Influence of Salinity, Total Dissolved Solids, Conductivity, and pH on Corrosion Behavior of Different Morphologies of Pearlitic Steels

The investigation discusses the influence of salinity, conductivity, total dissolved solids (TDS), and pH on the corrosion behavior of different morphologies of pearlitic steels with coarse, fine, and very fine microstructures obtained by furnace-annealed, air-cooled, and forced-air-cooled, respectively. Immersion test of the heat treated steels was carried out in freely aerated 3.5% NaCl solution for 28 days. The study also involves the effect of the water parameters on the composition, fraction, and morphology of corrosion products. Increase in cooling rate decreases the interlamellar spacings, and this refinement has resulted in higher corrosion resistance. The formation of large number of well-distributed microgalvanic cells between cementite and ferrite lamellae of the fine pearlitic air-cooled steel enhances the corrosion resistance. However, further refinement beyond a limit decreases the corrosion resistance due to the entanglement and breaking of cementite lamellae of the forced-air-cooled steel as compared to the air-cooled steel. Therefore, the air-cooled steel with fine pearlite shows better corrosion resistance than the furnace-annealed and forced-air-cooled steels. Generally, salinity and conductivity increase with a decrease in pH with time, whereas the increased TDS and protective index (alpha/gamma*) as measured by the weight fraction of stable alpha-FeOOH over the other unstable gamma-FeOOH and beta-FeOOH have resulted in decreased corrosion susceptibility of the pearlitic steels irrespective of their interlamellar spacing between ferrite and cementite.