Protecting threads/features for hot dip Galvanizing?
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    Default Protecting threads/features for hot dip Galvanizing?

    I am looking for some ideas to protect threads or special features when hot dip galvanizing. In the past we have used 100% silicone and hi temp RTV, but these are only good for 400 degrees. The acid bath is 840 degrees. Every time we would have to chase the threads. I called up to our galvanizer and he said to use either liquid nails or hi temp grease. I did find a select few "greases" but most are rated for 400-500. I found an extreme temp anti seize good for 3,000 degrees. Considering the main difference between grease and anti seize is oil vs. metal, I am going to give this a shot. But considering we have we have a bunch of 1 1/2 deep threaded holes on this upcoming run, I don't want to take the chance and cross my fingers. I also think liquid nail would require chasing threads...more than I would want to.

    Yes, there is putting bolts on the holes, but that can get sketchy with small holes.

    Does anyone have secrets they would like to share?

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    well this is a difficult one to awser without seeing the coating defects.

    a few of the possible issues are

    thread not cleaned properly before being coated.

    the galve bath hasn’t been cleaned or filter well enough,(circulating crap that’s sticking to the thread)

    Depends on the size of the object that’s being dipped, (this is difficult to explain) galve thickness is determined by the mass of the object. if its a large object compared t the size of the thread, then the thickness in the thread is going to be too much. only way out of this is double dipping witch gets expensive.

    U can calculate a thinner coating by the time frame of the dip. (however this shortens the product life.

    the list could go on depending on the type of defect it is.

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    I don't have a lot of galvanizing done. But what I have found so far is that they will screw up just about anything you give them. So I quit trying to protect anything. I just drill and tap features after the galvanize. There ends up being some bare metal, but with so much zinc around, we don't really see too much problem in the field.

    I have one vendor that puts 1/2" grade 2 bolts in the holes before galvanizing a part with 1/2-13" nuts welded to the assembly. Their thought is that the bolt can be removed post galvanize and leave a nice clean thread in the nut. Almost every time I get one in the field I end up breaking the damn bolt trying to get it out.

    I see you are in Minnesota ... who do you use for galvanizing? I have been using AZZ in Minneapolis. It's hit or miss. Got some simple weldments back with one entire face with no galvanize. And just last Friday they sent me a form to sign saying they were going to torch 12 holes in a trailer chassis that is made entirely out of channel. For flow reasons, they claim. I haven't figured out how they can have flow issues with channel.

    -Jim

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    Jim,

    We use AZZ. Most of the time this is structural, handrail and industrial spouting. With AZZ, you pay by the lb. So Any corner we try to chamfer or blow a hole. Especially handrail. We will vent every post and cross tubes where it is butt welded. 1. They claim the air pocket could explode. 2. to flow as much excess galv. we have had parts come back with corners FULL of galv. I am not 100%, but it looks like they dip in one position. They don't spin or rotate the part, so I can see how your chassis could fill in the corners, no matter what way the toe is positioned.

    These parts are going directly to the jobsite, hence the reason for properly prepping.

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    I'm still trying to wrap my brain around a 840 degree acid bath. Do you know the composition of the acid?

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    Probably a case of brain lag ;-)

    More like molten zink bath, ehhhh ?

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    I would not put a bolt in a threaded hole before galvanizing. You will have the dickens of a time getting it out as I found out on my boat trailer I had galvanized last year. My neighbor had some work galvanized that had 1/2" threaded holes about 3/4" deep. The guy at the galvanizing place told him to drive hardwood plugs into the holes from both sides. The wood will actually take the heat for a short period of time and even if it chars the carbon seems to keep the zinc from sticking. I know soot makes a good release agent so maybe that's why it works. It didn't really burn the wood that bad. They still had to be unscrewed with a pair of pliers and the threads were nice and clean.

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    Not sure if this will be helpful, but last year we had some combination irrigation/fence posts hot dipped. I bought a 1 inch pipe die to chase the pipe threads. We could not hardly start the die or turn it when we did. The welder that built the posts found that he could heat the threads and brush them with a steel brush and return the threads to usability and maintain most of the coating.

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    Quote Originally Posted by FredC View Post
    Not sure if this will be helpful, but last year we had some combination irrigation/fence posts hot dipped. I bought a 1 inch pipe die to chase the pipe threads. We could not hardly start the die or turn it when we did. The welder that built the posts found that he could heat the threads and brush them with a steel brush and return the threads to usability and maintain most of the coating.
    Just make sure the welder or whoever isn't breathing the fumes from the molten zinc - that's seriously bad news for your lungs.

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    We hot dip a lot of our product (anti-terrorist barriers). These are 5,000-6,000 pound weldments. Use the RTV in the tapped hole and then have to dig it out and run a tap through it to clean up the threads. Used to put the bolt in the holes, then you have to take a torch to the bolt to melt the zinc as you try to turn it, all the while breathing the fumes. We may want to try the wooden plugs. As the OP notes, it's a pita.

    John

    j8761-photo-dscf8856.jpg

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    Just a thought, when you apply a high temperature dissimilar liquid metal to a close fitting joint between two steel parts, don't we usually call that 'brazing'? Probably not the best process if you want to get those bolts back out.

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    Paint can sometimes have the desired effect

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    Normal practice in plating zinc... If load per bolt is not excessive.

    Use ~.015 oversize taps.... They are available...

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    Quote Originally Posted by Winterfalke View Post
    Just a thought, when you apply a high temperature dissimilar liquid metal to a close fitting joint between two steel parts, don't we usually call that 'brazing'? Probably not the best process if you want to get those bolts back out.
    You're breaking the already 'brazed' joint (brazed in the dip tank) by heating above the melting point.

    John

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    I have been sending stuff out to be hot dipped since the early 80s- and I always just retap the threads, and often, redrill ordinary holes.
    I just figure the time and expense in.

    I am dubious that any rtv or grease would work all the time- which means, again, you are retapping threads.
    So I just skip the goo, and retap. Saves a step, and the disappointment of hoping you were ready to go.

    I would agree that pretty much anything that can be screwed up, galvanizers WILL screw up. Even good ones.
    Its molten metal- it finds a way.

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    To clarify my OP, the compound to be used is to prevent the acid penetrating any metal that is not to be galvanized. The Zinc bath is 840 degrees. Thus, the compound needs to withstand such temps to prevent any buildup.

    When using oversize taps, would this work for 1 1/2" deep threads? I can see 1/2", but deep threads have enough time to drain?

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    Just got parts back from galvinzer. The 3000 degree antiseize didn't work at all. The parts with the liquid nails were better, but we still had to chase every single thread. We plugged the whole hole, but it seemed like the acid bath shrunk it just enough for a little zinc to get into the threads.

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    In the immortal words of Chuck Cleaver, of the band the Ass Ponys-
    I hate to be the one who said I told you so, but I believe I might have told you so...

    Last Night It Snowed - YouTube

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    I bought several pounds of hot dipped 3/8 and 1/2 bolts at habitat for humanity. Surprised to see so many there. Found out the were rejects with the end of the threads totally coated with Zn. So bad they where full diameter at the ends for 3-4 threads. I had to use a die to get a nut started. Snapped the first one trying to start a nut on it.
    Bill D.


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