2 EXPERMENTAL. 2.1 Specimen Preparation. 2.2 Electrochemical Test PO-33.2

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1 SERVICE LIFE ASSESSMENT OF THE THERMAL SPRAY COATING APPLIED TO LNG VAPORIZER L'EVALUATION DE LA DUREE D USAGE DU REVETEMENT PAR PROJECTION THERMIQUE APPLIQUEE AU REGAZEIFIEUR DU GNL Yong-Cheol Kim, Ph.D. Senior Researcher Young-Geun Kim, Ph.D. Senior Researcher Youngtai Kho, Ph.D. Senior Researcher R&D Center Korea Gas Corporation 638-1, Ildong, Ansansi, Kyunggido South Korea, ABSTRACT Aluminum and zinc spray coatings are generally used for corrosion protection not only of large structures but also of automobiles and aircraft. The service life of thermal spray coating applied to LNG vaporizer was empirically measured and concurrently simulated by numerical analysis. A spray coating system, which was sprayed by aluminum zinc alloy on aluminum substrate and finally sealed by epoxy, was investigated. Its electrochemical properties were measured by various tests such as corrosion potential trends and polarization behavior. By the difference of the corrosion potential between the substrate and the coating layer, it is considered that the coating can cathodically polarize the substrate. After the considerable time the coating layer will be exhausted and be faced disbonding. In this work both qualitative and quantitative estimations of protective efficiency were carried out. With the distributions of the corrosion potential and the current density according to various disbonded area of the coating, the service life of the coating was assessed. These results were compared with the simulation data. RESUME Des dépôts aluminium et zinc sont généralement utilisés pour la protection de corrosion non seulement sur de grandes structures mais aussi sur les automobiles et les avions. La durée d usage du revêtement obtenu par la projection thermique et appliqué au regazéifieur du gaz naturel liquéfié (GNL) a été empiriquement mesurée et concurremment simulée par la méthode d analyse numérique. Un système de dépôt, qui a été déposé avec l'alliage d aluminium et de zinc sur le substrat en aluminium et finalement étanché par époxyde, a été étudié. Les propriétés électrochimiques ont été mesurées par les différents essais tels que les tendances du potentiel de corrosion et le comportement de polarization. A partir de la différence du potentiel de corrosion entre le substrat et la couche de dépôt, on considère que le dépôt peut protéger le substrat suffisamment et cathodiquement. Dans la proportion où le temps de service passe, la couche de dépôt sera épuisée et puis placée en face de son décollement. Dans ce travail les évaluations qualitatives et quantitatives de l'efficacité protectrice ont été effectuées. PO-33.1

2 Avec les distributions du potentiel de corrosion et de la densité de courant selon les différentes zones décollées du dépôt, la durée d usage du revêtement a été évaluée. Ces résultats ont été comparés avec les données obtenues par la simulation numérique. 1 INTRODUCTION Thermal spray coating of aluminum and/or zinc has been widely used on auto, plane, nuclear reactor and large scaled structures such as bridge [1-2] and vessel [3-4]. The structure studied was vertical heat exchanging panel, ORV(Open Rack Vaporizer) used for LNG (liquefied natural gas) vaporization. Heat source for the vaporization is seawater that flows from top of the panel to the bottom. The panel was made of Al-Mg alloy (AA 5083). The Al-Mg alloys corrode in pitting form in seawater. To protect the panel from pitting, Al-2%Zn alloy was flame-spray coated as sacrificial anode. The coating system is the effective protection method for the utility by the galvanic effect between the substrate and the coating. The Al-2%Zn coating layer has porous microstructure and corrodes rapidly in seawater. Usually, epoxy sealant was applied for maximum service life. As time goes by the utility operating the coating layer is generally and/or partially corroded and is ultimately occurred disbonding. Owing to these conditions of the coating layer its protective efficiency for the substrate is declined. According to various disbonded area of the coating the current density flow from the remaining coating to the substrate for protection was measured. With these data the residual service life was assessed. In company experimental measurement, numerical analytic investigation was carried out and these results were compared with the experimental data. 2 EXPERMENTAL 2.1 Specimen Preparation The specimen preparation used in this work consists of three steps, namely, blasting, metalizing and sealing. The surface roughness of the substrate plate ( mm) can be improved by utilizing the accelerated alumina grit. For metalizing of Al-2%Zn alloy the flame spraying method was employed. The coating thickness on the blasted surface was built up about 200 µm. The sealing process with epoxy can fill up the pores on the metalizing surface and in consequence the protective effectiveness of the coating can be enhanced. 2.2 Electrochemical Test In order to understand the electrochemical properties of the previously prepared specimen, the polarization curves were measured. A three-electrodes system of the saturated calomel electrode (SCE) for reference, graphite for counter electrode and the coating surface for working electrode was utilized. The measured surface area was 1 cm 2. The measurement was conducted in continuously air-blowed 3.5 wt% synthetic seawater. The both cathodic and anodic curves of the substrate were individually measured by potentiodynamical method with scanning rate 0.5 mv/sec from open circuit potential of Al (E corr (Al)) to 400 mv and +400 mv. The dominant anodic curve for the coating layer was measured with the same scanning rate ranged from 50 to +400 mv vs. E corr (Coating). PO-33.2

