Zirconium and zirconium alloys have been widely used in industrial equipment for their excellent corrosion resistance and machining properties. In order to remove the oxide scale and residual lubricating oil on the surface of the
zirconium tube, pickling treatment is needed in the production process of staggered pipe. In practical production, zirconium tubes are pickled by acid washing solution of
titanium tubes. Because the zirconium tube and the acid reaction is too strong, in more than ten seconds, the surface will appear over pickling phenomenon, so the pickling operation is difficult. In addition, zirconium alloy has a strong adsorption capacity for hydrogen, and over pickling will cause hydrogen to dissolve into the material, resulting in hydrogen embrittlement. Therefore, this experiment studied the zirconium tube to remove the skin pickling process, to explore the appropriate ratio of acid solution, to control the speed of pickling reaction, and find out the appropriate pickling time, in order to meet the needs of industrial production.
The experiment uses R60702
zirconium pipe with the specification of 19mmx1.65mmx6000 mm. The pickling production is in accordance with ASTM B523 standard. The pickling reagents used are industrial hydrofluoric acid and industrial nitric acid.
Pickling experiment scheme with different acid ratio
Hydrofluoric acid is the main component in the acid solution, adding nitric acid as an oxidant can effectively control the corrosion rate and hydrogen absorption of the acid solution, the reasonable ratio of the two is the key to control the pickling rate and effect.
At the same temperature and the same pickling time, pickling experiments were carried out with four different acid solutions.
Pickling experiment scheme with different pickling time
Selection according to the different ratio of acid pickling experimental results, the best ratio of acid, then under 30 ~ 35 ℃, respectively 40120240600 s pickling time comparison experiment, find out the suitable pickling time, make both the zirconium tube inside and outside surface is clean, and do not produce owe to wash or pickling time is too long, affects the production efficiency.
Effect of acid ratio on pickling rate and tube surface quality
Under the condition of pickling temperature of 32 ℃ and pickling time of 90 ~ 120 s, pickling experiments with four different acid ratios were carried out, and the results are shown as follows:
| No. |
Acid pickling phenomenon |
Zirconium tube surface quality |
| 1#< |
Reaction is slow with very little bubble reflection |
Residual oxide scale |
| 2# |
The reaction is moderate, and the bubbles are more and more dense |
Clean and bright and clean |
| 3# |
The reaction is intense, and the bubbles are many and fast |
Overwash, pockmarked |
| 4# |
The reaction is more intense, and the bubbles are more and faster |
Mild had |
|
As can be seen from the table, in schemes 1#, 2# and 3#, the content of nitric acid in the pickling solution remains unchanged. When the mass ratio of hydrofluoric acid, nitric acid and water is 1:45:54, the corrosion of the zirconium tube in the acid solution is slow, and the oxide scale cannot be cleaned within 120 s. With the increase of hydrofluoric acid content, the reaction of zirconium tube in acid solution is more and more intense, and the corrosion rate is accelerated. When the mass ratio of hydrofluoric acid, nitric acid and water is 3:45:52, the corrosion rate is moderate, the pickling operation is easy to control, and the phenomenon of overwashing is not easy to appear. When the mass ratio of hydrofluoric acid, nitric acid and water reaches 5:45:50, the reaction is very violent, a large number of bubbles are generated, and the outer surface is easy to appear over pickling. By comparing 2# and 4# schemes, it can be seen that when the content of hydrofluoric acid is certain, the content of nitric acid decreases, and the corrosion rate of zirconium tube speeds up, and local over-washing phenomenon is easy to occur.
The experimental results showed that the ratio of acid solution (the mass ratio of hydrofluoric acid, nitric acid and water was 3:45:52) in the second group was better. Moreover, reducing the content of hydrofluoric acid or increasing the content of nitric acid can effectively slow down the corrosion rate of the patent tube in the acid solution, making the operation easy to control, and not easy to appear over pickling.
Effect of pickling time on surface quality of pickling pipes
Under the condition that the mass ratio of hydrofluoric acid, nitric acid and water was 3:45:52 and the pickling temperature was 32℃, pickling experiments with four different pickling times were carried out, and the results were shown in the table. It can be seen from the table that after pickling time exceeds 120 s, with the extension of pickling time, the surface of zirconium tube will gradually appear over-pickling phenomenon. When the mass ratio of hydrofluoric acid, nitric acid and water is 3:45:52, pickling temperature is 32℃, pickling time is 120 s, a clean and smooth pickling surface can be obtained.
| No. |
m(HF):m(HNO3):m(H2O) |
Pickling temperature / ℃ |
Pickling time /s |
Zirconium tube surface quality |
| 5# |
3:45:52 |
32 |
40 |
Residual oxide scale |
| 6# |
3:45:52 |
32 |
120 |
Clean and bright and clean |
| 7# |
3:45:52 |
32 |
240 |
Local had |
| 8# |
3:45:52 |
32 |
600 |
Severely overwashed, pockmarked |
|
(1) in the zirconium tube descaling pickling, the best ratio of pickling solution is m(HF):m(HNO3):m(H2O)= 3:45:52.
(2) When the mass ratio of hydrofluoric acid, nitric acid and water in the pickling solution is 3:45:52, pickling temperature is 32 ℃, pickling time is about 120 s, the pickling effect of zirconium tube is better, and a clean and smooth pickling surface can be obtained.
