Which is more significant when it comes to the corrosion rates of iron in deoxygenated solutions, a lower pH, or a higher chloride concentration? And how are they related? does the former exacerbate the effect of the latter or vice-versa ?
chlorides will affect corrosion rate regardless of pH due to their ability to break down passive films, you can shift the potential of the metal enough to slow down the corrosion rate to negligible levels. The attack you get is different from acid and chloride...at least at the surface level. chloride attacks tend to concentrate in pits or cracks.
Thank you Antonio for your answer. It seems that the effect of chloride is independent of pH? (at a certain potentials in few milivolts difference from the corrosion potentials or open circuit potentials)?
Pl find the following link to get your answer: http://www.swcc.gov.sa/files%5Cassets%5CResearch%5CTechnical%20Papers%5CCorrosion/CORROSION%20STUDIES%20ON%20SS%20316%20L%20INLOW%20pH%20HIGH%20CHLORIDE%20PRODUCT.pdf
As per my opinion, lower pH aggravates chloride pitting too. The presence of chloride is always associated with pitting type of corrosion but the rate of pitting corrosion rate is not governed solely by the chloride rather pH also plays a role here too.
Thank you Sebastián and Mohammad, your answers are about what I expected actually.
The following experiments are performed to judge the effect of the change in chloride ions concentration and the pH change:
[1] Keeping time duration and temperature constant, perform FCT (ferric chloride test) by keeping the iron/steel piece immersed in FeCl3 solution and increasing its concentration. As FeCl3 concentration increases, pH decreases and the corrosion rate also increases.
[2] Perform the same experiment; keeping all conditions same but decrease time of immertion to half the number of days.CORROSION RATE BECOMES APPXO. 65-67% of the fist experiment.
[3] Perform the same experiment; keeping all conditions same but decrease the temperature to half (C^0).RATE BECOMES APPXO TO 25-30% of the first experiment.
[4] Perform the same experiment; keeping all conditions same but keep the iron piece immersed in a mixture solution containing NaCl+HCl solution mixture. The corrosion rate increases ENRMOUSLYTHAN THAN THAT of the fist experiment.
[5] Perform the same experiment; keeping all conditions same keeping the iron piece immersed in a mixture solution containing NaCl+HCl solution mixture. But concentration of NaCl is increased and of HCl is kept constant to decrease pH. RATE OF CORROSION BECOMES MORE THAN THAT OF FOURTH EXPERIMENT.
[6] Perform the above experiment; keeping all conditions same but reduce the time of immersion to half the number of days. RATES OF CORROSION REMAIN APPXO. EQUAL TO THAT OF ABOVE EXPERIMENT.
[6] Draw two graphs; one pH vs corrosion rate and other chloride concentration vs corrosion rate.
The conclusions and their reasons are explained as
[a] The corrosion rate increases as pH decreases because increase in H+ attacks Fe more vigrously.
[b] Increase in temperature increases the corrosin rate because reactivity increases.
[c]The addition of chloride ions has a noticeable behavior on corrosion in the presence of acids because this ion breaks the passivity of the iron beyond its certain concentration and penetrates deep to cause pits in the iron.
So more the concentration of the chloride ions and higher the acidity (lower pH),the higher is the corrosion rate, BOTH COMPLIMENT EACH OTHER UP TO A CERTAIN CONCENTRATION OF THE CHLORIDE IONS BEYOND WHICH THE CHLORIDE IONS PREDOMINATE OVER pH.
1. pH, Chloride concentration and dissolved oxygen concentration all affect the risk of corrosion, as indicated by:
Shi, X., Nguyen, T.A., Kumar, P., and Liu, Y. A Phenomenological Model for the Chloride Threshold of Pitting Corrosion of Steel in Simulated Concrete Pore Solutions. Anti-Corrosion Methods and Materials 2011, 58(4), 179-189.
2. The addition of NaCl into an electrolyte will slightly affect (decrease?) its pH reading, as the newly added cations and anions participate in the water chemistry.
3. At the active corrosion site, the release of Fe cations (in the case of iron and steel) will also affect the pH of the adjacent electrolyte.
4. Inside the pits, the localized pH can be much higher than what's measured in the solution, due to the cathodic half-reaction.
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