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Anonymous Poster #1

Dacromet

05/14/2017 3:54 AM

can the dacromet coated bolts be used instead of hot dip galvanized bolts for steel bridge construction? Thanks in advance for answers.

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#1

Re: Dacromet

05/14/2017 4:53 AM

This would be a question for the engineer that designed the bridge.

Dacromet is an excellent coating containing both zinc and aluminum flake. It goes on much thinner the hot dip galvanizing there for does not offer as good of corrosion protection as hot dip.

It does have advantages though, being a thinner coating, there aren't the dimensional issues in threads that can be seen with hot dip. It is also cured at about 260c while hot dip is the temp of the molten zinc, 415c or higher, that could be a problem in hardened bolts.

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#2
In reply to #1

Re: Dacromet

05/14/2017 8:02 AM

GA

The design engineer on this bridge is the one to ask. They are also the only one with the authority to say if this coating is sufficient.

I can think of many bridge locations and locations on many bridges where Dacronet can be a suitable coating for a fastener. I can also think of locations where this coating will be insufficient to protect a critical fastener from the elements. The AP has not informed us any of the critical information the design engineer should know to make an informed choice.

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#13
In reply to #2

Re: Dacromet

05/15/2017 6:01 PM

Thanks, coatings are my business.

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#17
In reply to #13

Re: Dacromet

05/16/2017 9:47 AM

Any coating applied to any fastener of my experience is better than no coating at all for extending the service life of the fastener, but like beauty, coatings are only skin deep.

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#15
In reply to #1

Re: Dacromet

05/16/2017 9:11 AM

I think it might also depend on where in the world this bridge is installed. If in the middle of the Atacama desert, no problem should arise, but if on the coast of Brazil, yes problem.

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#16
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Re: Dacromet

05/16/2017 9:34 AM

It's connecting two uninhabited Aleutian islands. The Alaskan crab fishermen want the bridge to block the illegal fishermen from fishing in their international waters.

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#19
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Re: Dacromet

05/19/2017 2:09 AM

That would be a third "bridge to nowhere". The other two were over Knik Arm near Anchorage, and Tongass Narrows at Ketchikan.

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#3

Re: Dacromet

05/14/2017 5:32 PM

PLEASE tell us where this bridge is. I am sure not of us ever want to go across it. (I for one do not wish to cross a bridge designed by anonymous people on the internet - ever.)

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#4

Re: Dacromet

05/14/2017 6:42 PM

Yes! They can be used...and you can trust that an answer from an anonymous source on the internet is always telling you the truth!

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#5
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Re: Dacromet

05/14/2017 9:25 PM

Who cares at all about the truth! An anonymous forum is where anyone can blame James K. Polk for the mess we are in today. Had Henry beaten him, who knows what wars may have been averted and what festering sores would have needed lancing sooner or never.

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#6

Re: Dacromet

05/15/2017 2:56 AM

Yes, provided the individual making that decision is prepared to underwrite both the technical and the commercial risks involved in so doing.

Please read about the first Tay Bridge disaster, which ended the career of Thomas Bouch.

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#10
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Re: Dacromet

05/15/2017 11:11 AM

Good Story.

However, this isn't exactly the same comparison. You're comparing an entire bridge design to just one component of the bridge.

The coating is a factor and each coating has a different wear resistance.

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#12
In reply to #10

Re: Dacromet

05/15/2017 12:32 PM

...which is why a change to the design merits being passed through the design review process and formally adopting prior to being undertaken.

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#7

Re: Dacromet

05/15/2017 5:33 AM

You need to use Structural bolts A325 ASTM or equivalent and the proper washers and nuts for that bolt. The design engineer is the final word on this question. Ask him.

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#8
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Re: Dacromet

05/15/2017 8:51 AM

In some designs even A490 bolts are used. The coating or lack thereof is up to the design engineer and should be followed to ensure the design life of the structure is not jeopardized.

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#11
In reply to #8

Re: Dacromet

05/15/2017 12:09 PM

I have been around and around on this. E. g., the ASTM 490 thing:

https://www.galvanizeit.org/education-and-resources/resources/technical-faq-dr-galv/a490-bolts

I go with metallized (mechanically galvanized, but not from the standard tumbler approach, rather arc spray; costlier, but worth the price).

There is a duplex arc spray Zn-Al non-embrittlement method for threaded fasteners that applies a combined two-layer coating a bit thicker than a single hot-dip pure Zn coating, but only with the added thickness on the highest stress and corrosion potential portions (exterior of nut and bolt head facets), which are also prone to the installation rupture of the applied plating. But with installing and removing thread blanks during the duplexing process, it ain't cheap, but still a lot cheaper than stainless, especially if you need major installed yield strength material.

Stopping the propagation of corrosion under nut and bolt heads to the threads at the bound interface of highest shank stress is a key objective. If you can spring for metallized Sprialoc nuts and heavy Series 300 stainless washers, this, too, helps. Finally, there is always the option of metallizing the whole lash-up after installation, but, again, not cheap and somewhat access limited.

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#9

Re: Dacromet

05/15/2017 11:08 AM

It would probably be fine, however it is more costly. Projects, such as building a bridge, are on a budget that is determined by the costs of previous projects. They know what the material costs, the manpower needed and the time needed to complete it. Throwing in something like the additional cost of a more expensive coating on a bolt, would require a great deal of justification to contractors to allow it. They are going to run with the known methods. It wouldn't be a big deal if it was just a few bolt, but we're talking about several thousand bolts and those thousand add up. From what I see in this article, Dracomet has about the same lifespan as Nickle Plating which is about 1000/hrs. https://www.anochrome.com/dacromet-coating/

Hot dipped Galvanizing life expectancy is about 30 years.

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#14

Re: Dacromet

05/15/2017 8:32 PM

When I find myself in the southwest desert. I'm constantly amazed at the lack of corrosion on infrastructure. I'm in a place where rust is often the only structure left of the original.

Location 3

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#18

Re: Dacromet

05/16/2017 10:09 AM

How do you know that any coated fastener that works or vibrates loose will not suffer abrasion loss of the protective coating and then subsequently corrode away even faster as the single exposed anode of steel on the entire structure?

Somebody has a learning curve they need to be on, and quickly, or else they will have some 'splaining to do later.

IF this is a salt spray kind of environment near sea water, then you had best get on your learning horsey very quickly. I suspect the thinner coating will not hold up to the rigors of the environment, and thus you should go back to a timed test standard bolt.

Why are bolts even needed in this bridge? Is it because of the type of bridge?

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