3 According to various disbonded area of the epoxy sealed spray coating the protective current density flow from remaining coating to substrate was measured. The scheme of the specimen is shown in Figure 1. Substrate (50~95%) Coating (50~5%) Substrate (50~95%) Coating (50~5%) (a) Figure 1. Specimen for the measurement of protective current owing to coating disbond: scheme for (a) the relative percentage of disbonded coating area (50~95 %), (b) the protective current flow from remaining coating to substrate The relative percentage of the coating disbond out of the total measured area (6 cm 2 ) was increased from 50 % to 95 % (Figure 1 (a)).)). The separated substrate and coating were immersed at 25 C in 3.5 wt% NaCl solution. The coating part for working electrode (WE) and the substrate for counter/reference electrode (CE/RE) were connected electrically. In this electric circuit the potential 0 V vs. RE was supplied potentiostatically and the current flow between WE and CE was recorded according to variation of the relative area. 2.3 Numerical Analysis Since the corrosion potential of the spray coating in seawater is several hundred mv negative compared to the substrate Al alloy, it plays a role of a sacrificial anode against the substrate. Developing into blister of the coating, the protective efficiency for the substrate by remaining coating can be declined. In this work a boundary element method (BEM) was used for the numerical analysis of the practical areas protected by coating. Since only the divided the boundary elements of a system are calculated by BEM, it has an advantage in reducing element numbers. The corrosion analysis with BEM method had been conducted by many researchers because of the speed and accuracy of the results[5-7]. For simulation commercial package (BEASY 7.0 Version, Computational Mechanics) was used. The disbonded area distributed at random sites with a random shape. The panel has corrugated surface to enhance heat exchange efficiency. As the thickness of electrolyte, seawater film, are rather uniform over the panel, we modeled the problem with simple flat geometry (Figure 2). (b) PO-33.3

4 Figure 2. Geometrical detail of the model for numerical analysis The thickness of the film is ignored as it is negligible compared to usual disbonded area size, several to hundreds mm. For the electrochemical properties of element surfaces (substrate and coating) the empirical polarization curves (Figure 3) was approximately divided with 3~4 straight sections. Both the anodic and cathodic curves and the conductivity of electrolyte (0.236 S/cm) were included by mat file into the Beasy program. 3 RESULTS AND DISCUSSION 3.1 Polarization Behavior By immersing a spray-coated specimen, a transitional behavior of its corrosion potentials was observed in the early stage of immersion. The time till the steady state of the potential was required about 67 hours for epoxy sealed coating. This is considered as the period for electrolyte permeation into irregular pores of a coating layer. For the electrochemical measurement of a porous surface such as thermal sprayed coating, it is necessary to be measured the electrochemical properties in steady-state region after a initial transition time by the trending of its corrosion potential. The measured polarization curve of two specimens (substrate and epoxy sealed coating) in 3.5 wt% NaCl is shown in Figure 3. Potential [V vs. SCE] Substrate(Al) Epoxy sealed coating Current density [A/cm 2 ] Figure 3. Polarization curves of thermal sprayed coating system in synthetic seawater (3.5 wt% NaCl); reference electrode saturated calomel electrode (SCE) PO-33.4

