US2171981A - Cleaning of metallic surfaces - Google Patents

Description

bVl-I VVIIII uulllvlvl "T" Sept. 5, 1939. w. HEIMBERGER 2,171,931

CLEANING PF METALLIC SURFACES I Filed Nov. 20, 1937 4 Sheets-Sheet 2 Ul U00 llUiUlUllUV Sept. 5, 1939. 1 w, H ER 2,171,981

CLEANING 0F METALLIC SURFACES Filed Nov. 20, 1937 4 Sheets-Sheet 3 Liai- 1 2 3 5 is i '6 cams/M1011 "/m T ig El I I I I Jrzwvz/ok 1 2 3 s e z ccnsMO/f 6 10 UIVUOO HUIUIUHUU Filed Nov. 20, 1937 4 Sheets-Sheet 4 Patented Sept. 5, 1939 umrso STATES PATENT OFFICE Application November 20, 1937, Serial No. 175,623 In Germany October 23, I934 10 Claim.

This application isa continuation-in-part of my application for U. 8. letters Patent Serial No. 14,801, filed April 5, 1935, for the Cleaning of metallic surfaces.

The present invention relates to the cleaning of metallic surfaces by pickling-and more especially to the pickling of iron. It is concerned with the treatment of metals such as iron preparatory to the coating of the metal surface with enamel or paint or another metal. It is an object of my invention to render the pickling process simpler and cheaper and less dangerous for the operator and to improve the quality of the pickled product.

Iron in the form of sheet or other roller products is not as a rule resistant against the action of the atmosphere or of acids. In order to produce stability, the surface of the metal must be efllclently protected'by painting or enameling or metallizing or like? measures. An efllcient protection is however'obtained only, if the protective coating is applied'to the clean metal.

In order to obtain clean iron surfaces, it is usual to remove adhering scale, rust or impurities 2'5 by means of acid solutions. These solutions, the

so-called pickling baths, consist as a rule of sulfuric acid orhyd'rochloric acid with a concentration of some per cent, for instance 10%.

It is further known that the pickling process =so-may be accelerated by an addition to the acids of salts which sometimes also purport to protect the bare. metal against the attack of the acid during the pickling process.

Thus it has for instance been proposed to treat stainless steel-alloys for the removal of scale with a solution containing a mineral acid and an alkali metal nitrate. The preferred concentration of both the acid and the nitrate was from about I to 5%, while :.the content even varied from 40 0.2% to about 40%.

, All these known pickling baths involve certain drawbacks which hitherto were considered necessary evils resulting from the concentration required fpr'an' eihcient pickling. Thus operation with relatively high acid concentration involves a certain danger to the operator and the acid vapors may even render the atmosphere almost unrespirable. The hydrogen developed may render the iron to he pickled brittle. An important 59 drawback is the fact that the pickling baths V gradually become richer with dissolved iron salts to such an extent that they can no longer be used whena clean surface is intended to be obtained by the pickling process. Perfectly clean ainscescannothevbt imda a l si i manner, but the blank under treatment must afterwards be scoured with sand and water in order to enable a satisfactory enamel coating to be produced thereon.

According to the present invention these draw- 5 backs are obviated, while the pickling process as such is at the same time rendered simpler and less expensive. I use principles hitherto not employed in theart of pickling by utilizing knowledge gained from the colloidal chemistry. I con- 1 vert the bi-valent iron salts formed in the pickling operation, by an addition to the pickling bath of oxidizing agents, into tri-valent soluble iron salts, which are then caused to partly change into colloidal ferric hydroxide or other 15 basic ferric compounds by suillciently diluting and heating the bath.

Several conditions must be fulfilled in order that this process can take place continuously. In the first line the acid strength must be very low, go much lower than hitherto used for the pickling operation. The acid strength must be low enough to allow the formation of basic ferric salts by hydrolytic action. I have found that for this purpose the concentration of the acid 5 during the operation proper must not exceed 0.1%, since insoluble iron compounds 'do not occur before the concentration of the acid has dropped to or below this value. The acid concentration must be the lower. the lower the tema0 perature of the pickling bath. I have further found that the content of the pickling bath at the beginning of the operation should not exceed about 1% sulfuric acid.

According to my invention it .is a further con- 85 dition for a successful operation to maintain the pickling bath at a temperatureabove 0., since the reaction mentioned which leads to the formation of insoluble basic iron salts, does not proceed appreciably below this temperature. I 40 prefer to heat the pickling baths to about 70-80 C. The higher the temperature, the higher is the acid strength .at which the insoluble iron salts start precipitating.

