Zr-based conversion coatings for multi-metal substrates

Abstract

In this PhD work, a new surface treatment based on the application of Zr-based conversion coatings by immersion in a Cu containing Zr-based conversion solution was investigated as a replacement of the traditional phosphating process for the automotive industry. Nowadays most of the cars are made of a combination of different metals, therefore, one of the aims of this work is to study the formation mechanism of Zr-based conversion coatings on different metallic surfaces. Prior to the immersion in the conversion solution, the metals are exposed to a surface conditioning treatment to remove the contaminants which might interfere in the formation of the film. Taking this into consideration, this PhD thesis is also focused on understanding the effect of the surface conditioning treatments on the deposition mechanism and kinetics of Zr-based conversion coatings on different alloys. In the first part of this PhD work, the formation mechanism and surface characteristics of Zr-based conversion coatings deposited on aluminium alloy 6014 (AA6014), cold rolled steel and hot dip galvanized steel were evaluated. This was done by combining Open Circuit Potential (OCP) measurements during the formation of the coatings and ex-situ surface analytical techniques. The results indicated that regardless of the metal, the deposition of the Zr-based conversion coating initiates and proceeds by the same mechanism. This process is initiated by the chemical dissolution of the oxide film by the free fluorides present in the conversion bath. This is followed by the deposition of the Zr-based conversion films, which is triggered by the alkalization of the metal/solution interface. Furthermore, the results also showed that the surface chemistry, elemental distribution and thickness of the resulting Zr based conversion coatings highly depend on the type of metallic substrate. The effect of convection on the formation of Zr-based conversion coatings on different metals was also assessed by implementing in-situ OCP measurements and Auger Electron Spectroscopy (AES) depth profiles. The results indicate that the conversion film thickness increases with the conversion bath stirring rate, but this increment depends on the metal substrate. The second part of this PhD thesis is focused on understanding the effect of the surface conditioning treatments on the deposition of Zr-based conversion coatings. A surface conditioning treatment can significantly modify the chemistry of the oxide film, such as the fraction of hydroxyl groups present in the outer part of the films. To investigate the influence of this parameter on the formation of the Zr-based conversion films, different model surface conditioning treatments were implemented in order to form oxide films with different hydroxyl fractions at similar oxide layer thicknesses prior to immersion in the conversion solution. The elemental distribution and surface chemistry of differently treated AA6014 and cold rolled steel specimens before and after the formation of the films were evaluated by means of AES depth profiles, Field Emission AES and X-ray Photoelectron Spectroscopy (XPS). The OCP as a function of time was recorded during the formation of the films in order to study the electrochemical reactions involved in the process insitu. The results show that for AA6014, higher hydroxyl fractions are beneficial for the formation of the Zr-based conversion coatings. The observed trend is linked to the effect of surface hydroxyls in the first stage of the formation of the Zr-based conversion coating, which is dissolution of the metal (hydr)oxides by the free fluorides present in the conversion solution. In addition, the results presented in this study indicate an inhomogeneous Cu distribution within the Zr-based conversion layers deposited on the differently treated AA6014. It was concluded that these variations in the Cu composition at the surface originate from the surface conditioning treatment and the Zr-based conversion treatment. The results presented in this part of this PhD thesis show that the largest impact of the surface hydroxyls on the formation of Zr-based conversion coatings is observed on AA6014, followed by zinc and for cold rolled steel no effect was observed.