The role of glycine in the iron-phosphorous alloy electrodeposition Natalia Kovalska a, b, * , Martin Pfaffeneder-Kmen a , Natalia Tsyntsaru c , Rudolf Mann b , Henrikas Cesiulis c , Wolfgang Hansal b , Wolfgang Kautek a, * a University of Vienna, Department of Physical Chemistry, A-1090 Vienna, Austria b Hirtenberger Engineered Surfaces GmbH, A-2552 Hirtenberg, Austria c Vilnius University, Department of Physical Chemistry, LT-03225 Vilnius, Lithuania abstract The influence of glycine on the iron phosphorous alloy electrodeposition was investigated by electro- chemical quartz microbalance (EQMB), in-situ external reflection FTIR spectroscopy, and electrochemical impedance spectroscopy (EIS) measurements. An increase of glycine concentration leads to a decrease of the iron-phosphorous alloy electrodeposition rate and an increase of hydrogen evolution. Strong adsorption of glycine species, such as H 2 (gly) þ , H(gly) ± or/and Fe(gly) þ , have been observed during the hydrogen evolution and the Fe-P deposition reaction. Due to the concurrent hydrogen evolution the pH attains higher values at the interface than in the electrolyte bulk (pH2.5). The formation of adsorbed Fe(gly) þ and of the chelate complex Fe(gly) 2 in solution avoids the precipitation of Fe(OH) 2 in the pH range between 2.5 and ca. 7 at the interface. The phosphorous content of the iron phosphorous alloy deposit increases with the glycine concentration. This is due to a lower deposition rate of iron caused by the adsorption of Fe(gly) þ , while the hypophosphite reduction rate to phosphorous increases. 1. Introduction The addition of additives to electrochemical baths can strongly influence the electrodeposition of metal alloy coatings. Additives may modify the deposition mechanism, the morphology, micro- structure, and physicochemical properties of the alloys. Recently, there has been enhanced interest in glycine as additive [1e5], such as in the plating of Fe [6], and of Ni-Mn alloys where the influence surface morphology and grain size was reported [7]. In the later system, the minimization of the grain size is important for nano- crystalline soft magnetic materials as well as for the increase of the corrosion resistance. Glycine is a bidentate ligand that can coordinate via its amino and carboxyl groups forming homonuclear, binuclear and hetero- nuclear complexes with metal ions. A study of the formation of Fe ion complexes showed that the cationic form of glycine (NH 3 CH 2 COOH) þ is present together with the zwitterion (NH 3 CH 2 COO) ± for pH < 4.0. At pH > 4.0, it exists as the zwitterion, and at pH > 9.0, as the anion (NH 2 CH 2 COO) - [1]. The Fe-P coatings become interesting in metallic foam and battery anode deposition [8e10], in magnetic [11 e14] and corro- sion inhibition applications [15e17]. Moreover, high strength, hardness, relatively high electrical resistivity, and advantageous mechanical properties may be achieved [10]. Glycine is known as a buffer in order to stabilize baths (e.g. avoid Fe(OH) x precipitation) due to its zwitterion form in the pH range of ca. 5e8 (pK a1 ¼ 2.3, pK a2 ¼ 9.6) [18,19]. It can also promote hydrogen evolution and thus lead to an increase of the phospho- rous content in the coatings [20]. The formation and participation of Fe hydroxide species due to the pH increase caused by the con- current hydrogen evolution during the Fe-Mo-P electrodeposition was repeatedly investigated. The behaviour of glycine on gold electrodes was studied by in- situ IR spectroscopy [4,5,21]. It was shown that, during cobalt electrodeposition, the pH has a drastic influence on the glycine deprotonation and the formation of Co glycine complexes [4,5,21]. The detailed mechanism of Fe-P electrodeposition has received limited attention. In the present study, a mechanistic investigation of the influence of glycine on the Fe-P electrodeposition is pre- sented applying in-situ techniques such as the Electrochemical Quartz Microbalance (EQMB), in-situ external reflection FTIR spectroscopy, and Electrochemical Impedance Spectroscopy (EIS).