Need yellow coating for fasteners with min.of 500 hour salt spray.

[RonMMG]

We have a customer that is looking for an attractive yellow finish for some of their fasteners. The current selection of finishes that are being used, include a Class 5 Mechanical Zinc with yellow di-chromate and another Class 5 with bronze chromate. Neither finish seems to hold up against abrasion during shipping and handling.
The finish will be applied to alloy bolts and large spring steel conical washers. Electro-plated finishes are not an acceptable option due to hydrogen embrittlement risks. This customer is looking for a finish that can acheive 500 hours salt spray protection and something that will have abrasion resistance after plating (must retain uniform yellow appearance though shipping, handling and installation).
Any recommendations?

 

[TVP]

An alloy plating of zinc-nickel with a bronze chromate conversion coating can achieve that type of corrosion resistance, but as far as I know, it is only available as an electrodeposited coating. However, I am by no means familiar with all plating suppliers, so you may be able to find a vendor that mechanically plates zinc-nickel. I recommend visiting the Suppliers section of the Products Finishing website:

http://www.pfonline.com/suppliers.html

 

[CoryPad]

Zinc alloy coatings can be mechanically plated. Try Mechanical Galv-Plating Corporation at:

http://www.mechanicalgalv-plating.com

 

[kenvlach]

A water-borne acrylic lacquer or polyurethane topcoat should solve both the abrasion and corrosion problems. Here is a listing (note: some are for use on wood):

http://www.masterpainter.com/mpi/approved/mpi103.htm

 

[CoryPad]

kenvlach,

Fasteners require thin coatings to prevent thread binding and time-dependent force loss. The materials to which you refer are not appropriate. Polymeric topcoats are used, but special materials (usually epoxy-based) are used along with specialized application processes to get thin, adherent coatings.

 

[kenvlach]

CoryPad,
I am sure you are correct for your presumption of conventional application methods for those materials.

I was thinking of very thin coatings known as anti-tarnish rinse coats used on metal plating (e.g., brass or nickel) and sometimes as supplemental (top) seals on chromates (atop a regular chromate ‘seal’). I used about one 5 years ago for an unusual application (plate iron onto brass parts, intentionally rust to match weathered sample, seal to preserve), “Miccro-Lac” or something similar, but can’t find the literature on MacDermid’s website (& no longer use that vendor). It was a one-component acrylic or PU, and, if I recall correctly, was diluted first with 4 volumes warm DI water, then to a final dilution of ~15:1, applied by dip coating & air dried overnight. The customer did salt spray testing to simulate (in their opinion) 5 years outdoor exposure in a park setting. I remember being impressed that an essentially invisible film (slight wet look) applied from an aqueous solution over rust could perform so well. The thickness was perhaps 0.5-1 micron (couldn’t measure due to the rust), which I didn’t think would interfere with threads.

ASTM B695-91, “5.2.5.2 Waxes, laquers, or other organic coatings may be used to improve lubricity…”
although I admit not having checked prior to your comment. Instead of trying to match the thread strength (which the chromate can’t do anyway), why not use something that gets out of the way like a lubricant?

ASTM B695-91 only requires the 5 micron Zn + chromate coating to last 72 hrs (to white corrosion). It is a bit challenging to handle 500 hours salt spray. I believe that it can maybe be done by switching to an alloy (with Al, Co, Fe, or Ni) and using a conventional rinse sealer for chromates. To be sure of maintaining the yellow color at 500 hours, and to have adequate abrasion resistance, I think a clear organic topcoat is also necessary. I don’t have thickness figures, but I think a rinse coating applied from a dilute aqueous solution may be thin enough.

There are seals available from the major vendors (Atotech, EPI, HeatBath, Henkel, MacDermid, Oakite, Pavco), some specifically made for yellow chromate on mechanically galvanized coatings, which can approach or reach the salt spray requirements. Many mention that they can be top-coated. And, some also sell the supplemental topcoat.

