Both 304 or 316 can be used for salt water applications.
304 is the most commonly stocked stainless material and is used widely in marine applications and many other applications.
316 is also used in marine applications but it is also considered a food grade stainless which gets exposed to acidic materials. Wine fermentation tanks will generally be made out of 316 as will restaurant counters, sinks, food processing machines and food storage.
316 has a higher nickel content which makes it a little softer. 304 is 10% stronger than 316 probably due to it’s slightly higher molybdenum content.
I researched stainless materials extensively before I made plates. My conclusion after talking to many experts and doing my homework was that 304 was perfectly suitable for diving products since it does not live in salt water for days or weeks at it time and gets rinsed after exposure.
I talked to the metal shop at Svendsen’s Marine in Alameda, CA which is a major yacht stainless shop famous around the world and they told me that all their base plates for railings are 316 but all the railing tube material is 304 (domestic) and they never have problems. Boat railings and other stainless components are exposed to salt water environment 24/7.
I found out that specifying domestic material is one of the critical details in stainless. Svendsen’s uses only domestic material for both 316 and 304. They explained that stainless must be certified as to the alloy when produced in this country so you know exactly what you’re getting. Whereas Chinese or imported stainless does not and there can be all sorts of impurities and excessive amounts of iron and other garbage in the alloy. They do not have to account for what’s in their stainless, you’re just expected to take their word for it.
I’ve done magnet tests on both imported and domestic stainless and all the Chinese stainless I’ve tested was highly magnetic for both their claimed 316 and 304.
The domestic 304 I use is 98% non magnetic which is correct for 304. There should be an ever so slight magnetism but almost nothing.
I built a rudder for a 57’ sail boat using domestic 304 for all components including the shaft, the internal construction of the rudder, and the attachment points on the skeg. All the stainless was properly zinced and after 15 years still does not show any signs of pit corrosion or problems, and that yacht (as most all yachts do) lives in the water and only gets hauled out once every two to four years.
There are a few ways of passivating stainless. You can remove ferrous metal contamination by dipping in acid, or you can manually remove the contamination by sanding or abrading. A great way to safely remove any iron from the stainless surface without affecting the other alloy components in the material is to use citric acid commonly found as a house hold canning product.
My products get so much sanding and finishing/polishing that I don’t bother with soaking.
I’ve never had any problems with any corrosion.
Using 316 only for dive gear is not true, that’s more based on hype and marketing. 316 is expensive and I think it’s overkill.
There’s nothing wrong with 304 as long as it’s domestic.
Domestic 304 is better than Chinese 316.
