STUDY ON GROWTH MECHANISM OF LANTHANUM SALT CONVERSION COATING ON GALVANIZED STEEL
Received date: 2009-08-21
Revised date: 2010-01-22
Online published: 2010-04-11
Supported by
Supported by International Zinc Association(No.IZA/CA/059)
Rare earth conversion coating was proposed as one of the most prospective alternatives to toxic chromate conversion coatings on some metals and alloys. In this work, the lanthanum salt conversion coating was obtained by immersing a galvanized steel in a lanthanum salt passivation solution modified with citric acid. Scanning electron microscopy (SEM), X–ray energy dispersive spectrometer (EDS), X–ray photoelectron spectroscopy (XPS) and infrared spectroscopy (IR) were used to investigate its structure and composition. Neutral salt spray (NSS) tests were used to eval- uate the anti–corrosive performance of the coating and the optimum range of treatment time. The results show that the lanthanum salt conversion coating modified with citric acid mainly consists of LaCit, La(OH)3/La2O3 and a trace of Zn(OH)2/ZnO. When treatment time is more than 10 min, the modified coating on the samples consists of two layers: the inner layer is compact and the outer lanthanum–riched layer is thin and poriferous. Cracks occur in the coating when treatment time within 10 s and are evolved during further treatment. The thickness of the coating obtained on galvanized steel that the immersion time is 1 min is approximately 250 nm, and increasing with treatment time within 60 min. The growing rate of the coating is faster than that of conversion coatings without modified with citric acid. The growth process of the coating can be divided into three stages: at the initial time complexes ions of lanthanum citrate were adsorbed on the whole surface of galvanized steel, then lanthanum citrate and lanthanum hydroxide are co–deposited on surface, and in the final stage there is lanthanum hydroxide mainly deposited on surface of the coating. The corrosion resistance of the coating is close to that of a chromate conversion coating. The optimum treatment time is 10 to 30 min.
Key words： rare earth conversion coating; galvanized steel; citric acid
KONG Gang , ZHONG Zheng , LV Jin-Tang , CHE Zhun-Shan . STUDY ON GROWTH MECHANISM OF LANTHANUM SALT CONVERSION COATING ON GALVANIZED STEEL[J]. Acta Metall Sin, 2010 , 46(4) : 487 -493 . DOI: 10.3724/SP.J.1037.2009.00546
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