Electrochemical investigation of the growth of anodic films on iron and ferrous alloys.
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Date
1994
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Abstract
An electrochemical investigation of the corrosion of iron and Fe18Cr based stainless steel alloys was
undertaken with particular emphasis on the nucleation and growth of surface films.
Chronoamperometry was shown to be a sensitive technique to investigate the initial stages of film
formation and growth. In a variety of acidic (pH < 7), alkaline and alkaline cyanide electrolytes,
providing dissolution of the substrate metal could occur rising current transients, similar to those
reported in electrocrystallisation studies, were observed when the electrode was stepped to the
appropriate potential. This indicated that at these potentials the surface film formed via the nucleation and growth of discrete nuclei. A significant aspect of this study was visual evidence of this nucleation
and subsequent growth of the film provided by scanning electron microscopy which supported the
electrochemical data and interpretation thereof.
Existing electrocrystallisation models were used to evaluate the experimental rising current transients.
While these models gave an indication as to the prevailing nucleation and growth mechanism, they were
found to be inadequate in describing anodic oxide formation on an oxidising substrate. A qualitative
model was proposed.
In acidic electrolytes, rising chronoamperometric transients were observed for Fe, Cr and Fe18Cr at
passive potentials and for FexCr (x = 16,18, 20,23% Cr) and alloys 444, 4732, 4733, 304L and 316L at
transpassive potentials. The transients were shown to be sensitive to variations of potential,
temperature, electrolyte and alloy composition.
A systematic investigation of the influence of temperature (20 0 C - 1200 C) on the chronoamperometric,
cyclic voltammetric and rotating ring - disc electrode behaviour of Fe in O.5M and 1.0M NaOH was also
undertaken. In alkaline electrolytes, the formation of a duplex surface film was proposed, with x-ray
photoelectron spectroscopy indicating that the protective base layer consisted of FeO while Fez03 and
FeOOH constituted the upper layer. Base layer formation was favoured with increasing temperature
and increasing hydroxide ion concentration of the electrolyte. Addition of OAM NaCN to O.5M and
1.0M NaOH had a marked effect on the electrochemistry of the system, with CN- inhibiting surface film
formation, particularly of the upper layer. A mechanism for the oxidation of Fe in alkaline and alkaline
cyanide electrolytes was proposed.
Description
Thesis (Ph.D.)-University of Natal, 1994.
Keywords
Metals--Anodic oxidation., Steel alloys--Corrosion., Iron--Corrosion., Electrochemistry., Theses--Chemistry.