Formation of passive film on steel-concrete interface
Author: Ana Maria
Author: Ana Maria
Within the environment of high pH, and in the absence of aggressive ions, soon after the start of hydration of cement, develops slowly a thin layer of passivation armor. Still in the fresh concrete begins the mechanism of the corrosion rate always decreasing until the dissolution of the metal becomes negligible (Brown, 2002; GAID 2004; HELENE, 1993; Tuutti apud BAUER, 1995; UZEDA, 1998). The mechanism passivation of armor off after the deposition of several layers and separates effectively the metal electrolyte (Brown, 2002; FEDERAL HIGHWAY ADMINISTRATION, 2004; THAN hematite, hematite, hematite, hematite, GAVEL; Rengaswamy, 1998). It is necessary to dissolve the metal that form the passive layer. It consists film laminated with magnetite (Fe3O4) adhered to the conductive surface of the steel covered by insulating layer of molecules of ferric oxide, hematite (Fe2O3), impermeable to ions, and of ferrous oxide (FeO), which are stable products of corrosion (CASCUDO, 1997; ST. JOHN; POOLE, Sims, 1998 cited in Brown, 2002; THANGAVEL; Rengaswamy,
1998). Above the passive layer may be the deposition of calcium ferrite, resulting from the reaction of Fe2 (OH) 3 with Ca (OH) 2 (BASIL, 1972 apud HELENE, 1993):2Fe (OH) 3 + Ca (OH) 2 CaO Fe2O3 + 4 H2O. Equation 2.1 Finally, the steel bars have also passivated are surrounded by the layer of Ca (OH) 2, from the reactions of cement hydration, because the Ca (OH) 2 contributes to the pH of the steel (CASCUDO, 1997; GENTIL, 1982; THANGAVEL; RENGASWAMY, 1998). The presence of sodium hydroxide (NaOH) and potassium hydroxide (KOH) dissolved in the pore water also helps to maintain the high alkalinity (HELENE, 1993; MEHTA Monteiro, 1994; SATO, 1998). The passivating layer is stable, provided that the pH value remains above 11.5, and since in the absence of chloride ions (MEHTA Monteiro, 1994). Once formed and in these conditions, no water and no oxygen can penetrate such a layer (Taylor, 1992).
Been shown that for the Fe-H2O system at 25 C and 1 atm, the diagram obtained by
potential, as measured by the standard hydrogen electrode versus pH shows a region formed by potential between +100 and -400 mV, typically found in concrete, with a pH between 12.5 and 13.5, in which the iron metal is in a position passivation (Pourbaix, 1974 apud HELENE, 1993; PANOSSIAN apud CASCUDO, 1997). A significant corrosion occurs only with pH values below 9 or above 14 (HOU, CHANG, Hwang, 2004).
Mappings Stratfull (1973 apud HELENE, 1993), made in the early 70s to the bridge deck with pathological manifestations typical action of deicing salts, formed the basis of ASTM C 876:1999, which provides general parameters to evaluate the corrosion of reinforcement.About the Author:
Ana Maria is Civil Engineer and expert in Concrete. Visit
Betoneira and see the
Betoneiras Usadas
Ações Judiciais Pertinentes Nas Relações Civis E De Consumo A glimpse into the magical world of Physics Great Events Require Outstanding Invitations A Bestseller: It Starts With Four Decisions A way to SEE the ICSE Internal Cleanse: Improve Your Life The Only Way to be Cool like James Bond How To Deal With Losing Mom At Your Funeral How To Start To use Your Clairvoyant Skills How Your Joint Venture Can Give You Peace Of Mind How To Pave Concrete Evenly With Concrete Pumping Do I Walk the Talk?