The formation of insoluble ferric hydroxide or 45 other insoluble basic ferricsalts in the form of colloids as attained under these conditions-of concentration and temperature does not yet secure any particular advantage. I therefore provide for the presence, in the pickling bath, of a 50 suitable electrolyte capable of coagulating these colloidal iron compounds in order to precipitate them from the solution in the form of a slurry, which may be filtered off. An electrolyte may generally be considered suitable for this purpose, if its anion differs from that of the pickling acid and its kation is less noble, in the sense of the electromotive series, than the metal to be pickled.

' An oxidant must be present in the pickling bath for converting the ferrous .ions into ferric ions. The electrolyte which serves to coagulate or precipitate the colloidal iron compounds, may be'identical with the oxidant and may for instance be an alkali metal nitrate.

In the practice of my invention satisfactory results are obtained if the pickling solution at the beginning of the operation shows for instanc one of the following compositionsz 1% sulfuric acid+2% potassium nitrate, or

1% nitric acid+2% magnesium sulfate, or 2% potassium persulfate+2% sodium nitrate.

I obtain good results also with other solutions, for instance with a mixture of nitric acid and sulfuric acid with magnesium sulfate, but I have found it advantageous inany case to have sulfate ions in the pickling bath, and this may be connected with the formation in the bath of F2(SO4)2O as insoluble ferric compound, as will presently be explained more in detail with reference to the drawings afflxed to this specification and forming part thereof.

In the drawings: Fig. l is a diagram showing the pH values of a sulfuric acid solution of different acid concentrations.

Fig. 2 shows diagrammatically the concentration of sulfuric acid at which, at different temperatures, basic iron salts start precipitating so that the solution will become turbid, while Figs. 3 to 10 show diagrammatically curves obtained by titrating baths with caustic soda solution, as will be explained more in detail farther 40 below. 1

First, however, I will develop the fundamental reactions on which I assume the new process is based and which may render intelligible the advantages attained.

If a piece of sheet metal or another blank is placed in a pickling bath according to my invention, i. e., a bath containing a suitable electrolyte, an oxidizing agent and a mineral acid of a strength not exceeding about 1%, this bath being heated to at least 50 0., part of the acid is consumed in the formation of iron salts. Under the action of the oxidizing agent these salts change over into colloidal ferric hydroxide, as soon as the acid concentration has dropped to a value at which the formation of colloidal ferric gyioxide is possible at the temperature of the While it cannot be ascertained with certainty, whether ferric hydroxide as such is precipitated 00 immediately or for instance a basic salt like F82(SO4)2O, the following equilibrium may be asumed to prevail in the bath during the pickling process according to my invention:

In the absence of metallic iron the insoluble or coagulated basic ferric compound would precipitate by hydrolytic action, until the solubility product is reached as indicated by the law of mass sulfate is consumed in a reaction, the equation of which may be assumed to be action. In the presence of iron, however, the. sulfuric acid formed by the hydrolysis of ferric ceed from the left hand side to the right hand side of Equation 1. It is true that the ferric sulfate of the left hand side of this equation may also react with the metallic iron present, for instance according to the equation 2Fe+2Fez(SO)a- 6Fe80 In but the ferrous sulfate thus formed changes, in

consequence of the oxidizing agent present, into ferric sulfate according to the equation 6FeS04+3H2S04+3O- 3Fe:(SO;) 3+3H2O IV From the Equations III and IV there results 2Fe +3H2SO4+30- Fe2 (S04) a+3H2O V be assumed to take place in the pickling bath:

.The principle of this process would not be changed, if in reality ferric hydroxide itself or another basic ferric compound were formed and precipitated, instead of Fe:(SO4):O. In any case the reactions, by which the iron is pickledaccordingto my invention, differ greatly from the process according to which pickling is hitherto carried out and which may be formulated as follows:

2Fe+3HzSO4+3O- Fez(SO4)3+3Ha0 VII and according to which no iron compound is precipitated, while the bath is continuously enriched with soluble iron salts.

When according to my invention colloidal ferric hydroxide is formed and coagulated or some other insoluble basic ferric compound is produced by hydrolysis, the anion of the dissolved, tri-valent iron salt is completely 'or partly set free, as may be guessed from the above equations, and it is capable of again participating, in the form of acid, in pickling reactions. A fresh portion of iron may therefore be dissolved. Since part of the colloidal ferric hydroxide is continuousl'y precipitated as insoluble matter under the action of the electrolyte present, shortly after the begining of the pickling process, i. e., after the blank has ben placed in the bath, a chemical equilibrium is established which shows the peculiarity that the concentration of the dissolved iron is kept very low and practically constant.

In the course of my investigations I have made a great number of experiments to ascertain the particular features of the new pickling process and I will now describe someof these experiments more in detail in order yto facilitate a complete understanding of my invention.

