H2O2 35% on Al 5083 at 150° gives 'limescale'

Hi,

In our production line we are heating h2o2 in a heater made of Alu 5083 to 150° for the disinfection of plastic bottles. After 6 months we needed to revise and clean our heater due to some sort of "limescale" formation. It only occurs at the beginning of the heater (I guess before evaporation).

Can someone help me figur out what this is?

Thanks in advance!

I'm guessing that you're diluting the peroxide with water. I'm also guessing that water is 'hard.'

When I see corrosion or deposition close to a junction, I suspect a step change in temperature, or an electrochemical reaction of the electrolyte, between the heater material and the feedline material.

As with the two previous posts, I was thinking that the oxide looks like porous alumina, but it could also may involve whatever cations are present in the water. Ideally one would be using demineralized water with little Mg, Ca, Si, which could cause formation of MgO, CaO, SiO₂, among others. Porosity/cracking of the oxide reduces/compromises the protective/passive nature.

Besides water chemistry, the other issue is the solubility, a function of temperature. What is the inlet temperature of the peroxide solution?

Thermal cycling could be another issue, related to the other two factors. Oxides of metals and the metals tend to have differences in thermal expansion. If the thermal expansion of hte metal is greater than the oxide, then thermal cycling can cause microcracking of the protective oxide.

Thanks for the response!
The inlet temperature is around 20 degrees.

My view is that it could be Al2O3. Aluminium has a small layer of Al2O3 by contact with air. It can only occur with h2o2 in liquid phase, which is what would explain why it’s only at the beginning of the heater. By corrosion and a strong oxidation with h2o2, Al2O3 will keep on growing.

What do you think?

If it's Al2O3, there should be serious pitting / material loss on the your aluminum substrate.

TomD1 said:

Hi,

In our production line we are heating h2o2 in a heater made of Alu 5083 to 150° for the disinfection of plastic bottles. After 6 months we needed to revise and clean our heater due to some sort of "limescale" formation. It only occurs at the beginning of the heater (I guess before evaporation).

Can someone help me figur out what this is?

Thanks in advance!

Is it more of a chalky powder, or is it a hard, somewhat tough deposit? If it’s the latter, I’m inclined towards it being hard water deposits. If it’s the former, accompanied by pitting of the aluminum part, it’s corrosion of the part.