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ASTM / ASME spec F1941 for Zinc Plating ... Which finish to choose?




December 9, 2010

Q. Hello everyone,

I am currently working on a project to classify fasteners for my company based on the ASME B18.24 [affil link] PIN Code System (the 18-digit code to describe virtually any fastener). I am getting really confused in the finishing section. My company uses fasteners in the most simple sense--like the kind of fasteners you buy in the bulk bin at the local hardware store. We do not specify specific plating specs; we simply use the general name.

For example, rather than specifying:
"ASME B18.2.2 [affil link] Hex Nut, 3/8 UNC-2B, SAE J429 Grade 5, ASTM F1941/F1941M-16 [affil link] Zinc Coating ZN/FE Type 5B"

...we simply specify:
"grade 5, zinc plated 3/8-16 nut"

Now, the problem I'm running into is that the ASME Spec lists about 240 grades of zinc plating! All I want is a single spec to call out, to get the basic zinc-electroplated fastener. My research is leading me to believe that ASTM F1941/F1941M-16 [affil link] (both inch and metric) are the two standards I need; but even those have over 200 grades listed!

So, to summarize, what I need to know is what Spec/Grade/Class/Type I should call out so that when I order a zinc-plated fastener...I get the basic $0.03 one, rather than some exotic $5.00 one. :)

Thanks everyone,
Grant

Grant Griffiths
CAD Designer - Hartland, Wisconsin



A. ASTM F1941/F1941M-16 [affil link] is for inch-based fasteners and ASTM F1941M is for metric fasteners, although the designations are identical between the two. [ed. note Sept 2016: the M version was withdrawn and there is now just ASTM F1941/F1941M-16 [affil link]]. For instance, you could probably use the designation Fe/Zn 3A for your purposes, which specifies zinc plating (not a zinc-alloy plating like Zn-Ni or Zn-Co), the 3 indicates a minimum thickness of 0.0001 inches, and the A indicates the chromate finish (clear, transparent colorless with slight iridescence). Changing the last letter to a B means that the chromate is blue-bright (transparent with a bluish tinge and slight
Iridescence) or C means that it is yellow (yellow iridescent). One of these should meet your needs.

Toby Padfield
Automotive module & component supplier - Michigan, USA
December 14, 2010


! Toby -- Thanks for boiling it down for me. That's exactly what I needed.

Thanks!

Grant Griffiths
- Hartland, Wisconsin, USA
December 16, 2010



Q. I received parts that required zinc plating per ASTM B633 [affil link] , the attached certs reference ASTM F1941/F1941M-16 [affil link]. Is there an acceptable way to demonstrate equivalence?

Peter Galsworthy
- Grand Rapids, Michigan
May 18, 2012




October 24, 2012

Q. Is there an error in the ASTM F1941/F1941M-16 [affil link] spec regarding salt spray hours for Fe/Zn 5E?

The chromate finish letter E designates a black color. It is my understanding that the darker colors are a naturally occurring process due to the hexavalent chromium and provide greater corrosion resistance.

Per Table A1.1 in the annex of ASTM F1941/F1941M-16 [affil link]:
Fe/Zn 5A = 6/12 hrs to white/red
Fe/Zn 5B = 12/36
Fe/Zn 5C = 48/72
Fe/Zn 5D = 72/96
Fe/Zn 5E = 12/blank

So why does E (black) only have 12 hrs to white rust and an omission on salt spray hrs to red rust?

My follow up question would be, if the fasteners are coated with trivalent chromite (adding the designation "T"), why do the salt spray hours in the ASTM spec differ between any color chromate designation? Why would Fe/Zn 5ET differ from Fe/Zn 5CT? Isn't the color achieved by adding a dye and the corrosive resistance achieved through a sealer or thicker chromate?

Thank you for your assistance.

Philip Crowson
Hardware distributor - Houston, Texas, USA




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