Evaluation on through-porosity of HVOF sprayed coatings. Ermittlung durchgehender Porositäten an HVOF-Spritzschichten

Transcription

1 Evaluation on through-porosity of HVOF sprayed coatings J. Kawakita, S. Kuroda and T. Kodama, Tsukuba/J Ermittlung durchgehender Porositäten an HVOF-Spritzschichten Für im Offshore-Bereich eingesetzte Korrosionsschutzschichten ist es notwendig, dass diese gegenüber Meerwasser undurchlässig sind. Obgleich eine HVOF-gespritzte Hastelloy-C-Schicht nur einen geringen offenen Porositätsanteil aufweist, ist es wahrscheinlich, dass Seewasser durch diesen hindurch den Grundwerkstoff angreift. In der vorliegenden Untersuchung konnte mit Hilfe der ICP-Emissionsspektrometrie die Durchlässigkeit von Schichten gegenüber gelösten Substanzen in HCl-Umgebung bestimmt werden. Diese Methode erlaubt auch, die Korrosionsbeständigkeit von Schichten mit unterschiedlichen Eigenschaften festzustellen. Durch elektrochemische Messungen konnten die Ergebnisse der ICP-Emissionsspektrometrie im Hinblick auf das Korrosionspotenzial ergänzt werden. Die Wechselstromwiderstandsmethode erwies sich im Hinblick auf die selektive Bestimmung von Durchgangsporositäten als ungeeignet. Beim Einsatz des Detonationsspritzverfahrens konnte nach dem Versagen der Schicht beobachtet werden, dass im Fehlerfall, bedingt durch die größere Schichtdicke, größere Anteile der Schicht auf der Substratoberfläche zurückblieben. Die Schicht-Substrat-Schnittstelle war in diesem Fall infolge der höheren Auftreffgeschwindigkeit der keramischen Partikel stärker ausgeprägt. 1 Introduction The ultra steel research project started in 1995 at National Institute for Materials Science. In this project, one of the research subjects is improvement of the corrosion resistance of a marine structure by coating of anticorrosion materials with a thermal spraying technique. The expected services of the sprayed coating are field mending of a damaged area of clad steel and alternative use to clad steel. Both impermeability and high corrosion resistance are essential for this type of coating. Therefore, High Velocity Oxy-Fuel (HVOF) spray was adopted because of the deposition capability of a dense coating as well as the comparatively small change in properties of a sprayed particle upon the spray process. So far, the coatings of HVOF sprayed SUS316L stainless steel and HastelloyC nickel-base alloy have been investigated with respect to the relation between the spray condition and their properties such as the porosity, the oxygen content and the corrosion resistance [1-3]. If a coating has some through-porosity, seawater permeates there and reaches the interface between the coating and the substrate. When the conductive solution meets together the contact part of different kind of conductive materials, the galvanic cell is formed. A combination of the noble coating and the less-noble substrate accelerates corrosion of the substrate, compared to the free substrate with the same surface area. Our coating of HastelloyC prepared by HVOF spray technique had a small amount of open-porosity, which was under the limit of detection for the mercury porosimetry. Furthermore, this measuring method could not be applied to the coating, if attached to the substrate. In this study, the through-porosity of the HVOF sprayed HastelloyC coating on SS4 carbon steel was evaluated with respect to its dependence on the coating thickness and on the combustion pressure upon spraying. To estimate the amount of the throughporosity, the ICP emission spectroscopy analyzed dissolved substances, which penetrated by way of the through-porosity after elution from SS4 in the corrosion reaction during immersion in hydrochloric acid (HCl). The amount of the through-porosity by the chemical analysis was compared with the result of the electrochemical measurements. 2 Experimental The coating of HastelloyC was carried out by HVOF spraying with a TAFA apparatus (JP-5, Concord, NH, US). The primary spray condition recommended by the manufacturer was adopted. This condition and the properties of the powder of HastelloyC were presented in the literature [1]. In this study, varied parameters were the combustion pressure and the traverse number of the spray gun. The combustion pressures were.68 and.86 MPa, and the former was the value recommended by the manufacturer. The substrates of SS4 and HastelloyC276 were 2t 5 1 mm in size. Before spraying, they were blasted using alumina and degreased ultrasonically in acetone. The coatings with 4 kinds of coating thickness were obtained by changing the traverse number. Properties of the resulting coating were listed in Table 1. Table 1. Properties of HVOF sprayed HastelloyC coatings Substrate SS4 HastelloyC276 Coating thickness 5~4 µm 4 µm Openporosity <.1vol% Oxygen content HP: 1.1wt% SP:.63wt% Note that SP and HP stand for the coatings prepared under the spray conditions of the standard (.68 MPa) and higher (.86 MPa) combustion pressures, respectively.