5 The aluminum substrate has the corrosion potential approximately 880 mv vs. SCE reference electrode and the current density of about 40 µa/cm 2. The corrosion potential of the epoxy sealed coating is measured a value about 230 mv more negative than that of the substrate. By this potential difference between the coating and the substrate, the coating layer can serve as a sacrificial anode and as a result of this characteristic the coating provides an extremely effective protection against substrate corrosion [8]. A new polarization behavior of the galvanic coupled materials is determined by the mixed potential theory [9]. 3.2 Calculation Results For the sake of qualitative evaluation for substrate protection due to coating disbond, a consumption rate of residual coating has to be considered in company with the variation of corrosion potential. A potential difference between substrate and coating acts as a driving force for a protective current flow from coating to substrate. The potential was maximum at the center of the defect as shown in Figure 4. Figure 4. Current profile (epoxy sealed coating, disbonded area 70%, sea water layer 5mm) The consuming current of the epoxy sealed coating due to 50 % coating disbond is below 17 µa/cm 2 (Figure 5). On the other hand the current density of 80 % disbonded coating is increased to 58 µa/cm 2. This value is three times larger than that of 50 % disbonding. With Faraday s law, the measured current densities can be converted in terms of corrosion rates 186 µm/year (by 50 % disbonding) and 635 µm/year (80 %). Generally considering on the base of the utilizing about 200 µm thickness, the residual coating in these cases can be exhausted wholly within a year. Therefore, according to the disbonding of coating over 50 %, the residual service life of the coating is limited within a year. Practically since the disbonding is localized, the actual life of the neighboring coating can be lengthened. Another possibility for explaining of the difference of the experimental and simulated values is existed. The utilized polarization curves of the substrate and the coating for simulation are measured potentiodynamically. By this method every points of the relation of potential and current density in polarization curve PO-33.5

6 can be measured in a short time dependent upon the scan rate. Namely, the measured current densities are possible to overestimate compared to current densities in stabilized moment. The experimental values are converted from the current densities that are recorded by a potentiostatic measurement for a long-term. Current density [A/cm 2 ] 1.8x x x x x x x x x experimental value simulated value Disbonded area [%] Figure 5. The changes of current density flow from coating to substrate for substrate protection due to coating disbond 3.3 Residual Service Life by Remaining Coating In order to assess the service life by remaining coating the consumption rates are calculated by previously explained method. Under the assumption that the residual coating thickness is 100 µm, the residual service life due to coating disbond can be converted. The results are shown in Figure 6. The dashed line represents a preparation time (about a half year) for remetalizing of the coating system. As a result of the experimental value, assuming that the coating is disbonded about 50 %, the remaining coating can provide sufficient protection more than 2 years. PO-33.6

7 Residual service life [yrs] Disbonded area [%] experimental value simulated value Figure 6. The estimation of residual service life of the remaining coating (about 100 µm; dashed line - preparation time for remetalizing of the coating system 4 CONCLUSIONS The electrochemical properties of the thermal spray coating (Al-2%Zn alloy) on aluminum alloy (AA 5083) were measured. From the results of polarization curves of the coating system and the difference in corrosion potential, it is considered that the substrate is successfully protected by the sacrificial action of the coating. It is shown that the permitted limit of the disbonded area for the protection of the spray coating could be estimated. With the increasing of the disbonded area of the coating, the current density flow from the coating to substrate for the substrate protection increased obviously. Assuming the 100 µm residual coating thickness and the 50 % coating disbond, the estimated residual service life of the coating is more than 2 years. 5 ACKNOWLEDGMENTS The authors would like to thank the Korea Gas Corporation (KOGAS) and the Korea Institute of Science & Technology Evaluation and Planning (KISTEP) for the funding of this work. 6 REFERENCES CITED [1] R.H. Unger, Thermal Spray: Advances in Coatings Technology, Orlando, Florida, USA, Sept. (1987), Pub: ASM International Materials Park, Ohio 44073, USA, p (1988) [2] K.J. Altorfer, Atmospheric Corrosion, Hollywood, Fla Oct. (1980), Pub: John Wiley and Sons, Inc. One Wiley, Dr. N.J. Somerset, p (1982) [3] D. Grasme, Thermal Spraying Conference (Thermische Spritz- konferenz), Aachen, Germany 3-5 Mar. (1993), Pub: Deutscher Verlag fuer Schweisstechnik DVS-Verlag GmbH., Aachener Str. 172, Postfach , Dusseldorf 1, Germany, p (1993) PO-33.7

8 [4] S. Kawahara and R. Sumida, R.Nippon Yosha Kyokai Shi (Journal of Japan Thermal Spraying Society) 31, (1) p , 31 Mar. (1994) [5] J. O. Dukovic and C. W. Tobias, J.Electrochem.Soc., p. 331 (1987) [6] N.G.Zamani,J.M.Chuang and C.C.Hsiung: Int. J. Numer. Methods Eng., 24, p (1987) [7] D. J. Danson, M. A. Warne,: Current density/voltage Calculations using Boundary Element Techniques, NACE Conference, Los Angeles, USA (1983) [8] K. Smolka, Welding and Cutting (Schweissen Schneiden), 8, p. E (1988) [9] M.G. Fontana, Corrosion Engineering, 3rd Ed., McGraw-Hill Book Company (1987) PO-33.8