Susceptible to intensified corrosion? - welding 316 SS to Carbon Steel (ASTM A36, A572 Gr. 50)

[ggeorgia]

I am designing a subsea structure with the purpose of providing cathodic protection to other assets in a brackish water environment. The components include an anode sled and a clamp with a cable connected to the sled for continuity purposes (clamp will be mounted to asset intended to receive cp protection). These structure will be fabricated using mostly carbon steel (A36 or A572) and coated with a standard epoxy coating typical for subsea applications in the Gulf. In order to provide continuity between the clamp and sled via cable there are 2 choices at the connection point (bolt/all-thread to thru-hole on cable lug, secured with a nut):

1.) leave the internal threads (designated for a fastener) on the carbon steel frame uncoated uncoated--thus exposed to the seawater. A 316 SS fastener would make contact with these threads. Keep in mind that the entire system would be protected by the current provided by the anodes (Al-Zn) on the structure.

2.) Alternatively, there could be a small piece of 316 SS all-thread welded to the carbon steel structure. All of the carbon components would be coated, but the 316SS piece would be uncoated of course. Given the galvanic dissimilarity between 316SS and carbon steel, would the weld be subject to intensified corrosion in comparision to merely making contact with carbon (as seen in option 1) ?

I wasn't sure if there was a chemical reaction along the weld seam that would invite intensified corrosion or some other sort of synergistic effect to consider, so I thought I should ask to see what others' thoughts were. Since the cathode-to-anode ratio (316 SS to carbon steel) is small, I thought that the concern of disimilar metals shouldn't be a problem. Personally, I'd like to go with option 2 since there's no risk of exposing uncoated carbon steel. Thanks guys.

 

[OGMetEngr]

I would recommend a welded connection for CP continuity over a bolted solution. The carbon steel / stainless steel materials, including the weld, will all be polarized by the anode and protected. A bolted union only invites the potential for a break in the circuit, among other things, and will be less reliable than a welded connection (assuming the weld is done correctly).