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

[Tools]

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...

[Renoir]

Great explaination. Thanks for the post.

[JBat]

Good post Larry.

Just one nit to pick, galvanic corrosion occurs with the yoking of dissimilar metals, as in a sacrificial anode system used in cathodic protection. The metals must have different electrical potentials for there to be a flow of current. In gas pipelines, for instance, we attach magnesium galvanic anodes to steel pipelines. Current flows from the cathode (the pipeline) to the anode (the magnesium) through the metallic path (the wire connecting the two), then through the electrolyte (the dirt) back to the cathode. The mag corrodes and the pipe doesn't is a simplified way to describe it. The process whereby the corrosion is dramatically slowed is called polarization. Basically, the whole thing is a battery.

What you describe is more in line with atmospheric corrosion. In the absence of any of the four components of a corrosion cell, anode, cathode, metallic path or electrolyte, you cannot have a galvanic system. Since the watch isn't submerged in the electrolyte, and there aren't dissimilar metals involved, it's atmospheric corrosion.

[Tools]

John,
The slight amount of galvanic reaction is from the metals and minerals (sodium, chloride, iron, magnesium, etc.) in the body oils and secretions as well as freely in the atmosphere.....These cause small pools of activity under and around the O-ring and resultant pitting..

Of course as I mentioned, normal corrosion at the area occurs as well; perhaps predominately..

[JBat]

Originally Posted by Tools

John,
The slight amount of galvanic reaction is from the metals and minerals (sodium, chloride, iron, magnesium, etc.) in the body oils and secretions as well as freely in the atmosphere.....These cause small pools of activity under and around the O-ring and resultant pitting..

Of course as I mentioned, normal corrosion at the area occurs as well; perhaps predominately..

I can see how that would happen, Larry. I deal with this stuff every day and tend to think of things in terms of how large scale CP systems work.

[Jackxv]

Especially chloride containing water from the swimming pool is very harmfull.

After swimming in a pool clean up with warm water, soap and brush. It's even worse than sea water.

Jack

[Clogger]

Originally Posted by JBat

Good post Larry.

Just one nit to pick, galvanic corrosion occurs with the yoking of dissimilar metals, as in a sacrificial anode system used in cathodic protection. The metals must have different electrical potentials for there to be a flow of current. In gas pipelines, for instance, we attach magnesium galvanic anodes to steel pipelines. Current flows from the cathode (the pipeline) to the anode (the magnesium) through the metallic path (the wire connecting the two), then through the electrolyte (the dirt) back to the cathode. The mag corrodes and the pipe doesn't is a simplified way to describe it. The process whereby the corrosion is dramatically slowed is called polarization. Basically, the whole thing is a battery.

What you describe is more in line with atmospheric corrosion. In the absence of any of the four components of a corrosion cell, anode, cathode, metallic path or electrolyte, you cannot have a galvanic system. Since the watch isn't submerged in the electrolyte, and there aren't dissimilar metals involved, it's atmospheric corrosion.

Top explanation and I was thinking exactly the same thing.

However and I could be wrong as my knowledge is a little rusty (pun intended). What I believe the OP is refering to is called inter granular corrosion and pitting corrosion.

Also if movement in the gasket is occuring this could also be refered to at some level of fretting corrosion.

[Johny]

Originally Posted by JBat

Good post Larry.

Just one nit to pick, galvanic corrosion occurs with the yoking of dissimilar metals, as in a sacrificial anode system used in cathodic protection. The metals must have different electrical potentials for there to be a flow of current. In gas pipelines, for instance, we attach magnesium galvanic anodes to steel pipelines. Current flows from the cathode (the pipeline) to the anode (the magnesium) through the metallic path (the wire connecting the two), then through the electrolyte (the dirt) back to the cathode. The mag corrodes and the pipe doesn't is a simplified way to describe it. The process whereby the corrosion is dramatically slowed is called polarization. Basically, the whole thing is a battery.

What you describe is more in line with atmospheric corrosion. In the absence of any of the four components of a corrosion cell, anode, cathode, metallic path or electrolyte, you cannot have a galvanic system. Since the watch isn't submerged in the electrolyte, and there aren't dissimilar metals involved, it's atmospheric corrosion.

hi j welded a few anodes on in my time.

[Highhorology]

I have heard that the gold in a two tone actually acts as a catalyst, in some instances. Any truth to that?

