Protecting screws from rusting 3

[bibayoymicabayo]

Hi everyone!

we are currently using DIN912 Class 12.9 screws in a project we are working on, those that come in a black finish. Our customer asks us to clean the screws, so the bare metal is exposed and we end up with rusty hardware. To solve this, we are passivating the screws for this batch, but for the near future we are buying screws with some anti-corrosion treatment. But that raises several questions to me...And unfortunately we cannot switch to stainless steel screws because all the design and analysis have been made considering these screws, that is out of the question.

Which options do I have? Is there any major difference between zinc-plating and passivating the screws (in terms of corrosion resistance)? Does anyone know any supplier of this kind of hardware?

Thanks in advance

 

[MintJulep]

Uh. Passivating is sort of meaningless for a carbon steel screw and offers zero corrosion protection.

12.9 screws can be zinc plated, but you need to be concerned with hydrogen embrittlement.

You might investigate one of the aluminum-zinc flake coatings, such a Dacromet.

 

[mcgyvr]

Why the requirement for cleaning/exposing bare metal from the customer?

What is the environment?
Expect time before white/red rust?
Many "common" finishes on steel fasteners can only last 100+ hours before rust develops in a salt fog environment/test.

 

[ctopher]

In addition to above...every screw should have some type of grease added to it before installation.

Chris, CSWA
SolidWorks 14
ctopher's home
SolidWorks Legion

 

[CoryPad]

The best solution is a zinc-rich coating such as those provided by Magni (e.g. Magni 565), D[ö]rken (e.g. Delta-Protekt KL + VH), or NOF Metal Coatings (e.g. Geomet 321).

Zinc plating can induce hydrogen embrittlement and has lower performance than the zinc-rich systems mentioned above.

 

[berkshire]

bibayoymicabayo (Aerospace)
You could look into Sheradizing, ( Sheradising) your components. This is a mechanical zinc plating process, where the parts are tumbled in zinc powder and sand.
B.E.

You are judged not by what you know, but by what you can do.

 

[Screwman1]

If the seating torque has been developed for dry bolts with no finish, make sure that you DO NOT add grease to any parts without taking into account the greatly increased clamp load that will result from the reduced friction.
I second CoryPads list of finishes; those are the most common high performance commercial coatings.
I would recommend against electroplating PC 12.9 parts, the high hardness in combination with a corrosive environment is an ideal set-up for either hydrogen embrittlement or stress corrosion failures.

 

[MikeHalloran]

Passivation includes a soak in hot acid.
The acid will dissolve a carbon steel fastener, in not a lot of time more than it takes to dissolve the carbon steel retained on the surface of stainless steel. It goes way beyond merely not providing corrosion protection.
It is absolutely not appropriate for the fasteners you are using.

You've got a pretty severe cleaning process if it removes a proper black oxide. Consider a quick dip in Brakleen instead.
Or just send them out for one of the mechanical zinc coating processes.


Separately from that small issue, the cheapest, most reliable fastener is the one that was removed from the design completely. Should evolved versions be in your future, consider assembling parts with cam surfaces or mechanical crimps.

Example: mechanical fuel pumps became much more reliable when the typical eight screws were removed from the design, and replaced with a peripheral crimp. They became non-rebuildable, but typically didn't need rebuilding over the life of a car. I'm sure they got cheaper, but the end user never saw that.




Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

bibayoymicabayo-

When you say you plan to strip the existing black oxide finish and then "passivate" these class 12.9 alloy steel screws, can you clarify what you mean by "passivate"? The term passivation is typically associated with a chemical treatment used on corrosion resistant steels that uses acid to etch contaminants like iron from the material surface. And as MikeHalloran pointed out, this type of process is definitely not suitable for an alloy steel material. Is it possible that what you meant by "passivate" was instead a conversion coating like zinc or manganese phosphate?

For your future requirements, the best choice of corrosion protection for your class 12.9 alloy steel screws will depend on the environmental conditions the screws are exposed to. Coatings like zinc plating or zinc/manganese phosphate are low cost, but they only provide limited corrosion protection. There are newer types of coatings for threaded fasteners like IVD aluminum that provide excellent corrosion protection, but they are more costly.

And finally, as others pointed out be careful of hydrogen embrittlement issues with whatever coating process you decide on since your class 12.9 screws are made from high-strength alloy steel.

Good luck to you.
Terry

 

[cvg]

perhaps a less aggressive "cleaning" so the bare metal is not exposed? what exactly is being cleaned off the screws?

 

[boo1]

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Fluorokote

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Xylan

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Takecoat-1000

http://www.sandstromproducts.com/

Sandstrom