US3073726A

US3073726A - Removing mill scale from iron

Info

Publication number: US3073726A
Application number: US413450A
Authority: US
Inventors: Freud Herbert Manfred
Original assignee: Individual
Current assignee: Individual
Priority date: 1953-03-05
Filing date: 1954-03-01
Publication date: 1963-01-15
Anticipated expiration: 1980-01-15
Also published as: FR1085422A; DE1191657B; BE526720A; CH320340A; FR63845E; FR1075489A; GB787841A

Description

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6/800? lbs:

United States Patent 3,073,726 Patented Jan. 15, 1963 today when this duration has been reduced to a few 3,073,726 months as a result of technical advances.

REMOVING MILL SCALE FROM IRON Herbert Manfred Freud, dit Jean Frasch, 5 Blvd. du Sud-Est, Nanterre, France No Drawing. Filed Mar. 1, 1954, Ser. No. 413,450 Claims priority, application France Mar. 5, 1953 6 Claims. (Cl. 1343) The present invention relates to the treatment of metal surfaces. More particularly, the present invention relates to a process and composition for treating the surface of steel articles, e.g. in the form of plates, to provide for improved adherence of paint to the surface of the article to protect the same from the effects of corrosion.

It is known that mill scale, formed on steel in the course of rolling, forging or stamping under heat, constitutes an extremely harmful layer preventing good adherence of paint or other protective coating on the metal of the base, thus risking exposure of the metal base to destruction by corrosion.

The process of removing the mill scale from small tion of hydrogen by this attack (Me+HX=MeX+H) 5 which, accumulating under a certain pressure, starts to raise and detach the mill scale which falls to the bottom of the vat.

The addition to the acid bath of agents for inhibiting the corrosion of the metal serves the purpose of limiting the speed of attack of the steel which might be too extensive and produce a deterioration of the objects, either by a substantial diminishing of the material, or as the result of the so-called hydrogen embrittlement which is imparted to the piece, and which is manifested by a substantial reduction of the elastic properties of the metal.

When large objects are to be de-mill scaled, either of fixed type, such as bridges, pylons, and gasometers, or mobile type, such as a ship, for example, there is actually available only two methods, as follows:

(a) Either sand blasting or mechanical brushing, operations which are, very costly, very dangerous and impossible to carry out in certain cases, as, for example, in the vicinity of gear mechanisms (where very fine sand in penetrating the mechanism may scratch and adhere everywhere to the same) or in a petroleum refinery (where sparks produced by the sand blasting may start a fire). Moreover, and chiefly with respect to closed places, e.g., the interior of a ship, a great problem is involved in the removal of all the sand, which sticks to the walls and thereby interferes with good adherence of paint, or

(b) Natural exposure of the objects, and in particular of sheet metal, to corrosive atmospheric agents (especially at the seashore) for one or two years, which pro vides for penetration through the pores of the mill scale of the oxygen of the air and the atmospheric humidity for the purpose of attacking the iron underlying the mill scale which they transform to rust. The rust, occupying a volume 13 times greater than the metal from which it is formed, exerts a pressure on the mill scale which is so great that it is mechanically detached, in the manner of the hydrogen in the acid scouring bath.

De-mill scaling by natural exposure presents two major difficulties:

(1) It requires an extremely long duration, one to two years, a period which was not inconvenient some years ago when the construction of a ship, for example, required two to three years, but which is a great handicap (2) It involves the useless destruction of a large part of sound metal, because the rust does not form uniformly over the whole surface, but rather by regions often hollowed out in depth, so as to form what are called in technical terms cankers.

Another method which is theoretically valid consists in sprinkling the sheet metal or other objects coated with mill scale with a mineral acid or sea water in order to de-mill scale the same. Even if by this process one can accelerate the speed of de-mill scaling with respect to the natural exposure method, the experience of the inventor (as well as other numerous technicians who have tried this process before him) proves that this method is either of little elfect, or is harmful to the sheet metal for the fol lowing reasons:

(a) When an aqueous solution of a strong acid or sea water is applied on a smooth metal wall which is vertical or forms a ceiling, the solution due to its fluidity flows toward the bottom under the action of gravity. It remains adherent to the mill scaled metal only as a thin film having a thickness of the order of a few microns, containing a quantity of acid or salt which is insuliicient to effect electrolytic and hydrolytic reactions. If this operation is effected in the free air, in windy periods the thin liquid film is immediately swept off, in sunny periods it is immediately evaporated, and in rainy periods it is immediately washed oil, and since of the 365 days of the year it is either windy, rainy or sunny at least during 300 days, this operation has proved illusory.

