a) Smaller: For the equivalent heat transfer capability, a metallic immersion heat exchanger will typically be 3-4 times smaller than a thermoplastic (e.g. PTFE) heat exchanger. The smaller profile of metallic heat exchangers allows more obstruction-free room in the tank for pickling and helps to minimize the potential for damage due to mishandled work.

b) More efficient heat transfer: Metal heat exchangers have substantially higher thermal conductivity than do thermoplastics. This allows for more efficient heat transfer and faster heat-up time for comparable surface area. Also, every square inch of a metallic heat exchanger is available for active heat transfer.

c) More durable: Metallic heat exchangers have inherently higher strength and toughness than non-metallic materials, such as graphite and thermoplastics, and are less prone to damage during handling and in the harsh conditions of a pickling tank.

d) Easier to work with: Metallic heat exchangers are generally easier to fabricate or repair than alternative materials such as graphite or thermoplastics. Good ductility and workability allow for standard methods and equipment to be used in fabrication and repair.

e) Compatible with pressurized steam: Metallic heat exchangers are the most practical choice for use with pressured steam. Thermoplastic heat exchangers are generally not rated for higher than 35 psi steam pressure.

• From the caustic: The most commonly used caustic, diaphragm cell grade, can have up to 10,500 ppm chloride. These chlorides are dragged into the pickle tank with caustic residues.
  
• From pumping containment pit water into the acid tanks: The choice to pump containment pit water into the acid tank causes high-chloride spillage from the flux and caustic tanks to be put into the acid tanks. The pit must be divided so that the caustic and the caustic rinse will drain into one section, acids and acid rinses into another section, and flux into yet another section. Thus, drainage from these tanks is pumped directly back into the tanks from which it came.
  
• From chemicals added to the pickling bath: Inhibitors, accelerators, or fume-control agents may be a source of fluorides and chlorides as well as corrosive bromides and iodides. Some batches of raw sulfuric acid have been found to contain elevated levels of chlorides and fluorides.
  
• From chlorinated and/or fluorinated tap water: Evaporation of water from the heated pickle bath causes chlorides and fluorides to concentrate over time.

1. Use a low chloride grade of caustic: The cost low chloride or low salt membrane grade may be about 10 cents/pound higher than diaphragm grade caustic but should have chloride levels that are ~175 times lower (i.e. 60 ppm vs.10,500 ppm typical chloride levels).
   
   
2. Maintain low chloride and fluoride levels in the sulfuric acid pickling bath: Because sulfuric acid recovery systems allow for re-use of sulfuric acid, chlorides and fluorides have been found to accumulate in the pickling solution. Chlorides and fluorides are known to corrode zirconium and most other metals. Elevated chlorides can cause a localized pitting and elevated fluorides can result in general surface corrosion. Refer to Figure 5 for maximum recommended chloride levels for sulfuric acid pickling solutions. Fluoride concentration should be maintained below 10 ppm with levels below 5 ppm being preferred (see Figure 6). This data is based on laboratory analysis using reagent grade chemicals.
   
   NOTE: Zirconium heat exchangers in pickling solutions with elevated chloride levels do not always exhibit pitting corrosion. Some organics and other contaminants that build up in pickling solutions over time may have an inhibiting effect on corrosion and reduce the corrosion rate or reduce the potential for the onset of localized pitting corrosion. The limits in Figure 5, however, are considered to be a conservative approach. For existing zirconium heat exchanger installations where chloride levels are high and no corrosion is evident, chloride levels should at least be stabilized. Preferably, they should be reduced to levels below the limits in Figure 5. If chloride levels are allowed to rise, localized pitting corrosion may eventually occur.
   
   
3. Check chloride and fluoride levels in the acid tanks, rinse tanks, and caustic tank on a routine basis: Representative acid samples may be properly packaged and sent to a laboratory for analysis. The ATI Wah Chang Analytical Laboratory provides chloride, fluoride, and other inorganic chemical analysis services (see Analytical Lab article on page 3). If the chloride and fluoride levels are known, corrective measures can be instituted before corrosion problems occur.
   
   
4. Understand chloride and fluoride levels of all additives prior to use in pickling baths: This includes sulfuric acid, inhibitors, accelerators, fume control agents, caustic, and even water. Softened water can have high levels of chlorides. Municipal water may have chlorides and/or fluorides. If any additives are found to have high chloride or fluoride, reasonable alternatives should be considered. Again, ATI Wah Chang Analytical Laboratory provides chloride, fluoride, and other inorganic chemical analysis services as well as water analysis.
   
