Effects of pH Value of the Electrolyte and Glycine Additive on Formation and Properties of Electrodeposited Zn-Fe Coatings
Environmentally friendly and cyanide-free sulfate bath under continuous current and the corrosion behavior of electrodeposits of zinc-iron alloys were studied by means of electrochemical tests in a solution of 3.5% NaCl in presence and absence of glycine. The effects of pH on the quality of Zn-Fe coatings were investigated in order to improve uniformity and corrosion protection performance of the coating films. The deposit morphology was analyzed using scanning electron microscopy (SEM), and X-ray diffraction (XRD) was used to determine the preferred crystallographic orientations of the deposits. It was found that the uniformity and corrosion resistance of Zn-Fe coating films were strongly associated with pH of the coating electrolyte. To obtain the effect of pH on the film quality and corrosion performances of the films, the corrosion test was performed with potentiodynamic anodic polarization method. It was also observed that uniformity and corrosion resistivity of the coating films were decreased towards pH = 5 and then improved with increasing pH value of the electrolyte. The presence of glycine in the plating bath decreases the corrosion resistance of Zn-Fe coatings.
Zinc Alloy Electrodeposition for Corrosion Protection
t is known that the protective value of zinc is partially a result of its ability to form a protective film when plated on a base metal, such as steel. The zinc is sacrificial and corrodes I preferentially to protectthe steel. Although pure zinc coating provides some degree of protection for the substrate, in a severe corrosion environment, zinc is consumed rapidly. For this reason, it is usual practice to increase the thickness of the zinc coating. This is not commercially cost effective and the thick coating causes problems during forming and welding of the steel. Consequently, in the last 20 years, most industries, especiallythe automotive and building industries, began to look for an alternative process. The result was the development of zinc alloy systems, such as zinc-nickel,' zinc-c0balt,2~~~~ zinciron,',@ zinc-manganese,g,1° and the most current process that includes a combination of zinc-cobalt-chromium alloy" and zinc-iron-manganese alloy12 deposits. These alloy systems offer superior corrosion resistance and mechanical properties, weldability, paintability and thinnercoating. Such coatings are always high in zinc content, have less noble metal to maintainanodic protection ofthe steel substrate, and remain less active than plated zinc alone.',*,'3 The zinc in the alloy will still corrode sacrificially to protect steel, but at a much slower rate. The selection of a particular alloy process dependson the app1ication;this may includeease of controlling plating bath composition, the degree of corrosion protection it can provide and, last but not least, the operating cost. For this reason, great attention must be paid to selection of a suitable zinc alloy system. This paper points out some of the advantages and merits of each process and draws attention to their applicability for individual use. Many zinc electroplating processes are being developed for corrosion protection, and they contain various constituents (in addition to the main ions in solution) to control and increasethe life of the electrolyte. This paper reports on the investigation of four types of commercially used zinc alloy processes, namely zinc-nickel, zinc-cobalt,zinc-iron andzinc-manganese. Tables 1 and2offersome examples of zincalloysystems.Theseform the basis of this study and are a guide to the constituents, operating conditions and some of their industrial applications.
Substrate transformation phenomena during electrochemical nucleation of silver on polyfaced platinum single crystal electrodes—I. Temperature and overpotential dependence of the stationary nucleation rate
Abstract The initial stage of silver electrodeposition on platinum from 0.1 M AgClO 4 is studied under potentiostatic conditions. The current of growth of silver clusters is measured and the data are interpreted on the basis of the theory of progressive nucleation. Additional information is obtained from the dependence of the stationary rate of nucleus formation on temperature and overpotential.