US3440170A - Process for the cleaning of equipment - Google Patents

Description

ICC

United States Patent Patented Apr. 22, 1969 US. Cl. 252-147 6 Claims ABSTRACT OF THE DISCLOSURE trode, and is maintained at a value below 280 mv. by adjusting the quantity of stannous salt, whereby reduction of ferric ions to ferrous ions by steel in the equipment, and consequent corrosion, is prevented. The solution may also contain a complexing agent wh i c hfpim qlgblwmplexes with fernc ions, such as ammonium bifluoride", 51TH arreraaesfia'tfaim soluble complexes with ferrous ions. Scale such as mill scale is dissolved without corrosiop of the iron in the metal even when the solution is circu lated at a high flow rate and when it is heated to elevated temeperatures.

The invention relates to a process for the cleaning of equipment consisting wholly or partially of metal.

In the industry use is frequently made of equipment which, as a consequence of the medium with which it is in contact, has to be regularly cleaned. Examples of such equipment are steam boilers, pipes, reactors, heat exchangers, pumps, valves, distillation columns etc. With internal cleaning it is in some cases possible to carry out this cleaning operation by a mechanical method, such as by scaling, scraping, scouring, polishing etc. In order to carry out these mechanical operations, however, it is necessary to dismantle the equipment, whereupon the component parts may be cleaned separately.

It will be obvious that mechanical cleaning involving dismantling is time-consuming and therefore very expensive. It is also sometimes difficult and cumbersome to clean satisfactorily small spaces, which are difficult of access, such as bubble caps and rather long, in particular thin, pipes.

In view of the drawbacks involved in mechanical cleaning, attempts are being made to achieve the desired cleaning with chemical agents of such a type that the dismantling of the installation to be cleaned can be completely or substantially completely omitted. This object is achieved by the use of chemical cleaning agents which can be pumped through the equipment. This chemical cleaning method not only carries the advantage of a saving of time and labour but also improves the thoroughness of the cleaning.

The liquid cleaning agents used for this purpose are' usually inorganic or organic acids. According to the nature of the contamination and of the metal to be cleaned use is made in the chemical cleaning process of inhibited or uninhibited acids, such as citric acid, sulphamic acid, phosphoric acid, nitric acid, sulphuric acid and hydrochloric acid. For this purpose the use of hydrochloric acid is usually preferred especially due to its low cost, ease of handling and great capacity for dissolving many deposits and corrosion products. An inhibitor is usually added to the acid in order to counteract as far as possible the corrosive effect of these acids on the equipment to be cleaned. Examples of such inhibitors include hexamethylene tetra.- mine, alkaloids and quaternary ammonium bases.

In the practical application of said chemical cleaning methods, however, serious difliculties have been encountered. When said acids are used in fact corrosion occurs even in the presence of inhibitors-as a result of the formation of ferric ions originating from the iron-containing equipment to be cleaned and/or from the impurities containing iron compounds. This corrosion can even occur to such a large degree that after cleaning pitting and scouring are encountered in the metal of the cleaned equipment. This phenomenon occurs particularly with unalloyed steels and steels of low alloy, which are frequently used for example in steam boilers. The corrosion causes the formation of rough metal surfaces which in steam boilers lower the flow rate of the Water in the evaporator tubes, with the result that a reduction of the heat transfer and an increase in the wall temperatures in the installation may occur.

When combinations of a nobler and a less noble metal are present in the equipment to be cleaned, an intensified corrosive attack on the less noble metal is found to occur. It is indeed possible to reduce this intensified corrosion, for example by blanking off or removing the nobler metal of the metal combination from the apparatus before the cleaning operation, but it will be obvious that in practice this is at least inconvenient and often impracticable.

To this the following must be added. The corrosive attack appears to increase considerably when there is an increase in the flow rate of the cleaning acid. Thus, when the flow rate of the cleaning acid is raised, e.g. to above 0.5 m./sec., the corrosion of the metal to be cleaned increases greatly. This means that such flow rates will have to be avoided as much as possible in order to avoid damage to the installation which is to be cleaned. This, however, may not always be possible when the equipment to be cleaned is of a complicated design. In local constrictions within a tube system for instance, excessive rates offiow may easily occur, with the result that the corrosion at such a point can assume serious dimensions.

