There is nothing more frustrating than seeing little pits starting to appear on your watch. Thankfully it is very rare but corrosion pitting is to stainless steel as Kryptonite is to Superman. Stainless steel is a remarkable metal alloy, but as with all great solutions, there is always an Achilles heel. Knowing these weaknesses is necessary to get the best performance, and ensuring your watch remains pristine.
The basics of a watch case are straightforward, it needs to keep the movement in a safe environment. There are other secondary objectives but are secondary to this primary goal. These other considerations include not burdening the owner with excessive maintenance and being robust in day-to-day wear. On this front stainless steel is a winner because it achieves the goals at a reasonable price.
Stainless Steel Can Corrode
Stainless steel is an alloy of iron, carbon, and specific alloying elements. These specific alloying elements are chosen to prevent the steel from corroding and thus making it stainless. The key alloying elements for stainless steel are Chromium (Cr), Nickel (Ni). When the combined total of these two elements is more than 24% and at least 8% of either element, then the steel becomes corrosion resistant. Importantly, just as watches are water-resistant, stainless steel is corrosion-resistant. Water-resistant watches can leak outside their operating window so too can corrosion-resistant materials corrode in certain conditions. How does that happen?
The Corrosion Defenders
We have identified chromium and Nickel as the key components that provide corrosion resistance. What do these two alloying elements do to make the steel corrosion-resistant?
When Chromium comes into contact with oxygen, it oxidizes easily to form Chromium Oxide (Cr2O3). The benefit of Chromium Oxide is that it adheres to the surface of the metal. Over time it develops a thin film of Chromium Oxide that covers the whole surface. Oxygen is not able to penetrate through this protective oxide film. The result is that once the film is formed, no further oxidation (or corrosion) reactions can occur. This protective Chromium Oxide film is key to the corrosion resistance of stainless steel.
Why Is Chromium So Important?
The Chromium Oxide molecule is physically much larger than the chromium atom that it is created from. The chromium oxide, therefore, spreads over the whole surface of the steel. This is like bubbles in a bubble bath forming over the surface of the water. This means that one chromium atom reacting with oxygen can cover a relatively large surface area. This protects the whole surface of the metal even though some iron atoms will be exposed at the surface of the metal. The key is that the chromium atoms must be dispersed evenly throughout the atomic matrix and are not chemically combined (i.e. in its elemental form). In metallurgy this is described as the Chromium is held in a solid solution in the alloy atomic matrix.
If the Chromium is not evenly distributed throughout the atomic structure, it is possible that the Chromium oxide layer will not cover areas. Furthermore, if the Chromium is chemically bonded in the atomic structure, then the Chromium atom is not available to react with oxygen. This would result in a hole in the protective oxide layer, and that area will be susceptible to corrosion.
Back To Austenitic
When making a watch case, austenitic stainless steel is chosen for ease of manufacture, but the low carbon variant is always chosen. This is indicated with the “L” a the end of the ASTM designation. For example, “316L” or “904L” not only makes machining and finishing the piece easier during the manufacturing process but also improve its corrosion resistance. This is because Chromium has a high propensity to form carbides (Cr3C2) and would remove the Chromium from solid solution, reducing the corrosion resistance.
The Nickel acts in an almost identical way in creating an oxide layer. It has been found that adding Nickel to the alloy helps to make the oxide layer more stable. It also improves the performance of the resulting stainless steel in a high-temperature environment. Plus, Nickel does not form carbides easily, so its corrosion-resistant properties will not be impacted by the carbon content of the stainless steel.
For our purposes here, these simple principles will be sufficient to understand stainless steel corrosion, but it is a fascinating area of study that impacts our daily lives.
How Corrosion Works
The process of creating the protective oxide layer on the stainless steel is corrosion. The key aspect here is that the corrosion is instigated from a specific atom that creates the protective layer that eventually stops the corrosion process. When the process does not limit itself, then great damage and ultimately failure can result. On a watch, failure is when the corrosion impacts the visual or physical performance of the case. This is a very low failure threshold as it is visual rather than mechanical.
