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A Brief Analysis of Graphene Modified Anticorrosive Coating

浏览:318 发表时间:2018-11-25 16:34:21 来源:Graphene League

Preface:


Graphene as a new nanometer material and has a good barrier properties and shielding performance, but also with high conductivity, high strength. Combining graphene materials with traditional anticorrosion coating, graphene modified with anticorrosion has advantages of good effect of anticorrosion, low coating thickness, high adhesion, paint film is light weight, excellent resistance to salt fog performance. It is a good upgrade replacement of traditional anticorrosive coatings.


Corrosion has been one of the biggest challenges facing steel, metallurgy, construction, transportation and other industries. According to statistics, the direct economic loss caused by corrosion worldwide is about 2.5 trillion us dollars every year [1]. According to the statistics of the major consulting project "China's corrosion status and control strategy research", led by academician Hou baorong, the total cost of corrosion in China accounted for about 3.34% of the GDP of the year in 2014, totaling over 210 billion yuan, equivalent to the cost of corrosion of 1,555 yuan per Chinese person in the year.


The main components of anticorrosive coatings include film-forming substances, pigments, solvents and additives. Film-forming substances are the main substances that make the coating firmly adhere to the surface of the coating to form a continuous film, which is the basis of the coating and plays a decisive role in the physical and chemical properties of the coating. Pigment is an auxiliary film-forming material, giving paint a variety of colors, decorative effects on the object, the coating film rust, sun resistance, water resistance to chemicals play an important role; The function of the solvent [2] is to dissolve and disperse other components in the formula, which has a great effect on the properties of the coating. Additives [2] are small amounts of additives added to the coating to improve a certain performance of the coating and solve some technical problems, generally will not improve the anti-corrosion performance of the coating.


At present, anticorrosive coatings are generally divided into conventional anticorrosive coatings and heavy anticorrosive coatings, is an essential coating in paint coatings. Conventional anticorrosive coating is in the general conditions, such as the role of metal corrosion, protect the service life of non-ferrous metals; Heavy anticorrosive coating is a kind of anticorrosive coating that can be applied in a relatively harsh corrosive environment and have a longer protection period than conventional anticorrosive coating. [3]


 Graphene as a new nanometer material and has a good barrier properties and shielding performance, but also with high conductivity, high strength. Combining graphene materials with traditional anticorrosion coating, graphene modified with anticorrosion has advantages of good effect of anticorrosion, low coating thickness, high adhesion, paint film is light weight, excellent resistance to salt fog performance. It is a good upgrade replacement of traditional anticorrosive coatings. At the same time, the cost increase after adding graphene is far less than the anti-corrosion benefit brought by it. Therefore, graphene is expected to be a good upgrade alternative to traditional anti-corrosion coatings.


I. mechanism of graphene antisepsis


(1) graphene has good physical barrier to small molecules.





                                                                           FIG. 1 comparison of permeation results of GO and rGO films on water molecules, organic small molecules and gas molecules [4]

(2) extend the diffusion path of the corrosive medium: the diffusion path of the corrosive medium is extended and the service life of the matrix is increased.



                                                                           Figure 2 comparison of anticorrosive mechanism between ordinary epoxy zinc-rich coating and graphene-modified anticorrosive coating [5]

(3) an effective micro-conductive circulation path is formed with the zinc powder to effectively ensure that the zinc powder plays a protective role as an anode.


 Figure 3 action mechanism diagram of epoxy zinc rich coating [6]


FIG. 4 action mechanism of graphene-modified zinc powder coating [6]

II. Preparation of graphene-modified anticorrosive coating


(1) the prepared graphene was dispersed into the anticorrosive coating, with poor dispersion, poor interface bonding and easy operation.

(2) CVD directly deposits graphene on the protective matrix or deposits it on the base and then transfers it, with the following characteristics: small size, high cost, difficult transfer and difficult large-scale application.

(3) graphene was compounded with some polymer resins or other functional nanomaterials, and then added to the anticorrosive coating. Characteristics: improved dispersion and interface.


3. Main technical difficulties


III. Main technical difficulties

(1) dispersion of graphene. The preparation technology of graphene is not perfect and has many defects. It contains some functional groups and has high polarity. Therefore, it is easy to agglomerate and cannot effectively block the corrosive medium. The interface between graphene and coating components. Graphene has strong hydrophobicity and poor wettability compared with many substrates for coating preparation. Therefore, it is difficult to disperse evenly in anticorrosive coatings.


(2) the control of graphene addition is crucial. A small amount of graphene can optimize the resin assembly structure, improve the degree of order, and play an effective anti-corrosion role. The addition of excessive graphene will accelerate the matrix corrosion.

  FIG. 5 SEM comparison between graphene modified anticorrosive coating and ordinary epoxy zinc-rich coating [5]


Zhao has research on graphenes effect on resin assembly, as Fig 5, the cross section of Solvent epoxy resin with addition of graphene or without addition of graphene, because of graphene has adsorption toward polymers, can  form polymers wrapping graphene structure in samples with graphene addition, which makes the resin molecular when assembled into polymer materials is more orderly. Resin after curing also is more tighter and less defective than the test samples without addition of graphene



 Figure 6 influence of graphene dosage on the salt spray resistance of the film [6]


Sun etc. Research has shown that a small amount of the addition of graphene with zinc powder can form the effective micro conductive circulation path so as to effectively guarantee the zinc powder as anode protection, and the addition of graphene can lead to excessive micro conductive path formed between graphene itself, makes the corrosive medium forms a direct path between the substrate and the outside world, it will accelerate corrosion matrix.




(3) the electrical coupling between graphene and metal accelerates the corrosion at the graphene-metal interface.




Professor Alex Zettl of the university of California, Berkeley, has conducted a comprehensive study on the short-term and long-term properties of copper and silica based mtene coatings, showing that while graphene does provide effective short-term oxidative protection, it promotes more extensive corrosion over a longer time scale than initially exposed, unprotected Cu surfaces.


                                                                FIG. 7 comparison of graphene coating and bare copper corrosion and schematic diagram of copper surface corrosion [7]


 

Huang jiaxing from northwestern university, USA, while affirming the excellent barrier property of graphene, also emphasized from the perspective of electrochemical potential that it is negative to most metals and can promote the corrosion of the exposed graphene-metal interface. This can accelerate dangerous localized corrosion and severely weaken the coated metal. Any slight crack or scratch in the coating will accelerate the local electrochemical corrosion, greatly accelerate the corrosion rate in the exposed area, and reduce the strength and toughness of the metal and other properties. He suggests that insulation, such as polymers, could be inserted to cut the coupling between the current graphene and the metal. Graphene-polymer composite coatings are more durable, easier to apply and more scratch resistant. The coating should have well-dispersed graphene flakes to ensure low overall permeability to gas or liquid.



                                                                        FIG. 8 comparison of graphene coating and bare copper corrosion and schematic diagram of copper surface corrosion [7]



 

In addition, it is also necessary to consider how to establish a sound evaluation method, investigate the structure-effect relationship between the structure, properties, dosage and dispersion of graphene and coating protection performance, and clarify its mechanism of action.


Iv. Current solutions


For the existing technical difficulties, the current solutions are as follows :(1) improve the quality of graphene, reduce defects and reduce costs; (2) controlling graphene content to form microcircuits rather than electrical coupling; (3) graphene was modified to improve the dispersion of graphene in anticorrosive coatings; (4) improve the coating strength, adhesion, not easily damaged by scratch.


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