2 The open-porosity and the oxygen content of the coating were determined by the mercury porosimetry and the inert-gas fusion method, respectively. The coated plate was cut into pieces in a size of 2.5 cm square and was cleaned ultrasonically in acetone and ion-exchanged water, repeatedly. The through-porosity of the HVOF sprayed coating was evaluated by the chemical analysis and the electrochemical measurements. The coated specimens and the blasted bulk plate of both SS4 and HastelloyC 276 were connected with a stainless rod and were used as the electrode. The electrode surface except 2 cm 2 of sprayed face was covered with the silicon resin. This electrode was immersed at 3K in.5 mol dm -3 of HCl, which was adopted to avoid filling up the through-porosity by solid corrosion products and to include chloride ion with the same concentration as in seawater. At every predetermined time, 5 ml of test solution was sampled. The amounts of both iron (Fe) and nickel (Ni) elements dissolved in the sampled solutions were determined by the ICP emission spectrometry using the analyzer (SPS 3, Seiko Instruments Inc., Chiba, JPN). These elements were basis metals of SS4 (Fe) and of HastelloyC and HastelloyC276 (Ni), respectively. In addition, the corrosion potential of the electrode and its polarization resistance, R P, were obtained. The latter indicates the degree of corrosion resistance of the specimen, and was measured by the alternating current (a.c.) impedance method using the corrosion monitor (Model CT-5, Riken Denshi, Tokyo, JPN). The electrode potential was referred to the standard potential of the Ag/AgCl electrode in saturated KCl solution. 3 Results and Discussion 3.1 Dependence of through-porosity on coating thickness The dissolution of Fe element was caused mainly by permeation of Fe ion eluted from the SS4 substrate upon corrosion reaction by way of the through porosity of the HastelloyC coating. Accordingly, its amount corresponded to that of the through-porosity. The effect of the coating thickness on the through-porosity is shown in Fig. 1. The dissolved Fe amount is represented per geometric surface area of the exposed sprayed face. The curves of the HP and SP coatings with 5 µm in thickness are almost consistent with that of the bulk SS4 plate. Form this result, these coatings had numerous through-porosities and did not act as a shield. On the other hand, the Fe amounts of the HP (2 and 4 µmt) and the SP (4 µmt) coatings were smaller by about three orders of magnitude than those of the HP and SP ones (5 µmt). This indicated a significant decrease in through-porosity of these coatings and their sufficient shielding effect. The Fe amounts of the HP (1 µmt) and the SP (1 and 2 µmt) coatings are observed between two levels, as mentioned above. This result indicated that these coatings had the through-porosity to some extent. In comparison of the coatings with the same thickness, especially for 1 and 2 µm, the HP coating had a smaller Fe amount than the SP one, indicating a further closely packed structure of the HP coating than the SP one. If the through-porosity of the HastelloyC coating decreases, corrosion of the SS4 substrate and permeation of the dissolved Fe element are suppressed while corrosion of the coating is surpassed. Hence, the higher corrosion amount of the coating corre Elution amount (Fe) / g cm Elution amount (Ni) / g cm Fig. 1. Variation in amount of dissolved Fe element from HVOF sprayed HastelloyC coating with time immersed in.5m HCl at 3K. Full and empty circles represent HP and SP coatings. Cross mark is SS4 bulk plate. Coating thickness: ; 5, ; 1, ; 2, and ; 4 µm Fig. 2. Variation in amount of dissolved Ni element from HVOF sprayed HastelloyC coating with time immersed in.5m HCl at 3K. Full and empty circles represent HP and SP coatings. Cross mark is SS4 bulk plate. Coating thickness: ; 5, ; 1, ; 2, and ; 4 µm.

