Investigation of the Trivalent Chromium Process Conversion Coating as a Sealant for Anodized AA2024-T3

Abstract

The sealing of an anodic coating on AA2024-T3 with a trivalent chromium process coating (TCP-HF) was investigated. Specimens were anodized in 9.8 wt% H 2 SO 4 at 15 V DC (23 min) producing an oxide coating weight of ca. 1200 mg ft −2 and a thickness of ca. 5 μm. Elemental analysis revealed the sealant forms across the outer oxide layer and within the pores up to a depth at least approaching 1 μm. Anodic and cathodic currents in potentiodynamic polarization curves were suppressed by 5-10× and low frequency impedance (Z 0.01 Hz ) values were 5× higher for anodized specimens sealed with TCP (∼5 × 10 6 ohm-cm 2 ), as compared to the unsealed anodized controls. During continuous exposure to 3.5 wt% NaCl, Z 0.01 Hz values remained unchanged and greater than 10 6 ohm-cm 2 after 5 days for the TCP-sealed specimen and decreased by 10× for the unsealed anodized control after just three days. A 14-day (336 h) neutral salt-spray exposure produced no significant pitting, discoloration or major coating detachment on anodized specimens sealed with TCP. Overall, the results indicate that the TCP sealant increases the barrier properties of the anodic coating and functions to provide anodic and cathodic corrosion protection.

The Effect of Sulfuric Acid Anodization on the Electrochemical Properties of Aluminum Alloy AlSi10Mg Prepared by Selective Laser Melting

“…The 15 V used herein is a typical anodization voltage for aluminum and was used in our prior work with anodized AA2024-T3. 33 The current then declines over the next 3 min, trending toward a more constant value of 6-7 mA cm −2 for the duration of the anodization. During the ramp, the proportional increase in current density is because of the higher rate of oxidation with increasing voltage.…”

“…A DC power supply (Tenma) was used to control the applied voltage for anodization, as described previously. 33 The positive terminal was connected to the AlSi 10 Mg specimen (anode) with a wire and alligator clip. The cathode was a piece of SS304L plate with dimensions of 2.5 cm × 1.7 cm and a 0.1 cm thickness.…”

“…These parameters were selected based on prior work anodizing wrought AA2024-T3 to produce an oxide weight of ∼1000 mg ft −2 . 33 The anodized specimen was removed from the beaker and rinsed with ultrapure water. To fully remove slowly dissolving anodization smut, the specimen was immersed in ultrapure water for at least 20 min with periodic agitation.…”

“…The larger phase angle for the anodized specimens and the fact that the phase angle remains near −80°to lower frequencies is consistent with the formation of thick oxide coating with capacitive properties. 19,23,33 At the lowest frequencies for the anodized specimens, there is a significant increase in the low frequency impedance at Z 0.01 Hz with a value of ca. 10 6 Ω•cm 2 .…”

“…The changes in electrochemical properties brought about by the oxide coating were assessed using multiple electrochemical methods in naturally aerated 0.5 M Na 2 SO 4 + 0.01 M NaCl. 15,33 The present report contributes to the literature by providing a comprehensive study of the potentiostatically-formed anodic oxide on this SLM alloy and how the coating affects the electrochemical properties.…”