Pollution Prevention and Control Technologies for Plating
Section 4 - Chemical Solution Maintenance
4.7 MEMBRANE ELECTROLYSIS
This group of bath maintenance technologies was used by 5 (or
1.6%) of the respondents to the Users Survey. These technologies,
applied primarily to the maintenance of chromic acid solutions
such as decorative and hard chrome plating baths, chromic acid
etch solutions and chromic acid anodizing baths, compete with
ion exchange (Section 4.4) and ion transfer (Section 4.6) technologies.
All of the applications identified in the Users Survey involve
these solutions. Membrane electrolysis is also applicable to non-chromium
solutions, including those formulated with sulfuric acid and sodium
hydroxide, but are less frequently used for them.
Membrane electrolysis is related to the ion transfer technology
discussed in Section 4.6, however, commercial
membrane electrolysis units employ ion specific membranes rather
than the ceramic or polyfluorocarbon materials used in ion transfer.
With membrane electrolysis, an electrical current is passed through
electrolytes that are separated by an ion specific membrane. Two
reactions typically occur as the result of using membrane electrolysis:
(1) ions of a given species are electrically driven across a selective
membrane and (2) chemical changes occur (e.g., oxidation/reduction
such as the electro-oxidation of Cr+3 to Cr+6) at the electrodes.
The term membrane electrolysis is found in German literature (membran-elektrolyse)
(ref. 520). Some U.S. manufacturers of this technology prefer
to categorize their equipment as an electrodialysis technology.
This latter term however, is more often reserved for an electrochemical
device with alternating anion- and cation-exchange membranes (referred
to as stacks) that alter the concentration and/or composition
of an electrolyte simply as the result of electromigration (ref.
The primary function of membrane electrolysis, when applied as
a bath maintenance technology, is to lower or maintain at an acceptable
level the concentration of metallic impurities in plating, anodizing,
etching, stripping and other metal finishing solutions. This is
accomplished through the use of an ion exchange membrane(s) and
an electrical potential applied across the membrane(s). The membranes
employed with these technologies are ion-permeable and selective,
permitting ions of a given electrical charge to pass through.
Cation membranes allow only cations, such as copper, nickel, and
aluminum to pass from one electrolyte to another, while anion
membranes allow only anions, such as sulfates, chromates, chlorides,
or cyanide to pass through. Bath maintenance units can be configured
with only cation or anion membranes or both. The oxidation of
Cr+3 to Cr+6 occurs at the anode, in the same manner as high current
density dummy plating described in Section 22.214.171.124. The anode
material and/or coating must be properly selected in order for
this reaction to occur.
The most common applications for this technology within the plating
industry are the purification of chromium plating (especially
hard chromium) and chromic acid anodizing baths. These baths become
contaminated with various metallic impurities that are introduced
mostly as a result of: drag-in; anodic etching; corrosion of electrodes,
bus bars, racks, fixtures and parts; and reduction. The metal
impurities combine with the acid in the bath and form metal salts.
Eventually, the concentration of metallic impurities exceeds operational
limits and the baths must be discarded.
Hard chromium plating solutions are most often fouled by iron,
aluminum, copper and trivalent chromium. Iron is contributed during
the anodic etch of steel parts, which is usually performed in
the plating bath prior to plating. Etching prepares the surface
of the parts and improves adhesion. Iron is also contributed by
the corrosion of steel parts that have been dropped in the tank
and not retrieved. Aluminum is contributed to hard chromium baths
mostly from aluminum fixtures that hold the parts during plating.
Some facilities chromium plate aluminum parts, which also contributes
aluminum to the bath. Copper is added mostly from corrosion of
copper bus bars and fixtures. Trivalent chromium is generated
in chromium plating baths when hexavalent chromium (i.e., the
desired chromium species) is reduced. This occurs partly due to
the introduction of organic chemicals to the bath, but is mostly
due to an imbalance in the plating process. During plating, hexavalent
chromium is reduced to chromium metal on the surface of the part
(cathode). However, some hexavalent chromium is incompletely reduced
to trivalent chromium. Simultaneously, trivalent chromium is oxidized
at the anode to hexavalent chromium. An imbalance is created when
the surface area of the plated part approaches or exceeds the
surface area of the anode. This condition is most prevalent with
inside diameter (ID) plating since the anode, which is inserted
into the part, must be physically smaller than the part.
The deleterious effect of metallic impurities on the chromium
plating process is widely acknowledged, but the tolerable limits
are often debated. Metallic impurities are known to limit the
permissible cathode current density, resulting in slower plating
to obtain comparable deposit quality (ref. 369). Various defects
in chromium plated deposits, including burning, roughness, pitting,
and reduced adhesion, brightness and hardness are also attributed
to metallic impurities. The actual effects of metallic impurities
depends on many factors including the overall chemistry of the
bath, plating procedures used and the type of parts being plated
Various control limits for chromium plating baths are in use,
with a 4 g/l combined concentration of iron, aluminum and copper,
and other tramp metals as a common standard. Experts are in less
agreement over trivalent chromium control limits and many report
advantages from the presence of some Cr+3. However, a 4 g/l control
limit for Cr+3 is typical of that used in industry (ref. 370).
The effects of metallic impurities on chromic acid anodizing baths
and acceptable limits are not as well documented as hard chromium
plating. The key metallic impurities are trivalent chromium and
aluminum. Trivalent chromium is generated by the same methods
discussed for hard chromium plating (i.e., introduction of organics
and excessive cathode area in relation to anode area). One widely
recognized source recommends controlling Cr3 below 3 g/l to prevent
darkening of the anodic film. A 4 g/l limit is used by the Air
Force (ref. 384).
Aluminum is added to the bath by the electrolytic etching of the
parts during the process (in anodizing, the part is the anode)
and causes reduced efficiency. Chromic acid in the anodizing bath
is neutralized by aluminum dissolving in the solution. Knowing
the neutralizing effect of aluminum, the baths are typically controlled
by pH rather than direct aluminum concentration. This practice
is implemented by the Air Force, where the upper control limit
is pH 0.85 (ref. 384). In practice, a new chromic acid anodizing
bath is formulated by the Air Force with 48 g/l of chromic acid.
The control limits for chromic acid are 40 to 105 g/l. As chromic
acid is neutralized by dissolving aluminum, chromic acid additions
are made to keep the pH below 0.85. These additions increase the
chromic acid concentration of the bath. In theory, once the bath
reaches a chromic acid concentration of 105 g/l, the bath is discarded.
However, more frequently, the pH of the bath becomes difficult
to control at approximately 75 g/l of chromic acid and it is discarded
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