(b) In the hollows and on the flooring, on the other hand, there is a considerable accumulation of free acid or salt which favors the de-mill scaling operation, but which however may cause partial dissolution or strong corrosion of the sheet metal in places which may prove extremely dangerous, especially between two plates which are riveted, welded or bolted.

(c) The addition of wetting agents in the acid or saline solutions in order to favor the penetration of the latter between the mill scale and the sheet metal has the elfect of reducing the surface tension of these solutions to assist their spreading out, thereby to diminish the thickness of the liquid film, and as a result reducing the speed of the de-mill scaling process.

(d) The addition of thickening agents (such as gelatin or other sizing materials) to the acid or saline solutions has a double result, on the one hand to provide for the increase in thickness of the film (and thereby the quantity of acid and thus the speed and the effectiveness of the demill scaling procedure), and on the other hand to increase the over potential of the hydrogen (and thereby retard the electrolytic action of the mill scale iron pile," this word "pile being used in the present specification to designate what is usually called cell, and thus the speed and the effect of the de-mill scaling operation). There is, therefore, no advantage in adding any thickening agent with an adhesive base to the acid or saline solution. On the contrary, it is quite difficult to paint on a surface on which gelatin or adhesive materials have been applied due to their tendency to absorb water vapor, which raises up the paint which is applied.

It is an object, therefore, to provide for de-mill scaling metal surfaces by a process which avoids the above mentioned disadvantages.

Another object of the present invention is to provide a de-mill scaling material which can be applied on all steel or cast iron objects and which can effectively and rapidly remove the mill scale layer therefrom without an appreciable attack on the metal of the base.

Other objects and advantages will become apparent from the following description and the appended claims.

In particular, the invention concerns the provision of a paste material having a viscosity suflicient to enable the paste to be applied with a brush or with a spray gun (with the aid of a vessel under pressure) on vertical members or on a ceiling, the paste preferably having a thickness of between 0.01 and several millimeters without running ofi, that is, such as to remain at the same thickness as long as it is not removed. The paste, in accordance with the invention, has a suificiently strong action to provide for the de-mill scaling operation in a relatively short period of time, and of such composition to keep the sheet metal protected against all attack and irregular corrosion of the canker type, as well as all corrosion in the interstices between two plates which are riveted, welded or bolted.

The product provided in accordance with the invention is a paste material which comprises as essential components one or more oxidizing anions and an inert pulverized mineral charge (incapable of being attacked, particularly by acids) such as silicates.

The mineral charge in accordance with the invention is preferably composed of an insoluble silicate or of silica itself, such as clay or kaolin (aluminum silicate), talc (magnesium silicate), bentonite or ground shells (silica and calcium silicate). The mineral charge should be finely ground and moistened with water, in order to obtain a greasy paste so that it can be spread out in the manner of an oil paint. Ground quartz or silica which is entirely dehydrated is not suitable, since a paste made therefrom has the tendency to form lumps.

The function of the mineral charge utilized in the paste formed in accordance with the invention is not that of a thickening agent for the acid solution, but that of a porous vehicle or vessel, that is, to serve as a support for osmotic phenomena.

The function of the silicates contained in the charge conforming to the invention can be compared to that of a porous vessel in which a plant is grown. The porous vessel serves for the exchange of humidity and air between the roots of the plant (and the surrounding earth) and the surrounding atmosphere resulting from the osmotic pressure which is established through the vessel.

In accordance with this concept, the inventor has been able to show that the swelling colloidal charges capable 4a of forming a true gel with water, such as bentonite, are preferable to others due to their particular capability of serving as a porous vessel, that is, to absorb either the water provided by the atmospheric humidity, or other moistening agent, in swelling, according to the expression used in mineral chemistry, and the present invention preferably utilizes such charges.

Another property of such swelling colloidal charges is the possibility of obtaining the same viscosity of the paste with ditferent proportions of the liquid-solid components, according to the pH of the medium from which the first gel has been formed. Thus, for example, when the bentonite has been added to a sulfuric a cid solution in order to obtain a given viscosityfit willbe necessary to have 35% of bentonite and 65% of acid solution. On the other hand, if initially a gel 0 entontte is formed with water and the sulfuric acid is then added in order to obtain the same viscosity of the paste, and the same final proportion of the sulfuric acid with respect to the water, it will be necessary to have 25% of bentonite and 75% of acid solution (water and acid). It is thus possible to obtain the same viscosity in accordance with the method of preparing the paste with a lesser amount of the charge and increased amount of the ionizing and de-mill scaling dilute acid, and to vary as desired the viscosity of the paste by small additions of acid, water and various solutions, knowing that the addition of acid of low pH facilitates the liquefying of the gel, while water or solutions of higher pH facilitate the thickening of the paste.

