Self healing ability of silicate conversion coatings on hot dip galvanized steels

doi:10.1016/j.surfcoat.2011.03.088

Surface and Coatings Technology

Volume 205, Issue 19, 25 June 2011, Pages 4507-4513

https://doi.org/10.1016/j.surfcoat.2011.03.088 Get rights and content

Abstract

The self-healing ability of a coating is an important aspect for evaluating its corrosion resistance. Chromate conversion coatings show excellent anti-corrosion properties because of their compact microstructure and their strong self-healing ability. Silicate conversion coatings, which are potential replacement candidates for chromate conversion coatings, have good corrosion resistance, but their self-healing abilities have not been determined. In this paper, silicate conversion coatings are prepared by immersing hot dip galvanized steel sheets in sodium silicate solutions with SiO₂:Na₂O molar ratio of 1.00 and 3.50. The coatings are scratched with a blunt knife edge and corroded in a neutral salt spray chamber for certain lengths of time. The corrosion products in and near the scratched area are investigated by scanning electron microscopy and energy-dispersive spectroscopy. The self-healing ability of silicate conversion coatings is discussed. The results show that silicate conversion coatings have self-healing abilities under the experimental conditions. Higher SiO₂:Na₂O molar ratios lead to improved self-healing abilities. During the corrosion process, the silicate anions in the coating migrate to the scratched area, where a new conversion coating composed of Zn, O, and Si is formed. This delays corrosion in the scratched area.

Research highlights

► Single silicate coating on hop dip galvanized steel possesses self sealing ability. ► Self healing ability of the silicate coating depends on SiO₂:Na₂O molar ratio. ► Silicate anions migrate from the coating to the scratch during the corrosion process. ► A new conversion coating composed of Zn, O and Si is formed in the scratched area.

Introduction

Chromate conversion coatings are widely applied to metal post-treatments because of their high corrosion resistance and strong self-healing ability. However, because of their toxicity and carcinogenicity, hexavalent chromate use has been limited in many countries, especially in Europe [1]. In recent years, chromate-free passivation has become an area of active investigation [2], [3]. Sodium silicate has been used as an inhibitor for many years due to its effective inhibitory properties and low cost [4]. Silicate conversion coatings, which are mainly composed of zinc oxides/hydroxides, zinc silicate, and SiO₂, are also used to improve the corrosion resistance of zinc [5], [6], [7], [8]. Generally, silicate conversion coatings that possess good corrosion resistance are prepared from sodium silicate solutions with high SiO₂:Na₂O molar ratios (greater than 2.00) [6], [7], [8], [9], [10].

Chromate conversion coatings have unrivaled self-healing abilities, which are believed to arise from the migration of soluble Cr(VI) compound in the coating to a scratch or defect, where they are reduced to form a new protection layer [11], [12], [13]. Self-healing is defined as the ability of a material or surface to automatically heal or repair damages [14], [15]. Published reports show that silicate conversion coatings can fill the pores of zinc phosphate coatings and automatically repair damaged areas in the phosphate coating [16], [17]. Aramaki [18] prepared a self-healing protective film composed of an organosiloxane polymer containing Na₂Si₂O₅ and Ce(NO₃)₃ on a zinc electrode previously treated with a Ce(NO₃)₃ solution. A chemical passive film composed of Zn(OH)₂, ZnSi₂O₅, and Ce³⁺-Si₂O₅²⁻salts or complexes formed on the scratched surfaces, and preferential deposition of Si₂O₅²⁻compounds occurred at film defects, thus preventing pitting corrosion at the scratch. Hamdy [19] formed a silicate/cerate conversion coating on aluminum alloys and found that the cerate and not the silicate formed a protective coating on the scratch. Therefore, silicate plays a different role in forming a conversion coating at the damaged area of a multi-component system. Whether or not single silicate coatings have self-healing abilities has not been determined.

In this paper, the self-healing abilities of silicate conversion coatings prepared by immersing hot dip galvanized (HDG) steels in sodium silicate solutions of various SiO₂:Na₂O molar ratios are investigated.

Section snippets

Experimental

The same preparations of HDG steels and silicate conversion coatings with those in the literature [10] were adopted.

Self- healing ability of the silicate coatings

The silicate conversion coating was thin and transparent. No significant difference in appearance between the silicate-treated samples and the untreated HDG samples was observed. The grain boundaries of Zn on these samples could be seen under SEM. Fig. 1-1a, 1-2a, and 1-3a shows fresh scratches on the scratched HDG, M1.00, and M3.50 samples not subjected to the NSS test. These scratches are relatively smooth and flat, but some micro scratches parallel to the scratch are visible on the scratch.

Discussion

The study in this paper is a continuing search of the literature [10]. Previous results obtained by the authors showed that the corrosion resistance of silicate coatings significantly increased with increasing SiO₂:Na₂O molar ratios of sodium silicate solutions. The coating had the best corrosion resistance when the SiO₂:Na₂O molar ratio was 3.50. No white rust was observed on the surface of M3.50 in two spray cycles. The silicate coating resistance R_f and the charge transfer resistance of HDG

Conclusions

Silicate conversion coating, a potential candidate of chromate conversion coating, has better corrosion resistance, but its self-healing ability is not quite sure. In this paper, silicate conversion coatings are prepared by immersing hot dip galvanized (HDG) steel sheets in sodium silicate solutions with SiO₂:Na₂O molar ratio of 1.00 and 3.50. The coatings are scratched with a blunt knife edge and corroded in a neutral salt spray (NSS) chamber for certain lengths of time. The corrosion products

Acknowledgment

The financial support of the Fundamental Research Funds for the Central Universities (2009ZM0072) is gratefully acknowledged.

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Self healing ability of silicate conversion coatings on hot dip galvanized steels

Abstract

Research highlights

Introduction

Section snippets

Experimental

Self- healing ability of the silicate coatings

Discussion

Conclusions

Acknowledgment

Prog. Org. Coatings

Appl. Surf. Sci.

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Appl. Surf. Sci.

Trans. Nonferrous Met. Soc. China

Surf. Coat. Technol.