mist but not for ammonia or sodium cyanide. Ammonia and sodium cyanide both act as Lewis acids even though they have an alkaline PH and these 2 bad boys eat a lot of things. The reason why ammonia acts as an excellent cleaner is that it corrodes both organic and inorganic compounds.

Mike Cole