EIS investigation of the deposition of trivalent chromium coatings on Al 6063 alloy

Abstract

The influence of five variables (i.e. deposition temperature, time, bath pH, and concentrations of Cr 3? compound (KCr(SO 4 ) 2 ) and H 3 PO 4 ) on the preparation of Cr 3? coating on Al 6063 alloy was investigated using AC impedance spectroscopy (EIS) in 3.5 wt% NaCl. The optimal conditions were determined by AC impedance spectroscopy. The results indicated that the formation and quality of the coating were very sensitive to the deposition bath pH. A mechanism was proposed to explain the results. A simple model was derived and experimentally tested in terms of an equivalent circuit. Good agreement was found between the model predictions and the experimental results. The morphologies of the coated and uncoated electrodes were examined by scanning electron microscopy (SEM), and the results also support the proposed surface model.

Impedance spectroscopy studies of electroless Ni–P matrix, Ni–W–P, Ni–P–ZrO2, and Ni–W–P–ZrO2 coatings exposed to 3.5% NaCl solution

“…The corrosion resistance of alloy and composite coatings is determined using various methods, such as: the weight loss method (e.g., in NaCl solutions), the neutral salt spray test, the CASS test, polarization studies, and electrochemical impedance spectroscopy studies (EIS). EIS (and its modifications, e.g., dynamic electrochemical impedance spectroscopy (DEIS)) is an effective tool for investigating the mechanisms and kinetics of electrochemical (including corrosion) processes 22–24. Impedance spectroscopy is commonly used to study the corrosion behavior of electroless Ni–P coatings and in recent years it has been used as the principal investigative tool 1, 12, 25–27.…”

Investigation of the Ce-Mn conversion coating on 6063 aluminium alloy

“…The parameter ∆E was an evaluation of the passive range of the composite upon polarization, and it provided an indication of susceptibility to pitting. The increase of the E pit value and a broader passivation region of the coated sample indicated a better impediment to pit corrosion and suggested that anodic reaction (Al → Al 3+ +3e) could be blocked [18]. In the cathodic region, the lower slope in the presence of Ce-Mn conversion coating implied that oxygen reduction (O 2 + H 2 O + 4e → 4OH -) was difficult to occur [18].…”

“…The increase of the E pit value and a broader passivation region of the coated sample indicated a better impediment to pit corrosion and suggested that anodic reaction (Al → Al 3+ +3e) could be blocked [18]. In the cathodic region, the lower slope in the presence of Ce-Mn conversion coating implied that oxygen reduction (O 2 + H 2 O + 4e → 4OH -) was difficult to occur [18]. The polarization curve illustrated the Ce-Mn conversion coating was able to provide enough protection for the aluminium alloy surface.…”

Electrochemical Behavior During the Zirconium Conversion Coating Formation on AISI 1006 Steel

“…In this sense, many efforts have been made to reduce or abolish phosphatization and chromium (VI) chromatization from the production processes 4 by using, alternatively, silanes conversion coatings and chromium (III) chromatization 5,6 . However, phosphate conversion coatings are still present as pre-coatings in white lines, automotive vehicles and urban and office furnishing, as well as in electric components.…”