GB2048311A - Chemical pickling stainless steel - Google Patents

Abstract

The scale on the surface of a stainless steel article is removed by dipping in an aqueous solution containing nitric acid, hydrofluoric acid and from 0.05% by weight to 5% by weight of urea. The amount of the urea in the aqueous solution is advantageously adjusted so as to be present in a mol ratio to nitrous acid generated during the dipping, of less than 1. The effectiveness of the scale removal is increased if the scale on the articles is mechanically broken before dipping. Preferably the articles are dipped in a sulphuric acid solution before being dipped in the nitric acid - hydrofluoric acid solution containing urea. The dipping in the acid solutions may be repeated to produce particularly good results.

Description

SPECIFICATION Method of descaling stainless steel articles This invention relates to a method of descaling stainless steel articles e.g. scale-bearing stainless steel pipes, plates, bars and wires.

In producing stainless steel articles particularly stainless steel pipes, plates, bars and wires, it is often necessary to remove scale which is produced on the product surfaces at the time they are hotworked. Removal of these scales results in an improvement in the appearance and commercial value of the steel products and also smooths the surface of the articles and prevents the generation of flaws when used as intermediate materials for example by cold working, cold drawing, cold rolling, wire drawing or press-shaping.

For descaling stainless steel articles have hitherto been dipped in aqueous solutions containing 5 to 30% by weight nitric acid and 1 to 20% by weight hydrofluoric acid (hereinafter referred to an "HN03-HF solution").

Using such a technique an austenitic 1 8/1 8 Cr-Ni stainless steel can be satisfactorily descaled. If however a Cr-Ni austenitic stainless steel is high in Ni and Cr content, for example containing at least 12% by weight Ni and at least 20% by weight Cr, or a ferritic stainless steel containing at least 10% by weight Cr, or a stainless steel consisting of two phases of the above-mentioned austenite and ferrite obtained via heat-treatment, cannot be adequately descaled merely by being dipped in such an HNO3-HF solution.Such steels must therefore first be dipped for at least 60 minutes in a concentrated aqueous solution containing potassium permanganate and sodium hydroxide which has been warmed in advance to about 800 C. (this step being called a "permanganage treatment" hereinafter), and then dipped in the HN03-HF solution. A method of this type is discussed in the article "Wire, Cobury Germany" which was published in August, 1970. However, there are problems with this technique as well, and in fact a satisfactory result is hard to achieve with austenitic stainless steels which are high in Ni and Cr content, ferritic stainless steels, or stainless steels consisting of the two phases of austenite and ferrite. Particularly in the case of stainless steel pipes, the scale on the inside surface will often remain unremoved.Further, there is the problem that if the steels are dipped in the HN03-HF solution for a long time to achieve a satisfactory result, pitting and roughening of the stainless steel surfaces very likely will occur, which is of course not desirable.

In addition, the foregoing two-step treatment scheme is undesirable since the potassium permanganate solution contains manganese ions, and if such a solution is neutralized and discharged, a very significant environmental metal pollution results. To reduce the pollution effects, treatments such as reducing the permanganate ions to manganese dioxide, and then precipitating and removing the dioxide will be required, thus entailing enormous equipment and operating costs. Another significant drawback is that when the mixed solution of potassium permanganate and sodium hydroxide is warmed to about 800 C., a vapor is produced which is very irritating to human throats and eyes, and this acts to significantly deteriorate the working environment.

It is an object of the present invention to provide an improved method for the descaling of stainless steel products which will avoid the drawbacks noted above.

It is also an object of the present invention to provide an efficient pickling and descaling method for stainless steel products which will be more efficient than prior art techniques.

According to the present invention, the aforementioned permanganate treatment step can be eliminated entirely, whereas a fully adequate descaling is achieved, by immersion of the steel products in a nitro-hydrofluoric treatment solution (HNO,--HF solution) to which has been added a proper amount of urea. Stainless steel products which have been immersed in such a solution will be pickled very efficiently.

It has been found that by adding urea to an HN03-HF treatment solution such that the mol ratio of urea to nitrous acid in the HN03-HF solution is properly controlled, the pickling efficiency will be significantly elevated.

Further, it has been found that if the scale on the surface of the product is cracked prior to the pickling, the pickling efficiency will be elevated even more.

