Anodic Dissolution of Spheroidal Graphite Cast Iron with Different Pearlite Areas in Sulfuric Acid Solutions

The rate equation of anodic dissolution reaction of spheroidal graphite cast iron in sulfuric acid solutions at 298？K has been studied. The cast irons have different areas of pearlite. The anodic Tafel slope of 0.043？V decade？1 and the reaction order with respect to the hydroxyl ion activity of 1 are obtained by the linear potential sweep technique. The anodic current density does not depend on the area of pearlite. There is no difference in the anodic dissolution reaction mechanisms between pure iron and spheroidal graphite cast iron. The anodic current density of the cast iron is higher than that of the pure iron. 1. Introduction Cast iron is widely used for pipes in neutral environments such as water and soil. A lot of experimental studies of cast iron for practical use were conducted [1–8]. A form of corrosion unique to cast irons is a selective leaching attack commonly referred to as graphitic corrosion [9, page 89]. A graphite network forms a corrosion-resistant phase called a graphitic layer. Therefore, the corrosion resistance of cast iron is said to be higher than that of steel. That is not necessarily the case. Laque reported that the corrosion rate of cast irons is lower than that of a steel in the atmosphere above the sea [2]. However, Paris and Bruniere [1] and Horikawa et al. [10] found that the corrosion rate of cast irons is nearly equal to that of steel in water and in atmosphere. Furthermore, a book said that graphitic corrosion does not occur in spheroidal graphite cast irons, because the graphite network does not exist [9, page 89], while another author reported that it occurs with both gray and spheroidal graphite cast irons [11]. These inconsistent results come from the complexity of graphitic corrosion. To elucidate the mechanisms of corrosion of cast iron, intensive studies are required. In this paper, the electrochemical dissolution of ferrous matrix is discussed. Cast iron is composed of pearlitic matrix, ferritic matrix, and graphite particles. The area of pearlite influences the physical properties such as tensile strength, elongation, and hardness [12]. Pearlitic matrix is a lamellar mixture of ferrite and cementite. Thus, the length of interfaces between ferrite and cementite becomes large with increasing pearlite area. Fontana [9, pages 28–31] and Trethewey and Chamberlain [13] described in their books that grain boundaries are high-energy areas and are more active chemically. The book by Evans said that regions near the grain boundaries may possess electrochemical properties different from those of the grain interiors