Alloying for Corrosion Resistance
The primary alloying elements to increase corrosion resistance of steels are copper, chromium, silicon, phosphorus, and nickel. Broad categories of steel materials based upon alloying content include the low alloy steels, weathering steels, and stainless steels.
Carbon and Low Alloy Steels
For carbon steels, copper additions of 0.01 to 0.05% have the greatest effect for increasing general corrosion resistance, as seen in Figure 30. The relationship of the other elements on corrosion resistance is displayed in Figure 31. Small additions of chromium significantly increase tensile strength as well as increasing corrosion resistance leading to the high strength low alloy (HSLA) steels. Weathering steels is a term describing low-alloy steels with small additions of chromium, nickel, and copper. They can provide good service without any coatings in a non-marine atmospheric environment. Many inland bridge structures make use of weathering steels. Larger additions of chromium are required for a dramatic increase in corrosion resistance as previously mentioned.
Steels
Stainless Steels
Stainless steels contain 11 percent or more of chromium. The higher chromium content results in the formation of a chromium oxide protective film, greatly increasing the oxidation resistance of the steel. Stainless steels are most often exposed to a passivating solution to improve formation of the protective film.66 Corrosion resistance will generally increase with an increase in chromium content and decrease with an increase in carbon content. Stainless steels are excellent for oxidizing environments but are susceptible in halogen acids or halogen salt solutions. They are also susceptible to pitting in seawater.
Austenitic Stainless Steels
Austenitic stainless steels are the most commonly used class of stainless steels. They may be used in mild to severe corrosive environments, dependent upon alloying and are nonmagnetic compared with other steels.66 They may be utilized in environments with temperatures reaching 600ºC and for low temperatures in the cryogenic range. Almost all austenitic stainless steels are modifications from the 18Cr – 8Ni (304) alloy. Difficulty in processing stainless steels limits increasing concentrations of chromium. The addition of nitrogen has been found to be an austenite phase stabilizer which allows higher additions of molybdenum, up to about 6%, increasing the material’s corrosion resistance in chloride environments. Other additions which improve corrosion resistance to specified environments include high chromium alloys for high temperature service and high nickel alloys for inorganic acids. Table 22 represents a summary of the austenitic stainless steel alloys and their modifications in regards to corrosion resistance.
Martensitic Stainless Steels
Martensitic stainless steels have a much lower corrosion resistance than austenitic grades, and usually slightly lower than the ferritic grades. The martensitic stainless steels contain lower Cr and higher C concentrations compared with the other stainless steels. This structure results in a strong but brittle class of materials. They may be tempered to improve toughness, but to limited degree. Additions of nitrogen, nickel, and molybdenum at lower levels of carbon have been found to produce alloys with better toughness and corrosion resistance properties. The corrosion characteristics of the martensitic grades are summarized in Table 24.
Duplex Stainless Steels
Duplex stainless steels are two phase materials containing roughly equal amounts of ferrite and austenite phases developed specifically as a high corrosion resistant material. They contain high levels of chromium (20 – 30%), Ni (5 – 10%), and low carbon content (< 0.03%). They may additionally contain molybdenum, nitrogen, tungsten, and copper as modifiers to increase corrosion resistance in specific environments. Duplex stainless steels offer strength about double that of austenitic stainless steels, with increased resistance to chloride induced SCC and pitting. They are typically used in temperatures ranging from -60 to 300ºC. There are four primary alloys of duplex stainless steels used which are:
Iron-based Superalloys
Iron-based superalloys are also an extension of the stainless steels. They contain 20 – 30% chromium plus other alloying elements. They offer good corrosion resistance in a service temperature higher than the duplex stainless steels, but lower than nickel-based superalloys (up to about 815ºC). The cost of the iron-based superalloys is lower than nickel-based superalloys making them marketable in this service temperature range. Iron-based superalloys are used in structural components for furnaces, in steam and gas turbines, and in chemical processing equipment.