In view of the important advantages of chemical cleaning with respect to mechanical cleaning the first-named method is used on a large scale in practice in spite of the above-mentioned drawbacks. In order, however, to keep the degree of corrosive attack to some extent within acceptable limits it is often necessary to sluice the cleaning acid one or more times during the cleaning operation, and to replace it by fresh acid. This is an attempt to keep within certain limits the content of ferric ions in the cleaning acid which are responsible for the corrosion. Even when such a measure is taken the occurrence of corrosion is in practice often unavoidable. It thus appears that, when steel comes into contact with an inhibited hydrochloric acid bath which is in motion and contains ferric compounds, at a temperature as low as 20 C., 50% of the ferric ions are reduced within 2 hours to ferrous ions at the expense of the equivalent amount of steel, which goes into solution, or in other words, corrodes. At 50 C., a normal temperature for cleaning steam boilers, for example, this degree of corrosion is reached even within an hour. Since it may take some hours to fill a fairly sizable steam boiler with cleaning fluid by means of a circulation pump, strong corrosive attack has already taken place before it is possible to proceed to sluicing the cleaning acid containing ferric ions.

It can be seen, therefore, that serious corrosion problems are invariably encountered in practice in methods of chemical cleaning of equipment by means of a cleaning acid-even when cumbersome measures are taken, like the replacement of the cleaning acid by fresh acid.

According to the present invention it has now been found that the above drawbacks can be wholly avoided if a stannous salt, which is soluble in the cleaning acid, such as stannous chloride, is added to said inhibited cleaning acid and thereby inherently forms stannous upon dissolving. When a cleaning fluid of this type is used the corrosive attack--even at high rates of flow of the cleaning acid-is found to be wholly suppressed, while if an inhibitor is used any tin which happens to be precipitated does not impair the corrosion resistance of the metal to be cleaned either.

The invention therefore relates to a process for the cleaning of equipment consisting wholly or partially of metal which comprises treating the equipment with a cleaning fluid comprising an aqueous solution of an inhibited inorganic or organic acid which, in addition, contains a stannous salt soluble in this solution.

Stannous chloride is preferably used as stannous salt soluble in the cleaning acid and therefore the source of stannous ion and inhibited hydrochloric acid as cleaning acid. Depending on the quantity and the nature of the contaminations to be removed, concentrations of the hydrochloric acid of between 0.25% by weight and 15% by weight, appear to be very satisfactory. As to the quantity of the stannous chloride, in dependence on the concentration of ferric ions in the cleaning acid, and depending upon the redox potential as will be discussed hereinbelow, quantities of between 1 and 30 g. per litre of cleaning acid which therefore corresponds to a stannous ion concentration of between 0.53 and 15.8 grams per liter of against corrosion-can now be wholly omitted. The flow rate of the cleaning acid can be raised to high values without the corrosive attack exceeding acceptable limits. In order to gain the full profit from the present invention flow rates for the cleaning acid of at least 0.2 m./sec., and preferably of between 0.5 and 3 m./ sec. will be used, so that a rapid and effective cleaning can be obtained while avoiding a degree of corrosion of any substance.

The process according to the invention also provides protection against corrosive attack in equipment which contains combinations of noble and less noble metals, a protection which formerly could be only obtained by removing or blanking off the nobler metal concerned before the cleaning operation. It was thus established that by the use of the cleaning acid according to the invention the corrosive attack on the following metals or combinations of metals was wholly or substantially wholly suppressed.

Mild steel Carbon steel 50-60 1% Cr-Vz Mo-steel 5% Cr-Vz Mo-steel Copper Brass Aluminum-brass Tin-bronze C-upronickel -10 and 70-30 Inconel Nionel Nickel Monel Hastelloy 13 chromium steel 17 chromium steel Stainless steel, type 18% Cr-8 to 10% Ni The advantages of the present invention are evidenced very clearly in the cleaning of the frequently used steels which are unalloyed or alloyed with chromium and/or nickel and/or molybdenum, in particular steels alloyed with 15% of chromium and 04-06% of molybdenum. In particular steel with 1% of chromium and 0.5% of molybdenum displays the phenomenon that the hydrogen which forms during the corrosive attack diffuses in the metal and gives rise to hydrogen brittleness. High pressure steam pipes are often manufactured from this material, and the disadvantage is frequently experienced with the conventional chemical cleaning methods that the corrosive attack is accompanied by hydrogen brittleness in the cleaned material, with the result that the latter easily fractures due to its brittleness. This disadvantage also is wholly obviated by using the process according to the invention.

