Dear Dr. Askar Soltani,

the presence of chlorides in an aggressive environment (both in atmospheric exposure and in solution immersion) is one of the most aggressive ions towards materials as it is able to damage the passivation layer and create significant corrosion attacks, especially of localized type (the most dangerous). As for scientific tests, the concentration of 3.5% of NaCl (by weight) in aqueous solution is often used as, probably similar to the concentration of this salt present in sea water. For standard tests, more severe aggressive solutions are used (eg 5% in a salt spray chamber or even higher in other tests). This increase considerably increases the degree of corrosiveness of the solution and creates particularly aggressive conditions for the materials under test. The test conditions according to the regulations cannot be changed but, if I were to plan a search for a certain material under known conditions of use, I would try to use a solution, for laboratory tests, as close as possible to the real chemical (chemical composition, content of dissolved oxygen, etc.) and physical (temperature, static or flowing state, etc.) conditions.

My best regards, Pierluigi Traverso

Dear Dr. Askar Soltani,

the presence of chlorides in an aggressive environment (both in atmospheric exposure and in solution immersion) is one of the most aggressive ions towards materials as it is able to damage the passivation layer and create significant corrosion attacks, especially of localized type (the most dangerous). As for scientific tests, the concentration of 3.5% of NaCl (by weight) in aqueous solution is often used as, probably similar to the concentration of this salt present in sea water. For standard tests, more severe aggressive solutions are used (eg 5% in a salt spray chamber or even higher in other tests). This increase considerably increases the degree of corrosiveness of the solution and creates particularly aggressive conditions for the materials under test. The test conditions according to the regulations cannot be changed but, if I were to plan a search for a certain material under known conditions of use, I would try to use a solution, for laboratory tests, as close as possible to the real chemical (chemical composition, content of dissolved oxygen, etc.) and physical (temperature, static or flowing state, etc.) conditions.

My best regards, Pierluigi Traverso

Dear Dr Pierluigi Traverso

Thanks for your comprehensive answer. That's right that for simulation of the aggresive conditions that there is high chloride concentration we need to consider this level of NaCl solution. For corrosion experiments in oil industry, laboratory tests have been conducted based on the standard NACE 1D182 which considers 9.8% wt NaCl which is considerably higher than the academic studies with 3.5 wt% of NaCl, and, i am curious to find the reasons of this discrepancy.

As you said the best designed laboratory experiment is to simulate the real conditions including chloride ion concentration, oxygen concentration, temperature, velocity of the solution, TDS concentration and etc.

Dear Askar

The 3.5% NaCl (by weight) of Standard ASTM G44 is the threshold of the Cl- aggressiveness in seawater. So to answer your questions;

1- The difference in NaCl concentration for 3.5% wt NaCl (Standard ASTM G44) and in 9.8% NaCl wt of NACE 1D189 is because the content of Cl- not only higher than 3.5% wt NaCl, but also there is other aggressive ions in the sea water ,i.e.,SO4-2, present, see the attachment.

2-The reliability of the data production, in simulated corrosion studies, is dependent on which standard tests you follow. Otherwise use real seawater and measure its content as in the attached article.

Dear Askar Soltani

I hope that you take into consideration that the environments corrosivity comparison in terms of the chloride ion percent will fall off if these environments are different in their simulation field as ASTM G44 simulate the seawater corrosivity and NACE 1D189 simulate the oil-well environment corrosivity. Now, note that the test solution in ASTM G44 is a single phase fluid with only 3.5% wt. NaCl dissolved in distilled or deionized water conforming to the purity requirements of specification D 1193, while the test solution in NACE 1D189 is a complex of two phase making an oil/water emulsion fluid consisting of oil crude has purged with inert gas or with one corrosive gases such CO2or H2S and the brine purged with nitrogen to remove oxygen and then sparged with CO2 or H2S to simulate produced water. The synthetic brine is commonly 9.62% sodium chloride (NaCl), 0.305% calcium chloride (CaCl2), 0.186% magnesium chloride hexahydrate (MgCl2•6H2O), and 89.89% distilled water. The oil/water ratios most often chosen have been 50/50 and 10/90 oil-to-water. So it's clear that the brine is reflects the actual constituents of the oil-well brine as the crude oil represent an actual real well crude oil as so the oil/water emulsion reflect the actual well environments. Finally I enclose a copy of both practice ASTM G44 and NACE 1D189 for more explanations.

