Chromate-Free Corrosion Protection Strategies for Magnesium Alloys-A Review: PART I-Pre-Treatment and Conversion Coating

Abstract

Corrosion protection systems based on hexavalent chromium are traditionally perceived to be a panacea for many engineering metals including magnesium alloys. However, bans and strict application regulations attributed to environmental concerns and the carcinogenic nature of hexavalent chromium have driven a considerable amount of effort into developing safer and more environmentally friendly alternative techniques that provide the desired corrosion protection performance for magnesium and its alloys. Part I of this review series considers the various pre-treatment methods as the earliest step involved in the preparation of Mg surfaces for the purpose of further anti-corrosion treatments. The decisive effect of pre-treatment on the corrosion properties of both bare and coated magnesium is discussed. The second section of this review covers the fundamentals and performance of conventional and state-of-the-art conversion coating formulations including phosphate-based, rare-earth-based, vanadate, fluoride-based, and LDH. In addition, the advantages and challenges of each conversion coating formulation are discussed to accommodate the perspectives on their application and future development. Several auspicious corrosion protection performances have been reported as the outcome of extensive ongoing research dedicated to the development of conversion coatings, which can potentially replace hazardous chromium(VI)-based technologies in industries.

Keywords: conversion coating; corrosion; hexavalent chromium; magnesium; pre-treatment.

Figure 1

(a) Potentiodynamic polarization curves of the AZ91 alloy, with different surface roughness, tested in 0.5 wt.% NaCl [9]. (b) Hydrogen evolution of the pretreated AZ91 alloy in 3.5 wt. % NaCl solution [13]. Reprinted from [9] and [13] with permission from Elsevier.

Figure 2

The in situ observation of the corrosion morphologies of the AZ31 magnesium sample ground with (a) SiC paper (marked as AR+grinding-c); (b) polished up to 1 µm diamond paste (marked as AR+grinding-f) in 5 wt.% NaCl [14]. “AR” stands for “as-received”. Reprinted from [14] with permission from Elsevier.

Figure 3

H₂ evolution of AZ31 in NaCl 3.5 wt.% before and after a USSP process. The length mentioned after each case indicates the thickness removed by grinding using 2000 grit size SiC paper. Reprinted from [31] with permission from Elsevier.

Figure 4

Variation in the surface roughness with material removed for different inorganic (top) and organic (bottom) pickling solutions [61,62]. Reprinted from [61,62] with permission from Elsevier.

Figure 5

(a) The weight loss of an AZ31 Mg alloy immersed in H₃PO₄ solution with different concentrations for 60 s [83]. (b) The weight loss of an AZ31 Mg alloy in aqueous solutions containing 400 mL/L H₃PO₄ and different concentrations of HF/NH₄HF₂ [83]. Reprinted from [83] with permission from IOP, respectively.

Figure 6

Schematic representation of the surface after the various pre-treatment processes. “Activation” refers to acid pickling, “conditioning” implies alkaline treatment. Reprinted from [52] with permission from Elsevier.

Figure 7

(Top) Surface morphology of Mg alloy AZ91 and the EDS results of the Al_x(Mn,Fe)_y phase (a1–a4) before and (b1–b4) after the pre-treatment. (Bottom) Electron probe microanalysis technique (EPMA) mapping of the cross-section of phosphate conversion coating on the (a) untreated and (b) pre-treated AZ91. Reprinted from [110] with permission from Elsevier.

Figure 8

General overview of the most frequently used conversion treatments and corresponding products formed on the magnesium surface.

Figure 9

Predominance area diagrams for solutions containing 0.1 M phosphate as the function of concentrations of (a) Mg²⁺, (b) Mn²⁺, (c) Ca²⁺, and (d) Zn²⁺ ions and solution pH. Hydra-Medusa software was used to simulate the predominance area. This figure is inspired by [124].

Figure 10

Schematic diagram of self-healing mechanism of the scratched phytate-modified DCPD coatings in 0.9 wt.% NaCl solution (NS) (DCPD: dicalcium phosphate dihydrate coating; Phy: phytate). The self-healing mechanism is in order from (a–c). Reprinted from [170] with permission from Elsevier.

Figure 11

(a) Chemical composition of the phosphate conversion baths used in [150]. (b) The XRD patterns of the formed conversion coatings from the bath composition in (a) of AZ31. (c) Surface appearance and (d) hydrogen evolution of the AZ31 substrate coated with different phosphate conversion coating in (a) during immersion in Hank solution at 37 °C. Reprinted from [150] with permission from Elsevier.

Figure 12

Relationships between the TA/pH and corrosion protection effectiveness of PCCs derived from the published data. Each symbol represents a specific published reference. The information of references can be found in [209], where the figure is taken from. Reprinted from [209] with permission from Elsevier.

Figure 13

Optical photographs of (a) non-modified and (b) silane-modified cerium conversion coating before and after the cross cut tape test. Reprinted from [232] with permission from Wiley.

Figure 14

A schematic representation showing the titanate conversion coating formation on the AZ31 alloy: (a) the dissolution of Mg and Al as well as the discharge of hydrogen; (b) the formation of the porous layer composed of Mg(OH)₂ and MgF₂ as well as local precipitation of Si(OH)₄ and Ti(OH)₄ on top of the porous layer; and (c) the growth of the porous layer and the Si(OH)₄ and Ti(OH)₄ precipitates. Reprinted from [125] with permission from IOP.

Figure 15

Molecular structure of (a) phytic acid and (b) tannic acid.

Figure 16

SEM images of the phytate conversion coatings on the AZ31 alloy samples treated in phytic acid solutions, varying pH, phytic acid concentration, treatment time, and temperature. Reprinted from [324] with permission from Elsevier.

Figure 17

Surface morphology of AZ31, AZ31-TA, and AZ31-TA/Mg. (a) SEM morphologies, (b) tape test on AZ31-TA and AZ31-TA/Mg using Scotch tape. Adapted and reprinted from [328] with permission from John Wiley & Sons.

Figure 18

Grazing incidence XRD pattern of the AZ91 surface after 48 h in the reference, salicylic acid, EDTA, and NTA sodium salt solutions at room temperature. The data were shifted vertically for clarity. The inset shows the Gaussian polynomial fit of the NTA and EDTA patterns. Reprinted from [385] with permission from Springer Nature.