It is also to be noted that a special aspect of the present invention is the possibility of maintaining the composition of the cleaning acid, by a simple continuous electrometric control of the cleaning fluid, at such a level that throughout the entire cleaning process the certainty exists that no corrosion, or substantially no corrosion, will occur. Thus, when measured with a platinum electrode against a saturated calomel-KCI electrode, the redox potential in the presence of a corrosive quantity of ferric ions in inhibited hydrochloric acid of 10% concentration amounts to 200 to 400 mv., de ending upon the quantity of ferric ions; in the presence'of a quantitiy of stannous ions, su cient to counteract corrosion, the redox potential is below 280 mv. and in practice it is advisable to work below mv. By means of this electrometric control it is now possible to keep the re d 9gr p otemial @ianarrqi lmtsmrmmsai in to a P pweiiiifs iu by i itsihe. 2it ..in2t2.922 L of the l i l jil llt-QililliiOhQfiQ.Ifih q y at2iii i li 222 The temperature at which the cleaning operation is carried out is usually the normal ambient temperature. In the classical chemical cleaning process it was generally necessary to avoid as much as possible temperatures higher than the ambient temperature in view of the intensified corrosive attack at elevated temperatures. The cleaning fluid used according to the invention is characterized in that also at elevated temperatures there is no corrosive attack. The upper limit of the temperature to be used is only determined by the stability of the inhibitor in the inhibited acid, and this limit generally lies around 70 C. The present invention, therefore, now provides for the first time the possibility of carrying out the cleaning operation conveniently with a cleaning fluid at an elevated temperature, i.e. at a temperature of up to 70 C.

In a preferred embodiment of the present invention a cleaning fluid is used which in addition to an inhibited inorganic or organic acid and a soluble stannous salt, contains a complexing agent specific for ferric ions. It has been found that in that case the amount of stannous salt to be used can be considerably reduced; amounts of less than of the amount of stannous salt originally applied have been found to be suflicient for suppressing or substantially reducing the corrosive attack of the metal control of the redox potential as explained above and illustrated for the combination stannous chlorideammoniumbifluoride in Example IV.

EXAMPLE I The effect of the cleaning fluid to be used according to the invention can be seen from the data described in the following. The test results given in this example were obtained from tests which were carried out as follows.

Test plates from unalloyed steel (dimensions 60 x x 1.5 mm.), from which plates the grease had been removed and which had been scoured bright, were exposed in duplicate in beakers containing 0.5 l. of inhibited ironfree hydrochloric acid. As inhibitor use was made of a commercially available inhibitor based on quaternary ammonium bases in a concentration of 0.2% by weight.

The test plates were placed vertically on their long sides in small glass racks in the beakers. Beside each exposure in a stationary medium test plates were exposed in a moving medium having a rate of 0.2 1111. per second, which movement was obtained by means of an electrically powered glass stirrer. The plates were exposed for four periods, viz 8, 8, 8 and 24 hours, in the same fluid, unless otherwise stated. After each exposure the plates were weighed and examined for signs of pitting.

TABLE I [Medium: 10% HCl+inhibitor+10 g. of Fe+++ per litre; 20 0.]

Exposure Fluid stationary Rate of fluid 0.2 in. per sec.

Reduction v Reduction Period Hrs. of weight Remarks of weight Remarks 111 mg. in mg.

8 168 Surface slightly scoured, edges 1, 258 Distinct scouring particularly {(43 distinctly corroded. 1g; edges heavily corroded. 8 162 IIIII"'IIIIII 94 284 34 212 33 9 24 5.3% Corrosion visibly increased. 7 1, 482 Total over 48 742 1' 324 Surface 2. 3,685 mm. Surface a=3,685 mm. Surface b=3,595 mm. Surface b=3,685 mm.

TABLE II [Mediu.m: 10% HCH-inhibitor+20 g. of Fe per litre; 20 C.]

Exposure Fluid stationary Rate of fluid 0.2 m. per sec.

Mild Reduction Reduction Period Hrs. steel of weight Remarks of weight Remarks plate 1n mg. in mg.

618 Slight scouring, edges 2. 548 Rather heavy scouring of the surggg distinctly corroded. 2, face; edges heavily corroded. 429 173 360 50 353 50 510 Corr of the edges 61 332 intensified. 64 1, 920 2, 852 1, 769 514 Surface a=3,745 mm. Surface b=3,735 mm.

Surface a=3,745 mmJ Surface b=3,675 mm.

to be cleaned. This is more fully illustrated in Example IV.

The complexing agent should be specific for ferric ions in that it should not form a complex with the ferrous ions. This is essential for the purpose of the invention and another essential feature of the present embodiment is that an amount of the soluble stannous saltbe it a considerably reduced amount-should always be present.

Complexing agents which may be used include phosphoric acid or hydrofluoric acid or its salts of which ammoniumbifluoride '(NH HF has been found especially suitable. The relative amounts of stannous salt and complexing agent can easily be fixed at the desired level by When the inhibitor applied is replaced by other conventional inhibitors corresponding results were obtained, as also with the use of I-lCl in a 5% concentration.

From the above Tables I and II the following can be deduced:

(a) The corrosion increases by a factor of about 4 to 1 TABLE III tions possible, viz at high flow rates of the cleaning fluid which moreover has a temperature of 50 C.

EXAMPLE II Exposure tests of (a) rotating steel discs, mild steel (Insufliciency oi Sn++) (Very slight excess Sn++) Exposure Medium: Medium:

% E01 plus inhibitor+10 g. oi Fe=/l. 10% E01 plus inhibitor+10 g. Fe ll.

+6 g. of SnC1z.2H2O/l. g. of Sn0l .2H O/l. Rate 0.2 m./sec.; temp., 20 0. Rate 0.2 m./sec.; temp., 20 C.

Period Hrs. Plate Reduction 0! Remarks Reduction 0! Remarks weight, mg. weight, mg.

873 Distinct scouring, edges 70 870 slightly corroded. 7(5) 71 71 6 8 28 10 34 9 24 }Nc corrosion visible.

Surface a=3,520 mm. Suriace b=3,520 mm.=

(Excess Sn++) (Excess Sn++) Exposure Medium: Medium:

10% H01 plus 1nh1bitor+l0 g. of Fe ll. 10% 1101 plus inhibitor-H0 g. Fo /l.

g. of 81101:.21120/1. 50 g. of SIlCl2.2HzO/l. Rate 0.2 m./sec.; temp., 20 0. Rate 0.2 m./sec.; temp 2F 0.

Period Hrs. Plate Reduction 0! Remarks Reduction 0t Remarks weight, mg. weight, mg.

72 73 75 74 3 2 2 4 2 1 1 1 a }No corrosion visible g }No corrosion visible.

Surface a=3,620 mm. Surtace b=3,670 mm.

Surface a=3,700 mm. Surface b=3,580 mm.

first period of exposure.

It can also be seen from the above Table III that when with mill scale; (b) welded steel pipe, mild steel, inside diameter 19 mm., through which the fluid was pumped.

Medium: 10% HCl+l0 g. of Fe+++ per litre-l-inhibitor +small excess of SnCl .2H O.

Temperature: C. (:L2 C.) for all tests.

Exposure time: 15% hours.

TABLE IV Rate of fluid Ex sed Reduction Reduction Corrosion Material mild steel with respect to su ace in of weight of weight atter expressed in Remarks the metal, in mm. in mg. correction for mm./per

m/sec. scale in mg. annum 8. Disc with mill S0810 1- 1 10, 7 5 200 1- 3 swig entirely smooth and undam- 1 2. c5 10, 708 971 325 2.1 D6. Do i 3. 0 10, 708 958 312 2. 0 D b. Welded pipe 400 mm. in lengt 2. 3 654 2. 0 Do. b. Welded pipe 2,000 mm. in length 2 3 Do.

1 Average.

It is seen that under conditions which would lead to heavy corrosion with the known chemical cleaning methods, no or substantially no corrosion occurs.

