STUDY OF INFLUENCE CORROSIVE ENVIRONMENT ON CHARACTERISTICS PROTECTIVE COATINGS USED FOR LONG-TERM CORROSION PROTECTION OF STEEL SUBSTRATE Jitka PODJUKLOVÁ a,kateřina SUCHÁNKOVÁ b, Petr ŠRUBAŘ c,sylvie KOPAŇAKOVÁ d, Kamila HRABOVSKÁ e VŠB TU Ostrava, Ostrava, Čzech Republic, EU a jitka.podjuklova@vsb.cz, b katerina.suchankova@vsb.cz, c petr.srubar@vsb.cz, sylvie.kopanakova@vsb.cz, e kamila.hrabovska@vsb.cz Abstract Corrosive environment has a major impact on the life of steel structures (bridge structures, production shops, look-out towers, etc.), which constitute an integral part of civil engineering. The basic building material for these structures in most cases is steel, which unfortunately gradually degrades in contact with the surrounding atmosphere. This negative phenomenon can be avoided by the application of corrosion protection, most frequently by organic coatings. They are an indispensable part of surface finishes and great emphasis is placed on their quality. The choice of the protective coating system depends primarily on the expected or planned lifetime of corrosion protection. If we focus on the long-term corrosion protection, especially of bridge structures, we need to carefully consider the preliminary surface treatment. The most frequently used technology amongst numerous methods of surface pretreatment of bridge structures is blasting, by means of which it is possible to achieve the required surface quality. This means, in addition to cleaning of the surface of the base material, also the achievement of suitable micro-geometry and physical, or physical and chemical properties of the surface. The paper focuses on experimental verification of coating systems intended for application on steel structures with long-term exposure in corrosive environments. Samples blasted to surface preparation degree of Sa 2 with applied coating systems were exposed to artificial salt spray atmosphere and a corrosive solution, which is used for testing by the so-called accelerated immersion test. Following was the evaluation of degradation and adhesion of the tested coating systems. The experiment was concluded by ascertaining of the content of volatile organic compounds in paint matters using gravimetric method. Keywords: coating system, surface finish, steel structure 1. INTRODUCTION Degradation of steel structures can be prevented by protecting the surface of the base material. Quality of the surface finish depends on several basic factors. It is primarily the very surface treatment prior to paint matter application, properties of the paint matter used, the quality of the work performed, and thus compliance with technological and application conditions according to the paint manufacturer. Pretreatment of steel structures is realized mainly by blasting, in which material surface is cleaned from any dirt like scales, rust or other impurities. Blasting must be followed in the shortest period of time possible by the application of corrosion protection itself, since the surface after blasting is responsive and oxide layers are formed with time.
2. EXPERIMENTAL MATERIAL The experimental part of the thesis is based on samples of material KOSMALT - E 300T with dimensions 150 x 100 x 3 mm, whose samples were blasted with steel grit. These samples were supplied by VÍTKOVICE POWER ENGINEERING a.s. These are hot-rolled metal sheets, which is suitable for cold pressing. 1 Mechanical properties of steel KOSMALT - E 300T Element C Mn Si P S Al N Ti Content[%] 0,07 0,21 0,03 0,013 0,01 0,042 0,006 0,06 2 Chemical composition of steel KOSMALT - E 300T Yield strengthre [MPa] 300 450 Tensile strength[mpa] 350 550 Ductility A 5 [%] 28 Fig.1 Micrograph of the blasted surface (200x magnification, photographed with the NEOPHOT 2 microscope The reference samples were subject to surface roughness measurement according to EN ISO 4287. Evaluation of the substrate roughness was performed by means of MitutoyoSurftest SJ 301. The principle of measurement is based on scanning of the surface profile of the underlying material with a diamond stylus tip. The device setting was the following: profile filter λ c = 8 mm; measured length l = 40 mm, number of measurements 10. Arithmetical mean deviation of the considered profile Ra = 13.81 micron, the highest profile height Rz = 95.11 micron. Measured values are appropriate for the blasted surface. 3. EXPERIMENTAL COATING SYSTEMS Experimental works were performed with two types of paints REMOPLAST EP ZINK - this is a two-component epoxy primer containing zinc powder mixed with hardener based on polyamide, which is produced by Rembrandtins.r.o. The main property of coating is cathodic protection of steel surfaces, resistance to chemical, mechanical and climatic stress. REMOPLAST HS - KST FAST - is a two component epoxy paint with a high dry matter and low VOC content, which is produced by Rembrandtins.r.o. The paint contains anticorrosion lamellar pigments and is intended for cover applications on steel substrates.
