We are often approached by customers inquiring about the steels currently available in the wristwatch field. People would like to know which steel offers the highest corrosion resistance and hardness. As a general answer would indeed be possible but would leave some aspects neglected, we would like to discuss this topic further here.
For our new automatic models we chose an ultra-hard, highly corrosion-resistant austenitic stainless steel. Further below we compare this to us revolutionary material with other common stainless steels by a known test method for resistance to pitting corrosion (pitting resistance equivalent PRE). This resistance can be calculated using the respective alloy components using the following formula:
PRE = %Cr(Chromium) + 3,3 x %Mo(Molybdenum) + 30 x %N(Nitrogen)
This gave the following PRE values for the steels that were tested:
1. Watch stainless steel AISI 304 PRE value of about 20
2. Watch stainless steel AISI 316L PRE value of about 26–30
3. Stainless steel AISI 904L PRE value of about 35
4. Stainless steel for submarine construction, e.g. 1.3964/1.4566 PRE value of about 38
5. Stainless steel DIN 1.4456 (nickel-free) for medical implants PRE value of about 52
We think that the above values may serve to make a qualitative statement with respect to the corrosion resistance of our new steel without any further need for explanation. We will shortly go into more detail concerning this steel that will be used by us in the future and is offered by us in fully hardened condition with a surface hardness of 1600 HV (stainless steel 316L: 190–210 HV).
Real seawater resistance requires a minimum PRE-value of about 32. Sport watches made of conventional stainless steel should therefore be rinsed with fresh water after swimming or diving in the sea.