EXAMPLE III This example illustrates the cleaning of metal combinatrons with the process of the invention. Thest plates of difierent metals were combined with test plates of mild steel and the metal combination was exposed at 20 C. during 16 hours to cleaning acids of the composition and under the conditions set out in the following Table V.

TABLE V H l 101%,h M230 +125 3' Fe' /l plus 10% HCI+10 g. Fe /l. plus inhibitor; temp.- 0.; rate 0.2 m./sec.

Red. in Red. in Metal combination weight Remarks weight Remarks in mg. in mg. Mild Steel 3% }Steel has slight copper scale 3 }Metal unchanged.

:2 Steel has no copper scale, but brass shows dark discolouration i Do. :1; }Steel has no copper scale, but Al-brass has a black scale g Do. ig }Steel has greenish scale and Crsteel shows dark discolouration; alter scouring g Do. 205 slight pitting visible. 2

}Stee has greenish scale which can be easily removed 1 Do.

It will be seen that also in the case of combinations of Exposure of mild steel test plates in ferrous and ferric ions a noble with a less noble metal which present serious corcontaining inhibited hydrochloric acid rosion .difiicultiFs in the known chemical cleaning Composition of the cleaning fluid at the beginning of the rosion is essentially suppressed when applying the process 20 test mm of the invention. 1 0 n He! 7 57 1 re 0, EXAMPLE IV Inhibitor 0.3%, This example illustrates the use of a complexing agent vAmmoniumbifluoride (NH HF 10 grams, specific for ferric ions as additive in the cleaning fluid Stannous chloride (SnCl .2H O), 1.0 gram. and the considerable saving of stannous salt obtained Temperature 50 C.; rate of fluid 0.2 m./sec.

Addition of Redox Time after start 01 test run SnCh-2Ha0 potential Remarks in grams in mv.

0 hr Dissolved 10.4 g. mill scale present on mild steel plate, 1 hr. 15 min. 230

7 mild steel plates placed in bath:

1 hr. 30 min 1. During the whole of the exposure the redox potential stayed in the range in which (in the presence of the inhibitor) no corrosive attack takes place.

2. The total consumption of SnCia-2Hz0 was 1.15 gram which is about 10% of the amount required in the absence of the ferric ion complexing agent.

thereby with full maintenance of the eificiency of the The average corrosive attack of the test plates is 0.24 cleaning method according to the present invention. mm. a year. None of the test plates showed pitting.

Influence of fluoride on the redox potential of inhibited I clam:

1. In a process for the cleaning of metallic equipment gg ggg g g zi g varymg rams of ferrous Sulphate constructed essentially of iron containing metal by means of a recirculating aqueous corrosion inhibited acidic clean (A) 1 litre HCl 7.5%, 0.3% inhibitor ing solution, the improvement which comprises the steps (B) 1 litre I-ICl 7.5%, 0.3% inhibitor, 30 gram ammomf umbifluoride (1) contacting the surface of the equipment to be cleaned with 'a circulating aqueous inhibited acidic cleaning solution comprising (a) from about 0.25 to Further additions as indicated. Both test runs at 50 C.; rate of fluid 0.2 m./ sec.

A. Without fluoride B. With fluoride 15 weight percent of an acid selected from the group consisting of citric, sulfamic, phosphoric, nitric, sulfuric and hydrochloric acids, and (b) stannous ion in a concentration of from 0.53 to 15.8 grams per liter, the stannous ion being derived from a soluble stannous salt, and

Totally added: 5 grams ferrous, 1.3 grams ferric, 0.22 gram SnCl .2H O, i.e. less than 10% of the amount required in the absence of fluoride to keep the fluid free from ferric ions.

The bath without fluoride is corrosive.

The bath with fluoride is non-corrosive.

(2) adding a soluble stannous salt to the recirculating cleaning solution in a quantity sufficient to maintain the redox potential of the solution below 280 mv. as measured by a platinum electrode against a saturated calomel-KCl electrode, whereby the ferric ion con tent of the solution is controlled.

2. The process of claim 1, wherein suflicient soluble stannous salt is added to maintain the redox potential below 170 mv.