Corrosion of iron, or rusting, is an electrochemical reaction. This means that there is a flow of electrons with a change in the chemical composition of the materials involved. This process is harnessed in batteries. The movement of electrons from a chemical reaction is harnessed to power our phones or other electronic gadgets. When it arrives on our watches, it is not a welcome phenomenon.
The Corrosion Trifecta
Let’s start by looking at how this process works on a simple piece of iron. We have all seen the result of this process, brown rusted gates, and pipes. The brown rust is iron oxide (Fe2O3) created by a chemical reaction between iron, oxygen, and some moisture. Each of these components is necessary for iron to corrode. We can call these the corrosion trifecta.
The necessity to have oxygen in the system for iron corrosion to occur can be proved by submerging a piece of iron in a de-aerated salt solution and sealing it. The result will be no corrosion of the iron. Furthermore, if a piece of iron is placed in a beaker of distilled water and all oxygen is removed from the system, no corrosion will occur. If oxygen is then introduced into the system, then corrosion will start. Oxygen will quickly dissolve into the water, and corrosion will start. At this point, if salt is added to the water, then the rate of corrosion will increase. Adding salt increases the rate of corrosion as the dissolved ions facilitate the movement of electrons.
Corrosion and Stainless Steel
So how does this happen in stainless steel? On a piece of iron, the whole surface can corrode as seen in the picture above. This does not happen on stainless steel because of the protective layer of chromium oxide. Corrosion happens as pitting or lots of little holes on the surface of the stainless steel. How does this happen?
Each pit on the stainless steel surface should be considered its own individual corrosion site. Within this small area, the corrosion trifecta of oxygen, moisture, and access to an atom or two of iron is necessary. In most environments where we wear our watches oxygen is available, and a salty solution is always available in the form of perspiration. The final piece is the access to iron.
A Crack In The Armour
I can hear you all now saying that there is an oxide layer of chromium oxide protecting the surface, so the oxygen and salt solution cannot access iron. But what happens if there is some damage to the protective oxide layer? This is usually where the problems start.
If there is damage to a specific area and in a localized area on the surface of your watch, the corrosion process can start. Once started, provided that oxygen can continue to access the area and kept moist, then the pitting will continue.
Where is Your Watch Most Vulnerable to Corrosion Pitting
The most common location I have seen corrosion pitting is under the case back gasket. The question is, why is this the area that is most at risk? Well, it is where the trifecta can arrive in a corrosion microclimate and do its worst. In the case back area, a small piece of dirt can cause the oxide layer to fail. If it is held in the seal, there can be the constant movement that erodes the oxide layer. This area is particularly prone if the watch is not serviced regularly. Without regular servicing, the gaskets can dry out and hold the dirt. The flexibility in the gasket will allow a little movement of the piece of dirt, removing the oxide layer. A little bit of sweat can access the site, and with oxygen in the atmosphere, corrosion will commence. Over time the results can be quite dramatic.
What is key to remember is that this process starts at an atomic level and then accelerates over time to become visible to the naked eye.
There is another source of the same problem that can occur in older watches. If the stainless steel is not fully homogeneous (or the Chromium is not dispersed evenly), then in certain areas, the iron concentration is higher than it should be. This will result in the protective oxide layer not forming completely. This can allow pitting to occur over a localized area.
What Can Be Done
So what can we do to protect against the Achilles heel of stainless steel? Well, there is some good news. These days it is doubtful that modern stainless steels have any inhomogeneities, taking out one systemic problem.
Furthermore, most watches are made from 316L stainless steel, these include Molybdenum as one of the alloying elements. This element further increases the corrosion resistance of the final material. Anyone with a modern Rolex can sleep soundly as the 904L stainless steel they use includes copper in the alloy that further enhances the corrosion resistance.
Finally, from a user’s perspective, the best protection is care and maintenance. You should clean your watch regularly and dry it thoroughly. This will reduce the opportunity for the trifecta corrosion climate to occur. The most important preventative action you can take is to service your watch regularly to ensure all the gaskets on your watch are greased, clean and serviceable.
The properly greased seals will prevent dirt and water from accessing small crevasses that are the feeding ground for corrosion on a watch and provide another layer of protection to your watch.