ISSUE -
In typical Austenitic Steel reactor with agitator, there is a staining or colorization occuring in the region where NaOH (sodium hydroxide)+ water solution and air would be coming in contact with heated surface.
Top and Bottom portion of tank including agitator shaft, dip pipe are not affected.
The process of cleaning/neutralizing the reactor, post product batch is when the NaOH+water solution is used.
The NaOH+water (50%conc,) solution is 50-60% volume based filled in Austenitic steel reactor. The heating medium (steam) is supplied to the limpet area to boil the solution. With the continuous agitation during this process, droplet of these solution tends to settle on the hot unwetted surface of inner main body surface.
Once the droplet settles they quickly vaporized leading to high local concentration -NaOH scaling or localized deposition on surface results in corrosion like stain (red layer surface).
SUPPORTING DOCUMENT
The inner contact surface of reactor is certified SS316L Austenitic steel.
From metallurgical profile for Austenitic steel it affirms that it has good resistance to NaOH upto 50% concentration and temperature 93 degree Celsius.
Above 93 Degree Celsius the austenitic tends to exhibits unstable passivity and can suffer sever general corrosion.
QUESTION
The temperature has to be around 90 deg cel. as per requirement for cleaning the reactor with NaOH + water solution
The formation of stain like portion is in unwetted main body area only not on agitator profile nor inner top dish area.
The issue of red layer formation is with NaOH + water + steam Heating process only.
The issue of red layer formation is not formed under water + Steam heating process.
Request suggestion, on the implementation
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