Composition and corrosion behaviour of galvanised steel treated with rare-earth salts: the effect of the cation

doi:10.1016/S0300-9440(01)00250-8

Progress in Organic Coatings

Volume 44, Issue 2, April 2002, Pages 111-120

https://doi.org/10.1016/S0300-9440(01)00250-8 Get rights and content

Abstract

The corrosion performance and the composition of galvanised steel treated by immersion in nitrate solutions of rare-earth metals (cerium, yttrium and lanthanum) were evaluated by electrochemical techniques and surface analysis. The surface film consists of a mixture of rare-earth oxides/hydroxides, which hinders the corrosion reactions of the substrate by reducing the rate of both the cathodic and the anodic reactions. Although all the films provide corrosion protection, the effect seems to be more pronounced for lanthanum. The behaviour of the films formed on the surface is dependent on the rare-earth cation and on the treatment time.

Introduction

Metal pre-treatments are essential to ensure long-term performance of painted steel. The pre-treatment must act as an adhesion promoter and also provide corrosion protection. During the last decades, the most widely used pre-treatments utilise hexavalent chromium (Cr⁶⁺) compounds. However, due to increased environmental concerns and legislation limiting the use of Cr⁶⁺-based chemicals, new alternatives must be sought. Recent efforts have been focussed on the use of rare-earth (RE) salts as adhesion promoters and corrosion inhibitors. Currently, they are used in a wide range of applications, in areas such as metallurgy, glasses and ceramics. In the past 10 years, they have been successfully applied to control aqueous corrosion of several metals and alloys [1], [2], [3]. The most investigated REs are cerium, yttrium, lanthanum, praseodymium and samarium that were successfully applied to zinc [2], [3], aluminium [3] and steel [4], [5].

The pioneering studies in the field are those of Hinton and Wilson [2] and Hinton [3]. Those works report the use of cerium chloride for corrosion inhibition on zinc and aluminium and propose a cathodic mechanism to explain the formation of the RE oxide film. According to that mechanism, the cathodic reactions (oxygen reduction and hydrogen evolution) generate an alkaline environment that leads to localised precipitation of RE oxides and thus to the formation of the surface film.

Davenport and co-workers [6], [7] also investigated cerium deposition on aluminium alloys and showed that cerium can be oxidised from Ce³⁺ to Ce⁴⁺ in solution by dissolved oxygen and in a final step it precipitates as insoluble CeO₂ on the cathodic sites.

In spite of the efforts focussed on the use of cerium, other REs also show promising possibilities. Among these, yttrium and lanthanum have been investigated [3], [8] and the results suggest that they lead to increased corrosion resistance when applied to aluminium and its alloys and to steel [9]. Therefore, the possibility of pre-treatments based on these RE metals is also a field to be investigated.

The aim of this work is to characterise RE films deposited on galvanised steel surfaces by simple immersion of the substrate in RE (cerium, yttrium and lanthanum) nitrate solutions. The corrosion behaviour of the pre-treated substrates was evaluated under immersion in sodium chloride using the scanning Kelvin probe (SKP) technique, DC potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). The resulting films were also characterised using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).

Section snippets

Materials

In order to produce an RE surface film, cleaned and degreased hot-dip galvanised (HDG) steel substrates were immersed in RE nitrate solutions (0.01 M) at ambient temperature. Although long immersion times are not realistic in an industrial perspective, it was decided to test different immersion times, in order to obtain information on the deposition process. Therefore, immersion times of 10 s and 24 h were tested. After immersion, in the nitrate solution, samples were oven-dried at 150 °C during 50

Electrochemical measurements

The evolution of the open-circuit potential during immersion in the RE nitrate solution (Fig. 1) revealed an initial decay, probably due to dissolution of the native zinc oxides existing on the surface. Then the potential slightly rose in the cathodic direction followed by stabilisation. The potential shift in the cathodic direction is probably due to the development of an RE film on the surface. The time necessary to attain stable values was dependent on the RE cation, being around 15 min for Y

Discussion

RE salts are known to act as inhibitors of aqueous corrosion. Early investigations have shown that RE metals such as Ce, Y, La, Pr and Nd in the form of their chlorides improve the corrosion resistance of aluminium and its alloys during exposure to NaCl [12]. Electrochemical polarisation experiments demonstrated that these REs are able to hinder oxygen reduction reactions and consequently to act as cathodic inhibitors. The inhibition mechanism was also studied using the scanning reference

Conclusions

The formation of conversion layers on galvanised steel from rare-earth nitrate solutions result in a surface film that constrains the cathodic reduction of oxygen and the anodic dissolution of zinc, leading to a decrease of the corrosion rate.

The resistance to corrosion initiation under immersion in sodium chloride is dependent on the rare-earth cation: lanthanum seems to be more effective than yttrium and this one more effective than cerium.

The composition of the film is also dependent on the

Acknowledgements

The authors acknowledge funding from the ECSC programme (Contract No. 7210-C-066) and the support from POCTI. Our project co-ordinators, Dr. H. Edwards and Dr. D. Hammond from CORUS WTC, Port Talbot, UK, are also acknowledged for several interesting discussions.

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Composition and corrosion behaviour of galvanised steel treated with rare-earth salts: the effect of the cation

Abstract

Introduction

Section snippets

Materials

Electrochemical measurements

Discussion

Conclusions

Acknowledgements

J. Alloy. Compd.

Corros. Sci.

Corros. Sci.

Corros. Sci.

Corros. Sci.

J. Alloy. Compd.

J. Electrochem. Soc.