To Polarize or Not to Polarize: Practical Advice on How …
121
and Naugahyde
® . Increasing the polarization beyond this point decreases CE and
lowers strippability. The deposit structure turns to a coarse sand looking deposit and
then clam shells in cross section with polarization beyond the optimal range.
Relatively high polarization can cause dendrites. However, dendrites are more
typically associated with electrode misalignment. Misalignment or bent electrodes
create high current density regions on a cathode which in turn causes high polarization
and dendrites. Thus, dendrites are usually caused by poor maintenance and not excess
gelatin.
Pin holes are listed as a structure by several observers. Pin holes are caused
by the local accumulation of deposited impurities. This results in local corrosion
cells, which fosters hydrogen and zinc re-solution. Usually, pin holes structures
are more prevalent at lower polarization conditions (e.g. caused by impurities with
lower hydrogen overpotentials, Co, Ni, Ge, and Te) but they can be observed even in
structures that are consider higher polarization than the optimum values (e.g. coarse
sand structure).
When there are a combination of structures, it can be difficult to determine which
“lever” to pull to improve the deposit. It is recommended to take small steps (e.g.
small changes in gelatin) and observe the effects in a short time frame. It is important
to realize that due to the continuous nature of the cellhouse process, some cathodes
are initial growth and some are ready to be harvested that may have experienced
different conditions during the plating cycle. Looking at cathodes that have different
plating times will provide a more complete picture of whether a problem is starting,
has been occurring, or is ending.
Summary
The science of polarization/depolarization is generally accepted in terms of zinc
electrodeposition and hydrogen evolution. Applying that science to an operation
can be challenging. A processing person needs to understand the fundamentals of
polarization, but also cellhouse operations and changeable parameters. The most
effective way to understand and control a cellhouse is by “reading the metal”. The
metal will indicate where on the polarization curves the plant is operating. This
information can be used to decide which lever(s) should be used to manipulate the
plating conditions to produce the best conditions for a given cellhouse. The best
conditions will result in improved productivity and bottom line of the cellhouse. For
a given cellhouse, it may be decided that some CE will be sacrificed to maintain a
cathode structure that is readily stripped which reduces machine downtime.
Data need to be collected routinely and diligently. The morphology and growth
structure data should be correlated to the conditions that produced them. As the
relative impacts and intercorrelations of the conditions are learned, it will become
apparent whether the optimal conditions are a large target or rather small.
121
and Naugahyde
® . Increasing the polarization beyond this point decreases CE and
lowers strippability. The deposit structure turns to a coarse sand looking deposit and
then clam shells in cross section with polarization beyond the optimal range.
Relatively high polarization can cause dendrites. However, dendrites are more
typically associated with electrode misalignment. Misalignment or bent electrodes
create high current density regions on a cathode which in turn causes high polarization
and dendrites. Thus, dendrites are usually caused by poor maintenance and not excess
gelatin.
Pin holes are listed as a structure by several observers. Pin holes are caused
by the local accumulation of deposited impurities. This results in local corrosion
cells, which fosters hydrogen and zinc re-solution. Usually, pin holes structures
are more prevalent at lower polarization conditions (e.g. caused by impurities with
lower hydrogen overpotentials, Co, Ni, Ge, and Te) but they can be observed even in
structures that are consider higher polarization than the optimum values (e.g. coarse
sand structure).
When there are a combination of structures, it can be difficult to determine which
“lever” to pull to improve the deposit. It is recommended to take small steps (e.g.
small changes in gelatin) and observe the effects in a short time frame. It is important
to realize that due to the continuous nature of the cellhouse process, some cathodes
are initial growth and some are ready to be harvested that may have experienced
different conditions during the plating cycle. Looking at cathodes that have different
plating times will provide a more complete picture of whether a problem is starting,
has been occurring, or is ending.
Summary
The science of polarization/depolarization is generally accepted in terms of zinc
electrodeposition and hydrogen evolution. Applying that science to an operation
can be challenging. A processing person needs to understand the fundamentals of
polarization, but also cellhouse operations and changeable parameters. The most
effective way to understand and control a cellhouse is by “reading the metal”. The
metal will indicate where on the polarization curves the plant is operating. This
information can be used to decide which lever(s) should be used to manipulate the
plating conditions to produce the best conditions for a given cellhouse. The best
conditions will result in improved productivity and bottom line of the cellhouse. For
a given cellhouse, it may be decided that some CE will be sacrificed to maintain a
cathode structure that is readily stripped which reduces machine downtime.
Data need to be collected routinely and diligently. The morphology and growth
structure data should be correlated to the conditions that produced them. As the
relative impacts and intercorrelations of the conditions are learned, it will become
apparent whether the optimal conditions are a large target or rather small.