Comparison of rust removal effects of several acids

Manufacturing Technology Chemical Descaling Commonly used acids and rust removal properties Chemical rust removal There are many types of acids commonly used. Inorganic acid has high rust removal efficiency, high speed and low price. However, if the concentration is improperly controlled, metal “over-corrosion” will occur, and due to strong acidity, acid rinsing will not corrode the metal and affect the protective performance of the coating.

Phosphoric acid does not have this disadvantage due to its weak acidity. The effect of organic acid is mild. After the workpiece is treated, the surface is clean and the residual acid is not seriously affected, but the derusting speed is slower and the price is higher.

1) Hydrofluoric acid

Hydrofluoric acid is a weak acid and the strength of the acid is similar to that of formic acid. Commercially available hydrofluoric acid has a typical concentration of 30% to 50%. The main features of hydrofluoric acid derusting are as follows:

(1) It can dissolve silicon-containing compounds and has good solubility for oxides of metals such as aluminum and chromium. It is commonly used to etch workpieces such as castings and stainless steel.

(2) For steel and iron workpieces, low concentration hydrofluoric acid can be used for rust removal. A 70% aqueous solution of hydrofluoric acid has a passivating effect on steel.

(3) Hydrofluoric acid with a concentration of about 10% is weakly corroded to magnesium and its alloys, so it is often used for the etching of magnesium workpieces.

(4) Lead is generally not corroded by hydrofluoric acid; nickel is resistant to strong enthalpy in aqueous hydrofluoric acid solutions having a concentration greater than 60%.

Hydrofluoric acid is highly toxic and volatile. When used, it is strictly resistant to human exposure to hydrofluoric acid and hydrogen fluoride gas. The etching tank is preferably sealed and has good ventilation. The fluorine-containing wastewater can be discharged after being treated.

2) Hydrochloric acid

At room temperature, hydrochloric acid has a strong ability to dissolve metal oxides, while it dissolves slowly on metal substrates such as steel. Therefore, when rust is removed by hydrochloric acid, over-corrosion and severe hydrogen embrittlement are less likely to occur, and the surface residue of the workpiece after rust removal is less and the quality is higher.

The ability of hydrochloric acid to remove rust is almost proportional to its concentration. However, concentrated hydrochloric acid is highly volatile, especially when it is heated, which is likely to corrode equipment and pollute the environment. Therefore, the concentration of hydrochloric acid generally does not exceed 360 g/l, and most of them are operated at room temperature.

3) Nitric acid

Nitric acid is an oxidized strong acid. In low-carbon steel products, in 30% nitric acid, the dissolution reaction is very intense, and the surface is clean and uniform after rust removal. Medium and high carbon steel products and low alloy steel products, after derusting in the above concentration of nitric acid, the surface residue is more, and it needs to be in the liquid

Anode treatment is performed to obtain a more uniform, clean surface. Nitric acid is highly volatile, and when reacted with metals, it releases a lot of harmful gases (nitrogen oxides) and releases a lot of heat. Nitric acid is very corrosive to the human body. It should be protected during operation. The acid tank should have ventilation and cooling device.

4) Phosphate

Phosphoric acid is a medium-strength inorganic acid. Since monohydrogen phosphate and orthophosphate are hardly soluble in water, the derusting ability of phosphoric acid is low. To compensate for this disadvantage, dephosphorization with a phosphoric acid solution generally requires heating. The advantage of dephosphorization of phosphoric acid solution is that it can produce hydrogen embrittlement.

The energy is small, and a small amount of solution remaining on the surface of the workpiece can be converted into an insoluble phosphate protective film, which is suitable for derusting of welded parts and assemblies before painting.

Concentrated phosphoric acid is combined with a certain proportion of nitric acid, sulfuric acid, acetic acid or chromic acid to treat metals such as aluminum, copper and steel to improve its luster.

5) Sulfuric acid

At room temperature, sulfuric acid has a weaker ability to dissolve metal oxides, increasing the concentration, and can not significantly improve the solubility of sulfuric acid. More than 40% of the sulfuric acid solution has a significant reduction in the solubility of iron oxide, and more than 60% of the sulfuric acid solution is almost Insoluble iron oxide, therefore, the use of sulfuric acid as a derusting solution, the mass concentration is generally controlled at 8% to 30%. Increasing the temperature can greatly improve the rust removal ability of sulfuric acid, and is suitable for heating operation due to low volatility of sulfuric acid. The hot sulfuric acid solution has strong corrosion resistance to the steel matrix and has a large peeling effect on the scale. However, when the temperature is too high, the steel matrix is easily corroded, and the tendency of hydrogen embrittlement is increased. In order to avoid these hazards, the sulfuric acid descaling solution can be heated to 50~60 °C, generally not more than 75 °C, and appropriate corrosion inhibitor should be added. The iron salt accumulated during the descaling process will significantly reduce the descaling ability of the sulfuric acid solution, slow down the descaling speed, and increase the surface residue of the workpiece after derusting, and the quality will decrease. Therefore, the iron content in the sulfuric acid solution should not generally be More than 60g / l.

The combination of sulfuric acid and nitric acid can improve the gloss of the surface of the workpiece and slow the corrosion of nitric acid on the steel matrix.

6) Chromic anhydride

The chromic anhydride solution (chromic acid, dichromic acid) has strong oxidizing ability and passivation ability, but has low solubility to metal oxides, so it is generally used to eliminate residues after acid treatment and for passivation. Chromic anhydride is toxic and its sewage must be treated strictly.

7) Citric acid

It is a safe and non-toxic organic acid. It has good dissolving ability for iron oxide and is suitable for removing rust. As the concentration of solution increases and the temperature rises, the rate of dissolving iron oxide is accelerated. Citric acid has a strong chrome-carrying ability for metal ions such as ferric ions and divalent steel ions, and has a low corrosion rate to metals. Metals such as copper, brass, aluminum, chromium, and nickel are resistant to citric acid.

8) Oxalic acid

It is a common organic acid used for metal pickling. It has strong oxidizing properties. It slowly corrodes steel at normal temperature and forms an iron oxalate protective film when heated. It is suitable for the components of metal pickling and rust removing liquid and polishing liquid.

9) Sulfamic acid

It is a commonly used acid for metal safe pickling. It reacts slowly with metal, has low corrosion rate and does not generate hydrogen. By adding a suitable corrosion inhibitor, it can inhibit the corrosion of various metals such as steel, copper and stainless steel. Aminosulfonic acid has a low ability to dissolve iron oxide, and its rust removal ability is inferior to oxalic acid. The salt formed during pickling has high solubility and there is no problem of precipitation. Suitable for pickling and rust removal and scale removal.