Sounds like galvanized fitting. Most things you see that are galvanized are steel with a zinc coating. You weld the same as if you were just welding dissimilar metals, but since you have to insure high pressure, you should probably find out what kind of steel your working with, regardless if it's zinc coated or not. If it is a fine high chromium steel, then use something like a 308 ss and you'll be fine. Just lay a good bead in there first time, last forever.
The one thing about zinc coated steel and other metals coated with zinc or another forms of galvanization, they are basically electroplated and the zinc is scaley. You can grind the zinc out but you will take some steel with it. If you try to just weld over the zinc it will pop and backfire on you, bad if your tigging, can get away with it stick or wire. I grind some and then brush with 6" mounted wire wheel. The cleaner the galvanized the better.
Take Care
Steve
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