With its well-deserved credentials for chemical resistance and durability, stainless steel and particularly 316 grade stainless steel is a “go to” for the fabrication of ultrasonic cleaning tanks. Although it performs well in most applications, there are still those where stainless steel has its limitations. Stainless steel achieves its “stainless” characteristic by developing a very thin and tenacious layer of oxide on its surface when it is exposed to the oxygen in the atmosphere. This effect which is due to its chromium content of 18% or more, protects the surface and prevents further oxidation. It seems logical that anything that would compromise this oxidation layer would also compromise protection of the underlying steel alloy. As I have admitted repeatedly in this blog, I am, regrettably, not a chemist so must approach these things on a logical basis with what I can learn from the internet and personal experiences.
If one peruses the many compatibility charts found on chemical supplier’s and other websites (Cole-Parmer has a particularly good one), stainless steel ranks near the top in its range of compatibility against chemical attack. There are notable exceptions, however, including certain concentrations of acids including hydrochloric and hydrofluoric, many chlorides and chlorites including household bleach (sodium hypochlorite) and other “bleaches.” The one downfall of these chemical compatibility charts is that they don’t always provide possibly relevant details including concentration, temperature and the nature of exposure. For example, hydrogen peroxide in concentrations of 50% and above is rated “A” (excellent compatibility) with stainless steel – – or is it? Looking closely at a footnote, one discovers that the “A” rating for hydrogen peroxide only holds to a temperature of 120°F presumably deteriorating at higher temperatures. At concentrations below 50%, the compatibility rating slips to “B” (good compatibility) but the 120°F footnote disappears. This would prompt one to ask if the compatibility rating of a concentration less than 50% would also deteriorate at a temperature above 120°F. It would give me cause for concern if I intended to use a low concentration of hydrogen peroxide at a temperature of 120°F or above.
Another thing that is not clear in my mind (and may not be predictable) is if the compatibility rating applies if the particular chemical is part of a more complex formulation which includes other ingredients. Are there additive or cancelling effects? I would suspect this is a question for people who truly understand chemistry.
Finally, and most perplexing, is the effect of ultrasonics on chemical compatibility. Going back to that oxide layer that provides the chemical resistance – – will the intense ultrasonic energy created at the surface of an ultrasonic transducer disrupt that oxide layer and compromise its effectiveness allowing corrosion to occur? It seems logical. I was about to discard that thought until I found a little note on a cooking website (stainless steel is also extensively used in cooking utensils) that warned users not to add salt to water in a stainless steel pot before the water boils claiming that the salt laying undissolved on the bottom of the pot along with the heat applied would cause pitting of the stainless steel. Who Knew???
Personal Experience –
Cavitation erosion is a fact of life and will occur over time in any ultrasonic cleaner. Over the years, I have witnessed ultrasonic transducers that show extreme cavitation in as little as 6 months but I have also seen others that have been in use for 20+ years and exhibit virtually no cavitation erosion. In some cases the difference can be attributed to high temperature, extensive operating hours, improper use etc. but there are those where the pieces don’t fit together logically. The role that chemistry might play is often conjectured but seldom confirmed. Yes, we know situations where the transducer life expectancy is going to be short based on experience in other similar applications but even then there are exceptions – even under the same operating conditions.