Can anyone share some answer on the question? I read some articles which said that it boost the coating property but I also found others which said the opposite i.e., that ZnO caused brittleness and etc.
So, for the cases of coating containing zinc oxide and exposed to seawater. From my project. I found that coating containing zno does yield better structure even after being immersed for 60 days.
But, even more, Im interested in what actually happen. I mean the mechanism which induce such outcomes.
One more interesting finding from the project I have made is that, from the first 10 days to 30 days, I found that the ionic resistance of the coating with zinc oxide continue to increase significantly and will only drop slowly starting on the 40th day. Does anyone know how is this possible?
Thats all my question. Thank you.
ZnO will dissolve in low pH and high pH solutions. The solubility dip (minimum) is at about pH 9.5-10. It is known that chloride will attack ZnO and form Zn5(OH)8Cl2·H2O {Ref.1}. When you immerse the ZnO coating in sea water, ZnO will transform to Zn(OH)2 by the reaction ZnO + H2O = Zn(OH)2. Now you have a ZnO/Zn(OH)2 dual coating which increases the resistivity. With increase in the exposure time, the chloride concentration on the surface increases by adsorption. After reaching a critical chloride concentration, the reaction 5Zn(OH)2 + 2Cl- + H2O = Zn5(OH)8Cl2.H2O + 2OH- could occur {Ref. 1}. This locally increases the pH of the solution. This increase in the pH could cause dissolution of the ZnO and decrease the resistivity. This is what probably you see after 40 days of exposure.
Ref. 1. M. Mouanga, P. Berçot, J.Y. Rauch, Comparison of corrosion behaviour of zinc in NaCl and in NaOH solutions. Part I: corrosion layer characterization, Corrosion Sci. 52 (2010) 3984–3992.
When you refer to the ZnO nanoparticles or normal ZnO pigment particles used in anticorrosive coating application you have to consider that, when are in in contact with water and oxygen (some of the detrimental agent that trigger the corrosion of a metal) they act as a catalyst for the oxygen reduction and hence local pH shift to basic pH occurs in their surrounding environments. Then, the higher concentration of OH- cause the solubilization of Zn2+ cations that readily react with OH- ions forming insoluble Zinc hydroxides which increase the barrier properties of the coating against the corrosion by sealing the coating pinholes.
It is worth mentioning that ZnO, when exposed to the UV light, generates ion pairs electrons that migrate to the surface of the metal oxide and trigger the oxygen reduction.
When you refers to the “better structure” are you referring to the better barrier protection against the permeation?
Broadly speaking, the better barrier properties depend on the compatibility between the ZnO and the polymeric matrix; if it’s good, the diffusional path of the permeant molecules is more complex but on if it’s not, ZnO (as well as any other pigment) could generate higher density of pin hole and hence higher permeability). Same observation can be withdrawn for the mechanical properties.
The drops in the ionic resistance that you obtain after 40 days looks to me as the depletion in the inhibition of ZnO that naturally occur when the whole amount of ZnO is converted to the Zinc hydroxides species (similar result can be found in a previous work Chimenti, S.; Vega, J. M.; Aguirre, M.; García-Lecina, E.; Díez, J. A.; Grande, H.-J.; Paulis, M.; Leiza, J. R., Effective incorporation of ZnO nanoparticles by miniemulsion polymerization in waterborne binders for steel corrosion protection. Journal of Coatings Technology and Research 2017,14(4), 829-839)
Dear Dr. Wan Mohamad Ikhmal,
the corrosion rate of zinc in seawater is typically between 20 and 110 microm/yr, varying with location, lengh of exposure, type of zinc, etc.. It is generally higher at the beginning of the exposure and decrease with time. (Corrosion and Electrochemistry of Zinc Di Xiaoge Gregory Zhang (2013)).
Zinc coatings protect substrate against corrosion in two ways:
- By a barrier effect, i.e. they prevent oxygen and moisture from reaching the substrate surface.
- By giving cathodic protection at scratches, chips, edges etc. Zinc is a metal with a relatively low galvanic potential and a small tendency to corrode. However, the corrosion rate is low in most environments, since the surface of the coating is quickly covered with corrosion products, which subsequently protect against further corrosion.
The corrosion sequence in water is very complex and is influenced heavily by very small variations in the composition of the water. It is therefore difficult to give generally applicable rules. (http://www.nordicgalvanizers.com/foretag/Corrosion.htm)
I suggest you take a look at the following interesting paper:
Effect of zinc oxide in combating corrosion in zinc-rich primer
R.N. Jagtap, P.P. Patil, S.Z. Hassan
Progress in Organic Coatings 63 (2008) 389–394
With regard to the trend you have obtained from your tests (EIS, I think), zinc has probably carried out its anticorrosive power (protecting the substrate) better and better for up to 30 days. Only after 40 days of exposure, its protective power against the substrate began to decrease.
Best regards, Pierluigi Traverso
Thank you very very much to all researchers for taking your time to give me such a valuable informations. Im very grateful and appreciate all answers.
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