Best regards

Dear professor Mohammed

Well done job on descriping NACE solutions for sweet oil wells, i.e.,with CO2 gas

and sour oil wells ,with H2S gas, for down hole simulation of corrosion processes.

Dear Askar,

There are couple of reasons behind the simulated corrosion rate in presence of 3.5 % sodium chloride.

(1) Presence of chlorides in an aggressive environment

(2) Electrolytes itself the most abundant and common corrosive agent

(3) Dissociation of chloride from sodium is easy through the electron emission.

(4) Salinity is high

(5) Formation of corrosion products than the thermal  decomposition is easy in comparison to the other electrolytes. Hence weight corrosion rate is easy to make.

Ashish

dEAR Ashish Thakur ,

Thanks for your contribution. All of your mentioned reasons are correct but the puprpose of this topic is to find out the reason of the exact concentration of 3.5 wt%% (i.e. 35000 ppm) because we know that the salinity of sea water is different in every sea water around the world. I think the response of Khaled J. Habib is the exact response for this topic and that is the threshold of chloride ion's aggressiveness.

IN standard NACE 1D182 the concentration of NaCL is abouut 9.81wt% and other aggressive salts like CaCl2 and MgCl2 are also present in the solution and the standard solution in fillm forming corrosion inhibitor's tests is the solution based on this standard and not 3.5 wt% NaCl.

Dear Khaled J. Habib

Thanks for your contribution in this topic. You mentioned that 3.5 wt% NaCl is the threshold of the chloride ion aggressiveness and that's why most of the corrosion experiments have been done in this test solution.

Would you please share the reference for this expression ?

Regards,

Askar Soltani

Dear Mohammed Hussein j. H. Al'Atia hamm

I really appreciate your insight in corrosion experiments of the inhibitors in the oil industry. As you mentioned, for simulating synthetic brine and also the effects of the oil phase we need to conduct corrosion inhibitor tests according to standard NACE 1D182 and purge the pwater phase by a nutral gas like N2 and then saturate the test solution by CO2 in order to get the exact simulation because in this case the pH of the water would be around 4.

But the question is why most of the academic studies on corrosion inhibitor's investigations are being done in 3.5% wt of NaCl which is far from the real conditions for brine water in oil wells.

Be consider that 9.66 wt% of NaCl (96000 ppm of Cl-) is really really high and i cannot imagine this salinity. For example, in Persian Gulf in our gas-wells the chloride content is about 20000 ppm and it is far from the 3.5 wt% or 9.6 wt%. So, i think it is a conservative aspect of standard NACE 1D182 to consider this high level of chloride content in simulated solution. however, we know that as Mr Khaled J. Habib mentioned, the threshold of chloride ion aggressiveness is 35000 ppm and i asked him to share the reference of this reality..

I have attached the chemical analysis of the free water in one of our wells, you can see the concentration of chloride and other corrosive ions.

Dear Askar Soltani

I see you continue seeking answer for your question "why most of the academic studies on corrosion inhibitor's investigations are being done in 3.5% wt of NaCl which is far from the real conditions for brine water in oil wells."I understand your point of view and I share your view and can put my answer in the following points:

Based on the ionic mobility strength of chloride ions in solution, 3.5% wt of NaCl solution is more potent than brine water of 9.6% wt NaCl (NACE1D182) and less than your produced water of 2% wt of NaCl. This means that produced water of 2% wt of NaCl is more corrosive, but this is not the end of story as the produced water has complex formulation differed than NaCl solution alone.

In USA, the salinity of produced water may range from7.6 to 23.2% compared to a salinity of 3.2 to 3.6% for seawater and may statistically the range 9% to 10% as avarge salinity weight percent for produced water which is included in the practices of wheel test NACE1D182.

To my best knowledge, the practices NACE1D182 authorized the use of the sea water salinity for brine formulation in some cases.

The best thing to do is the wheel test in three brine formulations 9.6% wt NaCl, 3.5% wt NaCl and your local value of 2% wt NaCl. The results will be of great and vital look on the role of chloride percent in whole corrosion process and its rates and at the same time the results helps to take a valuable practical decisions.