I have carried out a series of tests in which samples of scaled iron sheet were pickled-with solutions of sulfuric acid and sodium nitrate of various concentrations and at various temperatures. crystallized sodium nitrate of commerce and sulfuric .acid of commerce wereused, which latter was diluted with tap water and adusted, by titration with n NnOH (pH values) with a foil colorimeter according to Wulfl, by means of which the pH values from 1.6 upwards may be ascertained with sufficient accuracy. The pH values were reduced to per cents of acid by means of the curve of Fig. 1 which is drawn according to the data published by Kordatzki.

The state of the pickling solutions may be ascertained by establishing titrating curves which fairly accurately show the quantities of bi-valent and tri-valent iron compounds present in the solution. Such titrating curves are established in the following manner: the starting point of the curve is represented by the pH value of the solution itself, the further values of the curve are obtained by gradually adding to ccrns. of the solution small quantities of a solution of caustic soda, determining the pH value after every such addition and plotting the values thus obtained as points of the curve. The course of the titrating curves is of interest for my purpose only up to the point of neutrality, i. e., pH=7. An acid solution which contains bi-valent and tri-valent iron salts, shows a titrating curve which has two well marked breaks-at about pH=3.0 and pH=6.3, each of which is followed by a substantially horizontal part of the curve. At the pH value of about 3.0 ferric hydroxide precipitates, while at the pH value of about 6.3 fer.- rous hydroxide is precipitated. The length of the horizontally extending part of the curve which follows these values, allows to guess the quantity of iron ionsof the corresponding valence present; the horizontal part of the curve is the longer, the more iron of the corresponding valence is dissolved.

The tests of this series were carried out in beakers with 200 ccms. solution. The heating, if

any, of the solution was performed in a water bath, During the individual tests the samples were movedfrom time to time.

The solutions were mixed in cold state. The pH value of the starting solution was drawn from the curve of Fig. 1.

In Test No. 1 the influence of an aqueous solution containing 0.1% H2SO4+2% NaNOs on black plate was investigated at 20 C. This solution was rendered turbid by the formation of insoluble ferric compounds only when ferric ions could form and when the soluton was neutralized to such an extent that Fe( H): could form. The titrating curves showed that this state is attained at a pH value of 2.9 to 3.1, corresponding to about Both the pickling effect of the cold solution on scale and the oxidimng efiect of the sodium nitrate added were very small and a clean ironsurface could not be attained with certainty. The titrations show that only very small quantities of iron compounds were formed.

Test No. 2 was carried out with the same concentrations, but at 65 C. In consequence of the higher temperature the time required for pickling was shortened, but the velocity of pickling was still rather low, due to the low acid strength of the bath at the start. The solution became turbid at pH=2.3-to 2.5 (corresponding to 0.03 to 0.02% H2804) or, if heated to boiling, at pH=1.9 to 2.0 (0.07 to 0.06% H2804) In the course of this test the diagram shown 10 lution containing 0.3% H2SO4+2% NaNO3. The

pickling velocity was about the same as in Test No. 1. Only small quantities of iron compounds were formed in the solution, as could be seen from the titrating curve. In spite thereof the sodium nitrate added needed several days for converting the dissolved ferrous compounds into ferric compounds. These ferric compounds are however not present in the form of colloidal Fe(OH) 3, since they proved to be easily reduced when the sheet metal was placed again in the solution. The solution became turbid, when the pH value reached 3.1. The clean-pickling effect of the solution was satisfactory.

Test No. 4, being very instructive, shall be reproduced in detail:

The solution contained 0.3% H:SO4+2% NaNOa. The temperature was 65 C. pH=-.1.3.

Time-of observation 2:00p. m. The sheet was placed in the solution. Developing gas could distinctly be observed.

2:04 p. m. The sheet was wiped, the scale showed to be already dissolved to a great extent. When caustic soda solution was added to a sample of the solution, a green precipitate was formed. (Such precipitating reactions were repeatedly performed in samples of the solution in order to investigate the state of the solution, and I will shortly indicate the color of thissample-precipitate in the following explanations.) pH=2.3. The solution had a light yellowish color.

2 :22 p. m. The sample-precipitate brown.

was dark 2:30 p. m. The sample-precipitate was brown.

2:32 p. m. The sheet was again placed in the solution, a slight development of gas could be observed. 2:36 p. m. The sheet was wiped; a light brown covering had formed on the plate, which could easily be wiped ofl. The sheet was perfectly clean save only small spots of scale.

2:40 p. m. The sample-precipitate was dark brown. pH=2.3, color of the solution distinctly yellow.

