Resumo:
The steel sheets coated with Zinc and their alloys are widely used in the automotive industry due they exhibit excellent corrosion resistance, together with good performances of formability and weldability. The corrosion resistance property is acquired through the galvanizing process; the coating layer protects the metal against direct contact with the corrosive environment.
Among the largely range of steels in the market, the most important are galvanized, called GI. The GI coating can be divided into two main layers, the first composed of intermediate intermetallic phases of iron-aluminum-zinc, called the intermetallic layer. Its formation and composition may vary according to the characteristics of the substrate used (steel grade and surface texture), the chemistry of the zinc bath and the process parameters used during its production. The second layer is formed mainly by the zinc itself.
The characteristics and properties of the GI coating have a direct relationship with the intermetallic layer formation, which is responsible for providing a dial between the metal substrate and the zinc layer, affecting the anchoring of the zinc layer itself. Thus, the intermetallic layer characterization becomes extremely important to the final characteristics of the total GI coating layer in the set.
The study aims to present differentiated metallographic preparation techniques used to characterize intermetallic layers in steel samples of GI coated produced by hot dip. The samples were collected at coils produced with two different steel grades. The different chemistries studied were: IF (Interstitial Free) and TRIP780 (Transformation Induced Plasticity) steels. Due a significant difference in their chemical composition (addition of alloying elements), this grades were chosen for evaluating the substrate composition influence of the intermetallic layer formation. The TRIP steels present higher complexity chemistry due to the addition of high levels of alloying elements, such as Si, Al, Cr and Mn.
The intermetallic layers were characterized for their morphology using different techniques of dissolution of the zinc layer. For the surface analyzes, it was used the immersion process in different chemical solutions, ultrasonic cleaning and metallographic procedures. For the section analyzes, were employed techniques based on different positions and angles during the bakelite preparation, providing greater amplitude of the field of vision of the interest region. The images were analyzed using a scanning electron microscope, and their chemical compositions were obtained by the energy dispersive X-ray spectrometer (SEM / EDS).
