Zr-based conversion layer on Zn-Al-Mg alloy coated steel sheets: insights into the formation mechanism

Abstract

Zr-based conversion layers are considered as environmentally friendly alternatives replacing trication phosphatation in the automotive industry. Based on excellent electronic barrier properties they provide an effective corrosion protection of the metallic substrate. In this work, thin protective layers were grown on novel Zn-Al-Mg alloy coated steel sheets by increasing the local pH-value at the sample surface leading to deposition of a Zr-based conversion layer. For this purpose Zn-Al-Mg alloy (ZM) coated steel sheets were treated in an aqueous model conversion solution containing well-defined amounts of hexafluorozirconic acid (H2ZrF6) and characterized after different immersion times with SKPFM and field emission SEM (FE-SEM)/EDX techniques. A deposition mechanism of Zr-based conversion coatings on microstructural heterogeneous Zn-Al-Mg alloy surfaces was proposed.

Review—Conversion Coatings Based on Zirconium and/or Titanium

“…Mechanical pre-treatment in the form of grinding and/or polishing may precede chemical pre-treatment but chemical pre-treatment is often applied without prior mechanical treatment. In some cases, the surface was not mechanically pre-treated but cleaned with ethanol 102 or acetone, 55 For rinsing, deionized water is usually used, although also tap water 92,94,106,112 was applied to make the procedure simpler for industrial processing. Due to environmental and health concerns related to toxicity and carcinogenicity of chromates, procedures that reduce the worker exposure and discharge into the environment were introduced.…”

“…33,34,36,37,47,51,52,55,59,61,62,[64][65][66]69,70,72,78,82,84,87,90,92,94,95,98,99,101,102,104,[106][107][108]110,[112][113][114] Elevated temperatures were also studied, i.e. between 30 and 55…”

Effect of Polarisation Mimicking Cathodic Electrodeposition Coating on Industrially Relevant Metal Substrates with ZrO2‐Based Conversion Coatings

“…In such coatings, it has been shown that copper from the pre‐treatment bath preferentially deposits on the intermetallic particles rich in Fe and Si on AA6014 . Surface‐element distribution also determines deposition of ZrO 2 ‐based conversion coatings on Zn–Al–Mg surfaces of metallic coatings . A larger hydroxyl fraction on the base material enhances the deposition of ZrO 2 ‐based conversion coatings, as shown by a greater thickness on AA6014, and a detailed in situ mechanistic study on zinc oxide .…”

Effects of Mg content on microstructure and electrochemical properties of Zn–Al–Mg alloys

“…During the last 30 years, Zn-Al-Mg alloys have attracted worldwide attention due to their excellent mechanical and corrosion resistance properties as sacrificial coatings to substitute traditional Zn coating for the protection of steel structures [1][2][3][4]. Zn-Al-Mg alloys can also be used in light metal structures due to their advantages such as low-density, high-strength to weight ratio and high-specific toughness.…”