Mechanism of fast growth of magnetite on carbon steel

Technical Report · Mon Jul 01 00:00:00 EDT 1985

OSTI ID:5387508

Macdonald, D D

The corrosion of and the growth of magnetite on carbon steel in chloride-containing aqueous solutions at temperatures from 200 to 270/sup 0/C and for times up to 400 hours have been studied using ac impedance and scanning electron microscopy techniques. Thick and porous magnetite films form in solutions which are acidic because of the presence of HCL or by hydrolysis of cupric, ferric, nickelous and ferrous chlorides. The reciprocal of the polarization resistance, which is proportional to the corrosion rate, increases with time (i.e., autocatalysis), but eventually tends to a constant value (i.e., a linear rate law). Autocatalytic corrosion is attributed to the gradual development of a highly aggressive solution within the porous film due to the hydrolysis of ferrous ions dissolved from the base metal and the transport of chloride ions from the bulk solution into the pores. However, the reciprocal decreases with time (i.e., self-passivation) when thin and compact magnetite films form in neutral or slightly acidic chloride solutions. The frequency dispersion of the impedance of the growing film has been successfully accounted for by a finite electrical transmission line model. Numerical analysis has shown that the total impedance is a sensitive function of the film geometry and of the inter-facial impedances along the pore wall and at the base of the pore. These analyses have demonstrated that the ac impedance technique is well suited for studying the corrosion of metals and the growth of corrosion product films in aqueous systems. 39 refs., 25 figs., 1 tab.

• Research Organization: Ohio State Univ., Columbus (USA). Dept. of Metallurgical Engineering
• OSTI ID: 5387508
• Report Number(s): EPRI-NP-4135M; ON: TI85920190
• Country of Publication: United States
• Language: English

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21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS
36 MATERIALS SCIENCE
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CORROSION DENTING
PWR TYPE REACTORS
REACTOR MATERIALS
STEAM GENERATORS
TUBES
CHLORIDES
CREVICE CORROSION
HIGH TEMPERATURE
INORGANIC ACIDS
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IRON ALLOYS
IRON BASE ALLOYS
IRON COMPOUNDS
IRON ORES
IRON OXIDES
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MINERALS
ORES
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
REACTORS
STEELS
TRANSITION ELEMENT COMPOUNDS
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360105 - Metals & Alloys- Corrosion & Erosion