Any difference between zinc and heavy zinc for carbon steel coating?

The difference between zinc and heavy zinc is the length of time and level of concentration of your zinc phosphate chemistry. Being this your product goes into the field and is "man-handled" while being amongst the earth's elements (heat, cold, rain, sleet, etc.), the objective is to protect the threads of your pipe from corrosion or "red-rust".

The difference between zinc and heavy zinc is dependent upon what level of protection you are requiring to meet this objective, which is not only going to be achieved by the zinc phosphate coating (mg/sq.in.), but the post-process of applying protective oils and lubricants during operation or installation.

Heavier coatings will achieve a longer life for absorbing and retaining the oils, as a thinner, less dense coating will respectively be less.

As for your question regarding brush or dip? Obviously any controlled process will be successful, however, if you are having to do this in the field, brushing would be the most effective way to accomplish your coating requirement. But either will achieve a good coating if pre-cleaning and processing is controlled and successfully adheres to the process specifications.

As for iron content in your process chemistry.  You can help to control or promote this by throwing steel wool in your process, similar to what you may see for "seeding" an electroless nickel bath or an aluminum etch or anodize solution. Contact your technical support technician for the chemistry process you are using and inquire of this and request a technical data sheet for your process. Also, you can always subscribe to Metal Finishing magazine and receive your copy of the "Metal Finishing Bible" or MF Guidebook.

Please let me know if this has been helpful  and feel free to contact me via private message or email to discuss further. Or, as I would like to thank you for utilizing finishingtalk.com, keep posting your questions right here!!!!