Corrosion inhibition for zinc plated fastener used for joining a 201 Stainless steel plate 3

[EtienneMunger]

Hello All,

On a transportation application, I have met the following situation and would appreciate insights:

The following situation is occuring: A zinc plated flanged nut (mild steel) is exhibiting rust after a number of years. This fastener is in direct contact to a 201 steel plate. The complete assembly is then in contact to an aluminum supporting rail. No rust is observed on the 201 steel plate while the zinc plated flanged nut (in contact to the stainless steel plate) is heavily corroded. In addition to this, customers are washing with waters the assembly, thus providing an electrolyte so the corrosion can occur.

I would like to validate with forum members the cheapest options available to prevent corrosion on the fastener and obtain additional recommandations. For us, changing the 201 steel plate is not really possible since the part is in service for a number of years.


The options we have identified so far are:
1) Adding a polymeric isolator ( a shim) simply to isolate the same zinc plated flanged nut.
2) Primer on top of the 201 stainless steel plate - zinc plated flanged nut
3) Stainless steel fastener (stud and flanged nut) in replacement of the zinc plated flanged nut and low carbon bolt.

Thanks all for your advices,

Etienne Munger

 

[CoryPad]

The zinc corrodes excessively when it contacts stainless steel due to he large potential difference between those two metals. Of your listed options, number three is the only good one. Option one is not suitable for structural joints due to stress relaxation in polymers, and option two will still allow corrosion at sharp corners and any areas with damaged coating (like where a socket contacts the fastener).

 

[hokie66]

Plated fasteners don't last long in an external environment, regardless of what they are in contact with. Agree with Cory that stainless steel fasteners are required with the stainless steel plate.

 

[Compositepro]

The essence of the problem is that the zinc plating on the nut is acting as a sacrificial anode protecting all the rest of the exposed surfaces. This small amount of plating will be gone very quickly. A stainless fastener is really the only practical solution.