Corrosion-resistant alloys, as their name indicates, are alloys with enhanced corrosion resistance. Some ferrous and many non-ferrous metals and alloys are widely used in corrosive environments. In all cases, it strongly depends on certain environment and other conditions. Corrosion-resistant alloys are used for water piping and many chemical and industrial applications. In case of ferrous alloys, we are talking about stainless steels and to some extent about cast irons. But some non-ferrous corrosion-resistant alloys exhibit remarkable corrosion resistance nad therefore they may be used for many special purposes. There are two main reasons why nonferrous materials are preferred over steels and stainless steels for many of these applications. For example, many of the non-ferrous metals and alloys possess much higher resistance to corrosion than available alloy steels and stainless steel grades. Second, a high strength-to-weight ratio or high thermal and electrical conductivity may provide a distinct advantage over a ferrous alloy.
Properties of Corrosion-resistant Alloys
Material properties are intensive properties, that means they are independent of the amount of mass and may vary from place to place within the system at any moment. The basis of materials science involves studying the structure of materials, and relating them to their properties (mechanical, electrical etc.). Once a materials scientist knows about this structure-property correlation, they can then go on to study the relative performance of a material in a given application. The major determinants of the structure of a material and thus of its properties are its constituent chemical elements and the way in which it has been processed into its final form.
Density of Corrosion-resistant Alloys
Density of typical aluminium bronze is 7.45 g/cm3 (UNS C95400).
Density of typical superalloy is 8.22 g/cm3 (Inconel 718).
Density of typical titanium alloy is 4.51 g/cm3 (Grade 2).
Density of typical aluminium alloy is 2.7 g/cm3 (6061 alloy).
Density of typical stainless steel is 8.0 g/cm3 (304 steel).
Density is defined as the mass per unit volume. It is an intensive property, which is mathematically defined as mass divided by volume:
ρ = m/V
In words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is kilograms per cubic meter (kg/m3). The Standard English unit is pounds mass per cubic foot (lbm/ft3).
Since the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance, it is obvious, the density of a substance strongly depends on its atomic mass and also on the atomic number density (N; atoms/cm3),
- Atomic Weight. The atomic mass is carried by the atomic nucleus, which occupies only about 10-12 of the total volume of the atom or less, but it contains all the positive charge and at least 99.95% of the total mass of the atom. Therefore it is determined by the mass number (number of protons and neutrons).
- Atomic Number Density. The atomic number density (N; atoms/cm3), which is associated with atomic radii, is the number of atoms of a given type per unit volume (V; cm3) of the material. The atomic number density (N; atoms/cm3) of a pure material having atomic or molecular weight (M; grams/mol) and the material density (⍴; gram/cm3) is easily computed from the following equation using Avogadro’s number (NA = 6.022×1023 atoms or molecules per mole):
- Crystal Structure. Density of crystalline substance is significantly affected by its crystal structure. FCC structure, along with its hexagonal relative (hcp), has the most efficient packing factor (74%). Metals containing FCC structures include austenite, aluminum, copper, lead, silver, gold, nickel, platinum, and thorium.
Mechanical Properties of Corrosion-resistant Alloys
Materials are frequently chosen for various applications because they have desirable combinations of mechanical characteristics. For structural applications, material properties are crucial and engineers must take them into account.
Strength of Corrosion-resistant Alloys
In mechanics of materials, the strength of a material is its ability to withstand an applied load without failure or plastic deformation. Strength of materials basically considers the relationship between the external loads applied to a material and the resulting deformation or change in material dimensions. Strength of a material is its ability to withstand this applied load without failure or plastic deformation.
Ultimate Tensile Strength
Ultimate tensile strength of aluminium bronze – UNS C95400 is about 550 MPa.
Ultimate tensile strength of superalloy – Inconel 718 depends on heat treatment process, but it is about 1200 MPa.
Ultimate tensile strength of commercially pure titanium – Grade 2 is about 340 MPa.
Ultimate tensile strength of 6061 aluminium alloy depends greatly on the temper of the material, but for T6 temper it is about 290 MPa.
