Hello everybody:

The Hydroelectric Power Plants have, among their auxiliaries, the hydro mechanical equipments such as: trash racks, inlet gates, draft tube gates, bulkheads and stoplogs.

In a general manner, the gates (skin plate, girders, trunnions girders, wear plates, struts, end frames) are manufactured with structural steel ASTM A36 or ASTM A572.

For seal plates and bolts, the material is 304 stainless steel.

In this context, with stainless steel bolts and nuts it exist a small cathode area relative to a large anode area (the structural plates) and a bimetallic corrosion should be a minor issue in the absence of an electrolytic; but, in the environment of hydropower plants, besides the occasional immersion, the rain water or the humidity of the air acts like an electrolyte.

From the document "Design of Spillway Tainter Gates", page 8-1
(U S Army Corps of Engineers.  Engineering and Design. EM 1110-2-2702    1 January 2000) it can be read:

"......Where dissimilar metals are in contact, rubber gaskets or equivalent insulators should separate them. Generally, this is not necessary for stainless steel bolts, because the area of contact between the bolt and structural steel is very small".

Is it or is it not necessary any insulation (paint, polymer, etc.) to avoid the direct contact between the threads of these materials?
Thanks in advance for your comments.
 

El que no puede andar, se sienta.

What is the expected lifetime?  Unless this is required to be totally free from even cosmetic corrosion for decades, it should be ok to use stainless steel bolts in this type of connection.

The worst that usually happens is that the structural steel sees slightly higher corrosion rates, but there is a lot of it so no one cares.
If you have locations that only get wet occasionally then you can have a situation where all of the steel corroding is under the bold heads resulting in loose connections.  In those cases coating the SS bolts would be a good practice.

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Plymouth Tube

Whether you specify it or not, the maintenance crew that has to disassemble, repair and reassemble this stuff will be applying Never-Seez or equivalent antiseize to stainless fasteners at the first opportunity, just so they have a chance of getting them apart someday without using a smoke wrench, and maybe have a chance of reusing a few of them.
The organic carrier and metal dust therein may affect the real outcome associated with your theoretical considerations.

 

Mike Halloran
Pembroke Pines, FL, USA

Galvanic corrosion is a function of the relative areas of the noble vs non-noble metals.  Turn this around.  If you had SS plates and CS bolts, the more noble SS plates would eat the CS bolts in short order in the presence of an electrolyte.  Your situation is obviously the opposite of that and you will experience accelerated localized corrosion amongst lots of other slightly less accelerated corrosion as EdStainless has noted.

The wording in the COE document is deceptive and should be studied very carefully - the words "equivalent insulators" is the catch phrase.  Even if you have a gasket, if you have a noble and sacrificial metal in close proximity and the ability for an electrical circuit to complete between them, the ions will flow through the electrolyte.

So unless you are able to completely isolate the two from each other electrically, gaskets, etc are band-aids not solutions.  Where this is absolutely necessary, there is specialized equipment to accomplish this.  Trust me, you don't want to go there for your application.

rmw

Hello everybody:

As always, the cavalry of the forum to the rescue.

Once again, I very much appreciate the kindness of all of you by giving your opportune comments. Thanks.

El que no puede andar, se sienta.