A. Nishikata et al., AN ELECTROCHEMICAL IMPEDANCE STUDY ON ATMOSPHERIC CORROSION OF STEELSIN A CYCLIC WET-DRY CONDITION, Corrosion science, 37(12), 1995, pp. 2059-2069
The corrosion rates of steels with different nickel contents (0, 2.5,
10 and 20 wt%) were monitored by the AC impedance method under a cycli
c wet-dry condition, which was conducted by exposure to alternate cond
itions of 1 h immersion in a 0.05 M NaCl solution and 7 h drying at 60
%RH and 25 degrees C. The corrosion rates of the ordinary carbon steel
and the 2.5% Ni containing steel were greatly accelerated by the wet-
dry cycles, while those of steels containing at least 5% Ni were only
slightly affected. AC impedance tests indicated that the addition of 5
% Ni greatly reduced the corrosion rates of the steel exposed to the c
yclic wet-dry environments containing chloride ions, in good agreement
with one year exposure tests in the atmospheric marine environments.
Within the wet-dry cycle for the carbon steel, a gradual increase in t
he corrosion rate and a shift of the corrosion potential to negative v
alues were observed at the initial stage of the drying period. These i
ndicated that the anodic metal dissolution rate was accelerated by a s
light increase in the chloride ion concentration. At the intermediate
stage of drying, the corrosion rate rapidly increased and the corrosio
n potential remained constant. This can be attributed to the accelerat
ion of both the anodic metal dissolution process and the cathodic oxyg
en reduction process which is usually controlled by the rates of O-2 t
ransport through the thin electrolyte film. At the final stage, just b
efore the surface dried out, the corrosion rate rapidly decreased, shi
fting the corrosion potential to the noble direction. At this stage, t
he anodic process was greatly inhibited. The corrosion testing system
employed in this study is very useful for the rapid evaluation of the
corrosion performance of metals in atmospheric marine environments and
for mechanistic study.