3.1. Paint application The paint matters REMOPLAST EP ZINK and REMOPLAST HS - KST FAST were applied using pneumatic spraying under a pressure of 3 bar with output nozzle diameter of 1.8 mm. According to the technical data sheet of REMOPLAST EP ZINK, the substrate was covered with wet film of a thickness of 70 micron and the REMOPLAST HS - KST FAST paint matter was applied in a wet film thickness of 110 micron, the application was performed in two layers. The measured dry film thickness of REMOPLAST EP ZINK coating was on average about 70 micron and that of REMOPLAST HS - KST FAST was on average 230 micron. 4. EXPERIMENTAL TESTS 4.1. Pull-off adhesion test according to ČSN EN ISO 16 276-1 Pull-off adhesion test was performed before the exposure of samples to corrosive chamber and after 720 h of exposure. The measured values of of pull-off strength are recorded and compared in the chart No.1. Chart 1 Chart of comparison of reference samples and the samples before and after the corrosion chamber exposure, and before and after the immersion exposure test It is generally stated that the threshold value for assessing the good adhesion of the coating system is 3 Mpa. The chart No. 1 clearly shows that the average pull-off strength in the two coating systems after exposure in corrosive chamber dropped down to the threshold value of 3 Mpa. This decrease could be caused e.g. by residues of blasting media, which manifested only after exposure. The results of characteristic fractures show a very poor adhesion of the priming matter, since in all cases there was a 100% adhesion damage between the substrate and the first applied layer. The results of the pull-off strength after immersion test are similar to the average pull-off strength after the corrosion test. Concurrently there was a decrease in the average pull-off strength below the limit value of 3 MPa in the priming pant matter REMOPLAST EP ZINK, which indicates poor adhesion of the substrate and proves 100% adhesion damage between the substrate and the first applied layer. 4.2. Cross-hatch adhesion test according to ČSN EN ISO 16 276-2 Cross-hatch adhesion test was performed before the exposure of samples to corrosive chamber and after 720 h of exposure. The substrate materials with applied REMOPLAST EP ZINK paint matter were subjected to cross-hatch test by a cutting tool with a spacing of 2 mm of the guide template. The guide template spacing in EP REMOPLAST ZINK REMOPLAST + HS - KST FAST system was 3 mm.
Chart 2 Chart of comparison of reference samples and the samples before and after the corrosion chamber exposure, and before and after the immersion exposure test The chart 2 clearly implies that after 720 h of exposure in the corrosion chamber the primer REMOPLAST EP ZINK showed average damage of the grid area of more than 5% but less than 15 %. The average damage of the paint matter REMOPLAST EP ZINK + REMOPLAST HS KST FAST, to the grid area exceeded even 65%. The exposure of the sample in the immersion test with EP REMOPLAST ZINK primer did not cause any substantial change in adhesion and the damage to the grid area is within 5 %. However, in the average damage of the grid area in REMOPLAST EP ZINK + REMOPLAST HS KST FAST system exceeded 65%. 4.3. Corrosion test in artificial atmosphere according to ČSN EN ISO 9227 The duration of exposure in corrosion chamber was at most 720 hours. During the test we carried out photographic documentation and evaluation of changes to the coating system in the interval of 0 h, 24 h, 48 h, 96 h, 120 h, 240 h, 720 h 480 h. The evaluation of the degree of blistering was carried out according to ČSN EN ISO 4628-2, evaluation of the degree of rust-perforation according to ČSN EN ISO 4628-3 and the evaluation of the degree of delamination and corrosion around the section according to ČSN EN ISO 4628-8. Table 3 Evaluation of the coating system REMOPLAST EP ZINK (Des. REZ) and REMOPLAST HS KS FAST(Des. RHK) during the corrosion tests in artificial atmosphere It is evident from the Table No. 3 that coating systems showed a very good corrosion resistance when exposed to corrosive agents in the corrosion chamber. The coating REMOPLAST EP ZINK, showed the first blisters already after 24 h exposure in the corrosion chamber. There was almost an insignificant number of defects resembling blisters of a size of 0.5 to 5 mm, which did not extent under following exposure, and there was no rust penetration.
