Marine material: corrosion form of aluminum

- Jan 11, 2019-

Aluminum and aluminum alloys generally undergo local corrosion in addition to extensive corrosion in a few media, such as alkaline solutions and phosphoric acid solutions. Common corrosion patterns: pitting corrosion, galvanic corrosion, intergranular corrosion, crevice corrosion, filamentous corrosion, layered corrosion, stress corrosion cracking and corrosion fatigue, etc. These corrosion forms may occur in 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 sudden occurrence of damage, most of them occur in the structure of the force, which has great danger of instantaneous destruction, which has aroused widespread interest in the academic and engineering circles and carried out more detailed and in-depth research, forming from experimental research to machine exploration. Wait a lot of work.

Pitting corrosion

Pitting, also known as pitting corrosion, is a highly localized form of corrosion in the metal that produces needles, spots, and pores. It is a unique form of anode reaction. Pitting corrosion was first caused by local batteries, and the pitting rate gradually slowed down with time. Industrial pure aluminum, that is, 1XXX series alloy, has high resistance to pitting corrosion, and 3003 alloy containing a small amount of Cu also has good corrosion resistance. The pitting rate of industrial pure aluminum decreases with the increase of its purity, especially when its Fe and Cu contents are low; the Al-Mg series 5XXX alloy also has good pitting resistance, and it is used in ships and marine facilities. It is more, so it can be called ocean deformation aluminum alloy. The Al-Mg-Si system 6XXX alloy has both a tendency to be etched and a tendency to intergranular corrosion. Hard aluminum and super-hard aluminum sheets should be coated with aluminum and Al-Zn-Mg alloy to prevent pitting.

The degree of pitting corrosion of aluminum and aluminum alloy varies with the medium. The pastoral atmosphere is lighter than that in the industrial atmosphere and the ocean atmosphere. The pitting corrosion in the water is more serious than in the atmosphere, and the surface state of the material is different. The pitting of the alloy of the layer is much lighter than that of the uncoated aluminum alloy.

Pitting corrosion occurs in the atmosphere, fresh water, sea water, and neutral aqueous solutions, and severe pitting can lead to perforation. Fortunately, the corrosion hole will stop developing when it reaches a certain depth. The corrosion resistance of aluminum is much higher than that of steel and galvanized steel, and the steel will continue to corrode, and the galvanized steel will accelerate the severe corrosion after the galvanized layer is destroyed. The degree of pitting is also related to the medium and the alloy. Experiments have shown that there must be an anion that destroys the local passive state, such as chloride ion and fluoride ion, in the aluminum alloy pitting medium. From the perspective of aluminum alloys, high-purity aluminum is less prone to pitting corrosion. Copper-containing aluminum alloys are most sensitive to pitting corrosion, while 3XXX and 5XXX alloys have relatively strong pitting resistance.

Intergranular corrosion

Intergranular corrosion is a selective corrosion that occurs along the boundaries of metal grains. The grain boundary is the mismatched zone between the metal grains. It is a high-energy zone and has stronger chemical activity. Therefore, the grain boundary is mostly faster than the grain. If the grain boundary is significantly more active, intergranular corrosion will occur.

Industrial pure aluminum is pitting in the atmosphere, fresh water and neutral solution, and does not produce intergranular corrosion; the 3XXX alloy and the 6XXX alloy with no excess Si formed outside of Mg2Si have no intergranular corrosion tendency because the MnAl6 phase and the Mg2Si phase are The potential of the solid solution is similar, and if there is excess Si, there is sensitivity to intergranular corrosion.

Al-Mg-based alloys of Al-Cu, Al-Cu-Mg, Al-Zn-Mg, Al-Zn-Mg-Cu, and Mg content of 3% have a tendency to intergranular corrosion. When the heat treatment is not proper, CuAl2, AlxCuxMg, MgZn2, Al2ZnxMg, Mg2Al3 and other phases precipitate on the grain boundary. Their potentials differ greatly from the grain itself and the grain boundary potential, forming micro-batteries, causing intergranular corrosion. In the alloy containing Cu, Al2Cu is a cathode, and the Cu-depleted region near the grain boundary is an anode, and dissolution occurs.

The degree of intergranular sensitivity can be effectively controlled or even eliminated by the processing of materials, the reasonable matching and selection of heat treatment parameters, and the control of the content of alloying elements causing intergranular corrosion. For example, the best artificial aging specification for 2A12 alloy sheet is 190 °C / (6h ~ 12h); for profiles is 190 °C / (6h ~ 12h); for 2A16 alloy sheet is 190 °C / 18h or 210 °C / 12h; 2A10 alloy wire It is 75 ° C / 24 h; 2A16 alloy wire is 190 ° C / 18h.

2A12-T4 alloy after aging at 190 °C / 12h, and then stabilized at 125 °C ~ 170 °C for a long time or stabilized at 200 °C or above, can eliminate intergranular corrosion; 2A16 alloy by 190 °C / 18h artificial aging and Stabilization treatment can eliminate the intergranular corrosion sensitivity; 2A06-T4 alloy material has no intergranular corrosion tendency after being stabilized at 200 °C, 250 °C or 300 °C.