3 sponds to the lower amount of the through-porosity. Therefore, the amount of the dissolved Ni element can be also used for evaluation on the through-porosity of the coating. As seen in Fig. 2, the result of the chemical analysis for Ni confirmed the effect of the coating thickness on the through-porosity, as mentioned concerning Fe. As for the gap between the maximum and the minimum of the dissolved amount, however, Ni was smaller than Fe because not a small amount of Ni was contained as an alloying element in SS4, as observed in the curve for SS4 bulk plate. The corrosion potential of the specimen is often used for evaluation of its corrosion resistance. In the case of Potential / mv vs. Ag/AgCl Fig. 3. Variation in corrosion potential of HVOF sprayed HastelloyC coating with time immersed in.5m HCl at 3K. Full and empty circles represent HP and SP coatings. Cross mark is SS4 bulk plate. Coating thickness: ; 5, ; 1, ; 2, and ; 4 µm. (R P -1 ) dt / Ω -1 s cm Fig. 4. Variation in integrated reciprocal of polarization resistance of HVOF sprayed HastelloyC coating between times immersed in.5m HCl at 3K. Full and empty circles represent HP and SP coatings. Cross mark is SS4 bulk plate. Coating thickness: ; 5, ; 1, ; 2, and ; 4 µm. the coated specimen, the abrupt change in potential suggests generation or increase of the throughporosity. As shown in Fig. 3, the potential curve of the specimen except the SS4 bulk plate corresponds to the behaviors observed in the results of the chemical analyses, i.e. the classification of the dissolution amount into three levels by the coating thickness and the abrupt change in dissolution amount at certain times. SS4 bulk plate should be excluded in this interpretation because it was not contacted with the HastelloyC coating as a different kind of conductive material. However, the potential value of the specimen is qualitative and does not indicate the true amount of corrosion. The polarization resistance, R P, indicates the degree of corrosion resistance of the specimen. When the reciprocal of R P is related with the corrosion rate, the integral of R -1 P between the immersion periods correspond to the corrosion amount. As shown in Fig. 4, the curve of the integral indicates the classification of the through-porosity by the coating thickness and the rapid increase of the through-porosity, which was found in the chemical analyses. In this case, however, the integral value indicated the whole corrosion amount of coated specimen. If there exists only a slight amount of through-porosity, each corrosion amount of the coating and substrate was unable to be divided. Elution amount (Fe) / µg cm Fig. 5. Variation in amount of dissolved Fe element from HVOF sprayed HastelloyC coating (4 µmt) with time immersed in.5m HCl at 3K. Full and empty circles represent HP and SP coatings. Rhombus mark is HastelloyC 276 bulk plate. Solid and dotted lines were SS4 and HastelloyC276 as substrate. 3.2 Effect of substrate on detection of through-porosity The comparison of the substrates revealed the corrosion process of the coated specimen at the presence of a small amount of through-porosity in addition to the

4 difference in corrosion resistance between the coatings under the different spray condition. As shown in Fig. 5, the coatings on the SS4 substrate had a larger amount of dissolved Fe element than those on the HastelloyC276 substrate, indicating occurrence of corrosion of SS4 and permeation of eluted Fe ion by way of the through-porosity. From this result, there exists the through-porosity even with 4 µm in thickness of the coating. As for the SP coating on SS4, the rapid increase in dissolution amount over 3 hours indicates the increase of the through-porosity of the Elution amount (Ni) / µg cm coating. In terms of the dissolution amount of Fe, the coatings on HastelloyC276 were larger than the HastelloyC276 bulk plate, and the HP coating was larger than the SP one. This was due to the poor corrosion resistance of the coating caused by deterioration of the sprayed particle upon the depositing process. In this case, the dissolved Fe resulted from an alloying element of the coating. The corrosion resistance of the coating was comparable to the dissolution amount of Ni. As shown in Fig. 6, there is obvious difference in corrosion resistance among