US2297670A - Method of protecting boilers and the like against embrittlement - Google Patents

Description

. following description.

Patented Sept. 29, 1942 I FYFICE METHOD OF PROTECTING BOILERS AND THE LIKE AGAINST EMBRITTLEMENT Wilburn C. Schroeder, Hyattsville, and Abraham A. Berk, Berwyn, Md.. assignors to the Government of the United States of America, as represented by the Secretary of the Interior and his successors in oflice N Drawing. Application October 31, 1938, Serial No. 237,982

14 Claims. (Cl. 210-23) (Granted under the act of March 3, 1883-, as

amended April 30, 1928; 370 O. G. 757) The invention describedgherein may-be manufactured and used by or for the Government of the United States for governmental purposes only without the payment of any royalty thereon.

This invention relates to a method of protecting boilers and the like against embrittlement, and more particularly to a method of treating waters or'solutions to be used in boilers and the like to prevent the embrittlement of the steel or other metal with which such waters or solution are in contact during the operation of the-desteel, iron or other metal of which the boiler is made.

Other objects and advantages of the invention will become apparent during the course of the As is well known, .embrittlement is the term applied to the intercrystalline cracking of boiler metal resulting from the deleterious action of boiler water thereon during the operation of the boiler. It has heretofore been believed that this type of failure in boiler metal was due primarily or exclusively to the action of sodium hydroxide solutions. However, we have discovered not only that solutions not containing sodium hydroxide will cause embrittlement, but also that the extent of embrittlement caused by boiler waters containing sodium hydroxide is not alone due to the presence of the sodium hydroxide, but is also affected by the presence of other compounds in -the solution. For example, our experimental work has demonstrated that the embrittling action of a solution containing sodium hydroxide may be greatly accelerated by the presence of very small amounts of sodium silicate.

Boiler waters normally contain a few hundred parts per million of sodium hydroxide and a smaller amount of sodium silicate. Since such dilute alkaline solutions will not cause embrittlement (a concentration of from 100,000 to 500,000

' parts per million of sodium hydroxide and a few hundred parts per million of sodium silicate being necessary-to cause intercrystalline cracking) it is generally assumed that the boiler water attains the high caustic concentration necessary to cause intercrystalline cracking as a result of evaporation in small spaces, as in riveted'seams or rolled-in tube ends. Our work has demonstrated that such concentration can take place in this manner. ,For example, if the dilute boiler water leaks or diffuses very slowly through a restriction toward a region of lower or atmospheric pressure, the water tends to evaporate, both because of its superheat and because of the heat supplied from the boiler metal, leaving a concentrated alkaline solution in the region of the restriction. Our experiments have shown that this process can concentrate boiler water of very low caustic concentration to more than 500,000 parts per million of sodium hydroxide.

While sodium hydroxide-sodium silicate solutions probably are most frequently responsible for intercrystalline cracking of boiler metal, we have discovered that this cracking may result from solutions of other compounds. For example, our laboratory experiments have demonstrated that concentrated sodium hydroxide solutions at temperatures below 200 or 250 0., containing even very small amounts of oxidizing agents such as sodium nitrate, potassium permanganate, sodium chromate, or salts of metals more noble than iron, such as salts of lead, copper, tin, nickel, cobalt,,arsenic and antimony, will cause embrittlement or intercrystalline cracking in stressed steel. Thus, it appears that the presence in boiler water of soluble compounds of metals such as lead, copper and the like, together with caustic soda, may play an important role in theembrittlement of boiler metal.

Moreover, our experimental work has demonstrated that it is not always necessary to have sodium hydroxide present in boiler water to produce embrittlement cracks. For example, a concentrated sodium nitrate soluition containing a small amount of a manganous salt will also produce this type of failure.

Although, under certain conditions, a water solution containing only one component may produce cracking, depending upon an adjustment of temperature, concentration, and metal conditions to provide a solution that will attackthe grain boundaries of the metal more rapidly than the crystal faces of the metal, our investigation has demonstrated that embrittlement or intercrystalline cracking is ordinarily produced by a water solution containing two components, one that will corrode the grain boundaries and one that will tend to protect the crystal faces. For example, with sodium hydroxide-sodium silicate solutions, the silica deposits a partially protective film over the crystal faces, leaving the grain boundaries relatively unprotected. In this case, the sodium silicate is the component tending to protect the crystal faces. However, the grain boundaries which are left relatively unprotected by the action of the sodium hydroxide to form a notch in the metal. If the material is under internal stress from cold work or under external applied stress, this notch widens and the entrance of fresh solution causes further penetration in the metal. When the notch is deep enough, the corrosion products exert bursting forces for the reason that they occupy more volume than the initial intercrystalline material, this action being analogous to the bursting forces produced by the expansion of water as it freezes in a deep crack in a stone. The entire process causes the corroding solution to penetrate rapidly into or between the crystal boundaries, resulting in the cracking of the metal.

