Why does the aluminum alloy surface protective film turn black after alkali washing during the forma

During the formation of the protective film on the surface of the aluminum alloy, alkali washing is carried out first to remove the oil and impurities on the surface, and then anodizing.

Process, the oxide film formed after anodizing is porous, and then put into the dyeing solution to dye black. This is a simple process of anodizing and blackening the surface of aluminum alloys. It is not blackened after caustic washing. This ingredient is not easy to determine. Certain transition metals.

It's black, or their chloride.

The fluoride is black.

For example, rhenium metal, which itself looks black, of course has a metallic luster, and it is insoluble in hydrochloric acid or hydrofluoric acid.

The reasons for the situation you are talking about are:

1.The surface of the aluminum is corroded by hydrochloric acid, hydrofluoric acid (aluminum fluoride has little solubility, but it is white) to reveal a certain alloy metal, which is black.

2.The aluminum alloy reacts with hydrochloric acid, hydrofluoric acid, to produce black salts, and these salts have low solubility and cover the surface of the alloy.

It is difficult to determine which ingredient it is, mainly because the detailed data of metal salts are difficult to find, especially the color. There are only common metals on hand, but their chloride and fluoride are not white.

Mechanical removal is generally not thorough, I tried with 25% sodium hydroxide.

solution, about 60, 20-30s to remove the oxide film, the surface of the aluminum alloy all blackened, in nitric acid.

Soaking in solution until the surface of the sample is completely clean, this method does not seem to completely eliminate the oxide film, is it the wrong time to soak in sodium hydroxide or nitric acid solution? Which one has done it, mention it, if there is a better way, please also point out, thank you.

After the aluminum is cleaned by caustic soda, the surface oxide film will corrode off, and then the surface will form aluminum hydroxide, and its surface will be blackened.

Caustic soda, also known as caustic soda, scientific name sodium hydroxide (NaOH), can chemically react with aluminum. If the aluminum product is slightly corroded by caustic soda, it can be restored to its original state by repairing part of the damaged part by plating. If the aluminum product is completely corroded by caustic soda, the material elements can be dismantled and carried out relevant chemical reactions to reverse and reborn, but the aluminum product cannot be restored to its original state.

Caustic soda is strongly alkaline and has strong hygroscopicity. Soluble in water, exothermic when dissolved, the aqueous solution is alkaline and has a greasy feeling; It is extremely corrosive and has a corrosive effect on fibers, glass, ceramics, etc. Hydrogen is released by reacting with metal aluminum and zinc, non-metal boron and silicon; disproportionation reaction with halogens such as chlorine, bromine, iodine; It neutralizes with acids to form salt and water.

Physical and chemical properties of aluminum

Aluminum is a silvery-white light metal. Malleable. The relative density of aluminum, the modulus of elasticity 70gpa, the Poisson's ratio. Melting point 660 . Boiling point 2327.

Aluminum is easily soluble in dilute sulfuric acid, dilute nitric acid, hydrochloric acid, sodium hydroxide and potassium hydroxide solutions, insoluble water, but can react slowly with hot water to form aluminum hydroxide.

Aluminum is a reactive metal, and the surface of aluminum in dry air immediately forms a dense oxide film, so that aluminum will not be further oxidized and can resist water. However, the powder of aluminium is extremely flammable when mixed with air. Molten aluminum can correspond violently to water, aluminum is amphoteric and highly soluble in strong alkalis and can also be soluble in dilute acids.

The above content is a reference to the waiter: Encyclopedia - Aluminum.