   
5. Consider chemical cleaning of zirconium heat exchangers: As a precaution, consider chemical cleaning of newly fabricated zirconium heat exchangers in a mixture of nitric acid-hydrofluoric acid. Chemical cleaning will remove embedded particles or surface contaminants rendering the zirconium more resistant to corrosion. Chemical cleaning also improves the surface finish, which may reduce the crystallization of ferrous sulfate monohydrate salts (see Tip #9). Chemical cleaning of zirconium should be performed in at least a 10:1 ratio of nitric acid to hydrofluoric acid to minimize the potential for hydrogen absorption. Because of the hazardous nature of hydrofluoric acid, a properly equipped facility should be used to perform the chemical cleaning. For questions related to nitric acid-hydrofluoric acid cleaning, please contact ATI Wah Chang Customer Service at custserv@wahchang.com or by calling 1-541-967-6977.
   
   
6. The heat exchangers must include a second concentric zirconium tube of larger diameter at the acid/air interface of the riser pipes: This outer tube section is known as a solution level protector, air cover, or air baffle as shown on the inlet and outlet pipes of Figure 3C. Solution level protectors are especially important when high pressure steam (e.g. 100 psi) is used as the heating medium. Acid residues on the pipe just above the solution level will concentrate to very corrosive levels due to evaporation of water. Solution level protectors provide an extra layer of protection to this vulnerable area of the heat exchanger as well as give a visual indication of corrosion as shown in Figure 7. Solution level protectors should be as long as possible, perhaps 8-12 inches or more, to account for the evaporative cycle of the tank. Solution level protectors should be inspected periodically to prevent heat exchanger leakage.
   
   
   

   FIGURE 7. “Sacrificial” solution level protector exhibiting pitting corrosion.

   
   
7. Mount heat exchangers so that the coils are always immersed: Evaporation of the pickling solution during off-shift and other down-times may cause the solution to drop to a level that exposes the heating coils to air. Acid residues on the exposed coils will then concentrate to very corrosive levels as they dry, similar to the phenomenon described in Tip #6.
   
   
8. Insulate heat exchangers from stray currents: Stray electrical currents can reach a heat exchanger via piping, tanks walls, hangers, or other metal objects. Because corrosion is an electrochemical process, metallic heat exchangers need to be electrically isolated from contact with other metals. To eliminate the potential of stray currents, a piece of pressure hose or a dielectric union should be placed between the heat exchanger fittings and the water/steam line. Consider using a PTFE insulator as shown in Figure 8. Also, insulate metal hangers that support the heat exchanger from contact with other metal objects.
   
   
   

   FIGURE 8. PTFE Insulation Couples. Photo Courtesy of Process Technology.

   
   
9. Avoid gray colored “monohydrate” crystal build-up on the coils: Build-up of ferrous sulfate monohydrate crystals is usually lower when using hot water heating versus high pressure steam heating. Techniques that have been used to minimize crystal build-up on the heat exchanger include:
   

   a) Maintaining a smooth surface on the heat exchanger, b) Bubbling or sparging of air beneath the heat exchanger c) Keeping the bath at a temperature where ferrous sulfate monohydrate is most soluble (i.e. 140-150¾F)

   If crystal build-up occurs, do not damage the surface of the zirconium by scraping, scratching, or marring when trying to remove the salts. These actions may embed iron particles or other contaminants that reduce the corrosion resistance of zirconium. Non-contact techniques such as soaking in hot water, pressure washing, or steam cleaning may be employed. Do not sandblast zirconium heat exchangers to remove crystal build-up or for any other reason. Sandblasting media may embed foreign particles in the surface that increase the potential for corrosion.
   
   
10. Protect the heat exchanger from damage using a barrier or shield: The heat exchanger should be protected to prevent it from being damaged. Consider having zirconium angle welded to the side of the unit as a shield. Other possible shielding materials include plastisol-coated steel, some grades of fiberglass, thermally-stable plastics, or even hardwood.

1. OUTLOOK, VOL. 1, NO. 4, Fall 1980, Zirconium Heat Transfer Coils Fight Corrosion in Sulfuric Acid System, by Te-Lin Yau
   
   
2. OUTLOOK, VOL. 9, NO. 1, Winter 1988, In Sulfuric Acid Pickling, Zirconium Continues to Provide Excellent Corrosion Resistance, by Te-Lin Yau
   
   
3. OUTLOOK, VOL. 25, NO. 1, First Quarter 2004, How Does Surface Condition Affect the Corrosion of Zirconium in Acidic Chloride Solutions?, by Steve Sparkowich & Te-Lin Yau
   
   
4. Paper # 319, Zirconium in a Sulfuric Acid Pickling Application, by Te-Lin Yau, presented at Corrosion ’88, March 21-25, 1988 in St. Louis, Missouri.
   
   
5. Corrosion Engineering Handbook, edited by Philip Schweitzer, Chapter 10, 1996.