Below are some vendors of chromates, seals and mechanical plating:

“Sealers
The Finishing Touch
Chromated zinc coatings provide an outstanding corrosion protection system. Economic and technical requirements of enhanced protection systems are becoming more and more demanding.
Sealers are used to improve over all corrosion protection. They are thin layers with a thickness between 0.5 and 5 µm. They penetrate into the chromate layer, stabilize it and seal any cracks and pores. At the same time, they increase the surface strength and meet the highest automotive industry corrosion standards
Corrosil
Corrosil is an inorganically based sealer, i.e. it is not a lacquer or a wax, and is therefore environmental friendly. Corrosil is perfectly compatible to the Atotech chromate system. It can be easily stripped from the racks and is ideal for use in basket and rack plating operations.”

http://www.atotech.com/start.php3?cl_my_id=21387

A chart for corrosion resistance of yellow chromated zinc (not including sealer, that is something you have to contact them for)

http://www.macindustrialproducts.com/yellow.html

“Henkel Surface Technologies offers a full line of Parcolene post rinses for multi-metal applications or a specific metal type. We offer both chrome and non-chrome post rinses for use after a conversion coating processes.”
[that’s about all they say on their website. Try to talk to Phil Johnson]

http://www.henkelsurftechna.com/index.cfm?location=9&pageid=2&group=2&category=34

The Pavco site gives chromate & chromate rinse sealer info:

Pavchrome Yelloy
· Specialty chromate which imparts a shiny yellow conversion coating on Zinc-Iron and/or Zinc-Cobalt plating.
· Produces a shiny film which gives over 200 hours to white salt corrosion.
· Salt spray: 200-300 hours
Pavco has about 5 sealers for chromate coatings, here are 2:

Superseal
· The most corrosion resistant post-plate coating in the plating industry.
· Can increase the salt spray protection to red rust by over 500 hours when applied over plated and chromated surfaces
· Simply add to the final rinse.
· Does NOT significantly add to the overall thickness of the plated part
Superseal W
· Includes a wax for torque tension purposes-- Ideal for faster applications where lubricity is required.
· Two-part system for total product control
· The most corrosion resistant post-plate coating in the plating industry.
· Can increase the salt spray protection to red rust by over 500 hours when applied over plated and chromated surfaces.
· Simply add to the final rinse.
· Does NOT significantly add to the overall thickness of the plated part
· NO added waste treatment headaches

http://www.pavco.com/index2.htm

Plating Systems & Technologies is a mechanical plater that offers chromate & sealer:
Proprietary sealants (such as PS&T Brand Hyperseal) can be applied over chromates to significantly enhance the protection of the chromates. The additional protection offered by such products is typically 100 to 200 additional hours, although exceptionally lengthy results have been reported.
The ability to apply leachant - sealants after the chromate conversion coating to enhance the corrosion protection of the process with little incremental expense. A zinc deposit of 0.0004" with a chromate and a leachant-sealant topcoat will get over 500 hours of ASTM B-117 salt spray; at 0.001"(one mil)process will deliver over 1000 hours.
© 2000 Plating Systems & Technologies. All Rights Reserved.

http://www.mechanicalplating.com/

E-Chrome 851-L — A yellow/bronze iridescent finish on plated cadmium and zinc. Liquid concentrate is used at 0.5% to 3% by volume in water at 65° to 90°F. The finish has outstanding resistance to salt spray, greatly surpasses the normal 96 hour salt spray resistance specified for yellow chromates on zinc. It will withstand up to 200 hours salt spray. The deposit is a pleasing finish in itself. It is an excellent base for paint. The finish may be dyed to various colors.

http://www.metal-finishes.com/zinc-chromates.html

[EPI]


Oakite has a general purpose seal; they don’t offer chromates for zinc:
Gardolene 60
Gardolene 60 is a specially formulated product that provides superior corrosion resistance on metal surfaces. It is applied on these surfaces after they have been treated with iron or zinc phosphates or treated with a chromate conversion coating.
Gardolene 60 contains ingredients that remove unreacted salts deposited by the conversion coatings or left by contaminated rinses on the metal surfaces. Most important, it produces a barrier to moisture which retards or prevents its penetration through subsequently applied organic finishes. As a result, paint adhesion is improved, corrosion is resisted or prevented and finished products have longer useful lives even if subjected to harsh environments. The final acidified chromic rinse can be applied to conversion-coated surfaces of iron, steel, galvanized steel, aluminum, galvalume and zinc. It enhances the corrosion resistance of conventional water- and solvent-borne systems as well as electrophoretic paint systems.

http://www.oakite.com/comdir/cditem.cfm?NID=180