High School Project / corrosion timed release...

[Canada Bob]

Hi,

I'd appreciate a bit of guidance connected with a school project that I'm working on.

I have built a device (about the size/shape of a basketball} to measure and record sea bed temperatures during different times of year.The device will be "anchored" on station tethered to a concrete block, but I need to produce a link between the anchor and the ball that will eventually completely corrode in say 3 months +/- a couple of weeks. What's the chance of a corroding link being 12 weeks +/- 2 weeks ?

I've looked at aluminum as being the most reactive (easily available) in salt water, but aluminum would most likely be too soft / not sturdy enough to hold the strain between the anchor and the ball, I'm looking at zinc as it's likely to be more rigid and quite reactive in salt water, wondering if a wrought iron might be better though, at least easier to work / form into a sort of chain link than zinc, I don't want to get into casting metal.

The ideal diameter (I guess) would be between 3mm to 5mm, thinking that if I went with aluminum I'd need 5mm but if I used a malleable iron 3mm might do, it's a balance between initial tensile strength and the rate of corrosion.

Thinking "outside the box" anyone know of a non metal biodegradable that I might be able to use ?

Bob.

PS: no problem to retrieve the device when it's released, it has a flag a LED strobe and a tracker on it.

 

[btrueblood]

Tricky. Possible, but not without some (a lot of ?) testing - you would need to have whatever material you used be very uniform in dimensions and breaking strength so that a few mils of corrosion difference between samples would not affect the timing beyond your spec. Couple that with the fact that (quoted from the article below):

"The corrosion rates of the copper base alloys, nickel base alloys,
steels, cast irons, lead, tin, lead-tin solder, molybdenum and tungsten
decreased with the concentration of oxygen in seawater, i.e.,
the corrosion rates were lower at depth than at the surface. The
corrosion rates of Ni-200, Ni-Cu 402, 410, K-500 and 45-55, Ni-Cr-Fe
X750, Ni-Mo2, all steels, grey cast iron and alloy cast irons decreased
linearly with the concentration of oxygen in seawater."

https://apps.dtic.mil/dtic/tr/fulltext/u2/743872.pdf

 

[Canada Bob]

Thanks for the above and for the link to the research done, over 100 pages hey, lots for me to take in there.
Thanks for taking the time, it's valued and appreciated.

 

[jmec87]

There will presumably be some varying forces on the link due to buoyancy and wave action. These forces could vary somewhat unpredictably (such as if a big wave comes along). You are not corroding until there is 0 thickness left, but until the remaining thickness will fracture due to the loads encountered.

Due to the variability in the corrosion rates and the unpredictability of the loading the link could see, I think corrosion until failure in 12 weeks +/- 2 weeks is very unlikely to be achievable unless you get very lucky.

 

[Canada Bob]

Understood and appreciated, the device will be in approximately 80 ft to 100 ft of water, some current I suppose, but no waves as such to contend with,the sphere doesn't have much lift / buoyancy, around 5 lbs understood regarding the fracture / failure point, thanks for the insights, appreciated. I'm also looking at non metal biodegradable plastics, PLGA could work but almost impossible to find in bar form, it seems still to be experimental.

Looks like there are devices out there already...

https://www.underseareleases.com/commercial.htm