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Internal Thread After Galvanizing

Internal Thread After Galvanizing

Internal Thread After Galvanizing

(OP)
Dear Members,

The fittings manufactured as per international standard dimension such as ASME or MSS can be galvanized for corrosion protection. Normally the small size galvanized fittings are used with internally or extenrally threaded end.
My question is whether the internal thread for fittings can be made after hop dip galvanizing or not.
If the internal thread is made before galvanizing, the galvanized fittings with internal thread can not be connected to other fittings or pipes with external thread due to galvanizing thickness.

I found out para.6.3 of ASTM A123 saying "Internal threads are not prohibited from being tapped or retapped after galvanizing." I interpret that internal thread can be made after galvanzing.

What do you think?

RE: Internal Thread After Galvanizing

Hi onrush

I read it the same as you, that you can tap the thread after galvanizing.

desertfox

RE: Internal Thread After Galvanizing

If this is for aerospace use I do not think it is allowed to tap after coating. There is always the option to tap the thread before plating using an appropriate oversize tap. However, as I recall there is a maximum oversize that can be used in order not to weaken the thread. Therefore, hot galvanizing probably will not fit for the job due the thick coating layer by hot galvanizing. In aerospace use an electroplating process is commonly used.

RE: Internal Thread After Galvanizing

The big problem with electroplating a thread is that the high stress areas on the flanks of the geometry are prone to flaking.  Having debris in the thread make-up leads to binding, you would eventually sieze the joint through galling.

In several other topics I have discussed liquid nitration, which is an absorption process to the parent material.  Case depths are usually 0.005/0.008 inch deep and hardnesses are in the order of HRc 50/55.  The process varies somewhat between vendors, I had some clown literally water quench the piece immediately after the salt bath in order to increase volume through his facility.  This is a no brainer, obviously thermal cracking in the root of the thread is an issue here.  So beware of the non standard and often extra steps buddy may insert in his opertations.

I prefer liquid over gas nitration because of the lower activation energies.  Gas nitration is usually done at a much higher temperature and makes the substrate material a bit annealled.  Also, liquid is oil quenched which adds to the lubricity of thread make-up whereas gas usually has a powder finish of various oxides due to the furnace application.  You can pick which is better for you.

So I would recommend the liquid nitration process, it does not vary the geometry of threading and adds the corossive resistance aspect to the joint, usually beneficial.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

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