A STUDY OF HYDROGEN EVOLUTION DURING AQUEOUS OXIDATION OF MILD STEEL
NAVAL RESEARCH LAB WASHINGTON DC
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Steel is useful as a construction material for boilers only because the product of the easy reaction between steel and water is an insoluble oxide, which forms as a thin adherent film. As boiler operating temperatures are increased, however, the endurance and integrity of this protective film are pushed past their limits. The chemical reaction by which the film is generated is 3Fe2 4H3O yields Fe4O 4H2, but in reality it is an electrochemical process. The substrate iron serves as the anode and the growing oxide film serves as a cathode the circuit is completed by water, which serves as the electrolyte, and by direct contact between the iron and its oxide. It was envisioned that if the specific places in this system where hydrogen was evolved could be determined, more would be known about the mechanism. An apparatus was devised to measure against time the flow of hydrogen toward the metal side of the system and toward the solution side of the system under various controlled conditions. Permeation of hydrogen from one side of the system to the other under a known pressure differential was also measured. It was found that in the presence of strongly alkaline solutions 15, hydrogen was evolved more toward the electrolyte, whereas with weakly alkaline ph 11 solutions, the hydrogen went almost entirely toward the metal. It was also shown that hydrogen very readily permeates the oxide film grown from strong caustic solution as compared with a film grown from a pH 11 solution. These facts substantiate the premise of a porous film and indicate that oxidation is taking place both within the pores themselves and at places where the solution in these pores is contacting metal.
- Physical Chemistry
- Properties of Metals and Alloys