Advanced characterization study of commercial conversion and electrocoating structures on magnesium alloys AZ31B and ZE10A

Abstract

The local metal-coating interface microstructure and chemistry formed on commercial magnesium alloys Mg-3Al-1Zn (AZ31B) and Mg-1Zn-0.25Zr-<0.5Nd (ZE10A, ZEK100 type) were analyzed as-chemical conversion coated with a commercial hexafluoro-titanate/zirconate type + organic polymer based treatment (Bonderite® 5200) and a commercial hexafluorozirconate type + trivalent chromium Cr 3+ type treatment (Surtec® 650), and after the same conversion coatings followed by electrocoating with an epoxy based coating, Cathoguard® 525.Characterization techniques included scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and cross-section scanning transmission electron microscopy (STEM). 2Corrosion behavior was assessed in room temperature saturated aqueous Mg(OH) 2 solution with 1 wt.% NaCl. The goal of the effort was to assess the degree to which substrate alloy additions become enriched in the conversion coating, and how the conversion coating was impacted by subsequent electrocoating. Key findings included the enrichment of Al from AZ31B and Zr from ZE10A, respectively, into the conversion coating, with moderate corrosion resistance benefits for AZ31B when Al was incorporated. Varying degrees of increased porosity and modification of the initial conversion coating chemistry at the metal-coating interface were observed after electrocoating.These changes were postulated to result in degraded electrocoating protectiveness. These observations highlight the challenges of coating Mg, and the need to tailor electrocoating in light of potential degradation of the initial as-conversion coated Mg alloy surface. IntroductionMagnesium and its alloys are of great interest for engineering applications ranging from functional uses such as biomedical implants to structural alloys to achieve automotive and aircraft vehicle light weighting [1][2][3]. In vehicle light weighting applications, the rapid corrosion of Mg, particularly when exposed to salt species under aqueous conditions, is a key issue [4][5][6][7][8][9][10][11][12][13][14][15][16].Alloying generally results in only moderate improvement in the aqueous corrosion resistance of Mg. Therefore, coatings are typically used to provide corrosion protection for Mg components [17][18][19][20][21].Coatings for Mg structural components generally utilize a multi-layer strategy, involving an initial cleaning step; a surface pre-treatment such as chemical or electrochemical conversion coatings, surface alloying, anodization, etc.; a second coating layer such as electrocoatings, platings, powder coatings, organic coatings, etc.; frequently followed by a final layer of sealant and/or paint . In single-and multi-layer coating concepts, the interface between the substrate alloy and the initial coating layer is critical to coating adherence and performance.Delamination or through attack in this region can allow the environment to access the underlying alloy and result in local corrosion and coating failure.Chemical conversion coatings are frequently used industrially as the ...

Review—Conversion Coatings Based on Zirconium and/or Titanium

“…90,94,95,105,106,127 When commercial baths contain trivalent chromium, the coating also contains Cr. 84,127 The Zr-based TecTalis coating was found to contain Zr and O at a ratio corresponding closely to stoichiometric ZrO 2 . 6 Among non-commercial conversion baths discussed in the literature, more variations in composition and ratio between reagents are found (Table I).…”

“…89,37,60 Cracks were observed at the coating surface, especially after long immersion times. Cracks can form during exposure to the vacuum of an SEM chamber because of dehydration and shrinkage, but also can form before introduction into the SEM vacuum during exposure in dry air 49,52,69,79,80,94,96,106,127 or during a curing step at 70…”

“…81 The majority of Zr/Ti based conversion coatings are in the category of thin coatings, between 10 nm and 50-80 nm. 6,[32][33][34]44,49,52,76,77,88,90,91,97,102,105,106,108,112 Coatings with thickness between 100 nm and 600 nm were reported as well, 48,81,94,127 and even in the micrometer thickness range from 1.0 μm up to 6 μm. 30,37,43,59,61,80,119,127 Thickness is related to other parameters such as substrate, pH, time of immersion and stirring, as discussed in Conversion bath parameters and Effect of substrate on coating formation sections.…”

A mechanistic investigation of corrosion-driven organic coating failure on magnesium and its alloys

“…3 Previously published works have evaluated the ability of electrophoretically deposited organic coatings (Ecoat) to protect of Mg alloys in accelerated corrosion tests. 4,5 It was proposed that corrosive electrolyte penetration and resultant formation of corrosion products at the coating-metal interface, such as magnesium hydroxide and hydrogen gas, caused delamination by physically forcing the coating from the metal surface. 4 A separate investigation showed that the presence of soluble inorganic salts at the metal-coating interface of polyester coated AZ91 Mg alloy caused osmotic blistering and subsequent corrosion-induced damage.…”

“…This present work expands on this previous investigation by employing a technologically important Zn-Zr/Nd based magnesium alloy (commercially available Electron TM 717 (E717), also known by other alloy designations as ZE10 or ZEK100). The promising mechanical properties of this particular alloy for certain automotive applications 8,9 has led to a significant present interest in understanding its corrosion behaviour, both in the presence 5 and absence 10 of an organic protective coating. Using model PVB coatings applied to E717 substrates, underfilm corrosion was initiated by applying a range of different chloride based containing salts to a penetrative coating defect, including NaCl, LiCl, KCl, CsCl, MgCl 2 , and HCl.…”