The proportions of the components of the paste, ac-

cording to the invention, can be varied within very great limits. A primary condition is the obtaining of such a viscosity that the paste can be applied on a ceiling or on a vertical wall without running, in a layer having a thickness of the order of 1 mm., but the proportions of the active ingredients are not critical, having effect only on the speed of action of the paste material.

The condition relative to the pressure-of the oxygen (p0 can be expressed by saying that these oxidizing anions have an rH greater than 41 and an r0 less than 0 by virtue of the following relations:

By way of an example of such oxidizingagents there may be mentione MnO; within the limits of ClO" at all pH f 10 at a pH between 1 and 1.5

; cl'2O7 at a pH between 1 and 1 i etc.

The rH-value is the logarithm, to the base 10, of the reciprocal of the hydrogen pressure which would produce the same electrode potential as that of a given oxidation-reduction system, in a solution of the same pH-value. The greater the oxidizing power of a system, the greater the rH-value.

The pH of the paste can be whatever at the moment its rH or r0 satisfies the condition described above. It can clearly be located between 0 and 10.

Certain oxidizing agents, such as hypochlorites or periodates, produce upon reduction salts which are good conductors and which enable the paste to continue its action, but certain others such as KMnO, produce insoluble products which make the iron passive and interrupt the action of the paste.

In this case, the invention provides for the addition to the paste of a salt or an acid soluble in a liguid which is a good conductor which avoids this passive action and has, besides, the advantage of aiding the formation of electrolytic piles of the type described below, in the case where the mill scale has a certain porosity. These salts or acids preferably have the following properties:

(a) Ionic activity greater than 0.01 expressed in the usual manner as concentration of the ion in solution in grams per liter.

(b) Reactivity on the iron (iron electrode potential in their aqueous solution less than 0).

(c) rH between 27 and 41.

They can be sulfates, acetates, pyrophosphates, of metals, preferably alkaline metals, for example,

of rH of the permanganate KMnO, are given below at different pH:

pH between 1 and 11 pH r0 rH in 1o 1! so a 10 47.5 s 10' 45 P9 10 42.5

The first solution is practically destroyed at the end of a day or two, the third has a duration of more than three months. Therefore, it is preferable to avoid the prolonged presence of strong acid ions.

However, this condition tending to do away with the use of strong acids in the pastes is valid only for reasons of transport and storage. It is not significant with respect to the activity of the paste. As a consequence, the procedure which provides for the introduction in the paste at the time of its employment of a strong acid or other electrolyte corresponding to the properties specified above, comes within the scope of the invention. This procedure, in fact, avoids the r isk of destruction of the paste, since the paste is used as soon as the addition is made.

As indicated above, the proportions of the components can be varied within very great limits. According to the concentration of the oxidizing agent, the paste is more or less active, but its action remains the same and therefore the proportions are not critical.

In the same way in accordance with the proportion of its constituents, the paste is more or less fluid, and more or less adapted to various types of de-mill scaling operations. The conditions of employment themselves being quite varied, and the layers of mill scale being very different in composition and thickness, the specifying of proportions of the components does not have very im portant significance.

However, it may be noted that the proportion of water of aqueous solution in the paste will be regulated as a function of the viscosity sought, which depends on the instruments used for applying the paste. This can be determined by one skilled in the art, and according to the instrument employed, i.e. a spray gun, a brush, painter knife, it will be necessary to adopt a certain viscosity from which the necessary proportion of the liquid can be determined. Besides, the user can always increase the fluidity at the time of use by the simple addition of water.

With respect to the proportion of the oxidizing agent, this can vary between 1% and 20% with respect to the total weight of the paste, according to the oxidizing agent adopted and the activity of the paste sought. Also, the electrolyte can be employed in the proportion of 1% to 30% according to its conductivity and its eventual particular activity in the electrolytic pile. But it is to be noted that these proportions are not critical.

