Coating for Bolts 1

[ligin18]

Hi All

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.

 

[dik]

What does Zn+Fluoropolymer coating mean? Is this a HDG part with a coating? or is it a zinc rich fluoropolymer?

Dik

 

[btrueblood]

or Xylan over Zn plating?

 

[ligin18]

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

 

[dik]

For a start, I would not consider electrogalvanised material for any use where corrosion was an issue.

 

[dik]

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.

Dik

 

[mk3223]

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.

 

[dik]

HDG gives far better corrosion resistance.

Dik

 

[SJones]

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.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Vladimir Mescerjakov]

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.

 

[dik]

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.

Dik

 

[SJones]

@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.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[dik]

Is ZN essentially the same as HDG?

Dik

 

[EdStainless]

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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Maui]

Dik, yes HDG is a zinc coating. HDG = hot dipped galvanized.

Maui

 

[rtmorgan]

For most recent offshore production applications, I have noticed our clients are now specifying 'XYLAR overcoated with XYLAN 1040 OR 1070'.

 

[MJCronin]

http://www.whitfordww.com/pdf/literature/xylar flier 2011.pdf

MJCronin
Sr. Process Engineer