Chemistry Reference
In-Depth Information
At the current peak, the critical current at which the effect of mass transport reaches
its maximum and the surface is covered with an oxide films, PS ceases to form.
Although diffusion has a strong effect in the electropolishing region, the current is not
fully limited by the diffusion process.
34,939
8.2.6
.
Chemical Dissolution during PS Formation
During PS formation at an anodic potential, the tip of pores dissolves electrochemi-
cally. The pore wall areas that are sufficiently distant from the tips where no holes are
available, dissolve only chemically and at a very low rate. The chemical dissolution
does not depend on potential but on the time of immersion and the total surface area
of the PS. Due to the large surface area of PS, a significant amount of material may be
removed by the chemical dissolution during the formation period of PS. Figure 8.18
shows that the amount of chemical dissolution increases with immersion time in the
HF solution and with decreasing HF concentration.
48
The dependence of the chemical
dissolution on time has been found to follow the equation
where
t
is time and
A
and
n
are constants depending on HF concentration and current
density. For 40% HF and a current density of the amount of chemical dis-
solution relative to that of electrochemical dissolution has been determined to be
This equation indicates that the amount of chemical dissolution relative to that of elec-
trochemical dissolution increases with time. This is reasonable, as chemical dissolu-
tion is proportional to the surface area of PS which in turn increases with time while