Also, it has been discovered that if a treatment with both an HN03-HF solution and a sulphuric acid solution in combination, the kinds of steels which are difficult to descale when using the nitrohydrofluoric solution treatment only will be pickled quite efficiently.

The invention will be better understood by the following description taken in conjunction with the accompanying drawings.

Figure 1 is a diagram showing the relationship between the moi ratios of (-NH2)2CO/HNO2 and NH3/HNO2 when nitrous acid and urea are added to an HN03-HF solution such that the mol ratio of (NH2)2CO/HNO2~ 1.

Figure 2 is a diagram showing the relationship between the mol ratios of (NH2)2CO/HNO2 and NH(NH2)2C0 when nitrous acid and urea are added to an HN03-HF solution such that the mol ratio of (NH2)2CO/HNO2 < 1 Figures 3 and 4 are diagrams showing the relationships between the descaling time and urea concentration for an austenitic stainless steel plate, Figure 3 representing the case when the urea is used in a low concentration and Figure 4 representing the case when the urea is used in a high concentration.

Figures 5 and 6 are diagrams showing the relationships between the descaling time and urea concentration for an austenitic stainless steel pipe after being worked with a straightener, Figure 5 representing the case when the urea used is in a low concentration and Figure 6 representing the case when the urea is used in a high concentration.

In accordance with the present invention, urea is added to an aqueous treatment solution containing about 5 to 20% be weight nitric acid and about 1 to 10% by weight hydrofluoric acid, representing a nitrohydrofluoric solution (HNO3-HF solution) to be used in a step of descaling a heattreated stainless steel product, such that the solution contains between about 0.0556 by weight and 5% by weight urea. The urea may be added in the form of a powder or in the form of an aqueous solution in water. As a result of the urea additions, the scale-removing properties of the solution become higher and the solution is thus much more effective in removing scale than when the stainless steel product is merely dipped in the same HNO3-HF solution not having the added urea.

The HN03-HF solution containing the urea is so effective in removing scale from steel products that the permanganate treatment step which was for some products absolutely required can be ommitted, and the time period required for immersion in the HN03-HFolution reduced. Indeed, even when the concentration of the liberated acid becomes reduced, the solution will still be able to effectively pickle the steel products immersed therein.

If urea is added such that the treatment solution contains at least 0.05% by weight, the descaling velocity will be accelerated. If urea is added such that the treatment solution contains more than 5% by weight, however, the descaling velocity will be reduced to a level lower than when no urea is added.

This latter result occurs because if too much urea is added, the urea will be adsorbed on the surface of the stainless steel and will then inhibit the dissolving reaction of the steel.

As the pickling proceeds, the urea will be consumed by reaction with nitrous acid. In order to maintain the descaling velocity accelerating effect, it is necessary to add urea such that the treatment solution contains about 0.05% to about 5% by weight urea.

Most preferably, in order to elevate the efficiency of the reaction of urea, the amount of urea to be added should be adjusted such that the mol ratio of the urea to nitrous acid in the aqueous solution of HNO3-HF does not exceed 1.

The temperature of the HNO3-HF treatment solution should be at most 700C so as to prevent the hydrolysis of the urea.

The believed mechanism of chemical reaction between the scaled stainless steel products and the inventive treatment solution is generally described in the following analysis.

The acceleration of the descaling process due to the addition of urea to the HNO3-HF solution is thought to result because during descaling, nitric acid reacts with the stainless steel, for example, as in 2Fe + 8HNO3 o 2Fe (NO3)3 + 2H2O + H2 + 2HNO2 to produce nitrous acid (H NO2). This nitrous acid will react with the urea us in the below mentioned reaction ( 1 ): 2HNO2 + (NH2)2CO e 2N2 t + CO2t + 3H2O (1) In such case, as seen in this reaction (1), nitrogen gas and carbonic acid gas will be generated.It is believed that since these gases are generated at a point mostly between the stainless steel and the scale, the scale will be pushed up by this generated pressure such that the descaling may be accelerated. Another possiblity is that although the formation of a passive film on the surface of the stainless steel by the nitrous acid at the time of the descaling would normally be a factor in obstructing the descaling process, the nitrous acid will be consumed by the reaction (1) sequence and, as a result, the formation of the above-mentioned passive film will be prevented and thus the descaling process will be accelerated.

