For the past few decades, the aerospace industry has been actively looking for measures to tackle the problem of localized corrosion of aluminium alloy 2024-T3. One such measure was the use of hexavalent chromium salts as corrosion inhibitors. However, recent studies have reflected the environmental concerns and health impacts associated with these compounds. Since then, there has been a quest for alternatives that can be used as corrosion inhibitors for AA2024-T3.

This thesis is aimed at studying these compounds by testing their inhibitive properties. Selecting an inhibitor from a set of compounds and recommending the best one is a crucial process. The research focuses on the screening of these inhibitors through various electrochemical techniques such as Potentiodynamic Polarisation(PDP), Linear Polarisation Resistance(LPR), and Electrochemical Impedance Spectroscopy(EIS).

The electrochemical experiments during the first 24-hour exposure of AA2024-T3 to inhibitors exhibited that 2,5-Dimercapto-1,3,4-thiadiazole acts as a corrosion accelerator, while Na-Mercaptoacetate and Mercaptobenzimidazole showed the most inhibitive performance among the tested organic compounds. On monitoring their nature during 24 hours, it was observed that they show stable polarisation resistance values after 6 hours. Among the inorganics, Cerium compounds were seen to have better inhibitive properties than the Lithiumcompounds. Additionally, surface studies such as Fourier Transform Infrared Spectroscopy (FTIR) revealed the presence of thiol/mercaptan and carboxyl groups on samples exposed toNa-Mercaptoacetate. X-ray Photoelectron Spectroscopy (XPS) showed oxides of Cerium on samples exposed to Cerium Nitrate. Finally, the correlations between the inhibition efficiencies calculated from different electrochemical testing methods were illustrated using Pearson’s correlation method.

For decades, developments towards a chromate-free protective system have been a crucial quest in the aerospace industry. In this search, innovation starts with a comprehensive understanding of the aircraft complex multi-material structures and the corrosion mechanisms involved. These complex structures, with aluminium the most dominant substrate, are used to optimize the strength to weight ratio, fatigue properties and operational performance in aircraft design. On the other hand, the use of multi-materials can cause an accelerated corrosion attack, called galvanic corrosion. This type of corrosion can be advantageously applied to galvanically protect a material but can also occur undesirably with serious accelerated degradation as result.

This thesis aims to increase our understanding of corrosion on AA7XXXClad alloys as stand-alone material, as well as in configurations relevant for galvanic corrosion with other metals. The understanding of these phenomena should support the definitions of new hypotheses on how these alloys can be better protected using chromate-free coating technologies. Two commonly used aluminium alloys in the aerospace industry have been investigated in this study because a selective galvanic stimulated dissolution of the cladding layer material was found after an accelerated corrosion test. This selective dissolution makes it difficult for the corrosion inhibitors to reach the exact site of corrosion propagation in the cladding layer, reducing the inhibition efficiency and allowing corrosion to propagate.

Since Zinc is the main alloying element in the substrate and cladding material, the role of Zn with respect to corrosion initiation and propagation was investigated first. Subsequently, the behaviour of clad alloys under galvanic corrosion conditions, and finally how inhibition under these conditions can be reliably assessed. This was performed by a combination of multiple electrochemical techniques and microscopic analysis. The results demonstrate that Zn plays a significant role in the dissolution of the cladding layer and may be held responsible for the selective dissolution observed. In addition, an experimental procedure was developed to measure the coupled galvanic parameters and to simulate the galvanic corrosion degradation in industrial desired timeframes. Although simulation of the phenomenon has shown to be promising with in-situ experiments, it was not fully observed. Furthermore, to test the performance of corrosion inhibitors, a procedure is developed with and without the use of coatings. In general, it can be concluded that limiting the cathodic reactions is of paramount importance to reduce the galvanic corrosion current.