coated fasteners for corrosion resistance

[clay87]

I am attaching two carbon-steel parts (pump columns) together using flanged connections. The application is underwater in brackish environment (let's say 3600 us/cm). In the past when we've used carbon steel parts in this application, we see some surface corrosion at the location of the B8 fasteners (after several years). This is not necessarily unexpected considering differences in galvanic cell potential and application. See pic:

I am considering using coated B7 fasteners (e.g. fluorokote #1) in an attempt to minimize part corrosion and fastener corrosion. Co-workers have questioned this approach as replacing one minor problem with a worse problem. Any thoughts?

Also, I have no way to avoid using the SS lockwasher but I don't feel like it will cause much problem given its small surface area.

 

[Screwman1]

The lockwasher is adding nothing to the performance of the joint in service- get rid of it and torque the fasteners to the proper level and they will never loosen. Otherwise use a locking nut design.
Other than that, one of the flourocarbon coatings will certainly do better than what you have there now.

 

[CoryPad]

I agree, remove the washer and use coated B7 fasteners. Fluorocarbon style coatings like Fluorokote, Xylan or Magni are good options.

 

[clay87]

Thanks for the responses so far.

Can you provide references that support eliminating helical lockwashers on rotating equipment (other than general google searches). We typically take credit for thread adhesives as an alternate method of threadlocking but I'm not sure it would be effective without metal contact.

 

[CoryPad]

NASA RP1228 Fastener Design Manual

You can search this site for lock washer and find numerous threads. Here is a good FAQ: faq404-1257

 

[1gibson]

Ok, I will say it: a spring lock washer can be used as a visual indicator that the nut has actually been tightened. Other than that, it is no better than a flat washer.