Electrochemical Characterization of Different Variants of a Commercial Trivalent Chromium Process (TCP) Coating on Aluminum Alloy 7075-T6

Abstract

The electrochemical properties (open-circuit potentials, anodic and cathodic polarization curve currents, and polarization resistances) were evaluated for AA7075-T6 alloys coated with three variants of a commercial trivalent chromium process (TCP) pretreatment coating. The coatings were formed on degreased and deoxidized aluminum alloy specimens. Measurements were made in oxygenated 0.5 M Na2SO4 and 3.5% NaCl. Comparison of coatings formed by immersion and spray was undertaken. The three coating variants were 650 chromitAL, versions E, V, and C. Similar concentrations of Cr were in all three coating baths but there were differences in Zr, Zn, S (likely as sulfate), and Fe among the three. TCP coatings formed by immersion exhibited electrochemical properties similar to those formed by spray. Overall, the greatest level of corrosion protection was provided by 650 E based on electrochemical data and results from a 14 d thin-layer mist (3.5% NaCl, 55°C) accelerated degradation test. The coating provides both anodic and cathodic protection in low-chloride electrolytes and functions as more of a cathodic inhibitor in high-chloride electrolytes. Rotating disk voltammetric data revealed the coating inhibits the reduction of dissolved oxygen by providing a diffusional barrier and possibly blocking sites for O2 chemisorption on the cathodically-active intermetallic phases.

Communication—Role of Trivalent Chromium on the Anti-Corrosion Properties of a TCP Conversion Coating on Aluminum Alloy 2024-T3

“…4,[7][8][9][10] There have also been other reports on another commercial version of TCP on AA2024-T3 and AA7075-T6. [11][12][13][14][15][16]18,19 The electrochemical data indicate that TCP provides barrier layer protection on the different aluminum alloys suppressing both anodic and cathodic currents near the OCP by 10-100x, depending on the alloy. Breakdown potentials, E b , are shifted positive by the TCP coating in chloride-containing electrolytes.…”

“…23 Recent published work has produced insight on the TCP coating structure and the underlying mechanisms by which the coating inhibits corrosion on various aluminum alloys. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]23 For example, we have reported on the properties of one commercial TCP coating on AA2024-T3, AA6061-T6 and AA7075-T6. 4,[7][8][9][10] There have also been other reports on another commercial version of TCP on AA2024-T3 and AA7075-T6.…”

“…Breakdown potentials, E b , are shifted positive by the TCP coating in chloride-containing electrolytes. 4,[7][8][9][10]18,19 In other words, the coating functions as both an anodic and cathodic inhibitor. The coating provides cathodic protection, in part, by serving as a diffusional barrier for O 2 , even though very thin (∼100 nm).…”

“…The coating provides cathodic protection, in part, by serving as a diffusional barrier for O 2 , even though very thin (∼100 nm). 4,[7][8][9][10][11][16][17][18][19] The coating consists of a bi-phasic structure with an outer hydrated zirconia layer rich in Cr(OH) 3 and an interfacial layer consisting of hydrated zirconia, some Cr(OH) 3 and KxAlF 3+x . [4][5][6][7][8][9][10] In this communication, we report on potentiodynamic polarization and rotating disk voltammetric data that show both conversion coatings suppress anodic currents equivalently but the coating with Cr(III) inhibits cathodic currents to a greater extent.…”

Hydrophobic Antioxidant Polymers for Corrosion Protection of an Aluminum Alloy

“…Yet AA2024 is highly susceptible to corrosion due to the presence of precipitates and second phases rich with copper. − One effective way to inhibit corrosion is via the deposition of protective coatings on the metal surface, which can serve as a barrier to the diffusion of water, ionic species, and oxygen, or contain inhibitors of corrosion reactions. Hexavalent chromium was widely used as the most effective corrosion inhibitor of aluminum in protective coatings, , but its carcinogenicity and toxicity prompted a search for efficient nontoxic inhibitors.…”