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triaceng1 (Mechanical) (OP)
23 Jan 09 20:34
I am designing a Grounding spool that is a Three part assembly. The spool center and and flanges are made out of aluminum, the flanges are bolted to the center spool by four bolts. If the bolts were Zinc plated would this cause corrosion problems between the screws and the outer flange?
CoryPad (Materials)
23 Jan 09 23:58
Are they steel bolts?  If yes, a better finish would be a zinc-rich multilayer coating like Magni 565 or the Doerken Delta Protekt series.  They are formulated to have excellent compatibility with aluminium structures.
bigghassan (Nuclear)
25 Jan 09 0:36
In any contact between zinc and aluminum if the aluminum is small relatively to Zinc and in presence of common electrolyte (water or soil) Aluminum will start to corrode.
In your condition apparently Aluminum are larger than zinc-plated area there so applying same coat to both side will help.       
 
Helpful Member!  strider6 (Materials)
26 Jan 09 3:10
zinc will corrode in contact with aluminum. if you have a look at a galvanic chart you'll see that the zinc is anodic vs aluminum. the amount of corrosion is a function of the area effect but also of the type of atmosphere..

"Under atmospheric conditions of moderate to mild
humidity, contact between a galvanized surface
and aluminum or stainless steel is unlikely to cause
substantial incremental corrosion"
this is taken from this doc:
Hot-dip Galvanizing for Corrosion Protection
http://www.galvanizeit.org/images/uploads/publicationPDFs/SP-SG-06.pdf
you can find also in the same document a table with Additional corrosion of zinc and galvanized steel resulting from contact with other metals.
hope this help

S
 

Corrosion Prevention & Corrosion Control
 

KENAT (Mechanical)
26 Jan 09 11:38
Don't have the galvanic chart to hand but if I recall correctly, zinc plated fasteners in Aluminum are better than Stainless fasteners wrt to the difference between them.

I know in Aerospace/Defense we'd often use zinc plated steel in contact with Aluminum in lieu of stainless.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

bigghassan (Nuclear)
27 Jan 09 8:12
You are correct in this case the zinc has more electronegative potential than the Aluminum.
I'M SORRY FOR MY MISTAKE

An

 
stevenal (Electrical)
27 Jan 09 11:26
One metal will always be sacrificial to another in the presence of an electrolyte. One must decide which metal to save. The thin zinc plating will quickly succumb, leaving the aluminum and steel to corrode next. Galvanizing on HDG bolts will last longer. If aluminum is massive with respect to the fasteners, consider using stainless bolts and letting the aluminum corrode a bit.
strider6 (Materials)
28 Jan 09 2:19
sometimes the corrosion can be acceptable, it depends on the criticality of the item, accessibility to make maintenance, the number of item affected by corrosion and also aesthetic...
so every case is different, there are general rules but the final decision is based on many different factors.

S.

Corrosion Prevention & Corrosion Control
 

JTreglio (Materials)
28 Jan 09 12:09
Why not have the bolts coated with IVD aluminum?

Jim Treglio

triaceng1 (Mechanical) (OP)
5 Feb 09 14:36
I would like to thank everyone for there replies, your insight has helped to me make a well infromed decision

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