Inasmuch as intercrystalline cracking can be produced by any solution that tends to corrode the grain boundaries more rapidly than the crystal faces, it is apparent that methods for preventing this style of failure are important, not only in boiler operation, but also in any case where a corrosive solution comes in contact with a metal surface. For example, digesters, autoclaves, pickling tanks, refrigerating and other metal equipment may be subject to this intercrystalline cracking. In certain cases, it apparently is necessary for the solution to concentrate in restricted spaces before it can cause appreciable damage. However, in other cases, for example, pickling tanks with their acid pickling baths, or digesters with concentrated alkali, the cracking may result from the action of the main body of the solution itself.

Certain methods have heretofore been suggested for the purpose of eliminating intercrystalline cracking of metal, as in steam boilers. One method that has been extensively used, and is recommended by the Boiler Code Committee of the American Society of Mechanical Engineers, depends on the maintenance in boiler water of certain ratios between concentrations of sodium sulfate and total alkalinity expressed in terms of sodium carbonate. In accordance with this method, the ratio of sodium sulfate to total alkalinity should vary as follows: at boiler pressures up to 150 pounds, the ratio should be 1:1; at 150-250 pounds pressure, 2:1, and at 250 pounds and over, 3:1.

While the above referred to method has been extensively used, we have discovered that intercrystalline cracking is not satisfactorily prevented by the use of sodium sulfate either in the concentrations recommended or in higher or lower concentrations. For example, we have found that sodium sulfate will not stop cracking silica film are attacked by the corroding' 25 intercrystalline cracking -impurities that may cause foaming or boiler water, it

ates and acetates, have also been suggested as embrittlement preventatlves or inhibitors. However, these are generally less satisfactory than sodium sulfate.

Because of the commercial importance of the problem of intercrystalline cracking, and the inadequacy of the methods heretofore suggested for combatting this type of failure in boiler metal and the like, we have conducted extensive experiments for the purpose of providing substantially improved methods of preventing embrittlement. As a result of our research, we have discovered certain materials that, when introduced into boiler water, are far more effective in preventing intercrystalline cracking than any heretofore employed or suggested for this purpose. The more important materials and methods for protecting metal against embrittlement are set forth below.

We have discovered that the addition to boiler water of lignin, lignin-containing materials, or decomposition, polymerization, or other derivatives thereof, either produced by thermal, chemical or mechanical means, will effectively prevent of the metal of boilers and the like. Such agents will prevent embrittlement at all caustic and silica concentrations, and regardless of the stress or change in stress in the metal.

Lignin material for use in prevent ng intercrystalline cracking may be obtained by conventional extraction methods from wood, corn stalks, or any other vegetable matter containing lignin. For example, waste liquors obtained from the digestion of wood during the manufacture of paper are particularly satisfractory for use as embrittlement preventatives. For example, the waste liquors from either the sulfite, soda or sulfate digestion methods of pulping wood may be employed, waste sulfite liquors bein especially useful since they generally are free from undesirable priming.

These waste liquors may be used in the dilute T form in which they are discharged from the pulp digesters, or they may be evaporated to a more concentrated solution, or even evaporated to dry powders.

While extracts of lignin from wood or other vegetable matter, such as the waste liquors referred to, are particularly suitable, it is not necessary to extract the lignin from the wood or other vegetable matter in order to secure the desired results. For example,'the vegetable matteritself may be dissolved in conventional solvents and the resulting solution dissolved in the boiler water to be treated.

Inasmuch as it appears from our experiments that any compound or derivative product of lignin will serve as an embrittlement preventative in that the lignin' is unnecessary material used be or contain pure lignin, although, if desired, the lignin material may be purified in any conventional manner prior to use for the intended purpose.

