Research

http://www.electrochemsci.org/papers/vol7/7054235.pdf (Good site about the Mg in stagnant and aerated solutions) Mg is very reactive metal because of which, its free element is not naturally found on earth, though once produced, it is coated in a thin layer of its oxide. The formation of this layer on the surface of Mg and its alloys does not provide full protection against corrosion, especially in chloride containing environments. This occurs as the Cl‒ ions from the surrounding environment penetrate the oxide layer and reach the surface of Mg then react with the metal substrate. The high corrosion susceptibility is also expected as a result of the high active potential of Mg. The presence of impurities and second phases within Mg alloys can be considered as active cathodic sites, which accelerate the local galvanic corrosion of the alloy matrix [11]. The presence of impurities such as iron, copper and nickel in low amounts increases the corrosion resistant of the Mg alloy. These elements if present in high contents will act as active cathodes with small hydrogen over voltage and result in dissolution of the Mg matrix [12-14]. This explains why the corrosion and corrosion protection of Mg in different aggressive electrolytes have been reported [15-19]. The objective of the current work is to compare between the effects of naturally aerated stagnant Arabian Gulf seawater (AGS) and 3.5% sodium chloride (3.5% NaCl) solutions on the corrosion of magnesium after its immersion for 1 hour and 6 days. The study was carried out using weight-loss immersion test for 600 hours along with SEM/EDX investigations and variety of electrochemical measurements such as polarization (CPP), current time (PCT) and electrochemical impedance spectroscopy. It is well known that AGS is a complex mixture of inorganic salts, dissolved gases, suspended solids, organic matter and organisms [20,21]. It has inorganic salts that include Na+, Mg2+, K+, Ca2+, and Sr2+ as well as very high concentrations of chloride (24090 mg/L), and sulfate (3384 mg/L) ions. AGS also has HCO3-, Br-, and F-, with total dissolved solids (TDS) of 43800 mg/L [21]. The presence of such high TDS concentration represents a very corrosive medium. Oxygen content also has a marked effect on corrosivity of seawater. At the same time, 3.5% NaCl solution has been also reported to simulate the percentage of chloride ion in the seawater and to be an aggressive medium towards many metals and alloys [22-41]. The weight loss (Δm, mg.cm−2) and the corrosion rate (KCorr, mg.cm−2.h−1) over the exposure period were calculated according to the previous studies as follows [40-43]: ) 1(A int fin m m m ) 2(A t int fin Corr m m K SEE WEBPAGE FOR PROPER FORMULAE Where, mint is the initial weight before immersion, mfin is the final weight after exposure to the test solution, A is the total surface area, 54.0 cm2, and t is the time of exposure in hours.