the coatings on HastelloyC276 and the HastelloyC276 bulk plate, compared with the result concerning Fe. The coatings on SS4 had a smaller amount of dissolved Ni than those on HastelloyC276. This was because corrosion of the coating was suppressed by acting of the SS 4 substrate as a sacrificial anode. The corrosion potentials of the specimens revealed their relative order of corrosion amount, depending on the substrate and on the properties of the coating, as shown in Fig. 7. The order of the corrosion potential corresponds to the result of the chemical analyses for Fe and Ni. The corrosion potential, however, did not describe the absolute relation of the corrosion amount between the specimens, as mentioned above Fig. 6. Variation in amount of dissolved Ni element from HVOF sprayed HastelloyC coating (4 µmt) with time immersed in.5m HCl at 3K. Full and empty circles represent HP and SP coatings. Rhombus mark is HastelloyC 276 bulk plate. Solid and dotted lines were SS4 and HastelloyC276 as substrate. (R P -1 ) dt / Ω -1 s cm Potential / mv vs. Ag/AgCl Fig. 8. Variation in integrated reciprocal of polarization resistance of HVOF sprayed HastelloyC coating (4 µmt) between times immersed in.5m HCl at 3K. Full and empty circles represent HP and SP coatings. Rhombus mark is HastelloyC 276 bulk plate. Solid and dotted lines were SS4 and HastelloyC276 as substrate Fig. 7. Variation in corrosion potential of HVOF sprayed HastelloyC coating (4 µmt) with time immersed in.5m HCl at 3K. Full and empty circles represent HP and SP coatings. Rhombus mark is HastelloyC 276 bulk plate. Solid and dotted lines were SS4 and HastelloyC276 as substrate. The integral of R P -1 of the specimen between the immersion periods corresponded to its corrosion amount, which included corrosion of both the substrate and the coating. Accordingly, the integral values seemed to be equivalent to the total dissolved amount of Fe and Ni obtained from the chemical analyses, as shown in Fig. 8. The rapid increase, observed in the dissolution curve of Fe for the SP coating on SS4 over 3 hours, was not observed clearly in the corresponding curve of the integral. This is because the increase in

5 corrosion amount of the less noble substrate together with enlargement of the through-porosity gave the suppression of corrosion of the noble coating. Accordingly, the change in corrosion rate was too small to detect. 5 Acknowledgement We are grateful to Dr. Y. Kobayashi for kindly providing a chance to use the analyzer for the ICP emission spectrometry. 4 Conclusions The through-porosity of the coating for HVOF sprayed HastelloyC was evaluated by the chemical analysis of the dissolved substances permeating by way of the through-porosity of the coating using the ICP emission spectrometry. The amount of the through-porosity depended on the thickness and the properties of the coating. The coating even with 4 µm in thickness had a slight amount of through-porosity, which could not be detected by the mercury porosimetry. This chemical analysis discriminated the substrate from the coating in terms of the corrosion amount. This fact brought about the precise determination of the corrosion resistance of the coatings prepared under the different spray conditions. From the results of the electrochemical measurements, the corrosion potential explained well the relative order of amount of the through-porosity and its abrupt increase. The corrosion amount assumed from the polarization resistance contained corrosion of both the substrate and the coating, and hence was not suited for evaluating the amount of the throughporosity. 6 Literature [1] Kuroda, S., T. Fukushima, M. Sasaki and T. Kodama: Microstructure and corrosion resistance of HVOF sprayed 316L stainless steel and Ni base alloy coatings. Proc. ITSC 2, Thermal Spray: Surface Engineering via Applied Research, C.C. Berndt, Ed., ASM International, pp.455/462. [2] Fukushima, T. and S. Kuroda: Oxidation of HVOF sprayed alloy coatings and its control by a gas shroud. Proc. ITSC 21, New Surfaces for a New Millennium, C.C. Berndt, Ed., ASM International, pp.527/532. [3] Kawakita, J., T. Fukushima, S. Kuroda, and T. Kodama: Corrosion behaviour of HVOF sprayed coatings in seawater. Proc. ITSC 21, New Surfaces for a New Millennium, C.C. Berndt, Ed., ASM International, pp.1137/1142.