[ALUMINIUM NETWORK] Aluminum and its alloys generally exhibit localized corrosion in addition to a small amount of media, such as alkaline solutions and phosphoric acid solutions. Common corrosion patterns: pitting corrosion, galvanic corrosion, intergranular corrosion, crevice corrosion, filiform corrosion, lamellar corrosion, stress corrosion cracking, and corrosion fatigue, which can occur in both ships and marine facilities.
Stress corrosion cracking and corrosion fatigue of aluminum alloys are caused by the combination of stress (tensile stress or alternating stress) and corrosion, and are more common in high-strength aluminum alloys. Due to the abrupt occurrence of the damage, most of them occurred in the structure of the force, which had a great instantaneous damage risk, aroused widespread interest in the academic and engineering circles, and conducted more detailed and in-depth studies, forming a discussion from the experimental study to the machine. Such a large number of works.
Spotted pitting, also known as pitting, is an extremely localized form of corrosion in needles, dots, and small holes in metals. It is a unique form of anode reaction. Earlier pitting was caused by local cells. As time went on, the pitting rate gradually slowed down. Industrial pure aluminum, ie, 1XXX series alloys have a high resistance to pitting corrosion, and 3003 alloys containing a small amount of Cu also have good corrosion resistance. The pitting rate of industrial pure aluminum decreases with its purity, especially when its Fe and Cu contents are low; 5xxx alloys of Al-Mg series also have good resistance to pitting corrosion, and they are used in ships and marine facilities. More, it can be called ocean deformation aluminum alloy. The Al-Mg-Si series 6XXX alloy has both a tendency of slight erosion and an intergranular corrosion tendency. Duralumin and superhard aluminium sheets should be coated with Al and Zn-Mg alloys to prevent pitting.
The degree of pitting corrosion of aluminum and aluminum alloy varies from medium to medium, and the rural atmosphere is lighter than that of industrial atmosphere and marine atmosphere. The pitting corrosion in water is more serious than that in the atmosphere. The degree of pitting on the surface of the material is different from that in the atmosphere. The pitting corrosion of the alloy of the layer is much lighter than that of the unclad aluminum alloy.
Corrosion occurs in the atmosphere, fresh water, seawater, and neutral aqueous solutions. Severe pitting can lead to perforation. Fortunately, a certain depth of corrosion hole will stop development. The corrosion resistance of aluminum is much higher than that of steel and galvanized steel. The steel will always corrode, while the galvanized steel will accelerate the violent corrosion after the galvanized layer is destroyed. The degree of pitting is also related to the medium and the alloy. Experiments have proved that aluminum alloy corrosion medium must also exist in the destruction of the local passive anion, such as chloride ions, fluoride ions. From the viewpoint of aluminum alloys, high-purity aluminum is less prone to point corrosion, aluminum alloys containing copper are more sensitive to pitting corrosion, and 3XXX series and 5XXX series alloys have relatively strong pitting corrosion resistance.
Intergranular corrosion Intergranular corrosion is the selective corrosion that occurs along the boundaries of the metal grains. The grain boundary is an interdigitation zone between metal grains. It is a high-energy zone and has a stronger chemical activity. Therefore, the grain boundary mostly erodes faster than the grain. If the grain boundary is significantly more active, intergranular corrosion will occur.
For 2XXX-based alloys such as 2024, CuAl2 precipitated between crystals during artificial aging, and a Cu-poor region formed at the grain boundary. The potential in this region was relatively negative, and the corrosion proceeded along the grain-depleted Cu region to form intergranular corrosion. The 7XXX-type alloy is a uniform solid solution after solution treatment. After an artificial aging treatment with a tendency to intergranular corrosion, MgZn2 with a potential of -0.86 V is precipitated between crystals, and an alloying element with a potential of -0.57 V is formed at the grain boundaries. The (Mg and Zn) regions, while the crystal grains themselves are solid solutions, have a potential of -0.68V. Obviously, because the potential of MgZn2 is relatively negative, corrosion proceeds along the intergranular.
Because the intergranular corrosion occurs inside the metal, it is often difficult to find and it will suddenly cause structural damage and even cause major accidents. In August 1981, a Boeing 737 cargo plane in Taiwan Province of China was crashed due to intergranular corrosion during a high altitude flight. After inspection, it was found that the aircraft was used to transport live fish and seafood and caused serious aluminum structure. Intergranular corrosion.
Industrial pure aluminum undergoes pitting corrosion in atmosphere, fresh water and neutral solution, and no intergranular corrosion occurs; the 3XXX series alloy and the 6XXX series alloy without excess Si forming Mg2Si have no tendency of intergranular corrosion because MnAl6 and Mg2Si phases are different from each other. The potentials of solid solutions are similar, and if there is excess Si, then there is sensitivity to intergranular corrosion.
Al-Cu, Al-Cu-Mg, Al-Zn-Mg, Al-Zn-Mg-Cu, and Al-Mg-based alloys having a Mg content of 3% tend to have intergranular corrosion. When the heat treatment is not properly performed, CuAl2, AlxCuxMg, MgZn2, Al2ZnxMg, and Mg2Al3 phases are precipitated on the grain boundary, and their electric potentials are greatly different from the crystal grain itself and the grain boundary potential, forming a microbattery and causing intergranular corrosion. In the alloy containing Cu, Al2Cu is the cathode, and the Cu-depleted region near the grain boundary is the anode and dissolves.
The degree of intergranular sensitivity can be effectively controlled or even eliminated by the material's processing technology, reasonable matching and selection of heat treatment parameters, and controlling the content of alloying elements that cause intergranular corrosion. For example, the best artificial aging specification of 2A12 alloy sheet is 190°C/(6h~12h); the profile is 190°C/(6h~12h); 2A16 alloy sheet is 190°C/18h or 210°C/12h; 2A10 alloy wire The temperature is 75°C/24h; for the 2A16 alloy wire, it is 190°C/18h.
2A12-T4 alloy artificial aging after 190 °C / 12h, and then stabilized at 125 °C ~ 170 °C for a long time or stable treatment above 200 °C, can eliminate intergranular corrosion; 2A16 alloy by 190 °C / 18h artificial aging and Stabilization treatment can eliminate the sensitivity of intergranular corrosion; 2A06-T4 alloy material has no tendency of intergranular corrosion after being stabilized at 200°C, 250°C or 300°C.