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Tonymech
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Dacromate on a Stainlees Steel Bolt?

08/08/2013 10:41 AM

Dacromet, I understand, is a baked on mix of zinc and aluminium. We have been offered a Dacromet coated stainless steel bolt to fix aluminium louvres. I know both 304 and 316 work well with aluminium, so why add Dacromet? Tony

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jesw55
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#1

Re: Dacromate on a stainlees steel bolt?

08/08/2013 10:48 AM

Hi Tony,

Could it be to stop the threads binding/

Best regards,

John

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BruceFlorida
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#2

Re: Dacromate on a stainlees steel bolt?

08/08/2013 11:50 AM

Anti-seize lubricant is also my best guess. We have some stainless steel rivet nuts that are tin plated. Lubrication was my best guess there.

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lyn
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#3

Re: Dacromate on a stainlees steel bolt?

08/08/2013 11:51 AM

I was taught to NEVER put Aluminum and Stainless together.

Use the coated hardware.
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JPool
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#4

Re: Dacromate on a stainlees steel bolt?

08/08/2013 1:46 PM

In essence, the top 3 answers are all correct.

Dacromet is a zinc rich coating with some dry film lubricating properties (which they are using it for in this case). I have used it myself and will tell you it is over rated and troublesome (adhesion, flaking) and serious over kill for use on S.S. because it's main function is corrosion protection on carbon steel. Ford recently awarded us several million dollars in business to coat carbon steel ballstuds with a thermally cured MoS2 dry film lubricant that were all previously coated in Dacromet and it's water based version, Geomet.

We blast and coat tens of thousands of pounds of 304 and 316 S.S. nuts, bolts, screws, washers, etc, with dry film lubricants containing MoS2, Graphite, W S2, PTFE, among others for 2 reasons. #1 obviously the lubricity and torque control and #2 to provide a barrier to prevent galvonic reactions between dissimular metals.

We also coat aluminum, for lubricity and steel for above reasons and corrosion protection. With carbon steel we do use Zinc and Aluminum rich coatings but never on stainless.

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Tonymech
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#5
In reply to #4

Re: Dacromate on a stainlees steel bolt?

08/08/2013 7:33 PM

Thank you JPaul and others for such comprehensive replies.

We have also on the same coastal area, a job using large Colourbonded steel louvres with galvanised steel frame inside and need bolts and self drilling screws. What type would be best?

Tony

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JPool
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#6
In reply to #5

Re: Dacromate on a stainlees steel bolt?

08/09/2013 9:08 AM

You can use the Dacromet coated bolts, I just think you may be paying more than necassary for the protection you need. I wouldn't be able to recommend a brand name seeing as you are on the opposit side of the world from me and we are a coating manufacturer and applicator, but I would use stainless coated with a molydisufide (MoS2) based coating.

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JCase
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#7
In reply to #4

Re: Dacromate on a stainlees steel bolt?

08/09/2013 9:47 AM

Dissimilar materials use coating for corrosion protection. (as stated above).

http://en.wikipedia.org/wiki/Galvanic_corrosion
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ndt-tom
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#8

Re: Dacromate on a Stainlees Steel Bolt?

08/09/2013 12:03 PM

Since nobody else has chimed-in this tidbit...

...further-to lyn's post :
("NEVER put Aluminum and Stainless together")

zinc has a very low melting point, and metallic zinc particles can diffuse into stainless steel causing embrittlement and cracking (google liguid metal embrittlement).

The OP states the bolts are : "to fix aluminium louvres", but, gives us NO clue as to how big / heavy the louvers are, in what ENVIRONMENT they are used (exhausting fumes?!?), or whether the environment poses risk of a fire around said louvers, and what hazards might be posed IF the bolts were subjected to cracking (due to liquid-metal embrittlement).

Thus ... many more questions must be answered before the Dacromet coating can be either "accepted" or "condemned" as a viable fastener.

Just my two cents ...

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Tonymech
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#10
In reply to #8

Re: Dacromate on a Stainlees Steel Bolt?

08/09/2013 5:18 PM

Thank you for your further insight. I was aware of hydrogen embrittlement (my main concern) but not liquid metal embrittlement.

The louvres are for either sun shade and/or ventillation of plant, such as around roof aircon. They are under 10kg/m. The fire and fume hazards are minimal.

Tony

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lyn
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#9

Re: Dacromate on a Stainlees Steel Bolt?

08/09/2013 12:11 PM

This is totally OT, but it may be of interest to some who are posting here:

Controlling Surface Chemistry and Morphology
to Promote or Prevent Adhesion

Live Presentation:
Thursday, August 15, 2013, 2:00 pm ET

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BruceFlorida (1); JCase (1); jesw55 (1); JPool (2); lyn (2); ndt-tom (1); Tonymech (2)

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