Corrosion and Inhibition Studies of Copper in Aqueous Solutions of Formic Acid and Acetic Acid

Corrosion behaviour of copper has been investigated in different compositions of formic acid and acetic acid at 30°C by a potentiostatic method. The maximum corrosion rate was found in 2&40mol/o formic acid and in 20 mol/o acetic acid in aqueous solution mixtures. The corrosion rate depended on the concentration of either acid. Formic acid is observed to be more corrosive than acetic acid. The metal exhibited active-passive behaviour in the concentration range of 30_70mol/o of HCOOH acid in the solution mixture. A short passivity range of potential with a high passivity current density was observed for the metal in the solution mixtures of HCOOH acid while in solution mixtures of acetic acid the metal exhibited only active dissolution. Some organometallic compounds, viz. BuzSnCIz, PhSnC13, Ph2SnC12, Ph3SnCI have been subjected to inhibition studies in the aqueous solution mixture (20 mol/o) of either acid. Among the inhibitors used Ph,SnCI functions as a better inhibitor in both acids. A strong interaction between the inhibitor and corroding surface of copper is speculated due to adsorption of the inhibitor. INTRODUCTION Organic acids constitute a group of the most important chemicals used in industry. The acids are produced more as precursors for other chemicals than for end use as organic acids. Acetic acid is the best known member of the group and is produced in the largest volume, but other organic acids are also important for the preparation of compounds used in routine compounds from aspirin to plastics and fibers. Corrosion by organic acids is complicated not only because there are numerous acids to be considered but also because the acids typically are not handled alone but as a process mixture. They are even sometimes used as solvents for other chemical reactions. Though organic acids are weakly acidic they provide sufficient protons to act as true acids towards most metals, because most organic acids are neither oxidizing nor reducing to metals, such as copper, which does not displace hydrogen from acids. A high corrosion rate will be encountered if acid is free of air and other oxidants. In the absence of oxygen or other oxidants, copper is probably the most widely used material for handling HCOOH and CH,COOH acid. The electrochemical study of metals in organic solvent containing different amounts of water indicates that water plays a significant role’ on the corrosion behaviour in formic acid solution. Substantial corrosion studies for different metals and alloys in these acids have been reported in the literature.2’3 Despite the wide use in industry and for handling purposes of these acids the corrosion studies of copper are scarcely known on the entire composition range of formic and acetic acid. In view of the above, the corrosion Manuscript received 14 June 1993; in amended forms 14 July 1993 and 14 February 1995.