Corrosion - S/ST 316

[Matt_Eng]

Hi Eng-Tip users,

For a small passion project I would like to machine a contact lense case, since I am a long-time user of contact lenses and use up a lot of disposable cases.

I figured S/ST 316 would be best for this based on its properties and cost, but I have stumbled on the question on the suitability of metal in this application with regards to corrosion from the contact lense solution. Naturally, there is not much information out there on this particular subject. The most applicable studies I could find were based on marine applications, the issue here is that sea water contains 3+ % sodium chloride whereas contact lense solution is less corrosive and contains 0.9% and it seems that the velocity of the water over the metal will influence the rate of corrosion, which for marine applications is high whereas a contact lense case can be assumed 0. I was hoping there may be someone with experience who could advise me on this?

Some initial thoughts:
[ul]
[li]Would the corrosion rate of 316 containing static saline solution be low enough to be negligible?[/li]
[li]Is there a treatment/coating that would improve this whilst also being safe to use for this application?[/li]
[li]Is there another metal best suited for this application?[/li]
[/ul]

Appreciate any insight!

Thanks

Matt

 

[TugboatEng]

Even if stainless steel were tolerate the chloride levels, 3% is about the same as sea water, it's too easily sensitized to corrosion by other factors. It would be a poor choice here. Titanium is essentially inert in salt water and is non-hazardous to health.

Now I have a question for the experts. Nickel is classified as a carcinogen and is present in stainless steel. It's not harmful when it's tied up in an alloy but does corrosion release the nickel into solution?

 

[mrfailure]

Titanium is probably the only metal you could use to prevent the possibility of microbial growth (which I think is the real issue here). Because of lid stiffness, you would need a seal of some kind on the case lids for when you put solution and the contacs in.

This seems to me to be a non-problem: the plastic used is flexible and inert in the environment, perfect for the purpose. I don't know what you mean about these cases being disposable - I never seem to be able to get rid of old cases once I have them, as my medicine cabinet attests!

 

[EdStainless]

If you want to suppress microbial growth then you need to use a Cu based alloy or a Silver alloy.
The issue with Ni is free Ni, and there is none in SS. Even most people who are sensitive (aleric) to Ni will not respond to a 304/316 alloy.
Some of the higher Ni grades are an issue but the real problem is Ni plating.
The corrosion of 316 doesn't bother me from the saline, but cleaners may be another issue. And if you use screw caps then galling is real problem.
Using a Brass or Bronze of some sort might be a good option.
The alloy groups tested and approved by EPA are just about any alloy with 65% or more Cu.
There are others, read this:

https://cuverro.com/sites/default/files/oem_files/ip010lewis-2009.pdf

In fact using two different alloys (base and caps) would help more with preventing galling.

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P.E. Metallurgy, consulting work welcomed

 

[mrfailure]

Screw caps can be a problem as they can easily be lost, as separate items, defeating the purpose. You would have to tether them to the case in some fashion.

 

[Matt_Eng]

Thank you for the replies,

Specify the material to suppress microbial growth is a great idea..the only issue I see here is the corrosive properties of an alloy with high Cu or Silver. Having an alloy with 65% of these elements concerns me, should I be concerned? Maybe since the saline solution is static, this wouldn't be an issue. Edstainless, you said you wouldnt be concerned with corrosion, could you help me to understand the degree of corrosion i might expect to see? would it be over weeks/months/years?

The issue with the screw caps is also a great point, I have considered a clamping type design where the top/lid is hinged and clamps over the top. The sealing here would mean it likely can't be metal to metal, so something of a rubber-type seal that seats inside the lid and clamps ontop.

Titanium seems to be a strong choice from the comments above, although i have little experience using this. Is there somebody who could advise me on the magnitude of cost increase? The material price seems to be 2.5 that of 316 and seems to be tricky to machine

appreciate your input!

 

[EdStainless]

If all that you want is trendiness then use Ti, functionally it gains you nothing.
Many of the Cu alloys are very resistant to seawater corrosion, but pick carefully.
Copper.org is a great resource.
You may not have noticed but Cu and Brass handles and fixtures are coming back in hospitals because of the natural microbial resistance.
With silver it would just need to be a plated surface, but then you have to worry about the plating sticking well.
The corrosion issues with 316 would arise if the case sat unused for long periods. The saline would evaporate and leave a small amount of saturated brine (26% salt) and this would likely lead to some pitting attack.
A clamp lid with two simple o-rings as seals sounds like it could work.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed