Acid-Resistant Materials

(acidproof materials), metallic and nonmetallic materials that are stable toward the destructive action of acids.

Metallic acid-resistant materials include wrought and cast high-alloy steels; nickel-, copper-, and aluminum-based alloys; and certain pure metals, such as nickel, aluminum, copper, and lead. The use of titanium and its alloys has promising possibilities. Zirconium, tantalum, and niobium and their alloys are used in especially critical structures. The oxidation-reduction properties of the medium (redox potential), the nature of the anions, and the concentration and temperature of the acids are taken into account in the selection of metallic acid-resistant materials. In oxidative media, materials have been used successfully that form films of a chemical compound with highly protective properties upon reaction of the aggressive medium with the surface (stainless steels, nickel-chromium steels, aluminum and its alloys, titanium, and high-chromium or silicon cast irons). Over-passivation of stainless steels and nickel alloys occurs in strongly oxidizing media. Metallic materials used in reducing media should be highly stable thermodynamically. These materials include copper, nickel, nickel alloys containing molybdenum (has-telloys) or copper (Monel’ metals), titanium alloys, and alloyed molybdenum. Weakly oxidizing and weakly reducing media require the use of chemically complex alloys and steels alloyed with metals that will increase their ability to inhibit corrosion and raise their thermodynamic stability. These materials include high-alloyed nickel, molybdenum, and copper; stainless steels; nickel alloys with admixtures of chromium, molybdenum, and tungsten; and titanium with small amounts of palladium.

Nonmetallic acid-resistant materials are classified as either organic or inorganic. Among the inorganic materials, polymeric acid-resistant materials are being used increasingly. They include faolite, polyvinyl chloride, polyethylene, polypropylene, polycarbonate, and Teflons. Teflons are distinguished by the highest degree of chemical stability; they are not broken down even in strongly oxidizing media. The materials mentioned are used for the production of sheets, pipe, rods, and custom shapes. Many of these materials may be readily welded, cemented, and fiberglass-reinforced. Graphite and graphites impregnated with resins are used as acid-resistant heat conductors. The inorganic acid-resistant materials include acid-resistant ceramics, cast stone, glass, quartz glass, cermets, asbestos, porcelain, enamels, caulking materials, concrete, and cement.