Corrosion-Resistant Materials

metals and non-metallic materials capable of withstanding the destructive action of corrosive mediums. They are used in the production of instruments, piping, and hardware designed for use under exposure to acids, alkalies, salts, and corrosive gases. The resistance of a material refers to its ability to withstand corrosion in a particular medium or group of mediums. A material that is resistant in one medium may be greatly damaged in another. The ability of materials to resist oxidation at high temperatures in gaseous mediums such as air, oxygen, or carbon dioxide is called oxidation resistance. Oxidation-resistant materials include alloys of iron with chromium (stainless steel) and alloys of titanium, zirconium, molybdenum, and tantalum.

The oxidation resistance of iron-based alloys is increased by alloying with components that are capable of forming a protective oxide layer on the surface of the metal that impedes further oxidation. In addition to chromium, such components are silicon and aluminum. Nickel alloys, such as nimonic or inconel, are used in cases when high strength is required in addition to oxidation resistance.

The noble metals platinum and gold are resistant to oxidation in gaseous and many liquid mediums. In acidic oxidizing mediums such as nitric acid, chrome-nickel and chrome stainless steels are corrosion-resistant. The most widely used chrome-nickel austenitic stainless steel, IKhlSNIOT, contains 0.1 per-cent carbon, 18–20 percent chromium, 9–11 percent nickel, and 0.35–0.8 percent titanium. Titanium or its substitute, niobium, is added to eliminate a specific type of damage, intergranular corrosion. With a nickel content of 9–11 percent, steel has an austenitic structure, which provides high plasticity and suitability for technological processing, particularly welding. However, nickel is expensive and in short supply. Thus, in a number of austenitic stainless steels, nickel is completely or partially re-placed by manganese. Stainless steel containing only chromium is poorly suited to technological processing, although it has greater strength. For articles requiring a combination of high corrosion resistance and strength, chromium steels of the martensite class, containing 0.2–0.4 percent carbon and 12–14 per-cent chromium, are used. Steels with a 25 percent chromium content have high corrosion resistance but are weak and poorly suited to technological processing.

Iron and low-alloy steel (containing less than 2–3 percent alloyed components) are resistant to concentrated nitric and sulfuric acids. The resistance of steels under these conditions is determined by their capacity for passivation by the formation of thin but very dense oxide films on their surface. Alloying of steel with chromium increases this capacity. Steels alloyed with 25 percent chromium, 25 percent nickel, and 2–3 percent copper, as well as titanium alloys and lead, are resistant to hot sulfuric acid solutions. In mediums containing chlorides, austenitic stainless steels, as well as aluminum alloys, are subject to lesion corrosion and to a specific type of damage called stress corrosion. To combat stress corrosion (corrosion splitting), the nickel con-tent in steels is increased to 40 percent, or up to 1.5 percent copper is added. Titanium alloys and hastalloy, a nickel alloy containing molybdenum, are resistant in mediums containing chloride, including hydrochloric acid solutions.

Copper and its alloys, such as bronze and brass, as well as aluminum and aluminum alloys, are resistant to both natural fresh water and seawater up to 100°C.

Graphite, aluminosilicates, and pure silica are notable among inorganic nonmetallic corrosion-resistant materials. Quartz glass, in particular, is resistant in many mediums and is widely used in the production of chemical glassware. Various natural materials, such as andesite and basalt rock, are used for lining the metal bodies of equipment in the production of inorganic acids. Many organic materials, such as polyfluoroethylene resin (Teflon), polyethylene, and polystyrene, are resistant in many aqueous mediums, although they all may be used at temperatures not higher than 100°–200°C.

The corrosion resistance of materials may be increased by the application of protective coatings. Zinc plating, anodizing, aluminizing (coating with aluminum), nickel plating, chrome plating and enameling, as well as the application of organic materials (paints and varnishes), are used for protection from atmospheric corrosion. Corrosion inhibitors are widely used to retard damage to materials in corrosive mediums.