Caracterización de productos de corrosión de la cupla recubrimiento base cinc/acero formados en función de la composición y el pH del medio

ELSNER, CECILIA I.; DI SARLI, ALEJANDRO R.

Proceedings of the 3rd IAS Conference on Uses od Steel

Instituto Argentino de Siderurgia

Lugar: San Nicolás; Año: 2006 p. 1 - 9

Factors such as previous history, chemical composition, surface preparation, electrolyte concentration, pH, temperature, convective processes, aeration, exposure time, etc., play an important role in the development of electrochemical processes on metals and, therefore, on the values of their characteristic parameters: corrosion potential (Ecorr) and corrosion current (Icorr). Being a standardized, effective and low cost protective method for avoiding the corrosion of the steel substrate, the galvanic coatings are used when that is submitted to aggressive environments exposure. Either formed by pure zinc or any of its alloys, this coating type provides cathodic protection to steel not only because it is electrochemically more active than the substrate but also due to the barrier effect afforded by the layer of zinc corrosion products precipitated on the metallic surface. Related with the pure zinc behavior in aqueous solutions, some studies showed that this metal tends to dissolve and forming oxides/hydroxides whose properties are determined by the medium composition and pH. Also that in contact with sulphates, chlorides and carbonates, insoluble compounds whose stability strongly influences the zinc corrosion resistance may be formed. As the corrosion products derived from the exposure conditions differ in composition, structure, morphology, etc., it is important to characterize them in order to understand first, and to combat then, the corrosion process. The situation is even more complex by considering the zinc base alloys. In this case, the corrosion potential depends upon not only of the equilibrium potential of the zinc but also of each component of the alloy. However, the values corresponding to zinc only differ significantly when the alloy components concentration is high. On the contrary, the Icorr changes have a much more complex dependence because it is affected by both the nature and amount of these components as well as the alloy microstructure and intermetallic phases formation. Besides, the corrosion of several zinc base alloys is associated to a zinc preferential dissolution giving place to an increase of the other metals surface concentration and, consequently, to a significant time dependence of the Ecorr and Icorr values. Taking into account all of these factors, the aim of the present work was evaluating the aqueous electrolytes pH and composition influence on the formation of reaction products coming from the dissolution of either zinc or 55%Al-Zn base galvanic coatings deposited on steel sheets. Commercially available panels were masked with virgin wax, and then submerged into in 0,5M NaCl or Na2SO4 solutions at 1 £ pH £ 14. In such experimental conditions, the Ecorr of each panel was daily measured. The corrosion products characterization was performed through XRD, SEM and EDXS techniques. Experimental results obtained with both metallic coatings allow to infer that: 1) the corrosion products formed at 2 £ pH £ 14, mostly zinc basic carbonate, provided a high system stabilization along the immersion period; 2) a pH < 2, the initial strong coating dissolution was accompanied by hydrogen bubbles evolution; 3) the higher protective level observed at pH = 13 was attributed to the formation of corrosion products able of inhibiting the galvanic coating dissolution; and 4) the high electrochemical reactivity at pH = 14 lead to the total coating consumption.