In the case where the partial pressure of the oxygen of the oxidizing anions is less than 1 atmosphere (rH less than 41), the mere presence of oxidizing anions is not always suflicient, and the invention provides then for the addition of non-reducing mineral acids having a pH less than 2.5, and in particular, sulfuric acid. It appears that, in this case, the mechanism of the reaction is as follows:

In the course of the penetration of the acid (and in particular sulfuric acid) through the mill scale, it reacts in part with the layer of FeO between the metal and the mill scale with the formation of ferrous sulfate and in part with the iron itself with the formation of hydrogen and ferrous sulfate equally. It is initially due to this formation of ferrous sulfate and then to its oxidation to ferric sulfate and rust by the action of the oxidizing agent that the adherence of the mill scale to the metal is diminished, and the de-mill scaling operation rendered possible.

In fact, if by rapid oxidation of the iron or of the ferrous salts, such as in the presence of an energetic oxidizing agent, there is obtained the variety Fe o xH oa or goethite, there is produced, on the contrary, by a moderate oxidation (such as by the simple action of air and the acid or saline solutions) the variety Fe O xH O'y or lepidocrocite which reacts with FeO or the ferrous sulfate, forming the ferrosoferric Fe O xH O or the mill scale hydrate. In contrast to the variety of red rust or goethite Fe O xH Om which occupies a volume 13 times greater than the metal from which it has been formed, the variety of black rust or ferrosoferric Fe O xH O occupies a volume hardly equal to or triple that of the metal. The force of expansion and thereby the ease of raising up the mill scale is then greatly increased by the formation of the goethite instead of the ferrosoferric oxide, and the presence of oxidizing ions in the paste, produced a rapid oxidation of the ferrous sulfate and the destruction of the hydrogen, permits this result to be achieved.

The acid utilized can be any mineral acid having a pH less than 2.5, with the exception of an acid capable of reducing the oxidizing anion present in the paste (such as HBr, H SO H PO H PO etc.). As a practical matter, the acid which seems to be of the greatest interest, at least from the point of view of economy, is sulfuric acid, at a concentration varying between 5% to 50% .with re spect to the water contained in the paste.

The oxidizing anion can be of any type, so long as it is reduced by nascent hydrogen at ordinary temperature and can oxidize the ferrous sulfate formed by the action of H on the iron and eventually the layer of FeO, as for example the anions CrO Cr O' MnO N0 N0 C10 1 C105, etc. Those anions are preferable which aresiisceptible of changing color when they pass from the oxidized state to the reduced state, such as the anion CI'gO-f' which is orange yellow and which, by reduction, is transformed to the blue green cation Cr+++, or the anion MnOywhich is dark brown and which by reduction is transformed into the pale rose cation Mn++.

The paste, which in the absence of these oxidizing anions, is white or colorless, is strongly colored in the presence thereof, either to an orange yellow in the case of the presence of Cr O- ions, or to dark brown in the case of the presence of MnO and when it is applied on the mill scaled metal it shows the same colorations. In the degree that the acid penetrates through the pores of the mill scale, it attacks the ferrous oxide and the underlying iron, which it transforms into R250, and liberates hydrogen. These two products act as reducers of the oxidizing anion which they reduce to the state of a reduced cation, with clear change of color of the paste at that point. There will appear, in fact, for example, in the case of a paste containing the 0 0,- ion, and which is orange yellow (more or less rapidly in accordance with the porosity and the thickness of the mill scale), points or regions which are blue-green, which increase in proportion until the entire surface on which the paste has been applied is covered. The initial points which change color indicate the pores of the mill scale and the places where the acid begins to penetrate. If the change in color of the entire surface does not mean that the acid has penetrated everywhere (because the change of coloration can also be produced by the diffusion of the hydrogen ion through the paste and the local reduction of the oxidizing anion), it nevertheless indicates that the acid has penetrated through a great number of the pores of the mill scale and that it continues to carry out its action of dissolution on the FeO and Fe under the mill scale between two or more pores. It is then preferable to wait a few days after the change of color of the entire surface before proceeding to remove the paste by washing and to expose the thus Washed sheet metal to atmospheric agents for the purpose of forming rust.

It has beerijggnd, furthermore, that while the de-mill scalihg prdcess can be carried out fairly well with the aid of a paste having the composition described above, even bgtteg re s ults qan be obtained, thatis, agreater speed of de-mill scaling can be achieyed if there is added to the .Esisflahnacaffi ifiifi .5 .3, Nah i 'This can be achieved easily by selecting, for example, Na cr O or K MnO as the body containing the oxidizing anion.

The preferred proportions are of the order of 0.5 to 5% in weight with respect to the paste, but these limits may be exceeded without difiiculty.

