Activation and Repassivation of Stainless Steels in Artificial Brines as a Function of pH

Round 1

Reviewer 1 Report

In their manuscript the authors investigate the repassivation time scale of depassivated stainless steels for oil wells in electrolyte environment. Repassivation is studied as a function of the elctrolytes’ pH and the alloying composition of the steels.

Within the scope of the manuscript from my point of view only minor questions and issues remain.

1.) Figure 2: In order to enhance the comparability of the current densities given in the text (in µA/cm^2) the unit of the current axis should be µA/cm^2, as well. The unit of the potential axis seems to be Vsce, not mVsce.

2.) Page 5, line 166 ff: Which physical mechanism causes pitting prior to uniform corrosion?

3.) Page 7, line 221: Probably Figure 4 is referred to instead of Figure 5.

4.) Page 8, line 249: Can the experiments be carried out in a way that there is no time overlap between scratching and repassivation? This might enhance the reproducibility of the experimental findings, since they would not depend on the time scale of scratching.

5.) Page 8, line 253: Figure 7, not Figure 8.

6.) Can the authors estimate the material erosion (in mm per day/year) caused by a corrosion current density of 0.01 mA/cm^2? This might illustrate the meaning of “passivated” surfaces.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

In this work the authors investigated the kinetics of repassivation on different stainless steels in a chloride rich solution with experimental essays managing to reproduce the working conditions typical of the oil and gas industry. The present study is within the scope but to my opinion, some major aspects are missing and can be implemented in the text.

In the abstract, the scope of the work is not clear and even if the introductory part later describes the purpose of the work, I suggest the authors to put it also in the abstract to define the issue the want to discuss. 

In the introduction section the authors should specify the conditions they used for the tests (30000 mg/L of Cl-, anaerobic conditions,...)

The experimental part could be better defined. Did the authors perform the analysis on replicated samples? If they did, they should explicit the number of replicates in order to have a good data dispersion, a statistical validation and a significance of the measurements.

Did the authors measure the quantity of Oxygen inside the solution (in terms of ppm) at the beginning and at the end of each test? This parameter could play an important role in the corrosion behaviour of the different stainless steels and also modify the kinetics of repassivation. According to the content of oxygen, the cathodic reaction is shifted, and the general corrosion mechanism is modified. Also, generally the pitting mechanism is connected to the oxygen content.

When authors performed indentations with a diamond tip, they should describe the load, dwell time and penetration depth for each material in order to verify how the tip indentation influenced the surface of the material. 

Did the author calculated the ohmic drop from the immersion experiment? if so, it would be interesting to specify it in the materials and methods (if the information is in reference 17 maybe the authors should put it specifically in the text). 

In the results and discussion section a subsection should be added in which the authors performed surface analyses of the materials after the experiments maybe using SEM or AFM on samples both after polarisation tests and immersion tests. It would also be interesting to describe the diameter and depth of the pits for each alloy and the volume percentage on the exposed surface after the immersion test.  

Did the author find the range of perfect passivity for each material? it is important to define it in table 2 and further discuss it. 

It is not clear why the authors decided to focus on only one sample and change the pH. The authors should specify it in the text. 

In the kinetic sub-section the authors applied a log i= K log t model to describe the behaviour of the oxide growth with appear to be linear. Maybe it would be interesting for the authors to concentrate on the chance of the alpha according to the different subsystem and a further elaboration of the data would help in the description of the different systems. I suggest a description of all the conditions with a table in which every slope is displayed. 

The discussion section lacks some important information that is described in the conclusion. Maybe the authors should move the part of the concluding remarks in the discussion section and implement it to give a better idea of the work and its applicability to actual cases.

Comments to the text

Page 1 line 34. The authors should add a reference to the text. 

Page 2 line 48-50. Maybe the authors should move the sentence to the materials and methods since it gives information on the methods used in the experiments and not on the literature. 

Page 2 line 72-76: the experimental conditions should be specified in the section (neutral environment, presence or absence of oxygen,...). 

Page 2 line 94: the authors should specify the purge time. 

Table 1. the authors should specify in the table also the designation of the standard alloys used in order to have a reference from the text. 

Page 5 Line 156-158: authors should be careful when talking about oxygen reduction reaction when working on the anodic part of the system. Maybe it would be better to leave the sentence out of the text. 

Page 5 line 173: when the authors give an average, they should also give the standard deviation of the value. 

Figure 3. It is not clear where the pits are, and it is not easy to get it from the picture. The authors should replace the LOM images with more magnified pictures (or change them using SEM pictures).  

Figure 4. To better understand the behaviour of each system maybe it would be interesting to fix a parameter (in this case the pH) in order to develop a logical discussion. I suggest splitting the figure in two different pictures. 

Figure 5. I would suggest plotting the average and the standard deviation instead of 3 replicas.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

In this article, the authors explored the passivation and repassivation of different stainless steels as a function of pH. The study was carried out by using cyclic polarization measurements, immersion tests and scratch tests. The work is of interest in the field of corrosion. The experiments performed for the characterization of the steels are appropriate and well described. But I do recommend some further but very minor revisions before the paper could be accepted for publication.

The abstract should be improved and the type of steel investigated should be included.

In the Line 143 it is written Eb … breakthrough potential. While in line 148 it is written Breakdown potential (Eb).

In Figure 2, the potentiodynamic measurement of the steel 13Cr6Ni2Mo is difficult to follow due to the overlap with the line of the potentiodynamic measurement of the steel 17Cr4Ni2Mo. Can it be changed with a more intense colour and maybe thicker line?

Line 87: the acronym PREN must be explained. Pitting resistance equivalent number must be added.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors kindly followed the comments of the reviewer and the paper is now worth publication