A New Sacrificial Corrosion Protection Mechanism for High Performance Zinc/Aluminum Flake Coating Systems and Applications
This report describes how the corrosion-protection capability of a new zinc/aluminum flake coating system is supported by a sacrificial corrosion-protection mechanism.
By Shusaku “Shu” Ishikawa and Yasuharu Takayama, Corrosion Specialists, Surface Treatment Group, Engineering, Yuken Industry Co. Ltd., Japan
Editor's Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF Sur/Fin 2013 in Rosemont, Ill., on June 12, 2013. A printable PDF version is available by clicking HERE.
Zinc/aluminum flake coating systems used for the automotive industry must have a thin film with high corrosion protection performance to meet the demanding requirements. Numerous steps are required to achieve a high level of corrosion resistance, and therefore, it is essential to reduce manufacturing costs and increase productivity. A new zinc/aluminum flake coating system has recently been developed. The coating creates a robust and versatile thin film with uniform covering power, and it can achieve high corrosion protection while not requiring the conventional full course of dip/spin process steps. This report, using some application examples, describes how the corrosion protection capability of this system is supported by a sacrificial corrosion protection mechanism.
Keywords: Zinc/aluminum flake coatings; corrosion protection.
In wide use like zinc and zinc alloy plating, zinc/aluminum flake coatings are often selected for the surface treatment of steel parts in the automotive and construction industries. Among the parts manufactured in those industries, the surface treatment for small parts or fasteners requires a thin film because of the requirements of dimensional accuracy. In particular, as parts processed with zinc/aluminum flake coatings quite often need to satisfy stringent requirements for corrosion resistance, the key is to design a thin film in such a way that good corrosion resistance can be sustained with the thin film. Therefore, it is essential for us to understand better the corrosion resistance mechanism of the coating film.
There have been many studies on the corrosion resistance mechanism of zinc alloy plating systems requiring high corrosion protection. It has been widely accepted as a given that it is important to form a corrosion product or basic zinc chloride which works as an insulator, instead of conductive zinc oxide, in an environment where chlorine ions are present. On the other hand, there have not been many studies on the corrosion protection mechanism of the zinc/aluminum flake coating film.
Zinc sacrificial corrosion protection is the main corrosion resistance mechanism of the zinc/aluminum flake coating film. Therefore, in order to achieve high corrosion resistance, we can easily imagine that the corrosion product developed from the film plays an important role, just as with zinc alloy plating.
In this study, we examined how the coating film develops a corrosion product in a corrosive environment containing chlorine ions, and looked into the correlation with corrosion resistance.
Preparation of test samples
We used two types of basecoats with different coating components in each of the test samples, designated B-1 and B-2. B-1 is our original basecoat system, which can provide high corrosion protection when it is used with our topcoat system applied on top. B-2 is a new basecoat system developed to improve the corrosion resistance of B-1. In addition to B-1 and B-2, we used a combination of B-1 and the topcoat (B-1+T). Zinc nickel plating (Zn-Ni) was also included as a comparative material. Test coupons made of cold rolled steel (JIS-G3141) were first coated with each coating system using a bar coater. The basecoat was cured at 260 to 300°C, while the topcoat was dried at 100°C when it was applied over B-1. The processed test coupons were then cooled down to room temperature and left sitting for more than 8 hr before they were used for the test. The zinc-nickel plating we used for comparison was not passivated. Table 1 shows a summary of each test film.