304 SS for Marine Applications

[jevoltin]

It has been suggested that 304 SS that has been electropolished has the chromium brought to the surface making the 304 SS part more corrosion resistant. I am wondering if this is a reasonable alternative to using 316 SS for marine applications. To be more specific, I am thinking of applications on boats such that the parts are not in direct contact with the ocean water under normal circumstances, but they are in the marine atmosphere and exposed to rain, limited splashing, etc.

 

[unclesyd]

I would stay away from 304 S/S on or near the ocean. I've had nothing except poor results with 304 S/S on boat parts with every type finish available. I would stay with 316 S/S if at all possible, even this is no guarantee of long lasting part. You could probably get a better alloy, Duplex S/S, 6Mo, or maybe Titanium for what I think you will have in your 304 S/S part.

Check out the following website for information on saltwater corrosion.

http://www.marfas.com

Come back with a little more information about the usage.
Is this a one of or do you plan to make several?
What is the general shape?

 

[jevoltin]

The primary application is mounting brackets for various light-weight (less than 1 pound) assemblies. These would be production parts - so the quantities would be 100s initially and probably 1000s within a year. I'm inclined to continue using 316 SS because I have not had good experiences using 304 SS for marine applications. I just wanted to get some additional input on this idea of electropolishing 304.

 

[unclesyd]

I would not use 304 S/S as stated above. I don't think you will get any measurable improvement in the enviroment you describe with electropolishing 304 S/S.

I have lost some Quench Annealed and electropolished 304 S/S parts that we under very low stress, roller fishing guides.

 

[nickt1960]

Just to add weight to this discussion, in the offshore oil and gas industry the use of 304 for topsides applications is universally banned by all the majors. We not only require 316, but usually specify 316 with a guaranteed minimum 2.5% Mo content to ensure the required pitting resistance. Most of our instrument tubing usually comes as 317 (3% Mo) as it's easier for the suppliers to guarantee the minimum Mo content that way.

 

[TEV]

Electropolishing will only improve the corrosion resistance of austenitic stainlesses (304, 316, etc.) marginally, as it can eliminate surface features such as heavily cold worked surface metal, surface inclusions and impurities, and crevices due to folded metal or other effects. If the surface is of good quality and relatively free of the above effects, a thorough passivation will suffice. If you electropolish, you must still follow with passivation for optimal chromium enrichment in the passive surface. This info is derived from studies done on corrosion of high purity 316L stainless for gas supply systems for semi fabs, as presented at SEMI Standards meetings. There is quite a bit of info available about corrosion resistance versus chromium enrichment of the passive layer. If you need more info, I can supply.

Two things that will definitely enhance your corrosion resistance much more than EP is minimization of inclusions, particularly sulfides, and specifying higher molybdenum levels, as stated by nickt1960. Free machining grades of stainless contain sulfur near the max permitted, and are particularly susceptable to end-grain chloride corrosion attack, initiated at the interception of the sulfide inclusions with the end-grain surface.

 

[mcguire]

Good to hear from TEV. 304 just doesn't cut it in marine environment under even optimal conditions. But, 316 can be servicable. Chemistry is crucial. Obviously high Cr, Mo and N help. See the FAQ above. Low Mn, S, and O also help.
Avoid abraded surface finishes; they are death. Electroploishing is quite beneficial.
Rath Manufacturing makes a 316L tubing that has a proprietary addition that results in a critical pitting temperature of over 20C at 16%Cr and 2.0% Mo. This is nearly equal to 317. They haven't advertized it yet, but may accept orders.