Elsevier

Corrosion Science

Volume 51, Issue 9, September 2009, Pages 2107-2114
Corrosion Science

Corrosion of zinc–magnesium coatings: Mechanism of paint delamination

https://doi.org/10.1016/j.corsci.2009.05.042Get rights and content

Abstract

Due to their promising corrosion properties, metallic coatings containing magnesium are currently widely investigated for use as protective coatings for steel sheet. Particularly, alloying zinc coatings with magnesium results in a remarkable improvement of the corrosion resistance of the painted system. While some aspects of this improvement have been understood, the progress of the corrosive degradation of the alloy coating/paint interface has not been reported in detail. In this paper, the delamination of a model polymer from the intermetallic MgZn2 is described and a degradation mechanism proposed. Aspects for the design of stable interfaces are discussed.

Introduction

In order to protect steel from corrosion it is often zinc-coated, i.e. galvanized, and painted. For automotive as well as building applications, the zinc coating consists essentially of pure zinc, with some minor additions of aluminium and silicon in the case of hot-dip coatings. In the last two decades economical as well as environmental considerations together with increasing requirements for performance have led to large efforts to develop novel zinc-based metallic coatings with increased protective properties.

The corrosion properties of zinc coatings can be improved by the addition of one or more alloying elements [1], [2], [3], [4]. Very promising results have been obtained for magnesium as an alloying element. Indeed, painted zinc–magnesium coatings on steel sheet exhibit excellent corrosion protection, exceeding that of conventional, unalloyed zinc coatings by far and also that of many other alloy coatings. As a result, of all novel alloy coatings, zinc–magnesium coatings appear to offer the best possibilities to reduce the coating thickness and maybe even to omit pre-treatments.

The corrosion of zinc–magnesium coated steel sheet was mainly studied under atmospheric corrosion conditions and focused on unpainted material. The results show that the superior corrosion resistance is due to reduced dissolution of the coating as well as an enhanced cathodic protection of the steel, both caused by protective surface layers [5], [6], [7], [8]. However, some controversy still exists about the role of magnesium in the formation in the protective layers, especially on the zinc–magnesium coating itself. The different results suggest that the composition of the protective layer is highly dependent on the exposure conditions. Generally, after salt spray testing, no magnesium is found in the corrosion products and magnesium appears to promote solely the formation of protective zinc corrosion products [5], [7], while immersion may also lead to the formation of protective magnesium corrosion products [8]. Older corrosion studies show that the corrosion rate of immersed zinc–magnesium alloys is higher than that of zinc [9], [10], which leads to the conclusion that the presence of protective magnesium corrosion products during immersion is likely limited to very specific conditions, also in the case of zinc–magnesium coatings on steel. A recent study on the corrosion of zinc–magnesium alloys shows that protective magnesium corrosion products are formed after exposure in a climatic chamber simulating atmospheric corrosion conditions [11].

The knowledge regarding the corrosion mechanisms specific to painted zinc–magnesium coated steel sheet is very limited. We reported that the corrosion resistance of painted MgZn2, an intermetallic that is found in most of the alloy coatings, is extremely high at a scratch compared to pure zinc [12]. This behaviour could be attributed to the complete inhibition of cathodic delamination. In fact, the gradient of the potential between the defect (or scratch) and the intact metal-oxide/polymer interface was found not to permit galvanic coupling necessary for cathodic delamination [12]. The unusual potential difference itself is ascribed to the oxygen reduction properties, or more precisely to the electronic properties, of the different passive oxides in the defect and at the intact interface: in the defect, the passive layer consists of zinc oxide and at the corrosion potential oxygen reduction is therefore controlled by the transport of dissolved oxygen species through the electrolyte to the surface; at the intact interface, however, oxygen reduction is inhibited by a passive layer which consists predominately of magnesium hydroxide [13]. As a result, the potential of the intact interface is more negative than that of the defect, even though the material is passive in the former case. Normally, this situation can be expected to lead to a delamination mechanism of the anodic type. However, no investigation on that can be found in the literature.

In this paper, we discuss the corrosion of painted MgZn2 with a defect, under constant conditions of high humidity and an electrolyte covered defect. Special attention is given to the galvanic coupling between defect and intact interface. The results are supported by electrochemical measurements and immersion tests of unpainted material. The design of stable interfaces in general and in the case of zinc–magnesium coatings in particular is discussed and some simple design rules proposed.

Section snippets

Cathodic delamination, corrosion of metallic coated steel and possible role of alloying elements

Cathodic delamination is a corrosion mechanism that can lead to a particularly fast destruction of the metal/paint interface [14], [15], [16]. Cathodic delamination is caused by the electrode potential difference between a defect in the paint and the surrounding intact area. High metal dissolution in the defect causes a negative corrosion potential, while in the intact area the metal is passive and the potential usually more positive. For instance, for painted iron the potential in the intact

Passivity of MgZn2 and oxygen reduction properties

In order to study the electrochemical processes at the MgZn2 oxide/polymer interface we investigated the passivation and oxygen reduction behaviour of MgZn2 electrodes in borate buffer solutions [13], [21].

At low pH (pH 7.7), MgZn2 (and zinc) does not passivate and the corrosion potential is quite negative (−0.92VSHE for de-aerated conditions). In the moderately alkaline pH range (pH 9.2 and 10.7), MgZn2 passivates and undergoes pitting in the presence of chloride (see Fig. 1 for pH 10.7). The

Experimental

The intermetallic phase MgZn2 was produced by melting pre-weighed quantities of the two metals of at least 99.9% purity in an induction heater. The cast was cooled down over several hours under ambient conditions. It was used as cast and identified to be single phase MgZn2 with SEM/EDX and XRD. Pure zinc samples used were 99.99%. Samples were surface finished by grinding on 1200 microcut abrasive paper in de-ionised water, cleaned in an ultrasonic bath containing absolute ethanol and dried in a

Corrosion in the unpainted state

The corrosion of MgZn2 was studied by one day immersion in aerated 0.5 M potassium chloride solution (for details see Section 4). During the immersion the corrosion potential was monitored, after the experiment the corrosion morphology was studied with a scanning electron microscope and the corrosion products and passive layer were analysed with EDX and XPS. A mass loss measurement was carried out to determine the corrosion rate.

Starting from a value of −0.79VSHE, the corrosion potential reached

Summary and conclusion

In the case of MgZn2, no cathodic delamination is possible due to an adverse potential gradient between defect and intact interface. In contrast, delamination proceeds via an anodic type of delamination, which is triggered by the migration of ions at the metal-oxide/polymer interface. The rate of delamination (or undercreep) is very low, but not negligible over long term exposures.

In order to decrease the driving force for the migration of ions and the galvanic coupling, the potential gradient

Acknowledgement

Funding by the German Ministry for Education and Research is gratefully acknowledged.

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