Effect of Grain Refinement and Immersion Time on Morphology, Topography and Corrosion Resistance of CCC-Coated 7075 Al Alloy

Abstract

Chromate conversion coatings (CCCs) were synthesized on AA7075 alloy. The effects of sodium saccharin as a grain refining agent (GRA) and coating time on the coating morphology, topography, and alloy corrosion resistance were studied in this work. Morphologies and topographies of CCCs were examined using optical, scanning electron and atomic force microscopes. The corrosion behaviour was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy in 0.5 M NaCl solution. Major decreases in both coating grain size and corrosion rate occur at a GRA concentration of 1 g/l. Coating roughness increased to a limited nano-scale extent with increasing immersion time.Hardened aluminium alloys are extensively used in aerospace industries because of their excellent mechanical properties such as toughness and high strength due to presence of second-phase particles in the alloys. 1 In alloy group of 7xxx, the primary alloying elements including Zn, Mg and sometimes Cu are used for age hardening and damage tolerance, but they form a range of second phase particles that make them sensitive to localized corrosion. 2 Applying protective conversion coatings onto the alloy surface is one approach to improve the environmental performance as well as to increase paint adhesion. 3 Chromate coating is a extremely effective and widely-used type of conversion coating for high strength Al alloys in aerospace applications due to advantages such as like high corrosion resistance, good paint adhesion, low costs, simple application process and low dimensional changes. 4-12 A combination of pressures, including environmental regulations on the use and handling of chromates, increased service-life requirements of the fleet of airplanes, and the cost of preventive maintenance, has motivated aerospace industries to invest in efforts to find improved and environment-friendly methods of corrosion protection.

Mechanism of LDH Direct Growth on Aluminum Alloy Surface: A Kinetic and Morphological Approach

“…It appears that at the early stage of immersion, the LDH covers preferentially the interior of the grains but not the grain boundaries. This preferential nucleation on grains was reported previously for CCC formation on Al alloys. , However, there is overtime bridging of the conversion coating across the grain boundaries, allowing full coverage of the entire sample surface …”

“…This preferential nucleation on grains was reported previously for CCC formation on Al alloys. 57,58 However, there is overtime bridging of the conversion coating across the grain boundaries, allowing full coverage of the entire sample surface. 57 The LDH conversion layer grows with time, and the height profile of the topography increases from 60 to 600 s of immersion (Figure 3a).…”

Trivalent chromium conversion coating on AA2024‐T3 used in aeronautical and aerospace industry

“…The equivalent circuit chosen to model the impedance spectra is represented in Figure . It consists in a coating resistance ( R coat ), which models the conductive paths determined by the monopores in the porous layer, in parallel with a constant phase element, the double layer capacitance ( C bl ), which represents the nonideal capacitive behaviour of the barrier layer at the base of the large defects and cracks.…”

“…This is more pronounced for AA2024‐T3 treated with PreCoat A32 and previously polished. This behaviour is due to electrolyte penetration in the coating structure . The electrolyte, containing corrosive agents (in this case chloride ions), penetrates through small defects and micropores of the conversion layer, thus reaching the metallic surface.…”