Tlmeof observation 2:45 p. m. The sheet was again placed in the solution. A covering formed on the sheet, which was rinsed off when the sheet was moved. Below this covering however a light, adherent covering, the formation of which is very characteristic for my pickling process, was gradually produced. I 2:49 p. m. The sheet was wiped and after this treatment of about 12 minutes showed to be perfectly clean. The solution is dark yellow colored] and slightly turbid. Small quantities of a precipitate are present. p1'-L=2. 4.

3:00 p. m. A fresh plate was introduced. -A

rather thick covering settled on it. but was rinsed off when the sheet I was moved. I 3:04 p. m. The sheet was wiped- Only a very small part of the scale was dissolved. The solution was distinctly turbid.

3:06 p. m. The sample-precipitate was dark brown. pH=2.5. 2 ccms. H2804 (containing 100 grams H2804 per liter) were added; the solution be.-

came somewhat lighter.

3:14 p.m. The solution was not yet perfectly clear again. The sheet was again placed in the solution, a very slight development of gas could be observed. I

3:1 I m. The sheet was wiped and the scale was I found to be removed to a consid- I erable extent. At the places, which were free from scale already before the sheet was placed in the solution, a brown covering had formed. The solution was some what more turbid.

3:20 p.m. The sample-precipitate was dark brown to olive-colored. pH='2.4. 2 ccms. H2804 were added.

3:28 p. m. The sample-precipitate was dark brown to olive-colored. The solution was. greyish brownand dis- I I tinctly turbid.

:31 ptm. The sheet was again placed in posi- I tion. A brown covering formed.

3:34 p. m. The sheet was wiped. Only small remnants of scale were present. The solution was more turbid and of darker color. I 3:38 p. m. The sample-precipitate was dark olive-green. -pH=2.5. 2. ccms. H2804 were added.

-- 3:48 p.m. The sample-precipitate was dark olivesgreen. pH=2.3. 2 H2804 were added. 4:02 p. m. The turbidity of the solution had disappeared. .The solution was brown-olive-colored. The sampleprecipitate was dark brown-olivecolored. pH=2.1. 2ccms. 8:804

- were added. I

4:28 p. m. The sample-precipitate was dark 4 brown; pH= 1.9.

:33 p.m.. The sheet was introduced into thebath, a slight development of gas could be observed. I

Timeoi" observation I I I 4:36 p. m. The sheet was wiped. The last remnants of scale had disappeared, the sheet was perfectly clean after 5 this treatment of abqut 14 minutes.

4:41pm. The sample-precipitate was dark brown olive-colored. pH'=1.8.

4:45 p. m. A fresh plate was placed in the solution. A light covering was formed 10 which could be rinsed off.

4:49 p. m. The sheet was wiped, about half of the scale was found to be removed. The solution appeared slightly turbid. 4:51p. in. The sample-precipitate was colored dark olive-green. pH=1.8.

5:02 p. m. The sample-precipitate was dark olive-green. The sheet was again placed in the solution. A brown'20 covering formed. 5:06 p. m. The sheet was wiped, the scale showed to be not completely re- I moved. J

5:15 p. m. The sample-precipitate was dark olive-green. pH=2.2. The solution became turbid and a 'flocculent precipitate appeared.

5:21 p. m. The sheet was again placed in the solution.

:25 p. m. The sheet was wiped and, after this treatment of about 12 minutes, was found to be perfectly clean save for a few small spots of scale. The solution was very turbid, its color greyish brown.

5:37 p. m. The sample-precipitate was almost a pure green. pH=2.3..

The solution was left standing about 42 hours. v

light yellow. Aflocculent ocherous precipitate was present. 4 grams NaNO: were added, the solution was not heated.

Third dai! Time of observation :1 a. m. The sample-precipitate was green.

0 p. m. The sample-precipitate was green.

The solution was heated.

:05 p. m. The sample-precipitate was green.

4:30 p. m. The sample-precipitate was green.

pH=2.3. 2 ccms. E804 were 00 added. The precipitate was not dissolved.

5:00 p. m. The sample-precipitate was green.

pH=2.8. 2 ccms. H3804 were The solution was left overnight and allowed to 1 cool down.

added. The precipitate was not m :43 a. m.

a Fourth day Time of observation 8:30 a. m. The sample-precipitate was dark olive-green. pH=1.7. The precipitate in the solution was dissolved save for a small residue. The cold solution was light yellowish colored, the color changed to light brown on heating.

The titrating curve illustrated by Fig.

3 was established. V The sample-precipitate was brown olive-colored. 4 grams NaNO: were added. The titrating curve illustrated in Fig. 4 was established.

The color of the solution was brown olive. The solution wascontinuously heated.

