On 2018-08-08 14:48:54
Heat treatment corrosion and high temperature oxidation of aluminum alloy
How to prevent corrosion and high temperature oxidation of aluminum alloy in heat treatment
The heat treatment of aluminum alloy is usually carried out in the salting furnace or vacuum furnace. It often causes corrosion on the surface of parts, even black spots and holes. The corrosion forms of aluminum and its alloys are pitting corrosion, intergranular corrosion, stress corrosion, exfoliation corrosion, etc. No matter what kind of corrosion, except for the material itself, it is related to heat treatment and environmental factors.
The surface of castings heated in salt bath is often loose and corroded. Corrosion marks are often formed in the threads, grooves and small holes of aluminum parts. Therefore, it is necessary to control the content of chloride ion in the bath furnace less than 0.5%, the total amount of impurities in the nitrate bath less than 2%, and the total amount of sodium carbonate in the alkali less than 1%. If corrosion is found on the parts, potassium dichromate accounting for about 3% of the total mass of the salt bath shall be added to the furnace, potassium dichromate and nitrate shall be mixed according to 1:5, dried, and added at about 400 ℃ in the salt bath. Ensure that salt bath heating does not produce corrosion. When quenching large parts, in order to prevent the nitrate from adhering to the surface or groove of the parts, clean them immediately. The water temperature shall be controlled at 30 ~ 50 ℃, and the cleaning water shall be free of acid and alkali.
When heating in an air furnace, hydrogen corrosion is easy to occur due to high humidity and much water in the furnace. Bubbles on the surface, unclean cleaning of the part surface, residual grease and sulfide will also damage the integrity of the original oxide film of the aluminum part, make it decompose, penetrate into the inner layer of the oxide film and produce corrosion. In the coastal areas, chloride ions containing salt in the humid air enter into the furnace and corrode the aluminum parts more severely. Therefore, before heat treatment, parts should be cleaned and dried without water before entering the furnace. Harmful gas shall be discharged from the furnace and protective gas shall be added. The dew point in the furnace shall be low and there shall be no moisture.
For aluminum alloy with stress corrosion tendency, it is necessary to ensure uniform and rapid cooling of all parts, shorten the quenching transfer time as much as possible, and the water temperature shall not be too high.
During the heat treatment of aluminum clad material, the alloy elements will penetrate into the aluminum clad layer. The heating temperature should not be too high and the holding time should be short. If the process is improper and the heating times are repeated many times, Zn, Cu, Mg will diffuse to the aluminum clad layer, resulting in corrosion.
The oxide film and anodized film on the surface of aluminum alloy have protective effect. In the air furnace, 3G ammonium fluorate is added into the furnace per cubic meter, and the oxide film will form on the surface of the workpiece at high temperature to protect it.
In the current production, aluminum and magnesium alloys have different ways of heating protection. Magnesium alloy is protected by SO2. The furnace for processing magnesium alloy can't immediately process aluminum alloy, so attention should be paid in production.
Generally, the oxidation resistance of super duralumin is the worst, followed by that of duralumin, and that of low strength aluminum alloy is better.