This action is probably concerned with the mechanism of the formation of the rust by the pile effect between the metal and the mill scale. The anion SO goes toward the iron anode and forms FeSO the cation K goes toward the mill scale cathode and forms KOH-t-H. The potassium hydroxide thus formed reacts on the ferrous sulfate in accordance with the reaction:

The ferrous hydroxide thus formed is more easily transformed to rust by the oxygen of the air or the oxidizing agent contained in the paste (than the ferrous sulfate which requires for its transformation to rust both oxygen and water) according to the reaction:

In the case of metals which are strongly mill scaled, the invention provides, in the case of the two types of oxidizing agents described, for leaving the above paste applied at least one week, washing it off, allowing the action of the atmospheric agents to continue for some time until the piece is completely covered by rust, and then removing the rust by any appropriate means.

These procedures will be more fully described below and explained by means of the following examples, it being understood that the examples given below are set forth only for the purpose of illustration, and are not intended to limit the scope of the invention in any way:

Example 1 A solution is first prepared containing:

600 gm. water 30 gm. KMnO 70 gm. (NH SO which is poured into a vessel to which there is slowly added in order to form a uniform gel:

300 gm. of bentonite The pH of this paste is about 7, the r-H is 44, and its color is red.

The gel thus formed is applied to the surface to be demill scaled, either with the aid of a spray gun, or with a brush.

At the end of some time, there will be seen a change in color of the paste which becomes maroon, this indicating that the KMnO has released a part of its oxygen for the function of the electrolytic and electronic piles and is transformed to MnO The paste is allowed to react about 10 to 15 days at the end of which time all of the KMnO has reacted and the paste becomes useless. The paste is then removed by washing and finally cleaned off with a metallic brush to remove traces of the detached mill scale.

In the case where the mill scale is thick and uniform, the process in accordance with the invention is preferably carried out as follows:

The paste is applied on the mill scaled surface and it is allowed to react for a few days. The minimum period which the paste is left on the sheet metal should be 48 hours, but it can be left on for several months without having any accelerating or retarding elfect on the de-mill scaling process. Thus, when the paste is applied on marine sheet metal in the course of storing the same in the storage depot, it will be left on until the sheets are drawn out for use. When the paste is applied on the bottom of a ship already built, for example, it will be left on 5 to 6 days. At the end of 8 days it dries. It is then necessary to remove it by washing it with water, since it hinders the detachment of the mill scale disintegrated by the rust which should form on the metallic surface after several weeks of exposure to the air and which can be removed either by a jet of water or a moistened sponge.

On the places where the mill scale has been relatively thin, it forms blisters. But in the places where the mill scale is sufficiently thick and uniform, the free acid takes a substantially longer time to penetrate to the metal (in passing by the layer of FeO), which then reacts more slowly and produces liberated hydrogen which has a lower pressure.

On the other hand the layer of mill scale, being thicker, requires a greater pressure of hydrogen than a thin layer in order to be raised up. It is for these two conjugate reasons that when a thick layer of mill scale is met with, it is not detached from its iron base together with the paste.

In fact, if dry paste is on the mill scale, it sufficiently hinders the passage of humidity and atmospheric oxygen for finishing the transformation of the ferrosoferric salts formed into ferric salts which, by hydrolysis, produces the rust. After washing off the paste, there is found on the sheet metal the mill scale which is visibly intact, that is, it is black without a trace of rust. In reality, it is not as adherent or as compact as it was initially. This will be easily seen in scraping it with a knife or any blade, and in this way it is rather easy to detach small chips of mill scale, which is not possible to do on mill scale which is intact, that is, not treated by the paste.

The mill scale is then fissured and its adherence to the metal is considerably diminished. However, the ferrosoferric salts formed by the action of the oxidizing and ionizing agents on the iron and the ferrous oxide do not .occupy a volume sutficient to exert an adequate pressure on the mill scale for the purpose of entirely detaching it.

When the paste has been removed by wash water, the oxygen of the air no longer finds an obstacle to its passage through the pores of the mill scale to the ferrosoferric salts, which is oxidizes to ferric salts which themselves, under the action of the humidity, hydrolyze to rust Fe O xH O. The rust occupying a greater volume than the ferrous salts and the iron from which it is formed detaches the semi-adherent mill scale.

The period of exposure of the sheet metal to atmospheric agents after removal of the paste is from 1 to 3 weeks, generally 15 days, and in all cases a period necessary so "that the entire surface is covered by powdered rust, which can be removed by simple brushing with a metallic brush.

