COMPUTER-AIDED ANALYSIS OF GALVANIC CORROSION UNDER SIMULTANEOUS CHARGE AND MASS TRANSFER CONTROL

     The importance of corrosion is techno — economic ,i.e.reducing material losses resulting from corrosion of piping,tanks, metal components of machines, ships, bridges, marine structures, etc. in addition to improved safety of operating equipment, which through corrosion may fail with catastrophic consequences, and also conservation of metal resources and associated losses of energy ,water reserves ,and human effort.Because of practical importance of protecting industrial equipments from galvanic corrosion, the need arises to analyse the effects of variables, such as temperature, acidity, velocity,pressure and area fraction of metals on galvanic corrosion in deaerated acid media when the system is under activation control and in seawater under mass transfer control of Fe– Zncouple.For these reasons
computer program is developed which can be used for a general number or type of coupled metals with a range for pHof1, 2,3,4,and7,temperature of 25,40,and 60 0 C, pressure of 0.5, 0.75, 1, 3. and 5 atm, Re of 5000 - 50000 and area fraction of 0.1-0.9 of Fe. The program can also be used for free corrosion of metals.The results show that the free corrosion rate of Fe and Zn increased with increasing temperature and decreasing pH under activation control. At diffusion control free corrosion rate of Fe and Zn increased with increasing Re and pressure while temperature leads to decrease the corrosion rate of both metals via decreasing O concentration as dictated by limiting current density.2For galvanic coupling of Fe – Zn, the results showed that for activation control, decreasing pH and increases temperature lead to increase the corrosion rate of Zinc. Increasing area fraction of more noble metal (Fe) increases the galvanic corrosion rate of more active metal (Zn). No effect of Re on the galvanic corrosion under activation control is noticed.For diffusion controlled galvanic corrosion, the result showed that increasing Re and pressure lead to increase the galvanic corrosion rate of Zinc while the temperature increase leads to decrease the galvanic corrosion of Zinc. Slight effect of area fraction on the corrosion rate of Zinc is noticed.When the system is under mixed control (charge and mass transfer) the results revealed that increasing pH leads to decrease the corrosion rate for free corrosion and galvanic corrosion. For free corrosion increasing temperature leads to slight decrease of corrosion rate of zinc while increasing Re and pressure lead to considerable increase of corrosion rate of zinc.For mixed control galvanic corrosion increasing temperature increases the corrosion rate of more active metal for pH< 4 , and decreases the corrosion rate of zinc at pH = 7. No appreciable effect of both Re and pressure is noticed on the galvanic corrosion rate of zinc under mixed control for pH < 4 while for pH=7 the galvanic corrosion rate of zinc increases with Re and pressure. Increasing area fraction of iron increases the corrosion rate of zinc for pH < 4 while it affects the corrosion rate slightly at pH=7.Generally, results showed that corrosion potential is shifted to more noble (positive) with increasing Re and pressure,while it is shifted to more active (negative) with increasing temperature and pH, except in mixed control it exhibits both trends with pH increase.