However, as a result of detailed investigations made by the present inventors, it has been found that the nitrous acid and urea will react not only as represented by the above-mentioned reaction (1), but also in such a way as in represented by reaction (2): (NH2)2CO + HNO2 < N2 I + CO2 I + NH3 + H2O (2) This reaction (2) produces not only nitrogen gas and carbonic acid gas but also ammonia, such that when the mol ratio (NH2)2CO/HNO2 of the urea to nitrous acid in the HNO3-HF solution is at least 1, the reaction (1) sequence will not occur, but only the reaction (2) sequence will occur. The smaller the above-mentioned mol ratio (NH2)2CO/HNO2,the higher the rate of the occurrence of the reaction (1) sequence. Figure 1 shows the results of beaker tests made to investigate the relationship between the above-mentioned mol ratio (NH2)2CO/HNO2 and the mol ratio NHHNO2 of produced ammonia to the added nitrous acid by adjusting the mol ratio (NH2)2CO/HNO2 of the urea to nitrous acid so as to be at least 1, adding the nitrous acid and urea into the HNO3-HF solution and determining the amount of ammonia produced from the reaction. It is seen in Figure 1 that when the mol ratio (NH2)2CO/HNO2 is at least 1, irrespective of the value of this mol ratio, the mol ratio NHwHNO2 of the produced ammonia to added nitrous acid will be always 1.Therefore, in this case only the reaction in the form of reaction (2) will occur. On the other hand, Figure 2 shows the results of beaker tests made to investigate the relationship between this mol ratio (NH2)2CO/HNO2 and the mol ratio NH3/(NH2)2CO of the produced ammonia to added urea in case the mol ratio of the urea to nitrous acid is less than 1. It is seen from Figure 2 that with the reduction of the value of the mol ratio (NH2)2CO/HNO2, the value of the mol ratio NH3/(NH2)2CO will decrease. This means that when the mol ratio (NH2)2CO/HNO2 is less than 1, reaction (1) and reaction (2) both occur together, and that as the mol ratio (NH2)2CO/HNO2 decreases, the rate attributable to reaction (1) will become larger.

Given the mechanism of accelerating of the descaling process with the use of urea as described above, in order to elevate the descaling effect it goes without saying that it is desirable that reaction (1) in which 2 mols of nitrous acid react per mol of added urea and in which the amount of generated gas is larger, be favored more than reaction (2).

Therefore, when adding urea, it is better to adjust the mol ratio or urea to nitrous acid in the HNO3-HF solution so as not to exceed 1.

Now, the effect of the urea addition has been described above. If the kind of steel which is hard to descale, such as an austenitic stainless steel high in Ni and Cr content (containing at least 20% by weight Cr and 12% by weight Ni) or else a ferritic or martensitic stainless steel, is pretreated in advance to mechanically crack the scale, the penetration of the acid will improve and the effect of the nitrohydrofluoric solution treatment will be maximized. As a method of mechanically cracking the scale on a steel pipe, it is advantageous to work the steel pipe bearing the scale as it is with a straightener and, in the case of a steel plate, it may be shot-blasted or the like. (A straightener is a well-known machine used to straight stretch steel pipes - such machines have never been used until now for the purpose of cracking scales).

In the case of steel pipe, the higher the crushing rate (in percent), the better the result. But, if the rate is too high, the pipe will be cracked. Therefore, the rate must be at most 7%. The so-called crushing rate is derived from the following formula: Outside diameter of steel pipe -- Clearance between rolls Crushing rate (in /O) = x 100 Outside diameter of steel pipe If a ferritic or martenstic steel is dipped for at least 10 minutes in a HN03-HF solution, it will become rough on the surface. On the other hand, if it is not dipped for at least 10 minutes, it will not be totally descaled. Thus, a contradictory situation develops.

A ferritic or martensitic steel can be descaled substantially fuily within a short time according to the invention by cracking the scale as hereinbefore described and then repeating the sequence of steps of dipping the steel in a sulphuric acid solution, (preferably of concentration not more than 1 5%) and dipping it in an HNO3-HF solution containing at least 0.05% by weight to at most 5% by weight urea.

If desired the cracking of the scale may similarly be repeated.