Preferably after brushing, it is of advantage to utilize the light covering of rust spread over the entire surface, which may be phosphated in accordance with known procedures, in order to obtain a better adherence of the paint.

Example 2 There is first prepared a solution containing:

550 gm. of water 40 gm. of potassium permanganate 60 gm. of acetic acid 50 gm. of sodium acetate This solution is poured into a vessel to which there is slowly added with agitation in order to form a uniform gel:

250 gm. of bentonite 50 gm. of kaolin The pH of this paste is about 4, the rH is 46.5 and its color is also red.

'Ihis paste, like the preceding one, acts by the formation both of an electronic pile and an electrolytic pile. The electronic pile reduces KMnO to Mn0 as in Example l. The electrolytic pile, when the products penetrate through the pores of the mill scale, reduces the red KMnO, to white manganese acetate.

The application is carried out as in Example 1, and the period of reaction varies also from 15 days to 1 month, including the time of exposure to the air for rushing.

Example 3 There is prepared a solution composed of:

500 gm. of water 80 gm. of sodium hypochlorite 80 gm. of ammonium bisulfate This solution is poured into a vessel, and there is slowly added with agitation in order to form a uniform gel:

250 gm. of bentonite 100 gm. of silica The pH of this solution is about 6, the rH is 53 and the color is straw yellow.

The electronic pile functions then by the loss of the oxygen of the straw yellow NaClO and its transformation to colorless NaCl. The paste of straw yellow passes then to a white color. The mode of application and the action of this paste are the same as those indicated in Examples 1 and 2.

Example 4 There is prepared at first a solution containing:

450 gm. water 170 gm. H 80 66 Baum 30 gm. Na Cr O' This solution is poured into a vessel to which there is slowly added with agitation in order to form a uniform gel:

350 gm. of bentonite The preparation is carried out at a temperature of about 40 C. which is easily obtained without heating by the simple addition of sulfuric acid in water. Obtained at this temperature, the gel is relatively fluid, but thickens by cooling. It is of yellow color. The gel thus formed is applied to the surface to be de-mill scaled, either with the aid of a spray gun or with a brush. After a few minutes, there will appear blue green spots which increase and multiply in accordance with the porosity and thickness of the mill scale until the entire surface is covered. The time varies from a few minutes to a few days and cannot be specified, being determined solely by the particular sheet to be de-rnill scaled.

If the the pH of the paste is taken immediately after the entire surface has been colored blue green, it will be found that it is still less than 2, that is, that all the acid has not reacted by penetration through the pores of the mill scale. If the sheet covered by the paste which has turned green is left exposed for three to four days (three to four days after the complete change of coloration), and if the pH of the paste is taken, it will be found that it has gone up to about 4 in a practically uniform manner, that is, that all of the acid has already reacted, and that it can be removed by washing without risk of having the wash water remaining acid.

After washing it will be seen that the adherence of the mill scale on the metal has been greatly diminished and that it can be removed by scraping. But this operation is long and expensive, and it is preferable to leave the thus washed sheet exposed to the atmosphere for 10 to 15 days. Red rust appears after a few days and covers the entire surface. When this rust is sufiiciently abundant, it is usually brushed olf by a metallic brush or re moved by a paint knife by entire sheets, and painting can be started immediately afterwards, or after a preliminary phosphating treatment.

Example 5 A very thick gel is first prepared starting with:

350 gm. of water 35 gm. of bentonite This is then liquefied by adding a solution composed of:

190 gm. of water 160 gm. of sulfuric acid, 66 Baum 20 gm. of potassium permanganate The suspension thus formed is re-thickened by retransformation to a gel by the addition of:

200 gm. of bentonite 45 gm. of kaolin The gel thus formed is a dark violet color and is applied on the mill scaled surface.

The change of color of the paste is manifested by its turning from dark violet to rose or very clear brown, when the acid succeeds in infiltrating through the pores of the calamine to the metal and produces the liberation of hydrogen and the formation of ferrous sulfate, both oxidized by the potassium permanganate.

The mode of application, the duration of the action and the rusting are effected exactly as in the preceding examples.

