Use of 316L Stainless Steel in Marine Environment 2

316L marginal. I would expect you to incur pitting corrosion within 10 years. Perforation could then happen.
If you must use 316L, use laser-welded, ultra low sulfur material.
My recommendation would be to use 2205 instead. It should be easily available in tubing and it's a lot stronger.

Consider Inconel 625

I agree with mcguire. 316L is found to corrode (Pitting)in marine service. Use of 2205 is common in offshore industry. The UNS is 31803. Weld of 31803 can be done with 2209 electrode. This is basically duplex steel with ferrite ranging from 30 to 55. Pitting index between 40 and 50. Sandvik, Avesta are some of the suppliers. Youmay refer to CHEMICAL ENGINEERING HAND BOOK Perry forcorrosion rates of various material in different solution/condition. You may also perform Pitting test and crevice corrosion test as per ASTM G 48/36, to be sure of the life of the item.

Thanks for the responses.

I guess my original post wasn't very clear.

I have some 316L tubing already in place. So far (1 year in service) it looks OK.

I am currently planning to install more. I would prefer to use 316L if its fit for purpose and cheaper than the alternatives suggested.

Painting of existing 316L has been suggested. I am now advised that would be the wrong thing to do because of the possibility of crevice corrosion in the event of paint defects / degradation.

Next time I will use 6Mo. This time I would like to stay with 316L preferably unpainted and with flag in maintenance program to check for severe pitting corrosion.

Any final thoughts?

Why don't you set up a periodic inspection routine and at the first sign of pitting, re-passivate. This will re-start the clock on the corrosion. Be cautious not to abrade the surface or to allow rust contamination.

When you do look at upgrading, if 2205 duplex is used make sure to specify UNS S32205 in addition to UNS S31803. Both cover duplex 2205 and most material is dual certified to each number. UNS S32205 will ensure you get the molybdenum, chromium, and nitrogen at the upper range allowed by the original UNS S31803 chemistry. All three are keys to pitting/crevice resistance.

Also whether duplex 2205 or a 6Mo like AL-6XN are used bear in mind both are much stronger than 316L. Each have almost 2X the ASME allowable stress values in Section VIII Div 1 versus 316L. As a result there may be an opportunity to thin the wall of the hydraulic tubing to reduce weight and cost.

If you use 2205 in the future also ask that the PREN # be > 34, and the ferrite # 38-65

Are you able to get compression fittings in either 2205 or 6 moly stainless steels? I wouldn't recommend using 316SS fittings with either tubing- the difference in strengths will affect swaging.