In such procedure the time period of immersion of the steel in the sulphuric acid solution is made as long as possible to prevent surface roughening. Thus the dipping time in the sulphuric acid solution may be 5 to 10 minutes using a solution having a concentration of at least 10% by weight of sulphuric acid, and may be about 1 5 minutes with a sulphuric acid concentration less than 10% by weight. The dipping time in the HNO3-HF solution containing urea should. be as short as possible and may for example be 1 or 2 minutes.

The concentration is not particularly limited but in the case of the sulphuric acid solution, if the concentration of sulphuric acid is too high the treated material will be corroded. Thus the concentration should preferably be not more than 3%. The urea may be added to achieve a concentration of 0.05 to 5% by weight in the NO3-HF solution, which preferably should not contain the hydrofluoric acid at a concentration more than 3%. If the temperatures of the HNO3-HF solution and the sulphuric acid solution are too high, the treated material will be also corroded and therefore their temperatures should preferably be about 30 to 500C.

The reason why the steels can be best descaled by being repeatedly dipped is as follows.

Generally the scale of such stainless steel consists of an upper layer quite insoluble in acid and a strongly reactive lower layer located between the steel material and the scale. in descaling the steel, it is necessary to dissolve this lower layer.

If the steel is descaied with only an HNO3-HF solution, the lower layer will dissolve quickly but the steel material will also quickly dissolve and will be undesirably roughened on the surface (even though adequately descaled). on the other hand, sulphuric acid will dissolve the surface of the steel material without roughening it but will be slow in dissolving the lower layer of the scale.

In order to compensate for the merits and demerits of both acids, the dipping is repeated. That is to say, the material is first dipped and dissolved in sulphuric acid so that the acid may uniformiy penetrate between the scale and the steel material and is then dipped for a short time in the HN03-HF solution having had urea added thereto so that the part not dissolved with the sulphuric acid solution may be dissolved.

When these treatments are repeated, the descaling reaction will gradually proceed to descale the steel without roughening it on the surface.

EXAMPLE 1 Austenitic stainless steel plates A, B and C having the respective compositions in % weight in the following Table 1 were descaled with an aqueous solution containing 20% by weight nitric acid and 5% by weight hydrofluoric acid at a solution temperature of 500C and the respective descaling times were compared. The descaling time is the time needed for the scale to e removed based on an observation of the surface state of the various kinds of steel.

It should be noted that the above-mentioned kinds of steel A, B and C were all so low in Ni and Cr content that even when they were treated with a solution having had no urea added thereto, they did not require treatment with a permanganate solution, but when urea was added to the solution, the descaling was speeded up as mentioned below.

Figure 3 shows the results of the situation when low concentratiolis of urea were added to the HN03-HF solution. Figure 4 shows the results of the situation when high concentrations of urea were added to the HN03-HF solution.

As seen from Figure 3, when urea in small amounts was added to the treatment solution, the descaling completion time decreased. It was found that when the concentration of urea was made too high, the descalability decreased: As seen from Figure 4, when urea was added in amounts greater than about 5% by weight, the descaling time became much longer.

The surface of the stainless steel pipe descaled with the HNO3-HF solution containing urea was seen to be entirely clean.

As the descaling treatment continued, the acid concentration in the HNO3-HF solution decreased, the concentration of the dissolved metal increased, the so-called ageing of the solution occurred, and therefore the descaling capacity for the solution ultimately became zero. For example.

when the composition of the solution became 2% by weight HF, 3% by weight HN03, 20gel of dissolved iron, 4g/l of dissolved nickel, 4g/l of dissolved chrome and 0.5g/l of other heavy metals, even when the above mentioned kind of steel B was dipped for at least 1 hour in the solution at a solution temperature of 500C, it could not be substantially descaled. However, when urea was then added to this solution, .the steel could again be descaled. The results are as shown in Table 2, the meanings of the respective signs in the Table being given as notes in the lower part of the Table.

TABLE 1 Compositions of respective kinds of steel **

Kind of Steel C Si Mn P S Ni Cr Ti Mo Cu A 0.10* 0.8 1.3 0.04 0.03 11.00 19.00 0 0 0 B 0.05 0.9 1.6 0.04 0.01 11.20 18.3 0.5 0 0 C 0.06 0.8 1.8 0.05 0.03 12.2 18.3 0 1.5 1.5 * Values in percent by weight.