As stated above, we have discovered during our work that decomposition products obtainable from lignin or from lignin-containing vegetable matter are also effective in preventing embrittlement. For example, we have found that eugenol, vanillin, pyrogallol, guaiacol and their derivatives, such as products resulting from the decomposition thereof in water at boiler temperatures, will all prevent cracking.

Where lignin material of the character referred to above is to be employed in a boiler or the like in a case where the stress in the boiler metal 65 varies at the operating temperature of the boiler. Moreover, if the sodium hydroxide concentration reaches 400,000 parts per million, or is higher, the presence of sodium sulfate in the boiler water will not stop failure. Since it has been shown that this concentration may be reached or exceeded in riveted seams or rolled-in tube ends, it will be apparent that the use of sodium sulfate may be inadequate in preventing embrittlement.

Other chemicals, such as phosphates, carbonthat the effectiveness of the. lignin material in' preventing embrittlement can be increased by subjecting the material to a preliminary preheating or autoclaving at a higher temperature. In preferred practice, this pretreatment is conducted in an alkaline or acid medium. For example, the best results are secured by autoclaving the lignin material in an alkaline or acid solution at a temperature between 225 and 350 C., for several hours, say over night. We 'have found that such treatment makes the lignin material much more effective in preventing embrittlement at lower boiler temperatures, say at a boiler tem"- perature of about 150 C.

We have found that the addition of even very small amounts of lignin material tends to prevent embrittlement. the, treatment as positive as possible, we recom mend that the amount of lignin material maintained in the boiler water should be equivalent to at least 3% of the sodium hydroxide and should be greater at the higher boiler temperatures. In preferred practice, concentrations of lignin material in the boiler water .should be equal to, and preferably greater than, those shown in. the following table:

Minimum ratio by weight of lignin material to caus- Boiler temperature tic soda Linear interpolations may be used to determine desirable concentrations of lignin material at intermediate temperatures. Lower concentrations than those indicated in the above table will produce some beneficial results but may not prevent all cracking, and substantially higher concentrations, for example, concentrations up to equal parts of lignin and caustic soda, and in some cases even a higher concentration of lignin, may be employed without harm if desired.

An additional advantage of the use of the lignin material in boiler water is that it aids materially in the prevention of the adhering of scale to the boiler. For example, it has been found that when calcium carbonate, calcium phosphate, calcium sulfate, silica or other scale-forming constituents precipitate from boiler water containing lignin material, they carry down and become covered with a layer of the lignin material. This prevents the particles from banding together to form scale, thereby greatly aiding in the prevention of adherent boiler scales.

The further advantage of the use of lignin material in boiler water is that it aids in preventing However, in order to make.

corrosion. At boiler temperatures in either di- 7 lute or concentrated alkaline solutions the lignin material will adsorb, absorb 0r react with oxygen. This removes the oxygen from the boiler water and aids in preventing corrosion. Moreover, we have found that the lignin material also forms a protective film over the metal surfaces of the boiler, thereby further aiding in preventing corrosion.

In addition to th use of lignin and derivatives of lignin or lignin-containing vegetable matter, such as wood, as referred to above, we have discovered that the phlobatannins serve as embrittlement preventatives or inhibitors in boiler water, even when employed in low concentrations. Moreover, materials containing phlobatannins perform the same function. For example, quebracho and cutch (catechu) are very effective when employed in boiler water to prevent embrittlement.

While, as stated above, even low concentrations of the phlobatannins willhave a beneficial effeet in protecting boiler water against embrittlement, we have found that for best protection, the phlobatannin material, such as quebracho or cutch, should be maintained in the boiler water in concentrations equivalent to about 5% of the sodium hydroxide. Larger amounts do no harm. Indeed, where higher boiler temperatures are employed, larger amounts of these materials should be dissolved in the boiler water. The following table indicates the minimum ratios of the phlobatannins to caustic soda preferably employed in boilers operating at different temperatures:

Minimum ratio by 1 weight of phloha tannin material to caustic soda Boiler temperature trations than those indicated in the above table,

for example, concentrations up to equal parts of .phlobatannin material and caustic soda, and in some cases even a higher concentration of phlobatannin material, may be used if desired.

