3. Durability

　　 The main forms of building metal corrosion:

　　 (1) Uniform corrosion. The corrosion of the metal surface makes the section thinner uniformly. Therefore, the annual average thickness loss is often used as an indicator of corrosion performance (corrosion rate). Steel generally corrodes uniformly in the atmosphere.

　　 (2) Pitting corrosion. Metal corrosion is punctual and forms deep pits. The occurrence of pitting corrosion is related to the nature of the metal and its medium. Pitting corrosion easily occurs in media containing chloride salts. The maximum hole depth is often used as the evaluation index for pitting corrosion. Piping corrosion is mostly considered for pipeline corrosion.

　　 (3) Galvanic corrosion. Corrosion caused by different potentials at the contact of different metals.

　　 (4) Crevice corrosion. The metal surface often undergoes local corrosion in gaps or other hidden areas due to the difference in the composition and concentration of the medium between different parts.

　　 (5) Stress corrosion. Under the combined action of corrosive media and high tensile stress, the metal surface corrodes and expands inward into micro-cracks, often causing sudden breakage. The high-strength steel bars (wires) in the concrete can be damaged.

4. Hardness

　　 Hardness indicates the ability of a material to resist hard objects pressed into its surface. It is one of the important performance indicators of metal materials. Generally, the higher the hardness, the better the wear resistance. Commonly used hardness indicators are Brinell hardness, Rockwell hardness and Vickers hardness.

        Brinell hardness (HB): Press a hardened steel ball of a certain size (generally 10mm in diameter) into the surface of the material with a certain load (generally 3000kg), keep it for a period of time, after removing the load, the ratio of the load to the indentation area, It is the Brinell hardness value (HB), and the unit is kgf/mm2 (N/mm2).

　　 Rockwell hardness (HR): When HB > 450 or the sample is too small, the Brinell hardness test cannot be used and the Rockwell hardness measurement can be used instead. It uses a diamond cone with an apex angle of 120° or a steel ball with a diameter of 1.59 and 3.18mm, which is pressed into the surface of the material to be tested under a certain load, and the hardness of the material is obtained from the depth of the indentation. Depending on the hardness of the test material, different indenters and total test pressures can be used to form several different Rockwell hardness scales. Each scale is indicated with a letter after the Rockwell hardness symbol HR. The commonly used Rockwell hardness scales are A, B, and C (HRA, HRB, HRC). The C scale is the most widely used.

　　HRA: It is the hardness obtained by a diamond cone indenter with a load of 60kg, used for extremely hard materials (such as cemented carbide, etc.).

　　HRB: It is the hardness obtained by using a 100kg load and a hardened steel ball with a diameter of 1.58mm. It is used for materials with lower hardness (such as annealed steel, cast iron, etc.).

　　HRC: It is the hardness obtained with a 150kg load and a diamond cone indenter, used for materials with high hardness (such as hardened steel, etc.).

        Vickers hardness (HV): A diamond square cone indenter with a load of 120kg or less and an apex angle of 136° is pressed into the surface of the material. The surface area of the material indentation pit is divided by the load value to obtain the Vickers hardness value ( HV). Hardness test is the simplest and most feasible test method in mechanical performance test. In order to replace certain mechanical performance tests with hardness tests, a more accurate conversion relationship between hardness and strength is required in production. Practice has proved that between various hardness values of metal materials, there is an approximate corresponding relationship between the hardness value and the strength value. Because the hardness value is determined by the initial plastic deformation resistance and the continued plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance and the higher the hardness value.