Applications

As with any cleaning process, molten salt baths are used to remove some type of unwanted surface soil, contamination, coating, or other substance from a substrate to allow further processing or reclamation of the substrate. Due to the relatively high temperatures involved with molten salt processing (205 to 650 °C, or 400 to 1200 °F), substrates to be cleaned are restricted to those materials that are compatible with the operating temperatures of the various processes. Because these baths are also chemically active, the substrate must also be chemically compatible with the various molten salt systems. While most metals that are temperature compatible will also be chemically compatible, there are notable exceptions to this general statement. For example, magnesium and its alloys must not be processed in oxidizing salt baths because of the potent oxidation-reduction reaction that may occur at elevated temperatures. This would result in ignition of the metal and destruction of the component.

Paint stripping in molten salts is a simple immersion process and is applicable to a wide variety of organic coatings, including solvent-based, water-borne, cured powders and high-performance coatings such as fluorinated polymers. Depending on the type and thickness of the paint coating to be removed, the stripping reaction time can vary from several seconds to a few minutes. The operating temperature depends on the specific process used, but it normally falls in the range of 290 to 480 °C (550 to 900 °F). The lower-temperature processes are generally used to reclaim reject-coated products and on temperature-sensitive materials and components. The higher-temperature processes are used for stripping more robust components.

The higher temperatures are also used for "maintenance" paint stripping of hooks, racks, carriers, and similar fixtures that serve as extensions or add-ons to the conveyor system that carries components to be painted through the paint line. Hooks and racks generally hang down from an overhead conveyor system, while carriers generally "ride" on a floor track or floor conveyor. The components to be coated are affixed to the hooks, racks, and so on and are transported through the various coating operations such as surface pretreatment, coating area, and curing ovens. At the end of the line, the finished parts are removed and "raw" unpainted parts are placed on the fixtures. Because the hooks and racks may pass through the coating line numerous times between stripping operations, they may receive numerous layers of coatings.

Stripping is accomplished by a thermochemical reaction between the oxidizing molten salt and the organic portion of the paint. Alkali nitrate, usually present in an oxidizing salt bath, donates the oxygen required to allow the organic material to be completely oxidized to carbon dioxide while immersed in the bath:

During the course of the reaction, nitrate is chemically reduced to nitrite. Contact with atmospheric oxygen then reoxidizes the nitrite back to nitrate, helping to regenerate the bath:

Alkali carbonates are formed as a result of the stripping from the reaction between carbon dioxide and caustic alkalis present in the bath:

The alkali carbonates continue to increase in the bath until the bath becomes saturated with them. After saturation has been reached, the bath continues to react with any additional organics introduced. The additional carbonates, however, begin to precipitate out of solution in the form of sludge. Molten salt stripping equipment typically is designed with collection devices into which the sludge, which is denser than the host salt, settles for subsequent removal from the bath. Along with the alkali carbonates, the sludges may also contain insoluble inorganic pigments, fillers and so on, that were present in the original paints that were stripped.

Upon removal from the molten salt, the components are rinsed in water to cool them and to remove the thin film of salt residue present on the components. Additional post-treatments, such as acid brightening, neutralizing, and so on, are also commonly used to prepare the components for recoating.

Polymer Removal. The removal of solidified synthetic polymer residues is another common use for oxidizing molten salts. Synthetic fiber production involves the use of intricate dies or spinnerets and associated components such as filter packs and distributor plates. The molten polymer (for example, nylon, polyester, or polypropylene) is extruded through the spinneret under pressure to form the fiber strand. It becomes necessary to disassemble and clean the packs and spinnerets when blockages are present or when production schedules dictate a "changeout" of the packs. The chemistry involved is the same as described above for paint stripping.

Great care must be taken when cleaning spinnerets because of their delicate hole geometries, low root-mean-square (rms) surface finishes, and high intrinsic value. To clean spinnerets and screens of polymeric material, the initial salt composition should be essentially neutral. Buildup of alkaline reaction products ultimately leads to some attack (pitting) of the workpieces and can cause an accumulation of undesirable ions (for example, chromate) in the salt. The spinneret with its solidified polymer residues is immersed in the cleaning bath and a polymer is quickly and completely removed via thermochemical oxidation, without harming the spinneret's properties.

Casting Cleaning. The cleaning of castings with molten salt processes is applicable to both investment castings (lost wax) and sand castings. Investment castings are processed in molten salt baths to remove residual external shell and to leach out preformed ceramic internal coring. Sand castings are processed to remove binder residues and burned-in core sand. Salt bath cleaning is usually used after preliminary cleaning operations such as shakeout and mechanical blasting.

Investment Castings. In the case of investment castings, a small amount of external shell is usually still present after mechanical cleaning operations. Salt bath processing is then used as a scavenger to remove these residues. Relying on the reaction between silica present in the shell and caustic alkalis in the salt bath, the silica is converted into an alkali silicate that is soluble in the bath:

Within the bath's normal operating temperature range of 480 to 650 °C (895 to 1200 °F), the water formed during the reaction is released from the bath as vapor and is visible as a mild effervescence on the bath surface. Inert shell and core constituents such as zircon or aluminosilicates simply slough off the casting as the silica is removed from the shell or core.

