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Watching your chlorides

July 14, 2015 at 11:12 PM by Dr. Drang

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.

A form of stress corrosion cracking, known as season cracking, was discovered during the British Raj, when troops learned that the brass cartridges of their ammunition cracked during India’s monsoon season. It wasn’t because the cartridges got wet—ammunition was kept dry during monsoon season by storing it under cover in the company horse stables. This turned out to be a mistake. Horses produce a lot of urine, and the ammonia that wafted out of the urine mixed with the humid air and settled on the cartridges. The cartridges were in a state of high stress because of the way the brass was drawn and crimped onto the bullet. The combination of brass, ammonia, and high stress led to the cracking.

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.

The most famous example of residual stress is the Liberty Bell. Back then, it was not uncommon for large castings to crack as they cooled. Metals, like most materials, shrink as they cool, and because the different parts of a large casting can cool at very different rates, stresses develop as these zones pull and push on each other. The stresses in the first Liberty Bell weren’t large enough to crack it upon cooling, but it did crack the first time it was rung, as the stresses from impact added to the residual stresses.

Several years ago, I was part of a team that analyzed the failure of a huge cast hook that was used to lift offshore oil platforms off a barge and set them on their legs. If I remember right, the hook weighed 25 tons, and it snapped as it was lifting an 8 million pound platform. Cracking from high residual stresses played a big role in that failure, so maybe we shouldn’t be too critical of 18th century foundrymen.

Residual stresses, though, are not limited to large castings. The brass cartridges that cracked during monsoon season were small and got their residual stresses from the cold forming operations of drawing and crimping. And I’ve seen plenty of small polycarbonate moldings and PVC pipes that have cracked because of high residual stress and exposure to certain oils.

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. ↩

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