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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.7 MEMBRANE ELECTROLYSIS
4.7.1 Overview
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. 401, 423).
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 4.2.2.3. 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 (ref. 370).
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 (ref. 384).