Novel Low Temperature Galvanic Coatings

Abstract

A major influence on performance and lifetime of steel-based structures is corrosion. Some such structures include ground support equipment (GSE). GSE plays a vital role in the safety and efficiency of aircrafts and airports worldwide. GSE encompasses all support equipment at airports, necessary for the function and logistical operation of airports, air bases and other aviation facilities. According to the IMPACT study undertaken by the National Association of Corrosion Engineers (NACE) in 2013, the global cost of corrosion was estimated to be US$2.5 trillion, equivalent to 3.4% of the global GDP at the time. The corrosion of GSE can be attributed to a number of factors present including, but not limited to, moisture in air, salt particles in coastal winds and industrial pollutants from fuel. For these reasons, corrosion prevention through means of protective coatings is of utmost importance; as well as a strict programme of corrosion inspection and repair. Traditional means of protection against corrosion centre around Hot-Dipped Galvanising (HDG). This is the process of applying a protective zinc layer to steel in order to prevent corrosion of a component or structure. One of its major issues is cost. This is primarily due to the need to outsource galvanising to external contractors, especially for manufacturers that do not have the capacity or capability to undertake the process in-house. HDG also happens at high temperatures of ~450°C. This requires a great amount of energy and would be impossible to implement in-house for manufacturers. The repair of HDG sections is also a concern. This often sees the protective Zn coating removed and the section welded to repair. In this instance, the steel substrate is left unprotected and susceptible to corrosive attack. The common solution here is Cold Galvanising Coatings (CGCs) or Zn Rich Paint (ZRP). ZRPs are, however, laden with issues; poor adhesion, UV degradation and poor galvanic contact with the substrate. These shortcomings mean the service life of ZRPs and CGCs are limited.The current work focuses on developing a lower-temperature alternative centred around fusible alloys. The aim is to provide a corrosion-resistant coating that can bond with the substrate at lower temperatures, be applied on-site and offer galvanic protection. The fusible alloy is mixed with zinc powder to create a novel corrosion resistant coating. The coating may also be used for repair of galvanised GSE, which often sees welded areas left susceptible to corrosive attack. Microstructural analysis was carried out as a function of Zn loading weight as well as curing time and temperature. A systematic study on curing time and temperature identified the optimal processing parameters for the novel coating. Heating of below 250°C was used to melt the fusible alloy to envelop the zinc. This can be achieved using a conventional oven, while Near Infrared (NIR) heating was explored as a rapid heating option. The resulting optical and scanning electron microscope (SEM) images show that the fusible alloy acts as a matrix, while “islands” of Zn are dispersed therein to offer galvanic protection. The use of a metal matrix in the microstructure allows for superior electrical contact between the sacrificial Zn and the structural steel substrate, unlike that given by most CGCs and ZRPs. Electrochemical studies, such as Zero Resistance Amplitude (ZRA) and Open Circuit Potential (OCP), and accelerated corrosion tests, implementing the Scanning Vibrating Electrode Technique (SVET), have shown that the coating offers excellent galvanic protection to steel in a 1% NaCl electrolyte. ZRA galvanic corrosion tests demonstrated that current flow between components was negligible. This suggests no obvious self-corrosion in the coating, despite the nobility of fusible alloy components. Current OCP results show that adding Zn to the fusible alloy shifts the OCP from -425mV to -970mV in a 24hr test in 1%NaCl. This is comparable to that of Zn which is stable at -1V and shows promise when compared to bare steel at -700mV, meaning the coating offers galvanic protection to the steel. This is reinforced by SVET maps of coated steel samples, confirming that steel remains cathodic to the coating throughout tests, showing that performance improves as a function of Zn content. Adhesion tests, using the 0t method, show the peak performance comes at a known Zn wt% with cracks appearing and propagating as Zn wt% is increased. Overall; peak microstructure, corrosion protection and adhesion are achieved and demonstrated. The resulting product is a robust, protective, and dynamic coating; a step forward in the fight against corrosion of steel and protection of HDG.