Part fifteen: surface corrosion analysis and quality control method

Part fifteen: surface corrosion analysis and quality control method

Part fifteen: surface corrosion analysis and quality control method

Surface corrosion

For mill finish extruded products that have not undergone surface treatment, the surface is chemically or electrochemically reacted with external media, causing local damage to the surface, which is called surface corrosion.

The surface of the corroded product loses its metallic luster, and gray-white corrosion products are produced on the surface in severe cases.

The main cause of surface corrosion

1. The product is in contact with corrosive media such as water, acid, alkali, salt, etc. during production, storage and transportation, or is parked for a long time in a humid atmosphere;

2. Sometimes due to some reasons, the adding ratio of magnesium and silicon is not suitable during the casting process, so that ω(Mg)/ω(Si) is in the range of 1.0～1.3, which is much smaller than the optimal ratio of 1.3～1.7 (generally controlled within the range of 1.3～1.5). In this way, although the content of magnesium and silicon components is within the prescribed range (ω(Mg)=0.45% to 0.9%, ω(Si)=0.2% to 0.6%). However, some surplus silicon exists. In addition to a small amount of silicon in the free state, this part of the surplus silicon will also form ternary compounds in the aluminum alloy;

3. During the smelting process, although the addition ratio of magnesium and silicon is within the range specified in the standard, sometimes due to uneven and insufficient mixing, the distribution of silicon in the melt is uneven, and there are local enrichment and poor areas;

4. Control of various process parameters during extrusion, such as excessively high billets preheating temperature, improper control of metal extrusion flow rate, air-cooling strength during extrusion, aging temperature and holding time, etc., will easily cause silicon segregation and liberation, which will cause magnesium and silicon do not completely become Mg2Si phase, but some free silicon exists.

Solutions for surface corrosion:

1. Keep the surface of the product and the production and storage environment clean and dry;

2. Control the content of elements in the alloy;

3. Effectively control the alloy composition to ensure that the ratio of magnesium to silicon is within the range of 1.3 to 1.7;

4. Strictly control the parameters of each process (such as smelting, stirring, casting cooling water temperature, billet preheating temperature, extrusion quenching air cooling strength, aging temperature and time, etc.) to avoid segregation and dissociation of silicon, and try to make silicon It forms a beneficial Mg2Si strengthening phase with magnesium;

5. Pay attention to changes in weather, rainy days or high humidity, the extruded aluminum profile should be covered with rain cloth to prevent dust and rain from contacting the surface of the aluminum profile;

6. Timely aging treatment can reduce the appearance of spots, and porphyritic or pit-shaped corrosion of aluminum profiles.

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