Surender Maddela

Influence of Surface Pretreatment on Coating Morphology and Corrosion Performance of Cerium-Based Conversion Coatings on AZ91D Alloy

Abstract The corrosion protection of cerium-based conversion coatings (CeCC) formed on AZ91D magnesium alloy has been studied using potentiodynamic polarization measurements, electrochemical impeda...

A Comparative Study on the Corrosion Resistance of Cerium-Based Conversion Coatings on AZ91D and AZ31B Magnesium Alloys

In the present work a comparative study has been carried out on the surface morphology, electrochemical properties and corrosion performance of cerium-based conversion coatings (CeCCs) on AZ91D and AZ31B magnesium alloys. The as-deposited coatings consisted of a two layer structure: a continuous Mg/Al oxide transition layer of ~ 50 nm thick and a CeCC layer of ~ 400 nm thick. Potentiodynamic polarization and impedance spectroscopy results using a 0.6 wt% NaCl and 0.6 wt% (NH4)2SO4 electrolyte showed that cerium-based conversion coatings enhanced the corrosion resistance of AZ91D and AZ31B magnesium alloys compared to polished uncoated samples. Good correlation of electrochemical test and ASTM B117 salt spray was realized.

Deposition of Cerium-Based Conversion Coatings on Aluminum Alloy 380

Cerium-based conversion coatings were deposited on as-cast aluminum alloy 380 substrates by a spontaneous immersion process. In this study, the effects of rinsing temperature prior to immersion in the coating deposition solution were studied with respect to the surface morphology, electrochemical response, and corrosion resistance of the coatings. Panels rinsed at 25∘C prior to coating had large cracks and holes in the coating. In contrast, panels rinsed at 100∘C prior to coating had a uniform coating morphology with fewer, smaller cracks. Electrochemical testing revealed that coatings deposited on substrates rinsed at 100∘C had higher impedance (~80 kΩ·cm2) and lower corrosion current (~0.34 𝜇A/cm2) compared to coatings deposited on substrates rinsed at 25∘C, which had 10 kΩ·cm2 impedance and 2.7 𝜇A/cm2 corrosion current. Finally, ASTM B117 salt spray testing showed that rinsing at 100∘C prior to coating resulted in cerium-based conversion coatings that could resist the formation of salt tails for at least 8 days.

The Influence of Galvanic Current on Cerium‐Based Conversion Coatings on Mg, AI, and Galvanized Steel Couples

The influence of galvanic current on cerium-based conversion coatings (CeCCs) for magnesium (AZ91, AZ31), aluminum (6016), and electro-galvanized steel (EGS) couples has been studied using zero resistance ammeter (ZRA) measurements in prohesion solution. The galvanic current measured between magnesium-aluminum, magnesium-galvanized steel, and aluminum-galvanized steel couples correlated with significant changes in coating morphology and deposition rate. The ZRA galvanic currents (mA) were 0.02 for 6016-EGS, 0.38 for AZ91-EGS, 0.72 for AZ91-6016, 1.08 for AZ31-EGS, and 1.08 for AZ31-6016 couples. The corrosion performance of the coated couples was evaluated by ASTM Bl 17 neutral salt spray testing. Cerium conversion coated couples performed better in salt spray testing compared to uncoated couples. The correlation of galvanic current, cerium deposition, and corrosion performance will be discussed.

Effect of Thickness on the Morphology and Corrosion Behavior of Cerium‐Based Conversion Coatings on AZ31B Magnesium Alloy

Cerium-based conversion coatings (CeCCs) were deposited onto AZ31B magnesium alloy substrates using a spontaneous reaction of CeCl3, H2O2 and gelatin in a water-based solution. The coating thickness was adjusted by controlling the immersion time in the deposition solution. Prior to deposition, the AZ31B substrates were treated using an acid pickling in nitric acid and then an alkaline cleaning in sodium metasilicate pentahydrate. After deposition, the coated samples were immersed in a phosphate bath that converted cerium oxide/hydroxide into cerium phosphate. Electrochemical impedance spectroscopy, potentiodynamic polarization and neutral salt spray testing studies indicated that ~100 nm thick CeCC had better corrosion performance than ~400 nm coatings. Characterization of the CeCCs by transmission electron microscopy (TEM) revealed a three layer structure with different compositions.

Corrosion and Adhesion Properties of Cerium‐Based Conversion Coatings on Magnesium Alloys

Corrosion resistance and adhesion properties of cerium-based conversion coatings (CeCCs) on Al/Zn and Al/Mn magnesium alloys have been investigated. CeCCs were deposited by immersing magnesium panels in a cerium ion containing solution for 2 minutes. Corrosion resistance and adhesion strength of cerium coated magnesium panels were evaluated by electrochemical and pull tab testing methods, respectively. The CeCCs had very good adhesion to and significantly improved the corrosion resistance of magnesium alloys. Electrochemical test result showed that there was a measurable difference in corrosion resistance between the different magnesium alloys. Magnesium alloys with higher aluminum content had better corrosion resistance than alloys with lower Al content. The correlation of cerium deposition conditions, corrosion resistance, and adhesion strength are discussed.

Corrosion Behavior of Cerium-Based Conversion Coatings on Magnesium Alloys Exposed to Ambient Conditions

Exposure of CeCCs on AZ31B and AZ91D Mg alloys to ambient sunlight, temperature, and humidity was done to determine the effect on corrosion resistance. It was found that the CeCCs changed from pale yellow to almost translucent after 24 hours of sunlight exposure. The effect of the solar electromagnetic radiation on the morphological, chemical and optical properties of these coatings was investigated using SEM and UV-Vis characterization techniques. In addition, the corrosion performance of CeCCs before and after ambient exposure was studied by ASTM B117 neutral spray testing and electrochemical polarization measurements. In general, the changes in appearance did not adversely affect the corrosion performance of the coatings.