Best regards…..

Dear Mohammed Hussein j. H. Al'Atia

Thanks again for your comprehensive response. As a member of the oil industry in Iran, i need to get a deep insight into the test procedures according to the standards like ASTM and NACE. Since, most of the inhibitor performance tests in the research center of oil industry in my country is are being done based on the standard NACE 1D182 recommendations, so i tried to research in this subject and find out the best laboratory experiments for inhibitor performance tests.

What i found was to simulate the conditions in more harsh electrolyte and i found that the most aggressive concentration of NaCl is 3.5 wt.%, however i could hardly find a text book or a recommended code for that.

I have summarized my findings as attached. It is based on a master thesis which was done in Ohio state university under the supervision of professor Nesic which has a lot of papers for CO2 corrosion. As you can see in LPR and potentiodynamic polarization curves, the higher the concentration of NaCl (more than 3 wt.%) , the higher the corrosion rate. As you can see in polarization curves, the higher NaCl concentrations are able to retard both cathodic and anodic reactions and decrease the corrosion current density. The writer of that paper call it as "salt retarding" and it was very interesting for me.

As you mentioned the best way in inhibitor's performance test is to simulate the real condition from the salinity, pH value, CO2 and H2S partial pressures, shear stress and temperature points of view and in my case it could be the use of 2 wt.% of NaCl instead of 9.6 wt.%.

You mentioned that ion mobility of higher concentrations of NaCl is less than the lower concentrations. Would you please tell me why and explain more to clarify your expression regarding the higher mobility of 2 wt.% of NaCl compared to 3.5 wt.% ?

Because i think 35000 ppm of Cl- is the limestone and highest threshold of chloride ion aggressiveness and i think in this case the corrosivity of 3.5 wt.% of NaCl is higher than the 9.6 wt.% and 2 wt.% which could be related to the higher ion mobility in 3.5 wt% NaCl compared to other concentrations.

looking forward to get your detailed answers

best regards,

Askar Soltani

Dear Askar Soltani

Thank you for your real attempt to find out some practical facts around the corrosion testing engineering concepts. First of all, I'm pleased with the supportive paper by the Ohio University that you've attached as this study is clearly support my points of view included my last and former answer. However, it's clear now by full agreement that corrosion process rate is a nonlinear inversely trend function in terms of the solution salinity (NaCl %).

About your request for more explanations to clarify my expression regarding the higher mobility of 2 wt.% of NaCl compared to 3.5 wt.% , I'm pleased to enclose the work by the G.G.Tenchov, please look at fig4 and table 2, you will find a clear nonlinear inversely functions for the cation mobility and anion mobility of NaCl solution versus molar concentration at different temperature. This means that ionic mobility strength of chloride ions in solution, 3.5% wt of NaCl solution (ASTMG44) is more potent than brine water of 9.6% wt NaCl (NACE1D182) and less than your local produced water of 2% wt of NaCl, and once more this means that your local produced water of 2% wt of NaCl is more corrosive than 3.5% wt NaCl and 9.6 % wt of NaCl.

Again, the best thing for you to do is the wheel test in three brine formulations 9.6% wt NaCl, 3.5% wt NaCl and your local value of 2% wt NaCl since the output of these tests will extend the work of H. Fang and S. Nesic (Ohio University) and provide a solid base data to you for more test developing approach .I hope my comment and notes are helpful for you and for all the followers.

Best regards

https://www.researchgate.net/publication/299430649_EFFECTIVE_ION_TRANSPORT_NUMBERS_AND_ION_MOBILITIES_OF_CONCENTRATED_SODIUM_CHLORIDE_SOLUTIONS_AT_DIFFERENT_TEMPERATURES

Dear Mohammed Hussein j. H. Al'Atia

Thanks for your contribution. After searching a lot, finally i found that NaCl concentrations more than 3.5 wt% have less corrosivity in CO2 simulated solutions. But, there is still a BIG QUESTION for me and that is why 3.5 wt%?

