Study on Modified Liquid Polysulfide Rubber Bimetallic Salt-Spray-Resistant Epoxy Coatings

Round 1

Reviewer 1 Report

It is well known that Epoxy zinc-rich paint (coating) is widely used for anti-rust primers.   However, the coating is not suitable for underwater environments.   This is because of its brittleness and high zinc content, i.e., not be less than 60% in the coating.   So, the objective of the present work was to produce a new epoxy coating for not only corrosion resistance & anti-blister performance, but also to be flexible & adhesive coating on a steel substrate for underwater environment.   Consequently, a new epoxy coating was produced.   The new epoxy coating made of modified polysulfide liquid rubber (MPSR), epoxy resin(ep), titanium powder (Ti), zinc powder (Zn), and carbon nanotubes (CNTs). Five new epoxy coatings were synthesized based on changing the ratio of the bimetallic filler, i.e., Ti, Zn, and carbon nanotubes, namely; MPSR coatings including MPSR0 (mEP: mTi = 1:1), MPSR1 (mEP: mMPSR: mTi = 2:1:2), MPSR2 (mEP: mMPSR: mTi: mZn = 2:1:2:2) and MPSR3 (mEP: mMPSR: mTi: mCNTs = 100 2:1:2:0.06),  MPSR4 (mEP: mMPSR: mTi:mZn: mCNTs = not known).   Then the coatings were verified by FT-IR and NMR for their content and structure, respectively. The results of FT-IR and NMR correspond well to confirm the successful preparation of the coatings.     Then, the five coatings were applied on steel plates for different examinations.   The examinations & results of the five coatings were categorized as the followings:

1-Weight gain rate in soaking deionized water during 7,14, and 30 days.

The MPSR0 coating was recorded to have the lowest weight gain rate among the rest of the coatings based on its composition content.   In contrast, the MPSR2 coating was observed to have the highest weight gain rate among the rest of the coating.   However, the weight gain rate of MPSR3 and MPSR4 coatings were found in the middle among the rest of the coating.   This manifestation was explained because of the high value of the compactness the of the coatings. 

2-Corrosion rate measurement by the linear polarization method.

The corrosion rate recorded to increase as the composition contents of the MPSR0, MPSR1, MPSR2<MPSR3, and MPSR4 increase.   On the contrary, the open circuit potential was observed to vary in a diverse manner, not in agreement with the corrosion rate measurement.   

3-Electrochemical Impedance Spectroscopy (EIS) of the five coatings in 3.5% NaCl solution. 

The steel substrates that were coated with MPSR2 and the MPSR1 had the largest A.C impedance, at low frequency in Bode plots , followed by rMPSR4, rMPSR3, and rMPSR0.

4-Characterization of coating salt spray test   

Salt spry tests were conducted on the five different coated-steel substrates for a duration of 0h, 360h, 720h, and. 1440h.  The MPSR2 sample had the best effect on preventing corrosion diffusion and corrosion, followed by MPSR1, while the MPSR0 sample had the worst anti-corrosion performance.  This observation confirms the results of EIS.

5-Test of adhesion strength of coatings 

 The MPSR0 had the lowest adhesion strength and the MPSR1 has the highest adhesion strength. After MPSR1 was added with MPSR, the introduction of flexible groups avoided stress concentration and increased the coating adhesion strength from 6.543 MPa to 9.729 MPa with an increase of 48.7%.

The addition of zinc powder to MPSR2 caused the adhesion strength to deteriorate. Also, the introduction of carbon nanotubes into MPSR3 increased the bonding force between the coatings and the metal and improved adhesion.

However, due to the increase of pigments in MPSR4, even with the addition of a small amount of carbon nanotubes, the adhesive strength was still reduced. 

6-Coating immersion test

The MPSR2 coatings had good resistance to water immersion, in a 3.5% NaCl solution.   The mass ratio of added zinc powder is only 28.6%, which greatly reduces the use of metallic zinc.

The following issues need to be addressed:

1-In the Abstract on line 17, the terms "IR and NMR" should be written for what they are stand for before the abbreviation.

2-In page 3, on line 95 & 96 The term "modified polysulfide liquid rubber was repeated twice in the same sentence.

3-In page 3, on line 99, the composition of the fifth coating; MPSR4 (mEP: mMPSR: mTi:mZn: mCNTs) is not given. 

4-In page 8, the solution composition of the linear polarization test needs to be mentioned.

5-Im Page 9, on line 228, the statement " was the comparison electrode should be "was the counter electrode".

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Reviewer 2 Report

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