Tech Talk No.2 : 316L vs 904L (Grades of Stainless Steel)

This second in the 'Tech Talk' series will also be about watch materials, in this case, stainless steel.

A good majority of watch companies use 316L grade stainless steel for their cases. But, one brand in particular, insists on using 904L. So what's the difference? 

First, the dry stuff…

316L - Grade 316 is the standard molybdenum-bearing grade. The molybdenum gives Grade 316 high corrosion resistant properties and particularly high resistance to pitting (a form of extremely localised corrosion that leads to the creation of small holes in the metal) and crevice corrosion in chloride environments. Grade 316L is the low carbon version of 316. There is not much price difference between 316 and 316L.

Grade		C	Mn	Si	P	S	Cr	Mo	Ni	N
316L	Min	-	-	-	-	-	16.0	2.00	10.0	-
	Max	0.03	2.0	0.75	0.045	0.03	18.0	3.00	14.0	0.10

904L - Grade 904L is a non-stabilised low carbon high alloy austenitic stainless steel. The addition of copper to this grade gives it greatly improved resistance to strong reducing acids, particularly sulphuric acid. It is also highly resistant to chloride attack - both pitting/ crevice corrosion and stress corrosion cracking. It is also non-magnetic in all conditions. Further, it contains substantial content of the high cost ingredients nickel and molybdenum. 

Grade		C	Mn	Si	P	S	Cr	Mo	Ni	Cu
904L	min. max.	- 0.020	- 2.00	- 1.00	- 0.045	- 0.035	19.0 23.0	4.0 5.0	23.0 28.0	1.0 2.0

Now that the dry stuff's out of the way what is the main advantage of 904L over 316L? 

It's not a higher grade of steel, merely a "different" grade. All that extra chromium and nickel simply means that it has better resistance to corrosion and pitting than 316L and that it is a more heat treatable steel to harden, to be used in dies, punches, bending and blanking equipment. So in fact, 904L is a "tool steel" much like how Rolex were first made to be "tool watches". 904L steel is only more expensive because it is not as popular in general application as 316L. 

Nickel chunk (from Wikipedia)

Chromium crystal and 1cm3 cube (from Wikipedia)

So what about everyday wear as divers' watches? Well 316L is not actually designed for continuous submersion in sea water. But that's not to say it's not any good. It is still extremely resistant to corrosion, nearly impervious to atmospheric and other mild corrosive agents,  and it is more than adequate for things like boat railing and exposure to sea water spray.

So would this mean that if you really wanted to go diving, you'd strap a Rolex to your wrist? Unless you're planning to dive into sulfuric acid or sodium hydroxide, it won't make a lick of difference. Plus, the seals, the movement, and most importantly, your body, would've dissolved long before that.

Molybdenum in crystalline fragment and 1 cm3 cube (from Wikipedia)

Also, how many people that wear a diving watch actually go diving with it? I think the amount of sea water most of these diving watches see are in publicity shots and perhaps at events. Watchnerds don't jokingly refer to their watches as "desk divers" for nothing!

The Omega Seamaster Chronograph Divers watch uses 316L steel

You must also keep in mind that the increased amounts of nickel and chromium and whatnot that reduce the susceptibility to corrosion and pitting only occur at high temperatures (for example, boiling concentrated nitric acid), which is not very likely in an ocean anyway.

So, at the end of the day, as long as you rinse your watch off with fresh water after a dive, or, if your sweat happens to be particularly acidic, rinse the watch every so often, she'll be right mate.

However, the higher content of nickel in 904L may lead to an allergic reaction on some people. Something to think about… (edit- according to @L_Horloger most of the nickel in 904 is captive, and not free. There is more nickel able to react in the 316, so is worse for allergies)

The Rolex Submariner Divers watch uses 904L steel

To finish off, here’s a little comparison of different materials used on diver's watches and their resistance to pitting :

The unit of measurement is PRE (Pitting Resistance Equivalent) - obviously the higher the number the better.

32 PRE (or higher) is considered as salt water resistant.

316L has a value of 24 PRE

904L has a value of 35 PRE


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