Yes one of the main parameter is the test solution resistance. in this respect i will recall my previous answer on the Researchgate link; :https://www.researchgate.net/post/What_is_the_relation_between_corrosion_rate_and_solution_resistance

if we consider two concepts, corrosion process and corrosion rate measurement. At low solution resistance, the corrosion process goes on at the same mechanism as for high solution resistance in terms of electrochemistry fundamentals and its related reactions but it could be at different rates. Now to measure the rate of corrosion, the error with high solution resistance is always higher therefore the relation between corrosion rate and solution resistance is a technical relation. For example at any polarization potential we should consider the sum of activation polarization potential, concentration polarization potential and solution potential drop (IR) which is always caused by current flow through the resistance in the bulk of the solution. Now at lower solution resistance (R) the value of IR drop is neglected then the measurement of corrosion current (I) or corrosion rate estimation goes toward more accurate value and at higher solution resistance (R) the value of IR drop is considerable then the measurement of corrosion current (I) or corrosion rate estimation goes toward less accurate value . This solution drop must be compensated and if we do this compensation well, the effect of solution resistance on corrosion rate measurement will eliminated. In this case electrochemical test methods and the gravimetric (immersion) test method will give the same results.

Best regards

Best regards

Dear Abdelaisalam

If you mean by the immersion test is the determination of corrosion rate of an Al-alloy by the corrosion weight loss test, so the value of corrosion rate for the Al-alloy is naturally would not be the same when measured with a different technique, i.e., electrochemical corrosion test. The corrosion weight loss test is dependent on a direct conversion of mass loss , due to corrosion processes of Al-alloy in a solution, to corrosion rate by using Faraday law. In contrast, the measurement of corrosion rate, of the same Al-alloy in the same solution , by an electrochemical (DC or AC)test, i.e., linear polarization, Tafel test, EIS, an so on, is dependent on an indirect measurement, by an extrapolation/estimation/interpolation of obtained electronic data, of the corrosion processes of Al-alloy in the solution, to corrosion rate of such test. This is the reason behind the variation in the corrosion rate between the weight loss method an the electrochemical techniques.

Dear Abdelaisalam,

In general, there are many parameters that influence an experimental measure. Providing absolute data is very complex while it is much easier to make comparisons between materials of the same type in the same corrosive solution.

In particular, as far as aluminum alloys are concerned, we must always evaluate the pros and cons of the technique used. For example, electrochemical tests (accelerated tests) can represent an estimate of what happens in reality, also depending on the experimental conditions and the cell geometry. With regard to weight loss, the corrosion products that are formed are both soluble and insoluble (the latter present on the surface of the sample) that should be removed before the gravimetric measurement. Other complications come to light if we consider environments that can stimulate localized corrosion phenomena (pitting, crevice, etc.) for example in the presence of chlorides.

This also makes a careful assessment of this phenomenon necessary ....

Best regards PT

Dear Abdelaisalam Al-bakoosh

In my understanding, you measured corrosion rate in terms of mass change after immersion test, and it was compared with the corrosion current (Icorr) value by electrochemical method. In case of immersion test, corrosion product formed on the metal surface could suppress dissolution rate of metal, and Al corrosion product has effective barrier property in general. On the other hands, electrochemcial corrosion rate only refers the electrochemcial reaction on ideal metal surface. If corrosion rate determined by electrochemcial methode is higher than that by immersion test, you have to be considered the barrier property of corrosion product on metal surface. In opposit case, please check the etching condition for corrosion product removal (or how to determine the corrosion rate after immersion test).

Sincerely yours,

HS

The immersion test gives you a global corrosion rate during the measurement time. Electrochemical test gives you temporary (during measurement) corrosion rate. The global velocity result can be compared with electrochemical corrosion measurements conducted at the same time (it is necessary to integrate the curve showing changes in corrosion rate over time).

In case of weight loss, the calculated corrosion rate is an average value over the exposure period, assuming uniform corrosion on the whole surface. Its accuracy depends on the duration of the corrosion test, adsorption of corrosion products and/or inhibitor on the alloy’s surface, and ease and completeness of their removal before weighing. Localized corrosion induced by the IMPs in Al alloys may give some error in the calculated corrosion rate.

Electrochemical tests give you the corrosion rate at the exact particular time, and in case of Al alloys even obtaining the reproducible results might be a problem due to mentioned above IMPs. Also important is surface preparation.

You can read our paper (Article Corrosion Inhibition of Aluminum Alloy AA6063-T5 by Vanadate...