The color of the solution had become .light brown. The sample-precipitate was red-brown. The titrating curve shown in Fig. 5 was now established.

A fresh black plate was placed in the solution.

found-to be only wry little attacked. 2 ccms. H1804 were added. The sample-precipitate was brownolive. pH=.-1.7.'

The sheet was again placed in the solution. v

The sheet was wiped, nearly the half of the scale was found to be removed. The color of the solution was greyish-brown.

The sample-precipitate was dark brown.

p. m. The titrating curve shown in Fig. 6

was established.

The'soluticn was left over to cool down.

Fifth day night and allowed Time of observation 9 :os a. m. The sheet was again introduced into the heated solution. A covering was formed.

The sheet was wiped, the scale proved to be farther dissolved.

The sample-precipitate was redbrown. pH=2.3. The pickling bath was in good condition.

The sheet was again placed in the solution, a covering occurred.-

The sheet was wiped and showed only some stripes of scale. The sheet was again placed in the solution. A strong covering formed. The sheet was wiped. The. scale showed to be still further dissolved.

The sheet was again introduced in the solution and a strong covering formed.

9:11 a.in.

9:24 a. In.

i am a. m. I

treatment of about 21 minutes, showed to be perfectly clean save for small spot of scale.

The sheet was wiped, the scale was The results of Test No. 4 may be summarized as follows:

The pickling velocity was greater than in Test No. 2, but it was considerably reduced by a further addition of sodium nitrate. The pickling effeet of the solution is good and secures clean surfaces.

At the beginning of the test a sheet metal was first immersed into the solution for a short time and the solution was then allowed to stand until the ferrous compounds, which had first formed, were converted, under the oxidizing action of the sodium nitrate, into ferric compounds. The oxio'iz.ng effect of the sodium nitrate is only small, probably because of the small starting concentration of the acid. During the first part of the test apparently no colloids were formed, as may be concluded from the fact that the ferric compounds could always easily be reduced. Additions of acid during the course of the test are of no influence in this respect. An addition of sodium mtrate eifected avery slow oxidation, the course of which may be seen from the successive titrat- I in'g curves shown in Figs. 3 to 7.

An increase of the content of sodium nitrate A The sheet was wiped and, after this The sample-precipitate was 'red-.

during the test up to 6% caused the precipitate, which was formed during the first part of the test, but was dissolved by an addition of acid, to not be formed afresh, although the pH value at the end of the test was again 2.3 and only ferric compounds were present in the solution. The titrating curves show also, that no precipitate wasformed; for the content of dissolved iron compoundsin the solution is shown by these curves Y to be always about the same, A nitrate concentration of 6% is thus too high.

The slight brown color of the solution in the second part of the test cannot be attributed to theformation of colloids. since the increased content of sodium nitrate prevents the formation of colloids. The brown color may indicate the formation of Fe(NO1):. 1

Test No. 5 was carried through with the same starting concentrations as Test No. 4, but at 80 C. The clean-pickling eifect of the solution is satisfactory. The pickling velocity is increased due to the higher temperature, but a further std--v dit.on of sodium nitrate proved again to reduce the pickling velocity. The colloids which could form in the solution when it contained 2% NaNOa, disappeared almost completely, when the content was increased to 4%. An addition to the pick ling bath of 4% NaNOa may thus be considered as the highest quantity admissible at a pickling temperature of 80 C. The specific consumption of acid, calculated on the surface of the pickled sheet, only amounts to about one half of that found in Test No. 4. I

The eifect of the presencein the solution of ferric ions may be illustrated by two further tests. numbered No. 8 and No. 9, in both of which the pickling bath contained 0.7% HaSO4+2% NaNOa, giving a starting pH value of 1.02, while the temperaturewas C.

Tlsr No. 8 Time of observation 3:17 p. m. The sheet.was placed in the solution. A vigorous development of gas occurred, which became gradually stronger. 4

3 :21 p. m. The sheet was wiped, the scale showed to be dissolved almost completely.

3:22 p.m. The sheet was again introduced.

' The development of gas was considerably reduced, but became stronger when the sheet was moved.

3:26 p.m. The sheet was wiped and, after this 13:44pm. The sheet was wiped. It showed to be free from scale, but not clean at all; at the free air it tarnished with greyish-green color.

' 3:47 p. m. The sheet was introduced again.

3:51 p. m. The sheet, on which a very light grey covering had formed, was wiped and showed to now be well cleaned after this treatment of about 12' 3:56 p. m. The sample-precipitate was green.

' pH=1.6. The color of the solution was a light olive-green.

fresh sheet was introduced. A hardly appreciable development of gas occurred.