[toph]

you learn something new everyday
Larry

[snow_rocks]

Strapping a piece of zinc to the back of the Rolex might offset this corrosive activity!

[Safetrends]

Great post Larry!

[karmatp]

That is one fancy way of reminding me to wash my watch Larry.

[ROGERB]

Insightful information Larry...and not that this would matter but I believe the only part of a Rolex that is 904L (other than the SDDS) is the case, not the back or the bracelet??
The rest is 316L or so I have been led to believe..
R

[Tools]

Originally Posted by ROGERB

Insightful information Larry...and not that this would matter but I believe the only part of a Rolex that is 904L (other than the SDDS) is the case, not the back or the bracelet??
The rest is 316L or so I have been led to believe..
R

Case blanks are stamped from a sheet of 904L steel. The case center section stamp plug is further stamped to shape, machined, and becomes the caseback..

[sevykor]

All-gold models are safe :)

[SS Oyster]

Originally Posted by ROGERB

Insightful information Larry...and not that this would matter but I believe the only part of a Rolex that is 904L (other than the SDDS) is the case, not the back or the bracelet??
The rest is 316L or so I have been led to believe..
R

Anyone know if this is true? I thought the entire watch was made of 904L, caseback, bracelet included.

[GradyPhilpott]

Originally Posted by SS Oyster

Anyone know if this is true? I thought the entire watch was made of 904L, caseback, bracelet included.

It depends on the watch.

My Explorer (Z) has a 904L case (which I believe includes the case back). The bracelet is made of another stainless steel.

My DJ (V) and 14060M (random) are 100% 904L.

[astrobi_astrobi]

would you know if my Explorer 114270 K series is already made of complete 904L steel? Thank you.

[Nalk7]

Originally Posted by astrobi_astrobi

would you know if my Explorer 114270 K series is already made of complete 904L steel? Thank you.

I also have a k series 114270 and yes the case is indeed made of 904l, case back and bracelet are 316l though

[Fofo]

Thank you very much, Larry. It's information like this that makes this forum extremely interesting, useful and fun.

[JohnBoy]

Good write up, Larry! Thanks!

Any comments on the seal material? I know that most elastomers swell and fail in chloride environments (ionized solutions) - which would include sweat, swimming pool water, seawater, etc. As you note, these solutions tend to remain in/around the sealing crevices. Not only is it warm as you suggest, but as the water evaporates from the residual, an even more concentrated and active solution is formed. Thus, seawater might not be of sufficient concentration to cause failure while swimming, but it can get much worse if you lay on the beach for an hour before dipping into the ice chest for that cool one. The best elastomeric material for chloride resistance in industry was Viton A for many years, but I have no idea what Rolex might be using for their case seals. Any leads on that? Thanks again.

[Aspirin]

23.0-28.0% Nickel (Ni) for the 904L, Nickel doesn’t such element, which prohibit for skin contact according RoHS?

[Scott95]

A couple of other points about stainless: Generally more than 12% Cr is needed for the material to be considered truly stainless. Also the passive film requires oxygen to self-repair, so stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments. In seawater, chlorides from the salt will attack and destroy the passive film more quickly than it can be repaired in a low oxygen environment. Seems that the environment around the seal with sweat working its way in to the seal face through capillary action would be ideal for rust to start if the chrome oxide layer is damaged. But that layer is pretty darn hard!

[bondtoys]

Thanks tools.

That explains the corrosion spots on st. steel clasps as well.
I always wondered, how uncleanable spots at the inside of Fliplock elements can happen.

I guess now, that they touch each other and with a little rubbing, the corrosion starts.

[Etur]

I just want to ad one more potential issue that may cause SS corrosion - The surface contamination with the free iron.

Basically all finishing equipment like buffing and polishing wheels and polishing compounds should only be used to polish SS and should never be used to polish carbon or middle steel.


If polishing wheel is contaminated with the Iron from the previously polishing carbon or middle steel, it will transfer iron particles into the SS surface, now if combined with the sweat from the hand it will form a cell and cause galvanic corrosion.

[witch watch]

Originally Posted by Tools

..

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..

Really surprised that such a small soft part of the watch can help speed up the corrosion so easily.

[nim]

High nickel content in 904 may not be good for people with highly sensitive skin

[Nalk7]

Originally Posted by nim

High nickel content in 904 may not be good for people with highly sensitive skin

Just wear it on a NATO no biggie

[ec51]

Thank you for this explanation.