I have been doing several potentiodynamic polarization test on steel 304 grade and high sulfur 303 grade steel. The electrolyte is 0.25M H2SO4. The result is that 304 steel gives different Ecorr and icorr when aereated and deaerated while 303 gives same values for all condition. The corrosion rate is same at very low or high content of dissolved oxygen for 303 steel the PDP curve overlap for different dissolved oxygen contents in electrolyte). But except for sulfur content they both have similar composition.
Can anyone give me a reason for this type of behavior of 303 steel?
it depends on the polarization layer , in another word, SS304 requires oxygen to build the protective oxide layer, thus the concentration of dissolved oxygen has an impact on its electrochemical behavior
Yes dear
Oxygen reduction process usually run under diffusion control and there is limiting diffusion current for oxygen reduction which is normally about 10 μA/cm2.
Now in case of ss303 the critical anodic current density was higher than limiting diffusion current density for oxygen reduction. This means that the oxidation power of oxygen was not enough to spontaneously passivates ss303 in 0.25M H2SO4. Therefore, aerated and deaerated conditions give same values of corrosion potential.
Move now to ss304, you see the critical anodic current density equal or less than the limiting diffusion current density for oxygen reduction( i.e. less than 10 μA/cm2) so in this case aeration able to passivates ss304 in 0.25M H2SO4 and moves the corrosion potential to more passive value.
Thanks
Thank you Wadah Jawich and Mohammed Hussein j. H. Al'Atia for your valuable answer and explanations. This really cleared my doubts.
One thing you also notice that the pH value of your solution will be change for both aereated and deaerated conditions which has more effect on active - passive materials like austenitic Type 304 SS.
Yes Soumya Basak, you are right. In case of type 304 SS there was Change in pH but in case of type 303 SS the pH change was very less than in 304.
It will be very nice, if you can note all these values and also putting these values in a paper will give you more highlights. Hope your doubt has been cleared by all of this good answers. Any prob in future feel free to contact me.
I think we should pay particular attention to the chemical composition of the two types of steel.
Yes only difference in these 2 is the Sulfur Content. 303 has higher sulfur
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