Fig.2 Photos of samples before exposure in corrosion chamber and after 720 h exposure in paint matter REMOPLAST EP ZINK (desig. ZK8) and the samples of paint matter REMOPLAST EP ZINK + REMOPLAST HS KST FAST (desig. MV8) The REMOPLAST EP ZINK + REMOPLAST HS KST FAST system was damaged in the form of the first type of defects (blisters) after 48 h exposure in the corrosion chamber. The amount of defects was very small, almost insignificant, with a size greater than 5 mm. Subsequent exposure caused no further extension defects and rust penetration of the coating. 4.4. Accelerated submersion test according to the ČSN 67 3087 Accelerated submersion test was carried out in three cycles. Each cycle was followed by a visual evaluation. Table 4 EvaluationofthecoatingsystemREMOPLAST EP ZINK (desig. REZ) and REMOPLAST HS KST FAST(Des. RHK)duringthesubmersion test Exposure in a salt bath 0. Cycle 1. Cycle 2. Cycle 3. Cycle Coating System marking Degree of blistering Degree of rusting Delamination/Corrosion along the cut Blistering aroundt he cut REZ 0 (S0) Ri 0 (S0) 0/0 0 (S0) REZ + RHK 0 (S0) Ri 0 (S0) 0/0 0 (S0) REZ 1 (S4) Ri 0 (S0) 0/1 0 (S0) REZ + RHK 0 (S0) Ri 0 (S0) 0/1 0 (S0) REZ 1 (S4) Ri 0 (S0) 1/1 1 (S4) REZ + RHK 1 (S4) Ri 0 (S0) 0/0 0 (S0) REZ 1 (S4) Ri 0 (S0) 1/1 1 (S4) REZ + RHK 1 (S5) Ri 0 (S0) 0/0 0 (S0) Fig 3 Photos of samples before exposure to accelerated submersion test and after the third cycle in the paint matter REMOPLAST EP ZINK (desig.zk3) and the samples of paint matter REMOPLAST EP ZINK + REMOPLAST HS KST FAST (desig.mv3 The primer coating system REMOPLAST EP ZINK showed slightly worse results than the system designated as REMOPLAST EP ZINK + REMOPLAST HS KST FAST. The primer coating system presented a very small, almost insignificant number of defects resembling blisters with a size of 0.5 to 5 mm
already after 1 st Cycle of exposure in the submersion test. These defects did not extend and the coating was not penetrated by rust. The REMOPLAST EP ZINK + REMOPLAST HS KST FAST system was damaged in the form of the first type of defects (blisters) after the second cycle of the submersion test. The amount of blistering ranging from 0.5 to 5 mm was almost negligible. Further exposure within the immersion test caused blistering growth over 5 mm. 5. DETERMINATION OF VOLATILE ORGANIC COMPOUNDS USING GRAVIMETRIC METHOD To determine the content of volatile organic compounds we used gravimetric method with time intervals of 15 min, followed by measurement of weight loss of the coating systems during the curing process. This method is used determine the content of volatile organic compounds [g l-1], density [g cm-3] and dry matter [% by wt.] of the coating systems. 6. CONCLUSION Chart 3 Comparison of paint matter evaporation curves The corrosion test in the salt spray artificial atmosphere showed that after 720 hours of exposure the coating systems showed good corrosion resistance, because in the course of the test only a very small, almost insignificant number of defects resembling blisters of the size of 0.5 to 5 mm occurred. From the point of view of adhesion the coating systems showed a significant decrease to the average value of pull-off strength of 3 MPa, the characteristic fracture results indicate very poor adhesion of primer, since in all cases there was a 100% damage to adhesion between the substrate and the first applied layer. The cross-hatch adhesion tests showed a very good performance of the REMOPLAST EP ZINK coating system, where average damage of the grid area was higher than 5% but less than 15%. Submersion test pointed to very similar results, which were achieved after corrosion test in artificial salt spray atmosphere. Pull-off adhesion test pointed to a low value of pull-off strength, as there was a 100% adhesion damage between the substrate and the first layer applied. Cross-hatch adhesion test of the coating system REMOPLAST EP ZINK pointed out a better adhesion (damage to the grid area of up to 5%) than in the REMOPLAST EP ZINK + REMOPLAST HS KST FAST system. Overall, it can be stated that the coating systems showed very good corrosion resistance, but the adhesion decreased with the time o exposure. AKCNOWLEDGEMENTS This contribution was created under the support of the Czech Ministry of Education, Youth and Sport KONTAKT ME 08083.
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