Monday, September 18, 2017
A very interesting connection between Stainless Steel Sinks and Legionnaire's Disease
People think little of Stainless Steel on a day to day basis. I mean, it is a wonderful, class of alloy and surrounds all walks of our life. It is strong, durable and easily rolled, formed and otherwise manufactured into multiple shapes, sizes and devices. Add a little Chromium here or Vanadium there, mess with the Carbon percentages and you can extend the applicability, life or property of choice. Therefore, most people are at least a bit surprised when they find out that Stainless Steel too, is vulnerable.
Just today, I came across a rather interesting study, that makes a connection between Stainless Steel degradation and rust in sinks with Legionnaire's Disease. At first reading, this must seem quite odd, but if you consider that a Legionnaire's safety expert oversaw the study, the connection makes more sense. Well, let us take a look at both of these for a minute.
Legionella, a genus of gram-negative bacteria, has several species that have been known to cause the disease, with symptoms ranging from headaches to pneumonia, and rarely mortality. Luckily, under most conditions, it doesn't get transmitted from patient to patient. And apparently, the disease is present in bathtubs, sinks, showers and other places we wash up, shower, etc. I learn something new every day. I will provide a couple of reference links at the bottom, if you were not familiar with Legionnaire's Disease as well.
Now, as an Engineer, with enough experience on the applications side, I can speak with some strength on Stainless Steel. Everyone who studies Materials Engineering is taught that Chromium (Cr), when added to Stainless Steel can make it resistant to rust. However, when you try to balance its properties you can't just keep adding Chromium. There is an upper limit.
Passivation on the other hand, is a process, I like many Engineers became familiar with. Using diluted strong acids such as Nitric Acid or weak acids such as Citric Acid, Stainless Steel is made rust-resistant through the addition of a very thin layer of oxide that prevents further rusting. For those interested, there are ASTM standards and plenty of reading materials out there.
The study, performed over 3 years, found that when tested on rusted taps, Legionella survived quite well. Non-rusted stainless steel taps were not inclined to serve as breeding grounds. The same was true of brass. I tend to know a little bit about brass as well. My undergraduate education involved a heavy dose of industrial training. And, I was accustomed to encountering brass impellers in submersible and other pumps designed for use along Oceans and Seas. Perhaps it is the presence of Zinc, but whatever it is, brass is wonderful in that, it can also be machined easily, like cutting into butter. Understandably, brass is expensive!
Therefore, your options are limited. Expensive sinks, or ceramic sinks. Most ceramic sinks still have stainless steel fittings, such as drains, etc. Perhaps you can limit the alloy portion of ceramic sinks to brass. It will make for both great looks and infection prevention.
How about re-passivation? Is that a thing?
I am not sure. Also, the problem is that you can't expect people to check their sinks and use Citrate to passivate their Stainless Steel bathtubs, sinks etc. And what about large buildings with many such sinks, such as, say Hospitals? There is already a big battle going on with Hospital Acquired Infections, and this ups the challenge.
It is a bit unclear that there is a single solution, but it appears, material substitution and regular inspection and servicing are the minimum requirements here. Maybe on-spot passivation can be a thing. Perhaps nanotechnology based coatings and solutions will emerge. Only time can tell!
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1. Legionnaire's Disease: https://www.cdc.gov/legionella/about/causes-transmission.html
2. A slightly different take on Legionnaire's: http://www.mayoclinic.org/diseases-conditions/legionnaires-disease/home/ovc-20242041
3. An article on the study: https://www.newscientist.com/article/2145448-stainless-steel-sinks-may-up-your-risk-of-legionnaires-disease/
4. Image Courtesy, Pexels: https://www.pexels.com/photo/blur-bubbles-cleaning-close-up-545021/