4:05 p. m. The sheet was wiped. The main part of the scale showed to be removed.

4:06 p. m. The sheet was again introduced into the solution. After a few minutes a very slight brown covering was formed. I

4:10 p. m. The sheet was wiped. v Thescale was not completely dissolved, the parts 4:01 p.m. A

which were free from scale, were 5 also clean. 4:13 p. m. The sample-precipitate was green.

pH=2.3 The solution was clear,

- its color olive-green.

4:15 p.m. The sheet was again introduced and a slight development of gas occurred. A weak covering formed on both sides. I

4:25 p. m. The sheet was wiped, the scale showed to be not yet completely dissolved.

4:26 p.m. The sample-precipitate was green. pH=2.3. The solution was still perfectly clear.

4:31 p. m. The sheet was again placed in the solution. A fairly uniform covering formed on both sides.

4:35 p.m. The sheet was wiped and, after this treatment of about 24 minutes,- showed to be not perfectly freed from scale while the parts free from scale were also substantially clean. The pickling eflect of the solution seems to be exhausted.

4:38 p.m. The titrating curve shown in Fig. 8

was established.

5:05 p.m. The solution became turbid. The

sample-precipitate w a s g r e e n pH=2.3-

( Tlmeof- 1 TmrNoJ' observation Time of of gas occurred (this sample of sheet seemed to be somewhat less oxidized than the other ones).

5:28 p. m. The sheet. was wiped and, after this treatment of about 3 minutes, showed to be free from scale and fairly clean.

5:32 p.m. The sample-precipitate was green.

The solution was allowed to stand, while maintained at 65 C.. until the sample-precipitate became brown. 5:50 p.m. The sample-precipitate was dark brown. I 6:00 p.m. The sample-precipitate was brown. The solution was left standing over night and allowed to cool down.

Second day Time of observation 8:30 a. m. A fresh sheet was introduced. A

slight development of gas occurred."

8:50 a. m. The sheet was wiped and. was found to be perfectly clean save for a few spots. The solutionwas yellowishbrown.

8:52 a. m. The sample-precipitate was red- 'brown. pH=1.7.

8:59 a. m. The sheet was introduced again. A uniform brown covering was formed. The solution was clear, its color had become still darker.

9:01 a. m. The sheet was wiped and showed to be perfectly clean after this treatment of about 6 minutes.

9:03 a.m. The sample-precipitate was brown. pH=2.3.

red-

9:07 a.m. A fresh sheet was introduced. The

temperature was lowered to 53 C. A uniform brown covering formed. 9:11 a. m. The sheet was wiped and proved to be perfectly clean apart from small spots. The solution was clear and brown colored. 9:15 a. m. The solution was heated to 70 C. It became turbid and the turbidity 4 increased noticeably.- 9:18 a. m. The sample-precipitate was brown.

9:23 a.m. The sheet was introduced again. A

flocculent precipitate formed in the solution and a uniform covering on the sheet.

Time of observation 9:25 a. m. The sheet was wiped andshowed to be perfectly clean after this treatment of about 7 minutes. The precipitate settled down. 9:28 a. m. The sample-precipitate was dark brown olive-colored. pH=2.3. 9:50 a. m. The titratingcurve shown in Fig. 9

' was established.

The results of Test No. 8and 9 may be summarized as follows:

The acidstrength of 0.7% at the start caused in Test No. 8 a small reduction of the time required for the pickling, while the clean-pickling eifect is appreciably reduced. The solution became turbidionly at the end of the experiment, after the solution was kept about one hour at a pH value of 2.3. In Test No. 9 the solution was given suflicient time, at the beginning of the test, for the conversion of the ferrous compounds into ferric compounds; for this reason the cleanpickling eflect of the solution was satisfactory already with the flrst sample. The influence of the formation of colloids on the acid content of the solution could be ascertained in this test by figures. The influence is represented by a, distinct reduction of the pI-I value from 2.3 to 1.7, whereby Equation No. I or the corresponding equation is confirmed. The solution became turbid, and insoluble compounds were separated, when after the formation of colloids the pH value of the solution had again increased to 2.3.

The influence of too high an acid strength at the start is shown by Test No. 13, which may therefore also be detailed. In this test a solution containing 2% HzSO4+2% NaNOa was used and the pickling operation was carried out at 65 C.; the pH value at the beginning was thus 0.7.

Time of observation 9:30 a. m. The sample sheet was placed in the solution. A very vigorous development of gas occurred. After half a minute the sheet was wiped and more than three fourths of the scale showed to have already been j could almost completely be wiped 9:41 a. m. The sample-precipitate was brown.

9:42 a. m. The sheet was again introduced.

