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How to Prevent Heat Treatment Corrosion and High Temperature Oxidation Of Aluminum Alloy?

Aluminum alloy heat treatment is generally carried out in a nitrate furnace or a vacuum furnace. It often causes corrosion on the surface of the parts, and even black spots and holes appear. The corrosion forms of aluminum and aluminum alloys are mainly pitting corrosion, intergranular corrosion, stress corrosion, and peeling corrosion. No matter what kind of corrosion it is, in addition to the material itself, it is also related to heat treatment and environmental factors.

Castings heated in a nitrate bath often have loose and corrosion marks on the surface. Corrosion marks are often formed in the threads, grooves and small holes of aluminum parts. Therefore, it is necessary to control the chlorine ion content of the bath furnace to less than 0.5%, the total amount of impurities in the nitrate bath to less than 2%, and the total amount of sodium carbonate in the alkali to less than 1%. If the parts are corroded, add about 3% of the total mass of the salt bath potassium dichromate to the furnace, mix the potassium dichromate and the nitrate in a ratio of 1:5, and then dry, in the salt bath about 400℃ join in. Ensure that the salt bath heating does not produce corrosion. When quenching large parts, in order to prevent nitrate from adhering to the surface or grooves of the parts, clean them immediately. The water temperature is controlled at 30~50℃, and the cleaning water must not contain acid or alkali.

When heating in an air furnace, due to the high humidity and high moisture in the furnace, it is easy to cause hydrogen corrosion. The appearance of air bubbles on the surface, the parts surface is not cleaned, and the residual grease and sulfide will also destroy the integrity of the original oxide film of the aluminum parts, cause it to decompose, and penetrate into the inner layer of the oxide film, resulting in corrosion. Salt-containing chloride ions in the humid air in coastal areas enter the furnace and corrode aluminum parts more severely. Therefore, before heat treatment, the parts must be cleaned and dried without water before entering the furnace. Harmful gases should be discharged from the furnace, protective gas should be added, the dew point in the furnace should be low, and there should be no moisture.

For aluminum alloy with stress corrosion tendency, uniform and rapid cooling of all parts should be ensured, quenching transfer time should be shortened as much as possible, and the water temperature should not be too high.

When the aluminum-clad material is heat-treated, alloy elements will penetrate into the aluminum-clad layer, the heating temperature should not be too high, and the heat preservation time should be short. If the process is improper and repeated heating times are too many, Zn, Cu, Mg will diffuse to the aluminum clad layer and cause corrosion.

The oxide film and anodized film on the aluminum alloy surface have a protective effect. In an air furnace, 3g of ammonium fluoride is added per cubic meter of furnace, and an oxide film will be formed on the surface of the workpiece to protect it at high temperatures.

In current production, aluminum and magnesium alloys have different heating protection methods. Magnesium alloys are protected with SO2. The furnace that processes magnesium alloy cannot process aluminum alloy immediately, so care must be taken during production.

Generally, super-duralumin has the worst oxidation resistance, followed by duralumin, and low-strength aluminum alloys are better.