It is to be understood that the examples given above are only by way of illustration and not by limitation, the nature of the constituents and their concentrations being variable within the limits indicated in the description.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A method of removing mill scale from an iron body containing the same on the surface thereof, comprising the steps of applying to said surface a composition consisting essentially of a paste of an aqueous solution of a non-reducing acid electrolyte having in said solution in said paste a degree of ionization greater than 0.01 ion grams per liter, said solution containing at least one oxidizing anion having in solution in said paste an rH value greater than 27, the pH value of said solution in in said paste being numerically less than 2.5 at rH values between 27 and 41 and being numerically up to 10 at rI-I values greater than 41, said solution being free of any acid capable of reducing the oxidizing anions in the paste, and of at least one finely divided mineral substance inert to said solution and being distributed therethrough and dispersed therein in an amount sufiicient to form said paste with said solution, said finely divided mineral substance acting as a porous vehicle for said solution so as to cause controlled oxidation of ferrous ions to ferric ions and the formation of goethite which results in the loosening of the mill scale; maintaining said composition on said surface until said oxidizing anion is substantially completely reduced; and removing said composition after reduction of said anion, and the loosened mill scale from said surface.

2. A method of removing mill scale from an iron body containing the same on the surface thereof, comprising the steps of applying to said surface a composition consisting essentially of a paste of an aqueous solution of a non-reducing acid electrolyte having in said solution in said paste a degree of ionization greater than 0.01 ion grams per liter, said solution containing at least one oxidizing anion having in solution in said paste an rH value between 27 and 41, the pH value of said solution in said paste being numerically less than 2.5, said solution being free of any acid capable of reducing the oxidizing anions in the paste, and of at least one finely divided mineral substance inert to said solution and being distributed therethrough and dispersed therein in an amount sufiicient to form said paste with said solution, said finely divided mineral substance acting as a porous vehicle for said solution so as to cause controlled oxidation of ferrous ions to ferric ions and the formation of goethite which results in the loosening of the mill scale; maintaining said composition on said surface until said solution of said composition reaches a pH value numerically of at least 4; and removing said composition after reduction of said anion and the loosened mill scale from said surface.

3. A method of removing mill scale from an iron body containing the same on the surface thereof, comprising the steps of applying to said surface a composition consisting essentially of a paste of an aqueous solution of a non-reducing acid electrolyte having in said solution in said paste a degree of ionization greater than 0.01 ion grams per liter, said solution containing at least one oxidizing anion having in solution in said paste an rH value greater than 27, the pH value of said solution in said paste being numerically less than 2.5 at rH values between 27 and 41 and being numerically up to at rH values greater than 41, said solution being free of any acid capable of reducing the oxidizing anions in the paste, and of at least one finely divided mineral substance inert to said solution and being distributed therethrough and dispersed therein in an amount sufiicient to form said paste with said solution, said finely divided mineral substance acting as a porous vehicle for said solution so as to cause controlled oxidation of ferrous ions to ferric ions and the formation of goethite which results in the loosening of the mill scale;-:naintaining said composition on said surface until said oxidizing anion is substantially completely reduced; removing said composition after reduction of said anion and the loosened mill scale from the surface; exposing said surface to the atmosphere until the same is covered with rust, thereby loosening mill scale still remaining on the surface; and removing said rust and the loosened mill scale from said surface.

4. A method of removing mill scale from an iron body containing the same on the surface thereof, comprising the steps of coating said surface with a composition consisting essentially of a paste of an aqueous solution of a non-reducing acid electrolyte having in said solution in said paste a degree of ionization greater than 0.01 ion grams per liter, said solution containing at least one oxidizing anion having in solution in said paste an rH value greater than 27, the pH value of said solution in said paste being numerically less than 2.5 at rH values between 27 and 41 and being numerically up to 10 at rH values greater than 41, said solution being free of any acid capable of reducing the oxidizing anions in the paste, and of at least one finely divided mineral substance inert to said solution and being distributed therethrough and dispersed therein in an amount suflicient to form said paste with said solution, said finely divided mineral substance acting as a porous vehicle for said solution so as to cause controlled oxidation of ferrous ions to ferric ions and the formation of goethite which results in the loosening of the mill scale; maintaining said composition on said surface until said oxidizing anion is substantially completely reduced; and removing said composition after reduction of said anion and the loosened mill scale from said surface.

5. A method of removing mill scale from an iron body containing the same on the surface thereof, comprising the steps of applying to said surface a composition consisting essentially of a paste of an aqueous solution of a non-reducing acid electrolyte having in said solution in said paste a degree of ionization greater than 0.01 ion grams per liter, said solution containing at least one oxidizing anion having in solution in said paste an rH value greater than 41, the pH value of said solution in said paste being between 0 and 10, said solution being free of any acid capable of reducing the oxidizing anions in the paste, and of at least one finely divided mineral substance inert to said solution and being distributed therethrough and dispersed therein in an amount sufficient to form said paste with said solution, said finely divided mineral substance acting as a porous vehicle for said solution, said finely divided mineral substance acting as a porous vehicle for said solution so as to cause controlled oxidation of ferrous ions to ferric ions and the formation of goethite which results in the loosening of the mill scale; maintaining said composition on said surface until said oxidizing anion is substantially completely reduced; and removing said composition after reduction of said anion and the loosened mill scale from said surface.

