As per the valve data sheet (sour service)the coating for bolts shall be "Zn +Fluoropolymer coating".But vendor offer is HDG+Xylan 1070. Plz clarify which one is better in quality.
Bolting-ASTM A193 Gr.B7M /ASTM A194 GR.2HM Zn plated +FLUOROPOLYMER coated. But vendor offer HDG + XYLAN 1070(Green).As per datasheet our requirement is Zn plated with fluoropolymer coating. Instead of this vendor offer is hot dip galvanized with xylan 1070 coat
I've not used Xylan 1070, but, I've used Xylan products and they seem to provide corrosion resistance... not sure how much. On a couple of projects, I've spec'd Dacromet coating, but, for likely a less intensive corrosion environment. Xylan and Dacromet are relatively thin coat systems, and, I'm not sure how HDG fits into the scheme. As I've noted, I would not use electroplating zinc for any serious corrosion resistance. Depending on the environment, HDG provides good corrosion resistance for almost anything with a pH over 7.
IMO, unless the project specified the special requirements, two materials are equivalent sine the Zn coating is similar to the HDG but a thinner coating layer, and the xylan coating is a family of the fluoropolymer material with similar properties, i.e. tensile/impact strength, hardness, abrasion/chemical resistance etc.
The offered coating complies fully with the coating stated in the datasheet. In the absence of any specified requirement to support offered coatings with independent test data, there is not a lot to discuss further.
Xylan 1070 is the default standard coating for Blue FLUOROPOLYMER (Xylan) coating. Which is basically same thing as just specifying Xylan.
Coming from a bolting manufacturers background - never heard of HDG before Xylan, to be honest, I do not believe PTFE will stick on HDG... Standard process is - degrease by acetone, media blasting, clean surface, zinc phosphate treatment, pre-bake, apply Xylan, bake, apply Xylan again, final bake and game over.
To clarify:
HDG+Xylan - does not exist
ZN+Xylan - is standard.
Xylan 1070 - just means default standard Blue Xylan coating.
HDG is totally different to ZN coating process.
If your vendor is offering you HDG+Xylan it is probably lost in translation and someone translated "zinc coating" as HDG.
have to be careful with HDG coatings. With some coating material, a zinc saponification occurs (this is like a soap film) and it greatly interferes with the bond of the coating to the zinc.
@Vladimir Mescerjakov - thanks for firming up the HDG issue. I had not followed the changing statements closely enough, simply focusing on "Zn + Fluoropolymer" for which HDG meets the "Zn" requirement. HDG + Fluoropolymer does not appear in NACE Publication 02107 as a regular option. I was trying to point out that, with a poorly crafted specification, one tends to get poorly crafted offers.
You can apply the Zn two different ways, either electroplate it, or hot dip in molten metal.
HDG provides a more corrosion resistant coating, it is thicker and less porous.