Overview Of The Corrosion Resistance Of Common Special Metal Materials

Specialised metallic materials with high corrosion resistance and verified mechanical workability largely satisfy the corrosion resistance requirements of production facilities and extend equipment service life. The following text provides an overview of the corrosion resistance of common specialised metallic materials.

Titanium Material

Titan is a metal that readily forms a passive oxide film. In air and in oxidising or neutral aqueous solutions, a stable oxide layer forms rapidly even after damage. Consequently, Titan demonstrates high corrosion resistance in such environments. Owing to its passivation characteristics, Titan does not accelerate its own corrosion when in contact with dissimilar metals; however, it may increase the corrosion rate of these metals. For example, when Pb, Sn, Cu or Monel alloys contact Titan to form an electrochemical couple in a low concentration of non-oxidising acid, the corrosion rate of these materials increases while Titan remains unaffected.

The iron content in Titan affects its corrosion resistance in certain media. In addition to the raw materials, an increase in iron content often results from welding contamination caused by iron infiltration. Consequently, sections of the weld exhibit higher iron content, leading to non‐uniform corrosion. Given that iron contamination on the Titan contact surface tends to accelerate, especially in the presence of hydrogen, mechanical damage to the titanium oxide layer on a contaminated surface permits hydrogen to penetrate the metal. Depending on temperature, pressure and other conditions, hydrogen diffuses accordingly, which results in varying degrees of hydrogen embrittlement in Titan. Consequently, when using Titan in systems operating at moderate temperature and pressure or in hydrogen-containing systems, surface contamination with iron should be avoided.

Nickel and Nickel-based Alloys

Nickel tends to develop a passive surface. At room temperature, its surface is covered by an oxide film that protects it from corrosion in water and various salt solutions.

At room temperature, Nickel is relatively stable in non-oxidising dilute acids such as less than 15% hydrochloric acid, less than 17% sulphuric acid and many organic acids. However, the corrosion rate of Nickel increases significantly with higher concentrations of oxidising agents (FeCl₂, CuCl₂, HgCl₂, AgNO₃ and hypochlorite) and with increased aeration.

Nickel remains entirely stable in alkaline solutions at elevated temperatures and in molten bases.

The Monel alloy exhibits higher corrosion resistance than Nickel in reducing media and higher resistance than Copper in oxidising media. It remains highly corrosion resistant when oxygen is present in any concentration of hydrofluoric acid. However, its resistance to hydrofluoric acid decreases if the solution contains oxidising agents or contaminants such as iron or copper salts. In applications requiring hydrofluoric acid resistance, Monel alloy is used alongside platinum and silver.

Copper-Nickel

The corrosion resistance of Copper-Nickel is comparable to that of pure copper. In inorganic acids, particularly nitric acid, significant corrosion occurs. In hydrofluoric acid concentrations below 70%, in the absence of oxygen and below the boiling point, it remains corrosion resistant. Furthermore, Copper-Nickel is less susceptible to corrosion by inorganic acids, alkaline solutions and organic compounds.

In caustic soda or electrolytic caustic soda with a diaphragm, B30 (70-30 Copper-Nickel alloy) can replace pure Nickel in the manufacture of film evaporator equipment, particularly in evaporator components. This substitution extends service life and utilises 70 % less Nickel. Furthermore, B10 (91-9 Cu/Ni alloy) may replace pure Nickel in the manufacture of evaporator tubes and equipment. In addition, Copper-Nickel exhibits high resistance to seawater corrosion; consequently, seawater-cooled heat exchangers typically employ Copper-Nickel alloys such as B10 and B30.

Zirconium

Zirconium has better corrosion resistance than stainless steel, Nickel-based alloys and Titanium. Its mechanical and technological properties also render it suitable for the manufacture of containers and heat exchangers.

Owing to its high price, Zirconium has seen limited use in industrial production. However, with the development of the domestic chemical industry, Zirconium materials are increasingly utilised in highly corrosive environments. This trend extends equipment service life and reliability and yields improved economic benefits. The technology from Zirconium production to the design, manufacture and testing of equipment is now more mature, thereby supporting the wider application of Zirconium containers.

Tantalum

Tantal exhibits high chemical stability, chemical resistance and strong resistance to atmospheric corrosion below 150 ℃. It remains corrosion resistant even in polluted industrial atmospheres. Below 200 ℃, Tantalum shows high stability in both acidic and alkaline media, which exceeds that of Gold or Platinum.

Tantalum is not corrosion resistant in concentrated alkali. It is not resistant to solutions of potassium iodide or those containing fluoride ions. The corrosion of Tantalum is uniform and general; it does not react sensitively to minor damage and does not cause localised corrosion phenomena such as corrosion fatigue or stress corrosion cracking. This property of Tantalum permits its use as a coating and lining material.

Metal Composite Material

Although specialised metallic materials exhibit higher corrosion resistance, they are relatively expensive. This cost factor limits their widespread use. However, metal composite technology facilitates the application of these materials from another perspective.

The metal composite material is a new type of material that consists of several metals or alloying elements such as a, b, c, etc. The metal bonds formed on the surface ensure that the composite material exhibits equivalent or improved properties compared with the original monometallic materials. It is not solely metal a, b or c. It integrates the advantages of the individual components and compensates for the performance deficiencies of any single component. Metal composite materials optimise material design and reflect the principle of rational material utilisation.