Protection and Failure Mechanism Analysis of Zinc Coatings for Galvanic Protection

Zinc coatings have been used for many decades for various applications to protect against corrosion. These coatings provide a very effective and an economical way of protecting steel against corrosion.  Both substrate-coating interface and the bulk of coating play important roles in terms of galvanic protection capability, thus the transformation process of zinc particles and their interactions were analysed together with corrosion resistant properties. It is imperative to study the protection and failure mechanisms of these coatings related to corrosion.  

            Two zinc coatings; namely hot dip galvanised (HDG) together with a special commercial zinc rich paint coated on AISI 1020 mild steel substrates were selected for a detailed study of their protection/failure  mechanism. High resolution cross sectional analysis of samples at different exposure stages together with other corrosion and electrochemical data will help to unveil the story.

            Samples were exposed in an advanced cyclic corrosion test chamber according to ISO 14993 and were withdrawn at the end of each cycle for respective evaluation. Electrochemical evaluations were conducted using corrosion potential measurements and electrochemical impedance spectroscopy (EIS).

            Localised electrochemical details and anodic and cathodic activities were investigated by high scanning vibrating electrode technique (SVET). 

            Cross sectional analysis of samples at different stages of exposure were done by high resolution scanning electron microscope with and without backscattering techniques to scrutinise the protection and failure mechanisms of the coatings.

            Corrosion rates were calculated by chemical pickling followed by mass loss measurements whilst visual examinations were performed according to ISO 10289 whilst corrosion products were analysed by scanning electron microscope, Fourier Transformation   infrared spectroscopy (FTIR) together with micro Raman spectroscopy.

            Results will be discussed in detail in the presentation