Ultrasonic cleaning machine tank bottom gets pitting / corrosion

I am an jewellery manufacturer and I use the ultrasonic cleaning machine with temperature control up to 80 °C made of SS 304 AISI the tank bottom of the tank always tends to get pitting / corrosion.

Black spots are visible at the bottom with a crater / depth in micron.

let me know the reason why it happens and what is the solution for avoiding the same

thanks,

When stainless steel is produced, non metallic oxide and sulfide inclusions are formed and are present in the steel. If these inclusions are present on the surface there are weak points in the surface oxide film that can cause pitting.
Also when chloride competes as an ion the passivating film on the steel is not able to re-form, leaving the steel open for corrosion.
Is the water you use to make up and replenish the solution high in chlorides? Are there chlorides present in the cleaner? The MSDS will tell you. You also may want to upgrade the tank to 316 ss. Good Luck!