** Steels contain mostly iron.

TABLE 2

Concentration of urea 0 0.2% Kind of steel B C 10 X X Time (in minutes) 20 X X 30 X X 40 X A 50 X 0 60 X - 70 X 80 b 90 0 Notes: i): The scale was perfectly removed and the surface had a silvery luster.

O: The scale remained in the form of spots.

A: The scale was partly removed.

X : The scale was not removed at all.

EXAMPLE 2 During pickling at a temperature of 400C by using 20m3 of an HN03-HF solution of 5% by weight HF and 10% by weight HNO3, 300kg of urea were added all at once or suitably divided soars to be added at 1 5, 30 and 60 separate times and a stainless steel pipe having a composition corresponding to that of the kind of steel B was descaled to obtain results as are shown in the below mentioned Table 3.

TABLE 3

Amount of Amount of Treated surface added urea added solution area (in kg/time) (NH2)2CO/HNo2 (in kg) (in m2) At once 300 6 300 12000 15 times 20 0.4 300 20000 as divided 30 times 10 0.2 300 20000 as divided 60 times 5 0.1 300 21000 as divided As mentioned above, when (NH2)2CO/HNO2 < 1, the descaling effect was remarkable.

EXAMPLE 3 Each of austenitic stainless steel pipes D and E having the respective compositions as shown in Table 4 were heat-treated to have a scale on the surface and were descaled in the below described fashions (1) to (3) and the respective completeing times were compared.

TABLE 4 Compositions of stainless steel pipes

C Si Mn P S Cu Ni Cr Mo Ti D 0.13* 0.3 1.1 0.01 0.02 0.1 20.S 25.6 0 2 E 0.05 0.7 1.5 0.03 0.01 0.5 30 21.3 l 0.4 * Values given in percent by weight.

** Steels contain mostly iron Descaling manner (1): Each steel pipe was dipped for 2 hours in an aqueous solution containing 10% by weight potassium permanganate and 12% by weight sodium hydroxide at 800C. and was then dipped at 500C in an aqueous solution containing 10% by weight nitric acid and 5% by weight hydrofluoric acid.

Descaling manner (2): Without such pretreatment as in the above-mentioned descaling manner (1), each steel pipe was directly dipped at 5O0C. in an aqueous solution containing not only 10% by weight nitric acid and 5% by weight hydrofluoric acid but also 0.3% by weight urea.

Descaling manner (3): The pretreatment was omitted. Each steel pipe was worked at a crushing rate of 5% with a straightener and was then dipped at 500 C. In an aqueous solution containing 10% by weight nitric acid and 5% by weight hydrofluoric acid and made to contain 0.3% by weight urea.

The results were as shown in Table 5.

TABLE 5

Descaling manner Kind of steel (1) (2) (3) D 2 hours 40 minutes 20 minutes (pretreatment) + 45 minutes (pretreatment) + (Pretreatment) + 50 minutes It is seen from the results shown in Table 5 that when no urea was added to the HNO3-HF solution, the steel pipes could be effectively descaled with the permanganate pretreatment. When urea was added, however, not only the above-mentioned pretreatment could be omitted but also when the scale was cracked with the straightener, the descaling time could be reduced even further.

In the descaling manner (3), the urea concentration was varied and the descaling time was seen.

The results are shown in Figures 5 and 6. Each steel pipe was dipped in the HNO3-HF solution for up to 1 20 minutes (which was a time proper for the usual work) and the surface state was seen. When it was dipped in the HNO3-HF solution for at least 120 minutes, not only the working efficiency reduced but also such bad influence as, for example, the intergranular corrosion of the surface state of the product was high.

Summarizing the results of Example 3, it can be said that the proper amount of urea added to the HNO3-HF solution was at least 0.05% by weight to at most 5% by weight.

It should be noted that in Figures 5 and 6, the signs represent the following: 0 The descaled state in the corresponding time in the kind of steel D was not good.

A The descaled state in the corresponding time in the kind of steel E was not good.

The descaled state in the corresponding time in the kind of steel D was good.

A The descaled state in the corresponding time in the kind of steel E was good.

EXAMPLE 4 Each of ferritic and martensitic stainless steel pipes having the compositions shown in the following Table 6 was worked at a crushing rate of 25% with a straightener and was then descaled in the procedures shown in Table 7.

