Stainless Steel and Microbial Erosion

Stainless Steel and Microbial Erosion

Sonix 4 Corporation uses stainless steel in manufacturing the industry’s leading ultrasonic cleaning systems. As such it is important for our customers to have some level of education in regards to the makeup of stainless steel which will ultimately say something about the integrity of the metal when in the working environment.
Over time some users may have noticed the creation of holes within the stainless steel tank. These holes, or pits, can cause serious problems once moisture is allowed to leak into the electronic components of the ultrasonic unit. Once a pit is created, and the unit is run for any period of time, we can expect the hole to exponentially increase in size. This is due to the powerful cavitations that occur when a Sonix 4 ultrasonic unit is in operation. So the question that becomes important is, “How can I prevent pits from being created in the first place?”
The answer comes from the knowledge of the chemical makeup of stainless steel, and the dangers that the working environment can introduce into the ultrasonic tank. Stainless steel differs from carbon steel in the fact that it contains more Chromium. Carbon steel will corrode when it comes in contact with the air and moisture due to the active film of iron oxide (Fe(OH)3), or rust, on the surface. Stainless steel with Chromium is resistant to, but not free of this reaction. There is an active film of Chromium oxide (Cr(OH)3) on the surface of stainless steel, and this active film causes concern when in the working environment. In the instruction manual that accompanies the Sonix 4 product there is a list of solutions known to be harmful to the stainless steel tank due to their low pH, or acidic content. The easiest way to think about the reaction that happens to the surface of the stainless steel is with the net ionic equation of any acid base reaction (H+ + OH-  H20). From this equation we can see that the addition of any acid onto the basic surface (due to active film of chromium oxide) of stainless steel will indeed react. The product of the reaction can be visually seen over a period of time in the form of holes, or pits, in the tank.
There is yet another way that acid can be introduced into the tank other than direct addition of acidic solution. They way to think about this is cavities in your teeth. We spend lots of money and time cleaning our teeth to keep them free of bacteria because bacteria produce acids which will react with tooth enamel (Ca5(PO4)3(OH)). This is the basis for the Sonix 4 recommendation that regular cleaning maintenance is necessary to prolong the life of the ultrasonic tank. If the unit is used to sterilize, but is not cleaned after use then there are microscopic bacteria hard at work producing acid solution that will react with the stainless steel creating microbial erosion of the tank. Over time this microscopic process becomes more and more visible as the holes get larger. Furthermore, as stated earlier, once the smallest amount of erosion has occurred the ultrasonic cavitations will be cannibalistic in that they will aid in the rapid erosion of the ultrasonic tank.
So, in summary, “How can I prevent pits from being created in my ultrasonic tank?” Prevention primarily comes from the regular cleaning maintenance of the ultrasonic tank after use. The list of solutions not to be used in Sonix 4 ultrasonic units (Instruction Manual) should also be consulted.