Sand castings are cleaned using a method similar to that used to clean investment castings. Again, the principal reaction is between silica (sand) and the alkalis present in the molten salt. When cleaning cast iron, however, the process is usually performed electrolytically.

Incorporating direct current into the molten salt cast iron cleaning process allows simultaneous removal of sand, surface graphite, and scale. The casting to be cleaned is normally subjected to an initial reducing (cathodic) cycle to dissolve sand and produce an oxide-free casting. This procedure not only produces a casting that is free from any sand contamination, but also greatly improves the machinability (and machine tool life) of the casting by removing the tough, hard surface scale. The scale reduction also helps to expose any sand particles that may have been masked by scale at the metal surface; the now-exposed sand is then dissolved by the bath (Fig. 1a and b). To prepare cast iron surfaces (either as-cast or machined) for subsequent brazing, babbitting, or other metal coating operations, the electrolytic process becomes somewhat more involved.

Fig. 1 Schematic cross section of the surface of a cast iron component as it is modified by cleaning in a molten salt bath. (a) As-cast. Note surface scale, burned-in core/mold sand particles, and flake graphite extending to surface. (b) After first reduction cycle. Exposed sand particles have been chemically dissolved, while the original casting oxide has been electrochemically reduced. The original flake graphite is unaffected and intact at this stage of processing. (c) After oxidation cycle. The original flake graphite has been electrochemically oxidized to carbon dioxide. The entire exposed cast surface is now covered with a very thin, uniform layer of iron oxide. (d) After second reduction cycle. The cast surface is now free of all original cast scale, sand inclusions, and exposed graphite flakes. The final reduction cycle also removes the thin layer of iron oxide that was formed during the oxidation cycle. (e) After brazing. The braze metal uniformly "wets" the surface of the metal and freely flows into the surface voids previously occupied by graphite flakes.

Fig. 1 Schematic cross section of the surface of a cast iron component as it is modified by cleaning in a molten salt bath. (a) As-cast. Note surface scale, burned-in core/mold sand particles, and flake graphite extending to surface. (b) After first reduction cycle. Exposed sand particles have been chemically dissolved, while the original casting oxide has been electrochemically reduced. The original flake graphite is unaffected and intact at this stage of processing. (c) After oxidation cycle. The original flake graphite has been electrochemically oxidized to carbon dioxide. The entire exposed cast surface is now covered with a very thin, uniform layer of iron oxide. (d) After second reduction cycle. The cast surface is now free of all original cast scale, sand inclusions, and exposed graphite flakes. The final reduction cycle also removes the thin layer of iron oxide that was formed during the oxidation cycle. (e) After brazing. The braze metal uniformly "wets" the surface of the metal and freely flows into the surface voids previously occupied by graphite flakes.

The initial cleaning cycle usually incorporates a reducing cycle to remove sand and surface scale as described above. The polarity of the direct current is then reversed, effectively electrolytically oxidizing the casting. This converts any exposed surface graphite to carbon dioxide (Fig. 1c). To remove the thin, uniform layer of iron oxide from the casting formed by the oxidizing treatment, the current is once again reversed to produce a final reducing cycle. This results in a scale-free, sand-free, graphite-free surface ready for coating or joining operations (Fig. 1d). When joined, the brazing alloy uniformly "wets" the metal surface and penetrates the voids previously occupied by the graphite flakes (Fig. 1e).

The amount of foreign material removed from a given casting will vary widely from application to application. In the case of investment castings, it will depend on the size of the casting, how much preliminary mechanical cleaning (e.g., shot blast) the casting receives prior to salt bath cleaning, and the geometry of the casting itself. It may range from as low as a fraction of an ounce to several pounds. Likewise, the amount of material removed from a sand casting will depend on the amount of burned-in mold and core sand that is present after mechanical shakeout. These amounts are somewhat more predictable and usually fall in the range of fractional ounces to a few ounces for a typical cast iron engine head or hydraulic valve body.

Glass Removal. Molten salts are an effective medium for removing both solidified glasses and glassy coatings from metals. They are commonly used for cleaning glass fiber production equipment, such as spinnerets and spinner disks, and removing the glassy lubricants commonly used in high-temperature forging operations. Reactions involved are analogous to those for sand removal (see the section "Sand Castings" in this article).

Plasma/Flame Spray Removal. Oxidizing molten salt baths are effective in removing a variety of flame spray or plasma coatings. It is necessary to strip these wear-resistant and protective coatings when jet-engine components are repaired or rebuilt, when tooling and jigs are cleaned during plasma coating, or whenever these tough coatings are not wanted.

The stripping reaction usually involves both the metallic and carbide portions of the coating. Soluble alkali salts are formed by the metallic constituent, while the carbide portion is oxidized to from carbon dioxide. In the case of chromium carbide, the net reaction products are alkali chromates and alkali carbonates. The simplified reaction is as follows:

Analogous reactions take place with tungsten carbide. Stripping rates are quite rapid, with typical stripping times of 15 to 30 min being common to remove a "full-thick" plasma coating. The actual coating thickness depends on the coating process but generally ranges from a few to several mils (0.001 to 0.015 in.). Removal of worn coatings during rework or overhaul requires correspondingly less time.

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