Abstract
The occurrence of corrosion nowadays is significantly increased sincecorrosivity of the environment due to air, soil and water pollution is particularly highcompared to the past. Additionally, due to the increased use of metals in the variousfields of technology, as well as the use of metal structures of more minute dimensionswhich cannot tolerate the corrosive attack to the same extent compared to the heavystructures used in the past, there is a pressing need to understand and reduce if noteliminate the phenomenon of corrosion. It has been estimated that the annual damageof iron and steel due to corrosion is the 1/4 and according to others is about the 1/3 ofthe world production. The damage could be reduced in a large amount (about 25%), ifthe current scientific knowledge in materials’ protection is properly exploited.The use of organic coatings is the most important method for protecting metalsurfaces against corrosion. Due to the anticorrosion protection they provide to steelstructu ...
The occurrence of corrosion nowadays is significantly increased sincecorrosivity of the environment due to air, soil and water pollution is particularly highcompared to the past. Additionally, due to the increased use of metals in the variousfields of technology, as well as the use of metal structures of more minute dimensionswhich cannot tolerate the corrosive attack to the same extent compared to the heavystructures used in the past, there is a pressing need to understand and reduce if noteliminate the phenomenon of corrosion. It has been estimated that the annual damageof iron and steel due to corrosion is the 1/4 and according to others is about the 1/3 ofthe world production. The damage could be reduced in a large amount (about 25%), ifthe current scientific knowledge in materials’ protection is properly exploited.The use of organic coatings is the most important method for protecting metalsurfaces against corrosion. Due to the anticorrosion protection they provide to steelstructures, increase their operating time, saving particularly significant amounts, whilecontribute to the improvement of safety of industrial equipment. The application ofsurface coatings is the most common method of protection against corrosion. Theprotective effect of various types of coatings is due to either the induced separation ofthe metal from the corrosive environment and therefore the increase of the electricalresistance or the alteration of the anodic or/and cathodic polarization. The use ofepoxy resins as binders in industrial coatings is highly increased over the recent years,since these resins provide the best possible combination of anticorrosion protectionand mechanical properties when they are copolymerized with other suitablecomponents, such as amines or polyamides. The increased demand for coatings withexcellent technical characteristics have led to the use of composite polymericcoatings, reinforced with fillers which exhibit improved anticorrosive and mechanicalproperties.The present study presents the effect of the use of quartz and feldspar of Greekorigin, as inorganic fillers, in the mechanical and anticorrosive properties of epoxycoatings for steel specimens. The quartz and feldspar were used because they arematerials with high Mohs’ hardness. The substrate consists of common steel plateswith dimensions 10 x 10 cm and 15 x 20 cm. The substrate was prepared by usingepoxy resin of bisphenol type A. A polyamide of high viscosity was used as ahardener. The resin/hardener ratio was 200/50. The system was used either in thisform as a reference system or by adding quartz or feldspar powder with a grain size of70 μm or quartz with a grain size of 40mm, at concentrations of 15, 30, 45 and 60 %wt. The characteristics of quartz and feldspar used as fillers were the following: a)crystalline milky quartz with hardness 7 on the Mohs’ scale and density 2.6 g/cm3, b)sodium feldspar with hardness 6.5 on the Mohs’ scale and density 3.1 g/cm3. The steelspecimens, after the appropriate pretreatment, were coated with coatings with dry filmthickness equal to 180 μm (+/-) 12%. The specimens were exposed in a corrosiveenvironment which was either a solution of NaCl 3.5% wt. or a solution of NaCl 5%wt., and the temperature was 32 degrees C (salt spray chamber). The reference specimensused were steel specimens coated with pure epoxy resin. The increase of the fillerconcentration shouldn’t exceed a certain value, due to reasons of workability of themixture viscosity and also because, over a critical value (critical pigment volumeconcentration), the anticorrosive properties of the coating may deteriorate due toinsufficient coverage of the grains by the epoxy matrix. For these reasons, at firstmeasurements were performed to determine the number of quartz dust oil with a grainsize of 40 μm and 70μm and feldspar with a grain size of 70 μm, and through thosethe critical pigment volume concentration was calculated and the pigment volumeconcentration was tested for the aforementioned concentrations.The anticorrosion behavior of the coatings with a grain size of 70 μm wasestimated:- Through Electrochemical Impedance Spectroscopy (EIS), a method which allowsthe understanding of the contribution of the individual components involved in thecorrosive process of composite systems, such as the coatings under consideration.- These results were correlated with those of the water permeability assessed bymeasurements on the prepared free films with the same composition as the coatingsunder consideration by using the CUP TEST method (ASTM E 96-90).- Through visual observations, in accordance with the ASTM D610-85 (degree ofcorrosion) and ASTM D714-87 (bubble classification) standards of the specimensafter the exposure of the coated specimens for certain periods in a solution of NaCl3.5% wt. and in a salt spray chamber.The anticorrosion behavior of coatings with a grain size of 40 μm wasestimated:- By measurements of the time variation of the corrosion potential.- By measuring the corrosion speed by using the method of linear polarization.- Through visual observations, according to the ASTM D610-85 (degree of corrosion)and ASTM D714-87 (bubble classification) standards of the specimens after theexposure of the coated specimens for certain periods in a salt spray chamber. Twotypes of specimens were used. Specimens with untreated coating and specimens withcross engraving in the coating surface.The mechanical behavior of the coatings with a grain size of 70 μm wasestimated:- By determining their surface hardness by using the method of engraving by pencilaccording to the ASTM D3363 standard.- By measuring the Vickers microhardness according to the ASTM E 384-99.- By measuring the Shore D volume hardness according to the standard ASTM D2240-97.- By measuring the elastic deformation of the coated surfaces by pendulum oscillationdamping under König according to the DIN-53157 standard.- By measuring the resistance to abrasion according to the ASTM D 968-81 standard(reapproved 1991).- By measuring the mechanical strength by using the incident weight method ISO6272: 1993 (E).The mechanical behavior of the coatings with a grain size of 40 μm wasestimated:- By determining their hardness by using the method of engraving by pencil accordingto the ASTM D3363 standard.- By measuring the elastic deformation of the coated surfaces by pendulum oscillationdamping under König according to the DIN-53157 standard.The effectiveness of the various coatings regarding corrosion reduction and theirmechanical properties was evaluated as follows:(1) By comparing the specimens with coatings containing quartz or feldspar atspecific concentrations (15% wt., 30% wt., 45% wt., 60% wt.) to the referencespecimens (specimens with pure epoxy resin coatings).(2) By comparing the specimens with coatings containing filler.From the aforementioned measurements and comparisons, it is concluded thatthe coatings containing quartz with a grain size of 70 μm at 15% wt. exhibit improvedmechanical properties compared to coatings with pure epoxy resin and improvedanticorrosion behavior, while the coatings containing feldspar with a grain size of 70μm at 15% wt. exhibit improved mechanical properties and similar anticorrosionbehavior compared to coatings with pure epoxy resin. Moreover, the fact that theintroduction of feldspar or quartz by reducing the percentage of organic componentsof the coating reduces its cost and makes it more environmentally friendly(compliance to ISO 14000), proves that the use of such ceramic materials as fillers inorganic coatings and in amounts up to 15% wt. provides an improved coatingcompared to that of the pure epoxy resin. Also concluded that in spite of its highercost, quartz with a grain size of 40 μm improved the anticorrosion behavior comparedto coatings with pure epoxy resin occurs at percentages up to 30% wt. This isattributed to the better dispersion of the grains of the filler in the epoxy matrix.
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