Assessment of Chromate and Non-Chromate Conversion Coatings for Al Alloys Using Electrochemical Impedance Spectroscopy.

摘要

A study was conducted to evaluate whether a non-chromate conversion coating (Sanchem) for Al alloys (Al 2024 and 7075) could be a suitable replacement for the currently used chromate conversion coating (Alodine) without compromising corrosion resistance. Electrochemical impedance spectroscopy (EIS) and salt spray testing were employed to compare the corrosion behavior of coatings consisting of (a) pretreatment only (Sanchem or Alodine), (b) pretreatment (Sanchem or Alodine) plus primer (epoxy polyamide or waterborne epoxy), and (c) pretreatment (Sanchem or Alodine) plus epoxy polyamide primer plus polyurethane topcoat. Results indicated that the experimental impedance values provided a reliable estimate of the film integrity and corrosion protective capability, that the Sanchem conversion coating generally compared favorably with the Alodine coating, and that higher impedance values attributed to the application of topcoat of both alloys which were treated with either Sanchem or Alodine and epoxy polyamide are characteristic of low conductivity, good barrier type coatings. However, additional tests are required before the Sanchem can be recommended as a reliable alternative to the Alodine coating.... Aluminum alloys, Chromate coatings, Non-chromate coatings Corrosion resistance, and EIS.