Does anyone know what this coating is?

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A buddy of mine gave me some nuts he wants welded to some mild steel brackets. They are coated with a greenish coating. I have seen similar ones at tractor supply. Any input? I don't like messing with those coatings. Attached Images
Reply:Looks like a phosphate coating to reduce rust. I would Just weld over it. Dan D.Manipulator Of Metal
Reply:i wouldn't worry about it, if you were doing 50 of em i might.wear a half mask resp, or a filter if you really wanna be safe.
Reply:Try soaking them in vinegar it may remove it.
Reply:Looks like black oxide or a phosphate coating. Either way, safer to weld than standard zinc plated  fasteners! I believe black oxide is actually rated at being weldable. I know red oxide is!John 3:16(2) Miller Pheonix 456(2) Millermertic 252Dynasty 210DXHobart 210MVPDoringer D350 SA Cold SawScotchman 350LT Cold SawWebb 10x50 MillWebb 15x40 LatheGeka Bendicrop Ironworker
Reply:It is either xylan, or another solvent base polymer. Try wiping it off with lacquer thinner. Personally, i would just be sure to have good ventilation and weld it.
Reply:Grind clean
Reply:Zinc phosphate can turn greenish with age or in the presence of some lubricants."USMCPOP" First-born son: KIA  Iraq 1/26/05Syncrowave 250 w/ Coolmate 3Dialarc 250, Idealarc 250SP-175 +Firepower TIG 160S (gave the TA 161 STL to the son)Lincwelder AC180C (1952)Victor & Smith O/A torchesMiller spot welder
Reply:Standard thread? Throw them in your nut bin and go buy some without plating.---Meltedmetal
Reply:I have a bunch of plain steel heavy hex nuts (from McMaster-Carr) for such occasions. They're cheap and work well as long as high strength is not needed.The fasteners in the OP's picture look exactly like the grade 5 fasteners from Tractor Supply. I have never tried to weld them but they do have a plastic-y look.I would also be careful when welding grade 5 or 8 nuts since they are fairly high in carbon and may crack or become brittle if not welded properly.JohnA few weldersA lot of hammersA whole lot of C-clamps