Galvanic Corrosion

[vjr0512]

For one of the Offshore project, in the skid packages, there are Instrument Junction boxed ( stainless steel) fixed to the skid structure which is of Carbon steel but painted with a coating system as below

Inorganic Zinc silicate ( primer ) for 150 microns + Epoxy High build polyamide with micaceous iron oxide for 75 microns + Acrylic polyurethance Gloss for 60 micros .

Client is asking the contractor that since the Stainless steel junction boxes is fixed to the CS painted structure, due to marine atmosphere, galvanic corrosion is bound to take place and hence suggesting to provide teflon inserts between these two parts..

Whether the above scenario is likely possible even if there is a painting system in place on the Carbon steel structure?

Thanks to clarify

 

[SJones]

Coatings can always contain imperfections and will certainly degrade over time both of which could allow the corrodent to access the carbon steel of the structure. Since the carbon steel is the anode, and it will only be exposed at relatively small coating imperfections, the anode cathode (stainless steel) ratio is not favourable. Whether using 'teflon inserts' provides the necessary isolation of the two items needs to be evaluated in the context of the specific attachment and electrical arrangements.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[tbuelna]

Can you provide details of how the SS junction box is attached to the coated carbon steel skid, besides using some type of teflon insert?

When tightening a nut/washer in contact with a painted surface it is quite easy to damage the coating. The friction between the washer and the coated part surface created when the nut is tightened will easily shear even the toughest paint or primer.