Measurement of Adhesion Strength of Al Electroplating Film on AZ31, AZ61, and AZ91 Substrates
Authors: Onishi Masato , Ueda Mikito , Matsushima Hisayoshi , Habazaki Hiroki , Washio Kota , Kato Akira 
Published: February 2020

Introduction

Mg alloys recently have a great attention and are expected to be substitute materials for iron and aluminum in the fields of automobile and aerospace etc, due to the lightweight and high specific strength. The alloys are also used in the electronic devices since they have a high electrical conductivity and electromagnetic shielding properties. However these alloys have poor corrosion resistance because dense oxide film is not formed on the surface naturally. Therefore chemical conversion or anodization is required as a surface treatment of Mg alloys. However these treated surfaces show insulation properties, therefore the usage of the alloys for the electronics parts is limited.
Electroplating of Al on Mg substrate has some merits for corrosion protection. Because dense passive film is formed on the surface, and if galvanic corrosion occurs, the corrosion rate may progress slowly due to close standard electrode potential in both metals. Therefore development of direct electroplating technique is expected from industry.
In the previous studies, relatively good Al electroplating film was formed on AZ121 (12 mass%Al, 1 mass%Zn) alloy and relatively poor adhesion films were formed on AZ31 (3 mass%Al, 1 mass%Zn) and AZ91 (9 mass%Al, 1 mass%Zn). However few papers reported about relationship between adhesion strength and substrate composition of Mg alloys.
In this study, we aim application of direct Al electroplating for AZ31, AZ61, and, AZ91 alloys. As the first step of the study, relationships between adhesion strength of Al electroplating film and composition of substrate of Mg alloy or grain size of AZ31 alloy were investigated.

Experimental

The electrolyte was prepared by mixing of AlCl3and 1-ethyl-3-methylimidazolium chloride (EMIC) with a molar ratio of 2 : 1. Substrates of AZ31, AZ61, and AZ91 alloys were used to investigate dependency of Al concentration in substrate. AZ31 of average grain size with 12.3, 22.8, 39.5, 88.6, and 111 µm were used to investigate dependency of grain size. A pure Al plate was used as a counter electrode. These substrates were polished and ultrasonically washed by acetone, then polished again in the glove box before the electroplating. A pure Al wire was used as a reference electrode. Electroplating experiment was carried out in the glove box with a pure argon gas by current pulse electrolysis under the condition of frequency of 4 Hz, duty ratio of 80 %, current density of 1.7 mA cm-2, electric quantity of 15 C cm-2, and temperature of 283 K. The surface morphologies and cross-sectional view of electroplated films were observed by scanning electron microscopy (SEM). The adhesion strength of Al film for Mg alloys substrates were measured by PosiTest Pull-off Adhesion Tester.

Results and discussion

In the specimens of Al electroplating, silver white surface were formed on the every substrate of Mg alloys. The electroplated film had no space between Al electroplating film and AZ91 substrate, however there were small spaces in AZ31 and AZ61. The adhesion strength of Al electroplating films for AZ31, AZ61, and AZ91 were 1.0, 1.1, and 1.3 MPa respectively. Increase of Al concentration in Mg alloys, the adhesion strength increased. Mg alloys are consisted by Mg-rich α-phase and Al-higher content γ-phase (Mg17Al12). Then Al electrodeposits easily on γ-phase compare with α-phase, and it may be considered that adhesion strength of electrodeposits for γ-phase is stronger than α-phase. When Al concentration in Mg alloys increases, area of γ-phase Mg on the surface also increases. It results that the adhesion strength also increases with increasing Al concentration.
In the electroplating for several grain sizes of AZ31, adhesion strength of 12.3, 22.8, 39.5, 88.6, and 111 µm were 0.83, 0.91, 0.88, 1.02, and 1.03 MPa, respectively. It may be considered that the adhesion of the Al electrodeposits on grain boundary is not strong compare with the grains. The surface formed by lager grain size has low grain boundary. Therefore electroplating film on lager grain size containing low grain boundary may show stronger adhesion strength.