Avaliação do efeito da camada de oxalato de zinco na resistência à corrosão do aço galvanizado em concreto armado via técnicas eletroquímicas

Abstract

The corrosion of reinforced concrete steel bars significantly reduces the service life of struc-tures and can be accelerated in aggressive environments, such as chloride-rich atmos-pheres, where pitting corrosion may occur. Among the protection methods for reinforcing steel against corrosive processes, galvanization has proven to be an effective alternative, especially when combined with surface treatments such as hexavalent chromium (Cr⁶⁺ or Cr VI) solutions. However, several studies indicate that Cr VI is toxic and carcinogenic, mak-ing it necessary to develop alternative coatings. This study addresses a significant gap in the literature by focusing on the behavior of oxalic acid-treated galvanized steel in alkaline solutions (pH > 7), specifically for civil construction applications. In this context, dicarboxylic organic acid-based solutions have gained attention for providing effective corrosion protec-tion while being less toxic and more cost-effective compared to conventional treatments. This doctoral research aimed to evaluate the corrosion resistance performance of zinc oxa-late coatings applied to reinforcing steel in alkaline environments. For this purpose, galva-nized steel samples were subjected to a conversion treatment in oxalic acid solution (0.1 M), then exposed to an alkaline medium simulating the pore solution of concrete C30 class. The samples were also embedded in concrete specimens and monitored over controlled time intervals. The investigation combined morphological (SEM/EDS) and point chemical, mineralogical (XRD), and spectroscopic (FTIR) analyses with electrochemical tests, includ-ing polarization resistance and electrochemical impedance spectroscopy (EIS), on samples subjected to salt spray exposure to assess the evolution of the zinc oxalate layer and its influence on corrosion resistance. The results demonstrated that the formed layer exhibited an average thickness of 10 μm, delaying the onset of corrosion at the initial stages. However, over longer exposure periods, the alkalinity of the medium and the porosity of the concrete promoted coating degradation, this was observed in both the alkaline solution and the con-crete. Comparatively, treated samples showed an initial corrosion current density (Icorr ≈ 4.7 × 10⁻⁴ mm/year) similar to untreated samples (≈ 5.3 × 10⁻⁴ mm/year), while impedance spectroscopy indicated a charge transfer resistance (Rct) of up to 497 Ω after three months, decreasing to approximately 207 Ω after twelve months. Corrosion rates exceeded 0.1 mm/year in the treated samples, which, according to normative criteria, represents a severe corrosion grade. Therefore, it is concluded that zinc oxalate delays corrosion initiation; how-ever, its stability in alkaline media is limited, compromising the long-term effectiveness of the coating under prolonged exposure.