6. A method of removing mill scale from an iron body containing the same on the surface thereof, comprising the steps of applying to said surface a composition consisting essentially of a paste of an aqueous solution of a non-reducing acid electrolyte having in said solution in said paste a degree of ionization greater than 0.01 ion grams per liter, said solution containing at least one oxidizing anion having in solution in said paste an rH value greater than 27 and being selected from the group consisting of Crop Cr O MnO N0 N0 1 ClO C10 1 MnOp, C10, and- IO the pH value of said solution in said paste being numerically less than 2.5 at rH values between 27 and 41 and being numerically up to 10 at rH values greater than 41, said solutions being free of any acid capable of reducing the oxidizing anions in the paste, and of at least one finely divided mineral substance inert to said solution and being distributed therethrough and dispersed therein in an amount sufiicient to form said paste with said solution, said finely divided mineral substance acting as a porous vehicle for said solution so as to cause controlled oxidation of ferrous ions to ferric ions and the formation of goethite which results in the loosening of the mill scale; maintaining said composition on said surface until said oxidizing anion is substantially completely reduced; and removing said composition after reduction of said anion and the loosened mill scale from said surface.

References Cited in the file of this patent UNITED STATES PATENTS 1,428,084 Gravell Sept. 5, 1922 1,553,881 Siegel Sept. 15, 1925 1,729,765 Dinley Oct. 1, 1929 2,158,992 Cook May 16, 1939 2,220,451 Hunt Nov. 5, 1940 2,428,804 Terry Oct. 14, 1947 2,501,145 Smith Mar. 21, 1950 2,554,358 Burke May 22, 1951 2,672,449 Snell et al. Mar. 15, 1954 2,735,818 Cardwell et al Feb. 21, 1956 OTHER REFERENCES Metallic Corrosion Passivity and Protection, Evans Edward Arnold and Co., 1948.

Claims

1. A METHOD OR REMOVING MILL SCALE FROM AN IRON BODY CONTAINING THE SAME ON THE SURFACE THEREOF, COMPRISING THE STEPS OF APPLYING TO SAID SURFACE A COMPOSITION CONSISTING ESSENTIALLY OF A PASTE OF AN AQUEOUS SOLUTION OF A NON-REDUCING ACID ELECTROLYTE HAVING IN SAID SOLUTION IN SAID PASTE A DEGREE OF IONIZATION GREATER THAN 0.01 ION GRAMS PER LITER, SAID SOLUTION CONTAINING AT LEAST ONE OXIDIZING ANION HAVING IN SOLUTION IN SAID PASTE AN RH VALVE GREATER THAN 27, THE PH VALVE OF SAID SOLUTION IN IN SAID PASTE BEING NUMERICALLY LESS THAN 2.5 AT RH VALVES BETWEEN 27 AND 41 AND BEING NUMERICALLY UP TO 10 AT RH VALUES GREATER THAN 41, SAID SOLUTION BEING FREE OF ANY ACID CAPABLE OF REDUCING THE OXIDIZING ANIONS IN THE PASTE, AND OF AT LEAST ONE FINELY DIVIDED MINERAL SUBSTANCE INERT TO SAID SOLUTION AND BEING DISTRIBUTED THERETHROUGH AND DISPOSED THEREIN IN AN AMOUNT SUFFICIENT TO FORM SAID PASTE WITH SAID SOLUTION, SAID FINELY DIVIDED MINERAL SUBSTANCE ACTING AS A POROUS VEHICLE FOR SSID SOLUTION SO AS TO CAUSE CONTROLLED OXIDATION OF FERROUS IONS TO FERRIC IONS AND THE FORMATION OF GOETHITE WHICH RESULTS IN THE LOOSENING OF THE MILL SCALE; MAINTAINING SAID COMPOSITION ON SAID SURFACE UNTIL SAID OXIDIZING ANION IS SUBSTANTIALLY COMPLETELY REDUCED; AND REMOVING SAID COMPOSITION AFTER REDUCTION OF SAID ANION, AND THE LOSSENED MILL SCALE FROM SAID SURFACE.