The development of gas was moderate.

9:43 a. m. Thesheetwaswiped. Thescaleproved to be completely removed after this treatment of about 2 minutes, but

the iron plate was again dull grey and not clean.

Time of observation 7 9:49 a. m. A fresh sheet was introduced into the solution and again a vigorous development of gas occurred.

M 9:50 a. m. The sheet was wiped, the scale proved to be almost completely removed. 9:52 a.m. The sample-precipitate was brown. 9:53am. The sheet was again placed in position and again a vigorous development of gas occurred. 9:54 a.m. The sheet was wiped. The scale was not yet completely removed. 9:56 a. m. The sample-precipitate was redbrown. 9:57 a.m. The sheet was again introducedinto the solution and a very vigorous development of gas occurred.

I 9:58 a. m. The sheet was wiped and, after this 10:10 a. m. The sheet was again placed in the solution. When it was not moved, at some spots a light covering formed, which however disappeared on moving the sheet.

10:11 a.m. The sheet-was wiped. The scale showed to be not yet completely removed. The color of the solution was a light greenish-yellow.

10:12 a. m. The sample-precipitate" was redbrown. The pH value was now increased to 1.8. V

10:19 a. m. The sheet was again introduced and a vigorous development of gas occurred.

10:20 a. m. The sheet was wiped. The scale showed to be not yet completely dissolved.

10:22 a. m. The sample-precipitate was redbrown.

10:26 a. m. The sheet was again introduced. A slight covering was formed on some spots.

10:27 a.m. The sheet was wiped and, after this treatment of about 4 minutes, showed to be almost free from scale. Those spots which before showed a covering; now appeared clean, while the remainder of the sheet was grey.

10:32 a. m. The sample-precipitate was redbrown. A fresh sheet was introduced into the solutionand a light development of gas could be observed.

10:85 aim. The sheet was wiped. Only some of the scale was removed.

. 10:39 a. m. The sheet was again introduced. On

moving the sheet a vigorous development of gas occurred and a covering formed at some spots.

. 6. The greater the addition of sodium nitrate,

Time of observation 10:41 a. m. The sheet was ,wiped. The pickling eifect on the scale has very remarkably diminished, the sheet showed to be clean only at some spots. 10:44 a. m. The sheet was again introduced. n

moving the sheet a vigorous development of gas occurred. A covering formed. 10:46 a. m. The sheet was wiped and proved to be not yet perfectly free from scale after this treatment of about 7 minutes; those spots which were free from scale, were however clean also. The titrating curve of Fig. 10 was established.

Second day Time of observation 9:10 a. m. The sample-precipitate was redbrown. pH=1.8. The solution was perfectly clear.

By the increased acid strength on starting used in Test No. 13 the oxidizing effect of the sodium nitrate was promoted. A clean-pickling effect could not be observed at all at the beginning of the test and it occurred only gradually, after the pH value was raised to above 1.8 (H2804 less than 0.1). Now the picklingeifect on the scale showed to be very remarkably diminished, so that the fifth sample sheet was not yet free from scale after having been pickled 7 minutes. The titrating curve of Fig. 10 shows that a very, large quantity of dissolved ferric compounds had formed in the solution. The. light color of the solution however shows that no colloids were formed. At the beginning of the test colloids could not be formed, since the acid strength was too high. A formation of colloids could generally be observed only when the acid concentration had dropped to less than 0.1% H2804. When this concentration was reached, the quantity, of iron compounds formed was however already so large that the too high concentration of the ferric compounds prevented the formation of colloids. Thus a starting acid strength of 2% proved to be too high. p a

The experiments show under which conditions, in a pickling bath of sulfuric acid and sodium nitrate, iron may be pickled to become clean,

while at the same time soluble iron compounds separate, so that the content of dissolved iron 'compounds in the solution does not exceed a certain limit defined by the hydrolytic equilibriurn. These conditions, which secure the desired result may be summarized as follows:

1. The solution must be warmer than 50 C.

2. The starting concentration of sulfuric acid must not exceed 1 per cent.

3. The desired separation of insoluble iron compounds does not occur before the acid concentration has dropped to less than 0.1 per cent H2804. f

4. The oxidizing agent present in the solution must sumce to prevent the presence of any substantial quantity of ferrous compounds in .the soiution,'since otherwise the content in dissolved iron compounds cannot be prevented from increasing.

- 5. The clean-pickling effect also occurs only in the case that a sumcient quantity of oxidizing agent is present.

the smaller is the pickling velocity.

.7. The content of sodium nitrate must be less than 5 per cent.

8. The starting acid strength of the solution should not be too low, since theoxldizing effect of the sodium nitrate decreases with decreasing starting acid concentration.

