Etching Titanium with Hydrofluoric Acid and Nitric Acid Mixed Solution

1.          Theory

Hydrofluoric acid was first used in the glass industry to produce non-glare TV display screen. At present, hydrofluoric acid has been widely used in the solar energy industry, as well as the effective silicon processing chemicals in titanium and titanium memory metal industries. In practice, hydrofluoric acid is rarely used alone. It is usually mixed with another chemical agent to obtain a chemical solution for etching or surface treatment. The mixed solution of hydrofluoric acid and nitric acid is widely used, so we decided to use this method to study the effect of etching titanium.

Hydrofluoric acid (HF) is the main chemical agent for etching titanium. The engineering formula of etching chemical reaction is shown as follows:

In the case of etching titanium with hydrofluoric acid, titanium trifluoride and hydrogen will be produced. Although hydrofluoric acid is used alone

It can also corrode titanium, but it has some disadvantages when used alone. First, the hydrogen generated by chemical reaction is

Flammable and explosive substances, which may cause a potential fire hazard. Secondly, hydrofluoric acid is volatile, and chemical substances need to be added regularly to stabilize it. Third, the etching rate is relatively slow. Finally, the desired surface etching and polishing effect may not be achieved. To offset these shortcomings, we need to add nitric acid to hydrofluoric acid solution.

The reaction is shown as:

In this chemical reaction, the etching of titanium produces hexafluorotitanic acid, nitrogen dioxide and water. Using this mixed solution, hydrofluoric acid tends to be stable, does not produce hydrogen, and can accelerate the etching rate. The purpose of this evaluation is to record the effect of different chemical concentrations on the corrosion rate and surface polishing effect of the product when the titanium is corroded with hydrofluoric acid/nitric acid mixed solution. As the dry film is not suitable for the use of hydrofluoric acid, the side corrosion is not evaluated this time. Fluoride ions will penetrate the corrosion resistant film of the dry film and cause it to lose its adhesion. Lateral erosion will be evaluated in the future when necessary.

The material used for the test is grade 2 commercial pure titanium, with a thickness of 0.020 inch (0.508 mm) and 0.035 inch (0.889 mm), and the surface is moderately polished. Its chemical composition is: titanium – 98.9%, carbon - 0.08%, oxygen - 0.25%, nitrogen - 0.03%, iron - 0.3%, and hydrogen – 0.015%, with a density of 0.163 per cubic inch, hardness of Rockwell B68 to B80, and tensile strength of 40000 psi/min.

Before etching, titanium is prepared in the following way: cut the titanium raw material into 6 "x 6" titanium sheet, and then manually degrease the titanium sheet. Immerse the titanium sheet in phosphoric acid detergent for cleaning for two minutes, rinse and dry it. Take four points of each titanium sheet, measure the thickness with a micrometer, take the average value and record it, and then cover one side of each titanium sheet with a mask, so that this side will not contact with chemicals, Etch two titanium metal sheets at each chemical concentration. Then remove the mask, measure the thickness at four points again, and take the average value. Then divide the thickness of the etched material by the etching time to obtain the etching rate in microns or mils per minute.

The etching solution contains hydrofluoric acid (49%, specific gravity - 1.18), nitric acid (68%, specific gravity - 1.41), and water. During the test, the material shall be supplemented to adjust and maintain the acidity concentration. The etching is completed in the Chemcut equipment model 2315. There are four nozzles in the upper and lower grooves of the etching cavity of this model, which move parallel to the metal substrate transmission direction. The nozzle also oscillates perpendicular to the moving direction. Each nozzle consists of four 80 ° full cone nozzles, with a spray rate of 2.84 l/min (0.75 gal/min) at a pressure of 2.76 bar (40 psi). The metal panel will only be etched under the upper spray condition with the pressure of 2.07 bar (30 psi), the oscillation rate is 30 sprays per minute, and the etching temperature is 43.3 ° C (110 ° F). The effective length of the etching cavity groove of the etching line is 21 inches (53.34 cm).

The following figure shows the concentration of hydrofluoric acid and nitric acid for evaluation. 

The figure below shows the average thickness before etching, the average thickness after etching, the (material) loss of the average thickness, and the calculated etching rate of 3% hydrofluoric acid solution. 

The relationship between the etching rate and the concentration of 3% hydrofluoric acid can also be represented by the following figure. 

The figure below shows the average thickness before etching, the average thickness after etching, the (material) loss of the average thickness, and the calculated 5% hydrofluoric acid etching rate. 

The relationship between the etching rate and the concentration of 5% hydrofluoric acid can also be shown in the following figure. 

The figure below shows the average thickness before etching, the average thickness after etching, the (material) loss of the average thickness, and the calculated 10% hydrofluoric acid etching rate. 

The relationship between the etching rate and the concentration of 10% hydrofluoric acid can also be shown in the following figure. 

The figure below shows the relationship between etching rate and etching solution concentration. 

As shown in the figure above, the etching rate increases with the increase of nitric acid concentration. In the absence of nitric acid, the etching rate of hydrofluoric acid has a similar change.

After reaching the nitric acid level of 10%, the increase of etching rate begins to decrease, as shown in the figure below. 

The figure below shows the relationship between the nitric acid etching concentration higher than 10% and the etching rate. This table is applicable to hydrofluoric acid with different concentrations. 

When measuring the etched (titanium) metal sheet, it can be noted that with the increase of chemical concentration, the measurement results show more discontinuous changes, and the standard deviation of each group of measurement data can be plotted as follows. 

It can be seen that with the increase of nitric acid concentration, the etching speed increases, and the surface chemical reaction becomes more unstable and uncontrollable.

The second part of the evaluation is to check how various concentrations of chemicals affect the surface of the etched piece. The following photos show the side-by-side comparison of the surface etched with different concentrations of hydrofluoric acid and nitrate.

In the absence of nitric acid, the titanium surface is very rough and nodular. With the increase of nitric acid concentration, the surface roughness decreases, and finally a very flat (reflective) surface can be achieved. The following figure shows the contrast relationship between the solution with different chemical concentrations and the average roughness. 

2.         Summary

Etching titanium with hydrofluoric acid is a slow process, and it will produce by-product hydrogen and rough surface. Adding nitric acid to hydrofluoric acid can increase the etching rate, eliminate the formation of hydrogen and produce smooth surface. When the concentration of nitric acid is higher than 10%, the etching rate hardly increases. When the proportion of nitric acid is 15% and 20%, the etching rate begins to stabilize, but the chemical reaction becomes more unstable and uncontrollable. In addition, with the increase of hydrofluoric acid concentration, the amount of titanium oxide formed on the surface of titanium materials increases. This white oxide may be undesirable and difficult to remove. Because the corrosion resistant film (dry film) used this time is not suitable, the lateral corrosion characteristics have not been evaluated.

3.         proposal

In order to maintain a good controllable etching rate, we recommend using Grade 2 titanium raw materials. The chemical solution for etching is recommended to be a mixture of 3-5% hydrofluoric acid and 10% nitric acid.