Cathodic Protection of Austenitic Stainless Steel in splash zone 3

[dapatel]

We have some structural steel parts made of 304 and 316 stainless steel below the jetty deck in the splash zone. Should we protect these s steel parts with the cathodic protection system (using anodes) installed for the jetty piles which are c. steel.

Thanks in anticipation.

 

[mcguire]

It probably won't help in that the most severe pitting attack will take place when salt concentrates by evaporation above the water line, where the cathodic protection will do nothing. It will help below the water line.

 

[projacktowl]

Try cover up the splash zone with some corrosion resistant materials such as high strenght polymer or polymer matrix composites. It helps.

 

[SJones]

If the SS parts are in contact with the elements receiving CP then they will drain current too and, hopefully, be protected. This assumes that the CP designers accounted for the wetted area of SS as well otherwise the life of the anodes could well be shorter. 304 and 316 SS really won't perform too well in a splash zone environment (or marine atmospheric environment for that matter)and, if the materials can't be changed, then you should look at a suitable coating system.

Steve Jones
Petroleum Development Oman LLC
Muscat
Sultanate Of Oman

 

[AndrewLindsay]

You've really got to coat the stainless steel parts if they are not going to be permanently immersed.

There are a number of issues that you will have anyway with the selection of 304 stainless steel. As 304 does not have any molybdenum, its resistance to chloride pitting is going to be much less than that of 316. Typically for offshore and marine service, it is common to request 316 stainless steel with molybdenum towards the higher end of the typical grade, i.e. above 2.0%, and often higher than 2.5%, which is then getting into the realms of 317 stainless steel.

As for the design of the CP system, I would assume that the surface area of stainless steel is going to be greatly smaller than the surface area of steel. Therefore the current drain to the stainless is likely to be insignificant, and typical conservatism in the SP system design should be able to adequately cover these items. But please note that I have not reviewed the design of the system so I have made the assumption that the amount of stainless steel fittings is greatly smaller than the carbon steel that needs to be protected.

You will not get any protection of the 316 items when they are above the water line, so in those circumstances, you are looking at the relative corrosion resistances of the two stainless steels. For general pitting type corrosion, 316 is going to be better than 304 for the reasons stated above.

If the fittings are under lots of tensile stress, and they may heat up when out of the water, there may be a risk of chloride stress corrosion cracking. For 316 stainless steel, the critical temperature for CSCC to occur is somewhere above 50°C (different literature sources quote the minimum temperature to be either 50 or 60°C).

Regards

Andrew

 

[SJones]

To provide a starting proposal for a coating: thermal spray aluminium (TSA).



Steve Jones
Petroleum Development Oman LLC
Muscat
Sultanate Of Oman

 

[AndrewLindsay]

TSA can be a good coating, but as with all coatings, it may be difficult to get a good coverage items with small details. Likewise, it may be difficult to coat items that are already installed with TSA. Perhaps a high build epoxy, though not ideal, would give adequate service with a minimum of annual maintenance?

 

[FPQB12]

Nyalic clearcoat is an excellent coating for stainless. It provides envelope protection and eliminates the elecrolysis that causes the corrosion.

http://www.nyalic.com.

 

[mshimko]

Be leary of TSA/FSA (thermal/flame sprayed aluminum) in immersion, or even frequently wetted applications.

Once the topcoat is compromised, the FSA will quickly corrode under the remaining topcoat. This will protect the substrate - for a while, but eventually you'll have to totally remove all remaining FSA before represervation. If not topcoated, the FSA will corrode away even quicker, aided by mechanical action of the splash.

I'd also be leary of products pushed by sales people, as I suspect the previous poster is.