From watchuseek:
Re: 904L V 316L steel
Corrosion resistance of these austenitic Stainless Steels such as 316L and 904L are very similar for all practical purposes in a watch. The tests performed which show variances are performed in very corrosive chemical acids and bases. So for the purposes of diving in salt or fresh water there is very little if any difference that you will ever notice. But if you wear your watch in sulphuric acid or sodium hydroxide for instance you will have much better results with 904L. Of course the seals and internals of the watch would dissolve and oh yeah your body as well. The hardness and other physical properties of the two Stainless Steels are close enough as well so that you will never be able to tell the difference. 904L is more expensive only because it is not as popular in general application as 316L. 316L is used in thousands of marine applications from diving equipment to fixtures and hardware on boats. I have rarely seen any signs of corrosion on any of it in the marine or environment or industrial applications and most of those things were treated a lot rougher than anyones watch ever is. Increased amounts of Nickel and Molyebdenum reduce the suseptibility to chloride induced stress corrossion cracking and pitting which sounds impressive but these are things that occur at elevated temperatures thus not very likely in the ocean anyway.
To summarize, rinse your watch off after you dive and every once in a while anyway even if you don't dive it because your body sweat has chlorides in it also. And don't worry about the difference between 904L and 316L because in the application of a diving watch it just doesn't matter.
Vince.
