Evaluation of Corrosion Protection Performance of Zinc Coatings and Failure Mechanisms

Zinc-based solutions have been used for many decades to protect steel against corrosion. Having cathodic protection properties on top of the barrier protection, zinc has become an effective way of protecting steel. Hot dip galvanized techniques have a deep rooted history under this category although they have some disadvantages in terms of application process and productivity which limit their usage in modern applications. Zinc coatings and zinc-rich paints have come to the market aiming to overcome these disadvantages. However, the suitability of these coatings for various atmospheric and service conditions, vulnerability of their protection properties, especially cathodic protection characteristics, due to the influence of external factors like surface preparation and the application of top coats have posed some challenges. Proper methodology for corrosion evaluation and validation is a mandatory step towards solving these issues. The aim of this publication is to address the need of a corrosion characterization methodology to assess zinc-based coatings and also to discuss protection and failure mechanisms of zinc coatings based on selected case studies. A systematic methodology for evaluation and validation of corrosion protection properties of zinc coatings is introduced in this work. This methodology is based on scientific principles and international standards using standard and advanced techniques to assess both barrier and galvanic protection properties. Cyclic corrosion test has been used as the accelerated exposure and samples were withdrawn periodically for progressive evaluation. Barrier and galvanic protection assessment is based on protection rating, mass loss and subsequent corrosion rate measurements, electrochemical properties together with advanced electrochemical scanning techniques to investigate the anodic and cathodic activities. Another part of this work is to discuss failures of zinc coatings based on a selected case where zinc coatings were used as primers together with top coats. Corrosion products of zinc have been detected around zinc particles clusters and also at the substrate / coating interface at the early stage of accelerated exposure. This led to a compromise of the galvanic protection properties of the substrate by zinc and eventually caused the overall protection to fail. Top coat incompatibility and/or improper application were suspected to have led to this failure. Possible causes such as saponification of the zinc primer due to the drying of the binder in the primer coat without proper curing and the high porosity of top coat which can be detrimental to the zinc primer were analyzed in detail. Corrosion mechanisms were scrutinized based on electrochemical principles supported by scanning electron microscopy studies together with micro-Raman spectroscopy. The causes of coating incompatibilities and improper coating application practices and their influence on protection properties will be explained from a corrosion mechanistic point of view. Details will be explained in the presentation.