If these conditions are fulfilled, the following 7 results may be obtained according to my invention:

The operation with the highly diluted solutions does not involve any danger for the operator.

No hydrogen is combined with the metal, since in view of the oxidizing action of the liquor, the acid, when acting on the metal, does not split off hydrogen in an obnoxious intensity.

In view of the equilibrium maintained in the solution, the bath is not enriched with metal salts. The solution may thereforebe used almost infinitely without spoiling the product, and needs to be replenished only from time to time by an addition of acid or salt. When a solution 'of nitric acid and ma nesium may for instance be replaced, without an undue increase in magnesium ions, by an addition to the bath of sulfuric acid; or an addition of alkali nitrate may be used to strengthen the oxidizing effect of the bath without increasing its acidcontent. always be taken that the admissible concentration of acid is not exceeded. This does not ine serves as pickling bath, the sulfate ions consumed in the precipitating of the insoluble iron compounds When replenishing the bath, care mustvolve however any difilculty, since the basic precipitate present in the bath neutralizes any not too high excess of acid which should be added.

In the prior art the starting acid concentration is not distinguished from the acid concentration maintained in the course of the pickling operation. This difference is however important according to my invention, since the desired result is only obtained by a very small acid concentration during the pickling operation, while the starting acid concentration is preferably rendered high enough to secure that the required quantity of ferric ions is formed in the bath.

By treating iron with a pickling bath in ac- .cordance with my invention a pure metallic, perfectly clean surface is obtained, which is simultaneously passivated so that ,it will not be oxidized, when exposed to the free air, even after a long time. The surface is excellently suitable for receiving a protective coating of any kind.

The iron which is dissolved by the pickling action may be recovered in the form of basic iron compounds and may be utilized as pigment.

The pickling time is rather long, if my process is directly applied to strongly s'caled or rusted blanks. In this case I prefer to first free the blank from scale and rust by any known means and thereupon to clean-pickle it in a pickling solution according to this invention, for instance in an aqueous solution of 0.25% E04 and 2% potassium nitrate.

particularly iron with low carbon content which forms the material from which sheet metal and other rolled products are made.

Various changes may be made in the details departing from the invention disclosed in the foregoing specification without or sacrificing the advantages thereof. I claim:

1. The method of pickling iron, which comprises acting on the metal with an aqueous solution, heated to at-least 50 C., which contains free mineral acid in an amount sufliciently low to permitthe metal salts formed to be precipitated by hydrolytic action, an oxidant, and up to about 5 per cent of a salt, the anion of which difiers from that of the acid and the kation of which is less noble, in the sense of the electromoti've series, than the metal to be treated.

2. The method of pickling iron, which comprises acting on .the metal with an aqueous so- I lution, heated to atleast 50 0., which contains free mineral acid in a concentration of not more than 1 per cent, an oxidant in an amount sumcient to oxidize substantially all the dissolved ions of the metal under treatment, and a salt, the anion of which differs from that of the acid and the 'kation of which is less noble, in the sense of the electromotive series, than the metal under treatment, allowing the acid concentration of said solution, while it is in contact with the metal under treatment, to drop below 0.1per cent and maintaining it below 0.1 per cent during the pickling treatment.

3. The method of claim 2, wherein sulfate and nitrate ions are present in the solution.

4. The method of claim 2, wherein persulfate ions are added to the solution which contains nitrate ions.

5. The method of pickling iron, which comprises acting on an iron surface with an aqueous solution, heated to at least C., which contains up to 1 per cent sulfuric acid and up to about 5 per cent of an alkali metal nitrate, allowing the acid concentration of said solution to drop during its action on the metal under treatment, below 0.1 per cent and maintaining it below 0.1 per cent for clean-pickling said metal.

6. 'I'he method of pickling iron, which comprises acting on iron with an aqueous solution, heated to at least 50 0., which contains free mineral acid in an amount suiflciently high to attack the iron under treatment, but sufficiently low to permit ferric compounds formed to be separated in colloidal form by hydrolytic action, an electrolyte adapted to coagulate colloidal ferric compounds present in the solution, and an oxidant dissolved in an amount sufliciently high to oxidize substantially all ferrous compounds present, but not-so high as to prevent the separation of coagulated ferric compounds formed.

7. The method-of claim 2, wherein the treatment is performed at a temperature ranging between and C.

8. The method of claim 2, wherein the metal to be pickled, before it is treated with the said solution, is superficially freed from scale and rust.

9. The method of claim 2, wherein a plurality of mineral acids is present in the solution.

10. The method of claim 2, wherein the solu tion contains a plurality of salts.

I WALTER HEIMIBERGER.

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