* E. Slavcheva, G. Petkova, P. Andreev IEES – Bulgarian Academy of Sciences, Akad. G. Bonchev Str. 10, 1113 Sofia (Bulgaria) Inhibition of corrosion of AZ91 magnesium alloy in ethylene glycol solution in presence of chloride anions E. Slavcheva*, G. Petkova and P. Andreev The influence of chloride ions on the corrosion of AZ91 magne- sium alloy in water/ethylene glycol solutions and the inhibiting ef- fect of lactobiono-tallowamide (LTA) were investigated using elec- trochemical and surface analysis methods. Potentiodynamic polar- ization curves in aqueous solution of ethylene glycol (50:50w%) containing 0.1 g.L 1 chloride and up to 0.5 g.L 1 LBT were ob- tained at room and at elevated temperatures. The chloride anions showed a distinct deteriorating effect as they caused pit initiation and accelerated the dissolution of the tested alloy. The selected or- ganic compound demonstrated good protective properties against corrosion of AZ91 magnesium alloy and behaved as inhibitor of mixed type hindering both the cathodic and the anodic partial re- actions. It showed inhibition efficiency of 77% at relatively low concentration of 0.2 g.L 1 and was considered as a promising cor- rosion inhibitor. The mechanism of inhibition was discussed on the basis of the electrochemical impedance spectroscopy (EIS) and XRD analysis of the surface. 1 Introduction Magnesium and its alloys with aluminium, zinc etc. are widely used as a structural material for the automotive and aeronautic industry because of their very low density and high mechanical strength. Their corrosion resistance on the other hand, varies essentially with the medium to which they are exposed. Mg alloys are resistant in ordinary industrial atmospheres and are quite inert to many organic liquids like aliphatic and aromatic hydrocarbons and their derivatives. At the same time substances, which contain reactive polar groups are aggressive towards these alloys and may cause serious cor- rosion attack, which depends not only on the solute but also on the volume, movement and temperature of the liquid. In aqu- eous media the magnesium alloys corrode easily when even small amount of aggressive ions are present in the solution [1]. Chloride is considered to be the most aggressive anion both towards magnesium and aluminium and the effect of chloride ions and pH on the corrosion behaviour of Mg-Al alloys has been extensively studied [2 – 7]. Magnesium dissolution in aqueous environment generally proceeds by electrochemical reaction with water producing magnesium hydroxide and hy- drogen gas. The mechanism of the reaction involves some in- termediate steps as formation of Mg 2þ , MgO or MgH 2 . The process is relatively insensitive towards oxygen concentration and depends strongly on pH of the solution [2 – 5]. Several studies [8 – 11] have shown that the corrosion behaviour of magnesium alloys depends significantly on the microstruc- ture, particularly on the amount and distribution of different intermetallic phases as well as on the grain size, although the mechanism of the processes involved is not fully understood. For instance, there are different controversial explanations in the literature for the role of the b (Mg 17 Al 12 ) phase in AZ91 alloys and its effect on the galvanic corrosion. Although the usage of corrosion inhibitors is the simplest method for protection of metals in aqueous environment, its application for protection of magnesium alloys is not very popular mostly because of the relatively high rate of the pro- cess and the inclination of Mg to localised corrosion. How- ever, in some special cases of appliance of these alloys such as in the cooling systems of vehicles, the addition of cor- rosion inhibitors is strongly recommended. The inhibiting ef- fect of several inorganic substances towards corrosion of dif- ferent magnesium alloys in aqueous media has already been reported [12 – 14]. At the same time, there are only a few pa- pers [15, 16] devoted to the usage of organic compounds as corrosion inhibition of magnesium alloys in aqueous solu- tions. Our previous investigations [16] have shown that some of the derivatives of lactobionic acid have good inhibit- ing properties with respect to corrosion of AZ91 alloy in wa- ter/ethylene glycol solution. This acid includes in its molecule gluconic acid and galactose (4-0-b-D-galactonic-D-gluconic acid), two essential constituents of human nutrition and is en- tirely friendly to the environment. It is capable of bonding mi- neral ions and its salts can form complexes. The objective of this study was to determine the effect of chloride ions on the corrosion of AZ91 alloy in aqueous solution of ethylene gly- col and to study the protective properties of one of the deri- vatives of lactobionic acid – lactobiono-tallowamid (LTA). This compound possesses a long hydrophobic hydrocarbon chain which is expected to be favourable for the inhibition efficiency in aqueous media. Both the molecular structure of the compound and its ecological acceptability justified the choice of LTA as a potential corrosion inhibitor. Materials and Corrosion 2005, 56, No. 2 Inhibition of corrosion of AZ91 magnesium alloy 83 F 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/maco.200403829