EP is a polishing process of electrochemical anodic dissolution.
The process consists of an anode (workpiece), a cathode
and electrolyte. Generally, the electrolyte is composed of
viscous acid fluid. During the process, the hydrogen gas will
be formed from the cathode and oxygen gas will be formed at
the anode along with metallic dissolution, as shown in Fig. 1.
During the anodic dissolution, the dissolution rate at the anode
is the slowest and is the controlling factor. Therefore, the
electrochemical reaction is under diffusive mechanism. Due
to the diffusive mechanism, a viscous film will be formed
on the anode. The increase in resistance of the viscous film
prevents the electrochemical reaction efficiency from increasing.
Thus, it forms the plateau region, c–d, in the I–V
curve, as shown in Fig. 2. The leveling and brightening of the
anode (workpiece) surface takes place within this viscous
film. Finally, a passive film is formed. Because of selective
dissolution, the metallurgical compositions of the passive
film may be different from those of the substrate. It is the
passive film that enhances the corrosion resistance of the
workpiece.
Corrosion is a degradation phenomenon of a material
when under environmental chemical attack. Corrosion phenomena
of the stainless steel are composed of uniform
corrosion and localized corrosion (IGC). The uniform corrosion
is a macroscopic evaluation of the overall corrosion
of a workpiece under chemical attack. The localized corrosion,
a form of IGC, took place when the oxidation layer
was damaged and the substrate was exposed to the acidic
environment. It usually showed up as pitting or crevice
corrosion. This phenomenon is accelerated by the potential
difference between the exposed surface and the area that
are still under the protection of the passive film.
Both of the polarization resistance and the EPR analyses
were employed to investigate the uniform corrosion and localized
corrosion phenomena, respectively, of the SS 316L
stainless steel under various EP process parameters