<...Is welding of a galvanized pipe acceptable?...>

No. The welding fumes will be loaded with Zinc, which will affect the health of the operator carrying out the welding.

Assuming the pipe material underneath the galvanising is steel, what is in the pipe, its temperature and pressure?

Hello K.Muthu,

Can I suggest you do actually get a book for on-site work, as all these questions are relatively unconnected one with the next.

The cutting of the thread depending on what grade you are working to ASME, BS etc, each has a slightly different thread and thread depth, but, whatever the thread it will alway cut through the relatively 'surface layer only' of galvanising.

From my experience the pipe is usually threaded on the outer surface. But whatever the galvanising, when you cut a pipe to length you are left with bare black pipe. Check out the sites below and you may see a tape which can be applied to a joint after making. But note: The area of about an inch (~25 mm) from the end of the thread must be clean. So only ever clamp the pipe to cut and to thread it about 4 inches (~100 mm) from the end.

The type of thread I think you will find depends on the size, and the intended use of the pipe. What it carries and the type of 'atmosphere' the outer surface with be likely to have to live with.

You can read about galvanising on-line or get a book on it but, make sure you are judging like with like. Industrial galvanising and joint-making may be entirely different and a lot more 'substantial' than 'domestic' pipe.

Why are you so worried about this. Are you using an acid in the pipe, or is it for air, or water, or something else?

If you refer to a straight Galvanised coupling as below, the externally threaded pipe will be screwed into the internally threaded coupling, from both ways. So at that point the pipe after it is connected will have at least twice the thickness of the rest of the pipe at this joint.

Galvanized Pipe Fitting - Galvanized Coupling [2007-11-21]

Items Name: Pipe/tube fittings, like elbows(45&90), tees, corsses, nipples, sleeves, couplings, unions, bushings, plugs, caps, flanges, hex lock ...

Steelinking Galvanized Pipes Industry Ltd-Fitting Dept. [Province:Guangdong, China]

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The http://www.sparkelectrodes.com/erw-hot-dip-galvanized-steel-pipe.htm

Has details of pipes, pipe joints, and welding of Galvanised pipe.

1. Steel Pipes - Large Stockwww.cleveland-steel.com Tubes for structural & construction Over 40,000 tonnes in Stock UK

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Check out the various links below. The Manufacturing Process link is interesting.

I hope you will view all these sites and details and perhaps keep a written Document about this for future use?

Please note: I have spell checked my own typing only, no pasted address' and details.

Take care and I hope you find what you are looking for in my advice, good luck.

K;

I hope this helps some.

BST is also a tapered thread I believe the same 1/16th inch taper per inch of thread as NPT however it has a 55 degree included angle and different root and peek profiles. Both threads are available tapered or parallel (BSP / NPP) and both are about the same depth in either case. The 5 degrees difference is trivial if you consider the allowed variabilities in pipe diameter, thickness (respective of pipe schedule), and roundness (or should that be out of roundness?). Parallel threads are typically used for electrical or mechanical joints where you may wish to use more thread length. Tapered threads are used for fluid transfer as they compress some what during assembly and provide a more leak resistant joint. Straight threads should never be used with any gas applications (natural / propane) and typically are not advisable for the transfer of other fluids.

Welding of any pipe after it is threaded into a fitting as a means to prevent or stop leaks is a taboo. During the thread cutting process oils are used and typically during the thread connecting process some type of sealant is used. Thereby imparting impurities into the weld zone and creating an inferior weld. Rust and corrosion will do the same thing. Also even if everything were sufficiently clean you still have a compromised weld area due to the threads themselves. (Never weld on threads)

Typically welding is done with weld fittings. (look up "Weldbend" they are a major manufacturer of weld pipe fittings.) with either a butt weld, socket weld or slip joint. Threaded fittings are not suitable for welding.

Welding threads on galvanized pipe is a triple taboo. You have the above issues plus, zinc has a melting point of around 900 degrees Fahrenheit. When welded on, the zinc oxidizes (zinc oxide) and becomes poisonous in certain concentrations (trust me I have experience with this. and no it was not a mistake of my own). This is also self defeating as when you burn through the exterior coating to perform the weld you also burn off the internal coating behind the weld zone and compromise your corrosion resistance internally where it is needed the most.

The proper way of dealing with leaky galvanized joints unfortunately usually involves some disassembly followed by reassembly with fittings that are not corroded or simply making sure that during reassembly you use the proper amount, and type of sealant as well as tightening it sufficiently.

The galvanizing does not truly "penetrate" the surface of the pipe save for surface imperfections. The steel beneath has a melting point of around 2600 deg. Much higher than the zinc and therefor the two are in bond and not fused together. This means that deep gouges can also compromise corrosion resistance.

As far as the thickness of the zinc coating goes I do not believe I can be of assistance as what I learned about it some 30 years ago may no longer be applicable.

A galvanizing does not penetrate very much if any into the pipe and it is acceptable to thread galvanized pipe, And I have done this many manuy times in the past. i am not sure but all the galvanized pipe I have used the was threaded after it was galvanized. Threading it will remove some of the coating and it must be reapplied after the joint is made up. it does not affect the galvanizing on the interior of the pipe though and this is the normal reason most people use it. If the pipe was galvanized after it was threaded the additional thickness added with the coating would make it impossible to use standard fittings. I am not sure if they make a type of pipe and fittings with over sized threads in order to be able to galvanize the pipe after it is threaded live galvanized bolts but I have never seen it.

B. I am not sure about BSP threads but NPT threads are tapered and more metal will be removed from the end that the threads are started from.

C. Yes it is but to make a sound weld you must grind the galvanizing back in order to not contaminate the weld metal. I have welded galvanized piping to ASME B31-3 codes with x-ray quality welds many times but I 'd rather sand paper a wildcats tail than weld galvanized pipe after I got galvanize poisoning a few years back from welding on it. If you are thinking about welding on anything galvanized please look into the safety factors and PPE required to keep your welders from breathing the fumes.

D.If you go groove it through to the base metal it will rust. Galvanizing is a coating.

E. I am not sure of this but I would assume it is because of the costs and the fact that making the coating any thicker may not help with the corrosion resistance properties or it could be that it hinders the bond to the metal from being so thick. I would bet you could get the coating any thickness you want if you offered to pay extra for it.

F. Please start here for this request I think they may have some free pamphlets and such. http://www.galvanizeit.org/

One thing I found out years ago that I thought was interesting is that on hot dipped galvanized angle iron it is not possible to just cut out a angle section on one leg and bend it to make pipe supports or knee braces. Even if you grind all the galvanizing back to bare metal in the area it will still crack or break when you heat it up and bend it. I have never figured out why this happens and I have always heard that hot dip galvanizing does not effect the metal properties. I have bent hundreds of pieces of non coated metal shapes to make supports from without them breaking but never bent a galvanized piece that did not a least crack badly and some actually broke into. I also know that hot dip galvanizing heat treated studs will lessen thier strength because the heat from the galvanizing process will anneal the studs and they will no longer stand up to the specification.