Vol. 54 No. 4

Material Science and Technology of Japan, 54㸦2017㸧123㹼127

13Regular Paper
Development of Chromium-Free Corrosion Resistant Conversion
Coatings
Kaori URASAKI1,*, Shota TAKEMURA2, Tadashi DOI2, Yoshikazu TERANISHI2 andSatoshi KUWAHARA2
1

Bangkok Branch, Tokyo Metropolitan Industrial Technology Research Institute, Japan, MIDI Building, 86/6,
Soi Treemit, Rama IV Road, Klongtoey, Bangkok 10110, Thailand

2Surface Coating and Chemical Technology Group, Tokyo Metropolitan Industrial Technology ResearchInstitute, 2-4-10, Aomi, Koto-ku, Tokyo 135-0064, Japan

Received Jun. 12, 2017; accepted for publication Jul. 17, 2017Abstract
Chromate conversion coatings applied to zinc-plated steel for corrosion protection usually consists of various
chromium compounds that include hexavalent chromium (Cr6+). As the use of highly toxic and carcinogenic
chromium compounds has been regulated worldwide, the development of a chromium-free corrosion resistantconversion coating for zinc plating has become necessary. In this study, we used vanadium as an alternative tochromium, and successfully prepared vanadate conversion coatings and have found that it has corrosion resistance;
however, the corrosion resistance of the vanadate conversion coating was lower than that of the chromate coating.
Therefore, we studied improvement of corrosion resistance of the vanadate conversion coating by clarification of thevanadate conversion coating film composition and optimization of the solution composition. X-ray photoelectron
spectroscopy analysis of the coating surface showed that the vanadate conversion coating mainly consists of V2O3.
Furthermore, it was found that the NaNO3 concentration in the conversion solution significantly affected the coating
formation and the corrosion resistance. The concentration of NaNO3 was optimized to develop an optimum vanadateconversion solution and film composition.
Keywords: Chromium-free, Zinc plating, Conversion coating, Corrosion resistance1.

Introduction

a chromate conversion coating is applied to protect the surface of6+

Chromate conversion coating (Cr ) is a type of surface

the zinc plating. The chromate conversion treatment provides

treatment applied to zinc-plating to prevent the corrosion of steel

excellent corrosion resistance and is economical. However, the[1]. Various interference colors normally are obtained on

chromate conversion coating consists of hexavalent chromium

chromate conversion coating. And it is extreme thin film which(Cr6+), the use of which has been regulated worldwide. Therefore,

is composed by mainly chromium compound. In general, thetrivalent chromium (Cr3+) conversion coatings, which are lessthickness of the chromate conversion coating is only 100-200 nm

toxic, have been developed and used for practical applications

and that of the zinc electroplated layer is approximately 5-10 ȝP

[2]. However, trivalent chromium may be converted toFurthermore, the ionization energy of zinc is greater than that ofhexavalent chromium by some coexisting material. In addition,iron; therefore, zinc forms the corrosion product easily.

the cost of the trivalent chromium conversion solution is much

Consequently, the steel surface is protected from corrosion by thehigher than that of the chromate conversion solution. Therefore,

formation of corrosion products of zinc. This implies that zincthe development of a chromium-free conversion coating for zincplating performs as a sacrificial layer preventing the corrosion ofplating is required for industrial applications.

steel. However, surface of zinc plating easily discolor. Therefore,*Corresponding author: urasaki.kaori@iri-tokyo.co.th(123)