INVESTIGADORES
ELSNER Cecilia Ines
congresos y reuniones científicas
Título:
Caracterización de productos de corrosión de la cupla recubrimiento base cinc/acero formados en función de la composición y el pH del medio
Autor/es:
C.I. ELSNER; A.R. DI SARLI
Lugar:
San Nicolás
Reunión:
Congreso; 3rd IS Conference on Usese of Steel; 2006
Institución organizadora:
IAS
Resumen:
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.