Watching your chlorides

Although I don’t own an Apple Watch and don’t swim in the ocean (or anywhere else) for exercise, I enjoyed Craig Hockenberry’s post today on wet workouts with the watch. He gives a nice short explanation of why the watch’s touch screen doesn’t work when it’s wet and has some good tips for cleaning the watch after it’s immersed in salt water. Craig mentions removal of grit and protection against corrosion as reasons for thoroughly rinsing the watch in fresh water after a workout. I’d like to add a third reason.

Before I do, let me emphasize that I don’t think the phenomenon I’m going to talk about, stress corrosion cracking, is a significant threat to the Apple Watch. SCC is very well known to metallurgists and material scientists, and I’m sure Apple’s engineers have done a lot of testing and research to ensure that it isn’t a problem. But I’m a forensic engineer, and I like talking about how things can fail, even when those failure modes are only the remotest of possibilities.

The 316L stainless steel alloy Apple uses in the Watch is, as Greg Koenig said in this iMore article, quite resistant to corrosion. Your Apple Watch is extremely unlikely to develop corrosion pits or stains. But stress corrosion cracking, despite the word corrosion in its name, is a distinctly different phenomenon. SCC can break an object apart even as its surface remains bright and shiny.

Stress corrosion cracking comes from particular combinations of material, tensile stress, and environment, and usually manifests itself as a large number of small surface cracks. Under repeated or continuous exposure to that particular stress and environment, the cracks grow until one of them breaks through and the part fails.

Returning to the Apple Watch, 316L is known to be susceptible to SCC in a chloride environment,1 like salt water or salt spray, so two of the three requirements for stress corrosion cracking, material and environment, are met. What about stress?

Well, there certainly are no applied stresses worth worrying about. The watch basically just sits on your wrist, effectively unloaded. The only possible source of significant stress in the watch is residual stress. Residual stresses are, as the name implies, the residue of the processes used to manufacture a part. In metals, residual stresses typically come from casting or shaping the part.

It is, I think, on the stress side that Apple is preventing SCC. By keeping the residual stresses low—or by making sure the residual stresses at the surface are compressive rather than tensile—Apple is eliminating one of the three requirements for stress corrosion cracking, and that’s why Craig Hockenberry’s Apple Watch won’t split open, even if he goes swimming in the Pacific every day.

But he should probably continue to rinse it with fresh water, just to be safe.

  1. The Wikipedia article talks about aluminum alloys also being susceptible in a chloride environment, but I have no experience with that, so I’m going to talk about the stainless steel Watch only.