The evolution of the corrosion of iron in hydraulic binders analysed from 46- and 260-year-old buildings

Abstract

Corrosion patterns on samples taken on binder and rebars from two buildings, respectively, aged of 46 and 260 years old have been characterised by coupling different analytical methods at microscopic scale. Different corrosion patterns have been observed. The first one is constituted by the initial mill scale made of wustite, magnetite and hematite. The second one contains remains of this mill scale embedded in phases formed after aqueous corrosion: oxyhydroxides as goethite or lepidocrocite containing marblings of ferrihydrite, maghemite and magnetite. On the thicker and older layers, marblings were only constituted of magnetite and maghemite. It is proposed that the structural evolution of the pattern and their marbling is linked to wet/dry cycles and/or pH condition evolution during the corrosion processes.

Introduction

The understanding of long term corrosion mechanisms of low alloy steels in hydraulic binders is an important task in different application domains. Reinforced concretes are massively used in civil engineering. Buildings can be in service during several tens and sometimes several hundreds of years. Therefore, the present tendency is to build new structures that can challenge life-times longer than hundred years, especially in the nuclear industry [1], [2]. In a completely different domain, significant quantities of iron were used for the building of ancient monuments and are often embedded in hydraulic binders [3], [4]. Their corrosion behaviour has to be understood to improve strategies of conservation and restoration [5], [6]. To this purpose, several complementary approaches are used. “Short-term” laboratory experiments and studies on “long-term” analogues are combined in order to gather crucial parameters for modelling and benchmarking [7].

To reach these different goals, a scientific challenge is to perform a phenomenological modelling of long term corrosion, based on the understanding of precise mechanisms [7], [8]. To that purpose, the knowledge of the evolution of the corrosion patterns versus the age of the system is a crucial information. Several studies have been performed in laboratory to assess the “short-term” corrosion mechanisms [9], [10]. Electrochemical and gravimetric measurements have been carried out on steels immersed in different solutions simulating various concrete carbonation steps. The electrochemical results have highlighted that steel depassivation can occur at pH between 10 and 9.4 depending on the carbonate content and the redox potential of the interstitial solution. Moreover, analyses of potentiodynamic and potentiostatic tests have pointed out that the active corrosion mode of the system is controlled by a transport phenomenon through the oxide layer. Lastly, gravimetric measurements carried out for active corrosion in different solutions have shown that the average corrosion rate is decreasing with time. This is due to the formation of a porous oxide layer at the surface of the steels. Concerning the “long-term” approach, several corrosion systems (binders and steel reinforcements) were sampled on historical buildings of different ages. They were characterised by a combination of various structural and chemical microprobes [11], [12]. For buildings aged of several hundred of years, these studies demonstrates that, whatever the composition of the binder (plaster, mortar, etc.), the corrosion products have always the same morphology and are constituted of the same phases [13], [14]. Layers are mainly constituted of iron oxyhydroxides (goethite α-FeOOH or lepidocrocite γ-FeOOH) containing iron oxides marblings made of magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃), or a mix of these two phases.

For buildings built in more recent periods, several studies sometimes found another kind of corrosion pattern that seems to be denser. For example, Duffo et al. [1] has observed on a building aged of 65 years made of reinforced concrete, an internal zone apparently constituted of a mixture of maghemite and magnetite identified by Mossbauer spectroscopy. This author claimed that this zone was formed during the beginning of the corrosion period. On reinforced concrete beams, Poupard et al. [15] identified magnetite, wustite and goethite. These authors suggested that this kind of system corresponds to the initial state of the rebars and that wustite probably came from the manufacturing of the steel under high temperature. This “mill scale” is sometimes mentioned in the literature dedicated to corrosion in concrete as an initial layer [16], [17], [18]. For that reason, some authors tried to characterise it using various physico-chemical methods. Cook [19] studied it by Mössbauer spectroscopy and showed that the it is constituted of 53% FeO, 33% Fe₃O₄ and 14% Fe₂O₃. These phase proportions can be retrieved at room temperature in case of relatively fast cooling. Wustite constitutes the inner zone of the layer and hematite the outer one. Magnetite is located between these two layers. In most of cases cooling is sufficiently slow to provoke a partial or total disappearing of wustite that decomposes into a mix of magnetite and hematite [20].

After a given period, because of the modifications of the concrete chemistry (carbonation in that case) but also because of specific conditions (pre-cracked concrete or presence of defects at the steel/concrete interface followed by the succession of wet/dry cycles [21]), this initial layer progressively disappears. Some studies [18] made on systems constituted of reinforcements corroded in concrete for about 50 and 80 years identified the presence of the initial mill scale in some places and a goethite matrix embedding marblings made of maghemite and poorly crystallised oxyhydroxides as ferrihydrite (5 Fe₂O₃, 9 H₂O). The crystalline structure and chemical formula of this latter phase are still under discussion [22]. In the aforementioned study of Duffo et al. [1], the initial layer of magnetite and an external more porous zone made of goethite were observed.

Thus, the initial layer (formed during hot rolling or during the first stages of corrosion) progressively transforms into a system mainly constituted of goethite. Nevertheless the nature of the marblings embedded in the oxyhydroxide matrix is various: reactive phases (as ferrihydrite) or more stable one (as magnetite and/or maghemite). The physico-chemical conditions that lead to the formation of these diverse phases must be now investigated deeper. It is also crucial to verify if the marblings and the corrosion products observed on the very long term systems (i.e. formed after several centuries) could be the result of the evolution of the medium term (i.e. formed on several ten years). As a matter of fact, in the phenomenological atmospheric and concrete corrosion models, these marblings are considered to be involved in the corrosion process as they can be reduced during the wetting phase of the atmospheric corrosion process [23], [24], [25], [26], [27]. For these reasons the study presented in the present paper aims at characterising at the microscopic scale the corrosion systems of two buildings aged of 46 and 260 years. Corrosion layers are characterised and particular attention will be put on marblings in order to enlighten the corrosion phenomenon that occurred during very long periods.