THE EFFECTS OF FLUID FLOW ON THE PITTING CORROSION OF PURE IRON IN ACIDIC CHLORIDE MEDIA

The fluid flow effects on electrobright pitting of pure iron were investigated. A rectangular channel with minimized wall and entrance effects was used. The electrolytic solution consisted of an aqueous solution of 0.5 M sulfuric acid and 0.003 M sodium chloride. The experiments were carried out under continuous flow rate or were started under stagnant conditions, with the flow rate then stepped up to a preset value. The flow rate ranged between 0 and 80 centimeters per second. The metal studied was "as received," "annealed," and "strained and annealed.";The results indicated that the induction time was influenced only by the pretreatment given to the metal. The corrosion distribution on the metal surface was affected by the geometrical dimensions of the metal sample. A very weak relationship was found to exist between the pit diameter and the flow rate. Generally speaking, the pit diameter decreased when the flow rate increased. A sudden change in flow rate produced an instant increase in the corrosion rate inside single pits. The pits were subsequently passivated. These observations were explained in the context of a mass transfer boundary layer that developed on pits. The layer thickness depended on flow rate. A mathematical model based on a one-dimensional pit indicated that a salt layer film may exist inside pits. This layer controlled the corrosion process.