3. The process of claim 1, wherein the acid is hydrochloric acid.

4. The process of claim 1, wherein the soluble stannous salt is stannous chloride and is present in a concentration of from 1 to 30 grams per liter of solution.

5. The process of claim 1, wherein the corrosion inhibitor is selected from the group consisting of hexamethylene tetramine, alkaloids, and quaternary ammonium bases and the inhibitor is present in an amount of from about 0.2 to 0.3 weight percent.

6. The process of claim 1, wherein the circulating cleaning solution has a rate of flow through the equipment of at least 0.2 meter per second.

7. The process of claim 1, wherein the circulating cleaning solution has a rate of flow through the equipment in the range of between 0.5 to 3 meters per second.

8. The process of claim 1, wherein the temperature of the cleaning solution is between ambient and 70 C.

9. The process of claim 1, wherein the equipment is constructed of a metal selected from the group consisting of steel and steel alloys with at least one of the alloyed metals selected from the group consisting of chromium, nickel and molybdenum.

10. The process of claim 9, wherein the equipment is constructed of a steel containing from about 1 to 5 percent chromium and from about 0.4 to 0.6 percent molybdenum.

11. In a process for the cleaning of metallic equipment constructed essentially of iron containing metal by means of a recirculating aqueous corrosion inhibited acidic cleaning solution, the improvement which comprises the steps of (1) contacting the surfaces of the equipment to be cleaned with a circulating aqueous corrosion inhibited acidic cleaning solution comprising (a) from about 0.25 to 15 weight percent of an acid selected from the group consisting of citric, sulfamic, phosphoric, nitric, sulfuric and hydrochloric acids, (b) stannous ion in a concentration of from 0.053 to 15.8 grams per liter, the stannous ion being derived from a soluble stannous salt, and (c) a complexing agent for ferric ions selected from the group consisting of hydrofluoric acid and ammonium bifluoride, said complexing agent being present in an amount soluble in the acidic solution; and

(2) adding soluble stannous salt to the recirculating cleaning solution in a quantity sufficient to maintain the redox potential of the solution below 280 mv. as measured by a platinum electrode against a saturated calomel-KCl electrode, whereby the ferric ion content of the solution is controlled.

12. The process of claim 11, wherein the ferric ion complexing agent is ammonium bifluoride.

13. The process of claim 11, wherein both the soluble stannous salt and the ferric ion complexing agent are added to the recirculating cleaning solution in an amount suflicient to maintain the redox potential below mv.

14. The process of claim 11, wherein the redox potential is maintained at a value below about 170 mv. by the addition of stannous chloride and the ferric ion complexing agent.

15. An aqueous based cleaning composition for cleaning equipment consisting essentially of iron-containing metal consisting essentially of water, 0.25 to 15 weight percent of an acid selected from the group consisting of citric, sulfamic, phosphoric, nitric, sulfuric, and hydrochloric, from 0.053 to 15.8 grams per liter of stannous ion derived from a soluble stannous salt, from about 0.2 to 0.3 weight percent of a corrosion inhibitor selected from the group consisting of hexamethylene tetramine, alkaloids, and quaternary ammonium bases, and a ferric ion complexing agent selected from the group consisting of hydrofluoric acid and ammonium bifluoride present in an amount soluble in the cleaning solution.

16. An aqueous based cleaning composition for cleaning equipment consisting essentially of iron-containing metal consisting essentially of water, 0.25 to 15 weight percent hydrochloric acid, from 0.1 to 30 grams per liter of stannous chloride, from about 0.2 to 0.3 weight percent of a corrosion inhibitor selected from the group consisting of hexamethylene tetramine; alkaloids, and quaternary ammoniumbass, and ammoninum bifiuoride as a ferric complexing agent in a concentration of from about 10 grams to 30 grams per liter.

References Cited UNITED STATES PATENTS 1,460,395 7/1923 Vogel 252-148 1,678,776 7/ 1928 Gravell et al 252-147 1,773,247 8/1930 Williams 252-147 2,927,871 3/1960 Mancke et a1. 156-19 3,033,795 5/1962 Brevik 252-146 LEON D. ROSDOL, Primary Examiner.

W. SCHULZ, Assistant Examiner.

US. Cl. X.R.