Driving Performance: Innovative Passivation Technology for Cast Aluminum Alloys

Today, aluminum is finding new uses, particularly in the automotive industry, due to its excellent weight-to-strength ratio. However, the amphoteric characteristic of untreated aluminum makes its use in many industries, including automotive applications, impractical.
Chemical conversion coating is one of the most common methods to protect surfaces where conductivity is also required. There are two key types of corrosion impacting passivated aluminum surfaces with a conversion coating. The first and more common type is uniform corrosion, where the total surface is subject to corrosion due to an aggressive media. The corrosion protection against uniform corrosion is provided solely by the conversion coating. The second type can be simplified into the family of filiform corrosion, where the corrosion propagates under a protective layer by forming ”filaments” causing adhesion or seal failures. During filiform corrosion, the more voluminous nature of corrosion products pushes the protective coating (paint, KTL, or seal) away from the metal surface, causing adhesion failure of the protective coating or seal.
The driving force for both uniform and filiform corrosion is the electronegativity difference between the intermetallics consisting of the alloying elements and aluminum matrix, where both create galvanic couples enhancing the corrosion propagation. Among the electronegativity (how noble they are, like Cu) of the alloying elements other effecting parameters related to the intermetallic are the size and the distribution of the intermetallic in the aluminum matrix. Intermetallics with coarser size and more heterogenous distribution have negative impacts on the corrosion resistance. Furthermore, the grain size, roughness of the surface, and the porosity of the material are also important parameters, having positive or negative effects on the corrosion resistance.
In order to demonstrate the effect of the mentioned parameters on the corrosion resistance, two cast alloys close to the eutectic point have been selected due to their complex microstructure which has a negative effect on the corrosion resistance. The first alloy selected is EN AC 43400 with 9% Si exhibiting behavior similar to Hypoeutectic alloys with relatively less noble alloying elements. The other alloy is EN AC 46000 with 10% Sİ and 3% Cu exhibiting near-eutectic alloy behavior. Several pretreatment types have been applied to both alloys to demonstrate Cr+3 passivation:
- the differences in corrosion resistance
- the improvement of corrosion resistance by adjusting the pretreatment steps; creating a more suitable base for the conversion coating to take place



Biography

Dr. Can Akyıl currently works as Global Business Director of Mac Dermid Enthone and manages Light Metals Division in Mac Dermid Enthone Group, where he has been working for the last 13 years in various departments such as R&D, Production and Sales and Managing the Aluminum Business in Turkey. Before joining Mac Dermid Enthone 12 years ago he was part of the R&D team at Welding Alloys and oversaw the TR department for 3 years. Can Akyıl completed his bachelor's degree in Istanbul Technical University in Metallurgy and Materials Engineering and continued his master's thesis in the same department, where he worked on the electrochemistry and corrosion behavior of aluminum alloys. He has finished his PhD. in the same department at the beginning of 2019 on electrochemical behavior of tin alloys.