904L - Worth It ??? Case back Corrosion.. in Laymans Terms.

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I was recently asked... "Why did my 904L Stainless Steel watch pit inside the case back... I thought that 904L was immune to corrosion"

Well, the answer is complicated... but bare with me..

Stainless Steel is corrosion resistant because it forms it's own protective coating.. The Chromium content binds with Oxygen and forms a protective and invisible skin of Chromium Oxide, and this is difficult for the corrosive (rusting) affects of water to penetrate..

At the case back, we have a rubber gasket... This gasket flexes and creates movement, expansion/contraction, vibration, etc.; the gasket actually rubs off that film of chromium oxide protection where it touches..

So........ as we sweat, we provide the perfect electrolyte...salt water. Further, we secrete minerals and metals that mix with atmospheric contaminants as well....... we now have the perfect formula for some galvanic corrosion and normal corrosion to take place. As the salt water from our body seeps into and around the gasket, along with the body metals/minerals etc.... we have a soup just waiting to attack the metal when the rubber gasket rubs off that protective film... Electrolyte is also more active at higher temperatures...say body temps of almost 100 degrees.. Some folks also sweat a more acidic solution than others...

So, corrosion and pitting is always more likely to be found at points where the metals natural protective oxide coating is rubbed off and the elements are there to attack the now vulnerable bare metal....around the case-back O-ring..

Another good reason to give your watch a good cleaning periodically to remove that acidic electrolyte solution that has settled around the case-back O-ring..

Let's look at some of the common types of Stainless Steel:

Early Rolex cases were made from the 304L Stainless, commonly referred to as 18/8 Stainless and used in the medical and food industries. 304 Stainless is the most common stainless steel used:

304L blend:
AISI Type 304L is an Austenitic Standard grade Stainless Steel. It is commonly called AISI Type 304L Chromium-Nickel steel. It is composed of (in weight percentage) 0.03% Carbon (C), 2.00% Manganese (Mn), 1.00% Silicon (Si), 18.0-20.0% Chromium (Cr), 8.0-12.0% Nickel (Ni), 0.045% Phosphorus (P), 0.03% Sulfur (S), and the base metal Iron (Fe). Other designations of AISI Type 304L stainless steel include UNS S30403 and AISI 304L.
304L is commonly referred to as surgical grade stainless..


316L blend:
AISI Type 316L is an Austenitic Standard grade Stainless Steel. It is commonly called AISI Type 316L Chromium-Nickel steel. It is composed of (in weight percentage) 0.03% Carbon (C), 2.00% Manganese (Mn), 1.00% Silicon (Si), 16.0-18.0% Chromium (Cr), 10.0-14.0% Nickel (Ni), 0.045% Phosphorus (P), 0.03% Sulfur (S), 2.0-3.0% Molybdenum (Mo), and the base metal Iron (Fe). Other designations of AISI Type 316L stainless steel include UNS S31603 and AISI 316L.

You can see that the Chromium content is increased and the Molybdenum % is increased to inhibit corrosion is the passive film (chromium oxide) is removed... this grade is sometimes referred to as "marine grade" stainless

316F blend:
AISI Type 316F is an Austenitic Standard grade Stainless Steel. It is commonly called AISI Type 316F Chromium-Nickel steel. It is composed of (in weight percentage) 0.08% Carbon (C), 2.00% Manganese (Mn), 1.00% Silicon (Si), 16.0-18.0% Chromium (Cr), 10.0-14.0% Nickel (Ni), 0.2% Phosphorus (P), 0.10%(min) Sulfur (S), 1.75-2.5% Molybdenum (Mo), and the base metal Iron (Fe). Other designations of AISI Type 316F stainless steel include UNS S31620 and AISI 316F.

So it appears that 316L (Low carbon) has less carbon, higher % molybdenum - and this molybdenum is what provides it with more anti-corrosion ability..

this compares with 904L:

904L is an Austenitic Nonstandard grade Stainless Steel. It is composed of (in weight percentage) 0.02% Carbon (C), 2.00% Manganese (Mn), 1.00% Silicon (Si), 19.0-23.0% Chromium (Cr), 23.0-28.0% Nickel (Ni), 0.045% Phosphorus (P), 0.035% Sulfur (S), 4.0-5.0% Molybdenum (Mo), 1.0-2.0% Copper (Cu), and the base metal Iron (Fe). Other designations of 904L stainless steel include UNS N08904 and ASTM B625.

this contains even more Molybdenum for corrosion inhibition, higher chromium content (for the chromium oxide, a passive film that immediately forms to protect the metal from corrosion), and more nickel which is highly polishable...