Ultimate tensile strength of stainless steel – type 304 is 515 MPa.
The ultimate tensile strength is the maximum on the engineering stress-strain curve. This corresponds to the maximum stress that can be sustained by a structure in tension. Ultimate tensile strength is often shortened to “tensile strength” or even to “the ultimate.” If this stress is applied and maintained, fracture will result. Often, this value is significantly more than the yield stress (as much as 50 to 60 percent more than the yield for some types of metals). When a ductile material reaches its ultimate strength, it experiences necking where the cross-sectional area reduces locally. The stress-strain curve contains no higher stress than the ultimate strength. Even though deformations can continue to increase, the stress usually decreases after the ultimate strength has been achieved. It is an intensive property; therefore its value does not depend on the size of the test specimen. However, it is dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material. Ultimate tensile strengths vary from 50 MPa for an aluminum to as high as 3000 MPa for very high-strength steels.
Yield Strength
Yield strength of aluminium bronze – UNS C95400 is about 250 MPa.
Yield strength of superalloy – Inconel 718 depends on heat treatment process, but it is about 1030 MPa.
Yield strength of commercially pure titanium – Grade 2 is about 300 MPa.
Yield strength of 6061 aluminium alloy depends greatly on the temper of the material, but for T6 temper it is about 240 MPa.
Yield strength of stainless steel – type 304 is 205 MPa.
The yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning plastic behavior. Yield strength or yield stress is the material property defined as the stress at which a material begins to deform plastically whereas yield point is the point where nonlinear (elastic + plastic) deformation begins. Prior to the yield point, the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible. Some steels and other materials exhibit a behaviour termed a yield point phenomenon. Yield strengths vary from 35 MPa for a low-strength aluminum to greater than 1400 MPa for very high-strength steels.
Young’s Modulus of Elasticity
Young’s modulus of elasticity of aluminium bronze – UNS C95400 is about 110 GPa.
Young’s modulus of elasticity of superalloy – Inconel 718 is 200 GPa.
Young’s modulus of elasticity of commercially pure titanium – Grade 2 is about 105 GPa.
Young’s modulus of elasticity of 6061 aluminium alloy is about 69 GPa.
Young’s modulus of elasticity stainless steel – type 304 and 304L is 193 GPa.
The Young’s modulus of elasticity is the elastic modulus for tensile and compressive stress in the linear elasticity regime of a uniaxial deformation and is usually assessed by tensile tests. Up to a limiting stress, a body will be able to recover its dimensions on removal of the load. The applied stresses cause the atoms in a crystal to move from their equilibrium position. All the atoms are displaced the same amount and still maintain their relative geometry. When the stresses are removed, all the atoms return to their original positions and no permanent deformation occurs. According to the Hooke’s law, the stress is proportional to the strain (in the elastic region), and the slope is Young’s modulus. Young’s modulus is equal to the longitudinal stress divided by the strain.
Hardness of Corrosion-resistant Alloys
Brinell hardness of aluminium bronze – UNS C95400 is approximately 170 MPa.
Brinell hardness of superalloy – Inconel 718 depends on heat treatment process, but it is approximately 330 MPa.
Rockwell hardness of commercially pure titanium – Grade 2 is approximately 80 HRB.
Brinell hardness of 6061 aluminium alloy depends greatly on the temper of the material, but for T6 temper it is approximately 95 MPa.
Brinell hardness of stainless steel – type 304 is approximately 201 MPa.
Rockwell hardness test is one of the most common indentation hardness tests, that has been developed for hardness testing. In contrast to Brinell test, the Rockwell tester measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load). The minor load establishes the zero position. The major load is applied, then removed while still maintaining the minor load. The difference between depth of penetration before and after application of the major load is used to calculate the Rockwell hardness number. That is, the penetration depth and hardness are inversely proportional. The chief advantage of Rockwell hardness is its ability to display hardness values directly. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale.
The Rockwell C test is performed with a Brale penetrator (120°diamond cone) and a major load of 150kg.
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