Long-term protection of metallic parts from atmospheric exposure is an important challenge and one of the most frequent way to ensure their corrosion resistance is the application of sacrificial zinc alloys coatings [1]. Among the various zinc electroplating baths, the alkaline zincate solutions has gained a wide range of applications thanks to its simple bath composition, good dispersion and efficient coverage ability. Nevertheless, its efficiency is very low in absence of additives, which affects the performances of the coatings by modifying the crystal growth and therefore the structural and mechanical properties. This is a result from the competition between dihydrogen production and the reduction of the metallic species present in the bath.The present study deals with the optimization of ZnFe sacrificial coating on steel and aluminum substrate in the frame of the ATLAS project -Alternative TechnoLogies for improved Anticorrosion Solutions- managed by the IRT M2P. After the analysis of the electrochemical behavior of the ZnFe electrolyte patented by the Coventya company and the UTINAM institute, pulsed currents will be implemented to extend the versatility of the coatings. Indeed, pulsed currents are known to act on various parameters such as improving the faradic yield or modifying morphologies and structures [2].The determination of a first panel of pulsed sequences (average current densities, cathodic peak current, cathodic pulse time and off-time) have been based on transient curves analysis from a zincate bath. Three sequences have been selected as they resulted in very different patterns in terms of electrochemical behavior and coatings characteristics. These sequences have been replicated and compared to DC current in several electrolytes, from the zincate bath to the commercial formula. For each set of parameters, the faradic yield has been measured and compared to the hydrogen generated during the electrolysis distributed between the absorbed one (measured by hot extraction) and the dihydrogen released into the reactor atmosphere (measured by mass spectrometry with a dedicated set- up). It is interesting to note that the obtained values strongly depend on pulsed sequences as well as electrolytes composition. Coatings were systematically produced and characterized. Morphology was observed using SEM and microstructure determined from XRD diffractograms: preferential orientation and crystal lattice and crystalline phases. The latest has required a specific methodological development.Finally, the identification of nucleation parameters by model simulation has been undertaken to describe the mechanisms involved in the first steps of zinc electrodeposition [3].[1] B. Chatterjee, « Electrodeposition of Zinc Alloys », p. 31.[2] X. Zhang, K. Tsay, J. Fahlman, and W. Qu, « Journal of Energy Storage, 26 p. 100966 (2019)[3] A. Milche, Electrochimica Acta, 48 p.2903-2913 (2003)
Among the various zinc electroplating baths, alkaline zincate solution has gained a wider range of applications thanks to its simple bath composition, good dispersion and efficient coverage ability. Nevertheless, alkaline zincate solution cannot be used as such and it is necessary to introduce additives to control the process performances [1]. Otherwise, efficiency is extremely low, and the properties of the coatings are disappointing. Additives can modify the crystal growth and therefore the structural and mechanical properties, the corrosion resistance, and the appearance of coatings [2]. In the absence of additives, the main problem is low efficiency, resulting from the competition between dihydrogen production and the reduction of the metallic species present in the bath. Therefore, pulsed currents constitute an attractive way to eliminate the need for organic additives by significantly improving the Faradic yield. In this study, the influence of various pulsed current sequences versus the addition of organic additives is observed in the characteristics of the coatings as well as in the process execution. A key point in the determination of the pulse sequence is the study of the transient curves with a zincate bath. This allows for the identification of the most influential parameters on dihydrogen production, with direct consequences on the process mechanisms. By knowing the τD i.e. diffusion time preceding hydrogen reduction, it is possible to design pulse sequences with limited gas production. This is confirmed by mass spectrometry measurements which quantify the total dihydrogen production for each pulse sequence. Fig 1. The coating’s characterization consists in SEM analysis topographic pictures and preferential orientations and grain sizes determined through XRD. Results confirm the interest of pulsed currents versus organic additives for zincate baths. [1] J. L. Ortiz-Aparicio, Y. Meas, G. Trejo, R. Ortega, T. W. Chapman, et E. Chainet, « Effects of organic additives on zinc electrodeposition from alkaline electrolytes », J Appl Electrochem, vol. 43, no 3, p. 289-300, mars 2013, doi: 10.1007/s10800-012-0518-x. [2] L. Yuan, Z. Ding, S. Liu, W. Shu, et Y. He, « Effects of additives on zinc electrodeposition from alkaline zincate solution », Transactions of Nonferrous Metals Society of China, vol. 27, no 7, p. 1656-1664, juill. 2017, doi: 10.1016/S1003-6326(17)60188-2. Figure 1
Long-term protection of metallic parts from atmospheric exposure is frequently made by electrodeposited zinc alloys obtained from alkaline electrolytes because of their ease of use and fulfilling properties. The present study deals with the optimization of ZnFe sacrificial coating on steel and aluminum substrate in the frame of the ATLAS project -Alternative TechnoLogies for improved Anticorrosion Solutions- managed by the IRT M2P. It has been noted that coatings properties are linked to the electrolyte composition and the use of pulsed current acting as a reiteration of germination steps. In this frame, the modelling of a nucleation process followed by diffusion limited three-dimensional growth is an area of promising interest. The study of current transient responses to a potential step (chronoamperometry) is a relevant methodology [1]. This allows the determination of several parameters such as nucleation rate, nucleation density, and number of active sites. Different competing models are available, such as Scharifker and Hills [2], Scharifker and Mostany [3] or Bewick [4]. By using the model method i.e. the identification of the model parameters by error minimization, it is possible to reach an accurate description of the first layer growth and nucleation mode (instantaneous or progressive).[1] D. Vasilakopoulos, M. Bouroushian, et N. Spyrellis, Electrochimica Acta, 54(9), p. 2509-2514, 2009[2] Scharifker, B. & Hills, G. Theoretical and experimental studies of multiple nucleation. Electrochimica Acta 28, 879–889 (1983)[3] Scharifker, B. R. & Mostany, J. Three-dimensional nucleation with diffusion controlled growth. J. Electroanal. Chem. Interfacial Electrochem. 177, 13–23 (1984).[4] I. Danaee, « 2D–3D nucleation and growth of palladium on graphite electrode », Journal of Industrial and Engineering Chemistry, 19(3), p. 1008-1013, (2013)
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