Thanks for sharing the paper which had worked on the NaCl ion mobility and transport numbers in different concentrations. I read that unique paper and i enjoyed it.So, let me ask a question regarding the results of this paper:

Question 1) Conclusions of this paper indicated that in concentrated solutions, the ion transport numbers do not change with ion concentration. As it is clear, transport number equals to the fraction of the current transported by the anions (or Cations) to the total current. And also, transport number corresponds to the ion mobility and when it is constant in concentrated NaCl solutions it means that the ion mobility will remain constant. Would you please explain more about this conclusion and my idea ?

Question 2) In Fig.4 of that paper has been illustrated that the anion and cation mobility of NaCl solution decreases in higher concentrations. It means that, the higher the concentration of NaCl solution, the lower the ionic mobility. So, in this case we can say that in very diluted NaCl solutions, the corrosion rate will be more compared to higher concentration. For example, in this case the corrosion rate in 200 ppm of Cl- will be more compared to 35000 ppm, right ??

As we know, most of the academic laboratory experiments by most famous professors are being done in 3.5 wt% of NaCl. And, that's why i think 35000 ppm of Cl- is the highest threshold for the chloride ion aggressiveness.

So, I think the threshold of aggressiveness of chloride ion is 3.5 wt.% and the corrosion rate in 2 wt.% of NaCl (in my case) will be less than the 3.5 wt.%. However, i need to conduct laboratory experiments to make sure. I accept your point regarding the less corrosiveness of concentrations more than 3.5 wt.% but i couldn't find any reason for less corrosivity of concentrations less than 3.5 wt.% of NaCl. What do you think ?

To make sure i will conduct electrochemical measurements by RCE (Rotating Cylinder Electrode) in our laboratoy based on the concentrations of NaCl that you recommended and then i will share the results with you.

Best regards,

Askar Soltani

Dear Professor Juan Genesca

Very thanks for your contribution. I tried to find out the book " Robbin's book, Ions in Solution", But unfortunately i couldn't get it. Would you please share it if it is possible and legal?

And also, May i have your answer on following questions:

Question 1) Do you define 3.5 wt.% of NaCl solution for corrosion experiments for your students thesis? If your answer is yes, why 3.5 wt.% and why not to simulate the real concentrations ?

Question 2) In your opinion, What would be the reason for lower ion mobility in lower concentrations of NaCl ?

I think there is a threshold for NaCl concentration in which the chloride ion mobility in double layer in the electrode-electrolyte interface is in its highest level and in lower or higher concentrations it decreases.

Question 3) Is there any possibility for chloride ion, to carry more current by faster speed in a determined concentration as the highest level (i.e. 35000 ppm)?

I am confused by this and looking for the reason of this.

Your response or connecting me with your university's research team will be appreciated in advance,

Best regards,

Askar Soltani

Dear Askar Soltani

Borgmann et al. published In 1937 an composite data of several investigations about the effect of sodium chloride concentration on corrosion of iron in air saturated distilled water at room temperature .The corrosion rate increases first with salt concentration and then decreases, with the value falling below that for distilled water when saturation is reached at about 26% NaCl. The apex of corrosiveness of this salty solution was about 3%. Now this is the case still nowadays considered academically as the effective electrolyte for only laboratory testing for indexing and ranking the corrosion resistance of different materials and one of the applications of these facts is ASTMG44. Now, if you and other followers interested in standing on more details on the mechanism of corrosion by oxygen saturated distilled water – NaCl solution please look at item 2.5 pp9 in [1].

Now, the CO2 corrosion of carbon steel under effects of salt concentration is really different story. Han et al. reported decreased corrosion rates with an increase of salt concentration from 0.5 to 20 wt. % [2]. This means the apex of corrosiveness of this salty solution was below 0.5 %. Later on, Nesic et al. found that the corrosion rate of carbon steel in CO2 and NaCl environment significantly decreased with increasing NaCl concentration from 3 to 25 wt. % [3].

These are good supportive results to my previous comments in that CO2 satutated- NaCl brine water of 3.5% wt NaCl is more corrosive than of 9.6% wt NaCl (NACE1D182) and less than of 2% wt of NaCl.

Best regards

[1] CD8025_@_40_3.pdf (below)

[2]Article Effect of sodium chloride on corrosion of mild steel in CO 2...

[3] https://pdfs.semanticscholar.org/8b68/7d1db18f02d1a72b63e7aa9730a13a39c669.pdf

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