Corrosion Protection Of Aluminum Alloys By Surface Modification Using Chromate Free Approaches

Plasma electrolytic oxidation (PEO) is a surface treatment for the production of ceramic oxide coatings with great properties, such as high wear and corrosion resistance, on metal substrates, particularly aluminum and magnesium alloys. Formation of PEO coatings involves complex processes and mechanisms that are difficult to study. Currently, the PEO process is in a transition phase from research to commercial application, with a primary focus on the corrosion and wear protection of light alloys, and has recently generated interest as a promising surface treatment for biomedical applications. To justify the industrial application of PEO, a more systematic and in-depth study of the influence of various parameters on the process is required. The control of the PEO process to yield the desired morphology and microstructure for specific applications is a key requirement if the process is to be industrially applied. The aim of the research in this thesis is to study the influence of electrical parameters, so they can be optimized to produce coatings with enhanced properties mainly for tribological applications. Alumina coatings were deposited on 6061 aluminum alloy substrates in an alkaline silicate electrolyte using a unipolar pulsed DC current mode. The influence of processing conditions, mainly electrical parameters (applied frequency, duty cycle, and current density), on the formation, growth behaviour and properties of PEO coatings were investigated. Different characterization methods including scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffractometry, microhardness testing, electrochemical impedance spectroscopy, and linear polarization resistance measurements were used to study the microstructure, morphology and properties of the coatings. The correlation between the stage of the PEO process and the properties of the coating has been shown. The voltage-time response was found to be important since it provided readily measurable and useful information about these stages. It was found that the microstructure, morphology, growth rate, phase distribution and composition of coatings could be changed by varying the electrical parameters. To some degree corrosion performance could be tailored by adjusting processing parameters.

"These proceedings represent contributions to the Symposium on the Corrosion and Protection of Light Metals, held at the 204th Meeting of the Electrochemical Society, October 12th through 17th, 2003, in Orlando, Florida."--P. iii.

The System Approach Interface Engineering (SAIE) concept was employed to develop corrosion protection processes for aluminum (Al) alloys by application of a low temperature plasma interface engineering technique with a cathodic electrocoat (E-coat) as the primary layer coating. The SAIE concept emphasizes that the corrosion protection property of the coated system for Al alloys depends on the total system rather than any good corrosion protection component of the system. The cathodic E-coated SAIE plasma pretreatments on Alclad 2024-T3, 2024-T3 bare and 7075-T6 bare alloys showed excellent corrosion resistance property when tested by SO 2 and Prohesion salt spray tests. These systems out performed the conventional conversion coated controls, chromate conversion coated then Deft primer coated (CC Deft) and chromate conversion coated then cathodic E-coated (CC E-coat) in both the corrosion testes. The corrosion protection by SAIE systems depends on three major factors; (1) improved barrier characteristics of E-coat, (2) water insensitive adhesion of E-coat to plasma polymers deposited in a DC discharge and (3) creating a stable surface oxide layer by plasma treatment or chemical cleaning. Different chemical pretreatments were employed to create a stable barrier type aluminum oxide layer on the surfaces of the substrates prior to plasma polymer deposition. The surface analysis showed that these pretreatments depend on the type of alloy and surface chemistry. As received surfaces with acetone wipe and plasma cleaning of the organic contaminants was found to be best for Alclad 2024-T3 alloy. Chemical alkaline cleaning for 2024-T3 bare and alkaline cleaning followed by deoxidization for 7075-T6 bare alloy were necessary. The adhesion of the cathodic E-coat was improved by surface energy matching techniques by deposition of various plasma polymer films of trimethylsilane (TMS) and mixtures of TMS with O 2, H 2, and N 2 . The adhesion performance evaluated by the N-methylpyrrolidinone (NMP) test method showed that the low temperature plasma interface engineering technique could be used to create adhesion performance range for the organic paint on the metal surfaces from easily removable to coatings for life. The barrier properties of the cathodic E-coat were improved by the plasma polymer deposition on which E-coat was applied.