TABLE 6 Compositions **

Kind of steel Structure C S Mn P S Cu Ni Cr Mo Al x Martensite 0.3* : 0.5 0.6 0.02 0.02 0.1 0.3 12.9 0.1 0.2- Y ~1 Ferrite 0.1 0.6 0.5 0.03 0.02 0.15 0.2 12.7 0.2 0 * Values given in percent by weight.

** Steels contain balance of iron.

TABLE 7 Descalings

Procedure No. Manner (4) Dipped at 40 C. in an HNO3 - HF solution having had 0.3% by weight urea added thereto (5) Dipped for 10 minutes in 10% sulphuric acid at 400 and then dipped at 40 in an HN0a - HF solution having had 0.3% by weight urea added thereto (6) Dipped for 10 minutes in 10% sulphuric acid at 40 C. and then dipped at 400 C. in an HNO3 - HF solution having had 0.3% by weight urea added thereto and these steps were repeated Notes: In the above table, % is by weight.

The HNO3-HF solution contained 10% by weight nitric acid and 5% by weight hydrofluoric acid.

The dipping time and the number of repeated times represent times needed for the completion of the descaling.

TABLE 8

Procedure (4) (5) (6) Kind of steel X Y X Y X Y Time in minutes and 12 10 6 5 2 2 number of repeated times minutes minutes minutes minutes times times Surface state X X A A 0 0 0 . . . No surface roughening. Silvery luster.

A . . . Dull luster.

X... . Surface roughening.

As understood from Table 8, technique 6 provided perfect descaling without surface roughening.

Claims (21)

1. A method of descaling a stainless steel article comprising dipping the scaled article in an aqueous solution containing nitric acid and hydrofluoric acid, characterized in that the said solution contains from 0.5% to 5% by weight of urea.

2. A method according to Claim 1, wherein the temperature of the said aqueous solution is not higher than 700C.

3. A method according to Claim 1 or Claim 2. wherein the amount of urea in said solution is adjusted such that the mole ratio of urea to nitrous acid generated in the said aqueous solution during the descaling operation, is less than 1.

4. A method according to Claim 3, wherein urea is added to the said solution in a plurality of steps.

5. A method according to Claim 3 and Claim 4, wherein urea is added to the said aqueous solution in the form of a powder.

6. A method according to Claim 3 or Claim 4, wherein urea is added to the said aqueous solution in the form of a solution.

7. A method according to any of Claims 1 to 6, wherein the scale of the stainless steel article is mechanically cracked before dipping in the said aqueous solution.

8. A method according to Claim 7, wherein cracking of the scale of a scaled stainless steel tube is effected by working the steel product with a straightener as hereinbefore defined.

9. A method according to Claim 8, wherein the crushing rate as hereinbefore defined exerted by the said straightener is not more than 7%.

10. A method according to Claim 7, wherein the said scale cracking is effected by shot-blasting.

11. A method according to any of Claims 1 to 10, wherein the scaled stainless steel article is dipped in an aqueous solution of sulphuric acid before dipping into the said aqueous solution of nitric acid, hydrofluoric acid and urea.

12. A method according to Claim 11, wherein the immersion time in the said aqueous solution of sulphuric acid is longer than the immersion time in the said aqueous urea-containing solution.

13. A method according to Claim 12, whurein the immersion time in the said aqueous solution of sulphuric acid is from 5 to 15 minutes.

14. A method according to any of Claims 11 to 13, wherein the immersion time in the said aqueous HNO3-HF containing solution is from 1 to 2 minutes.

15. A method according to any of Claims 11 to 14, wherein the concentration of sulphuric acid in the said aqueous solution thereof is not more than 15%.

16. A method according to any of Claims 1 to 15, wherein the hydrofluoric acid concentration in the HNO3-HF solution is not more than 3%.

17. A method according to any of Claims 1 to 16, wherein the temperatures of said HN03-HF solution and said sulphuric acid solution are from 300 to 500 C.

18. A method according to any of Claims 11 to 17, wherein the sequence of steps of dipping in the said sulphuric acid solution and the said HNO3-HF solution, is repeated.

19. A method according to any of Claims 1 to 18, for descaling ferritic or stainless steel articles.

20. A method according to Claim 1, substantially as hereinbefore described, with particular reference to the Examples.

21. Stainless steel articles descaled by a method according to any of Claims 1 to 20.