An important advantage resulting from the use of quebracho and cutch for the. prevention of embrittlement is that these materials also serve effectively in the prevention of adherent scales and deposits. t

As pointed out above, particularly in the several tables set forth, best protection against embrittlement is obtained by usingthe organic materials specified in certain minimum ratios to the caustic soda present in the boiler water treated. Naturally, in order to secure the maximum benelit and security from the'use of these organic materials in the boiler water, it is necessary to have a convenient method for measuring their concentration. For this purpose we have employed: (1) the tyrosin reagent. (phosphotungstic and phosphomolybdic acids in phosphoric acid), and (2) the uric acid reagent (phosphotungstic acid in phosphoric acid). Both reagents react In addition to the organic materials referred to above as embrittlement preventatives or in hibitors, we have found that the depside tannins, such as oak tannins and chestnut tannins, will serve with a measure of success to prevent embrittlement when used in concentrations of at least parts of the depside tannin to 100 parts of the caustic soda present in the boiler water. This minimum concentration will serve for treating the water to be used in boilers operating at low boiler temperatures, but larger amounts should be used where higher temperatures are encountered. The following table may be employed in determining the amount of depside tannin to be employed:

Boiler temperature 10 to 100 10 to 100 13 to 100 to 100 Linear interpolations may be used to determine concentrations at intermediate temperatures. While higher concentrations than those indicated in the table, for example, concentrations up to equal parts of depside tannin and caustic soda. and in some cases even a higher concentration of depside tannin, may be employed without harm, lower concentrations will not prevent cracking.

As pointed out above, intercrystalline cracking may be caused by sodium hydroxide solutions containing compounds other than sodium silicate. For example, the presence of even small amounts of oxidizing agents, such as sodium nitrate or sodium chromate, will cause rapid cracking, especially if the alkaline solution is boiled or heated under low or atmospheric pressure. The amount of oxidizing agent required to cause this cracking is small. For example, in the case of sodium nitrate or sodium chromate in a caustic soda solution, cracking will result when the ratio of either of these compounds to sodium hydroxide is as low as 0.4 to 100. A wide variety of other oxidizing compounds in about the same ratio will also cause cracking.

Contrary to what might be expected, we have discovered. that higher ratios of these oxidizing compounds to the sodium hydroxide present in the boiler water will actually prevent intercrystalline cracking. We have discovered that by maintaining a concentration of an oxidizing agent in the boiler water equal to 2 or more parts of the oxidizing .agent to every 100 parts of sodium hydroxide, embrittlement may be prevented. Thus, oxidizing agents, such as soluble chromates and nitrates, notably sodium or potassium chromate 0r nitrate, are effective embrittlement preventatives or inhibitors when used in amounts equal to or greater than 2 parts for every 100 parts of caustic soda present in the boiler water. Greater amounts of the oxidizing agents may be employed if desired. In using soluble chromium compounds as embrittlernent inhibitors, these compounds form a chromium-containing film over the metal which is stable and will exist for many days. Accordingly, these compounds can be used for the treatment of boiler water by periodic rather than continuous introduction into the feedwater. This is highly advantageous since the cost of chromiumcontaining compounds would make the process quite expensive if continuous introduction of these compounds into boiler water were required.

We have discovered that protection against embrittlement can be secured by maintaining once each month for a period of 24 hours a sodium chromate, or equivalent chromium, concentration in the boiler water equal to 30 parts per million. Treatment for more than 24 hours, or more than once a month, or with more than 30 parts per million of a soluble chromate, is not harmful, and may be beneficial where badly embrittling waters are present. If desired, this treatment may be coupled with the continuous maintenance of a few parts per million of a soluble chromate in the boiler water to repair such imperfections in the protective film as may develop during the interval between the periodic treatments. If this procedure is employed, the amount of chromate maintained in the water should be equal to or greater than 2 parts for every 100 parts of sodium hydroxide present.

Lower concentrations of soluble chromates, or longer intervals between treatments, may be used with some beneficial results, but, in such case, the results are not as satisfactory as the results obtained following the procedure recommended above.

In addition to the compounds and materials referred to above as embrittlement inhibitors, we have discovered that sodium sulfite, when used in sufficient concentrations, will also serve to prevent embrittlement. This is also true of other soluble sulfites, such as potassium sulflte. Where a soluble sulfite is employed, it should, in any case, be used at concentrations greater than 10 parts per million and where the boiler water contains appreciable amount of sodium hydroxide, the soluble sulfite should be maintained in the boiler water in concentrations equal to or greater than 1% of. the sodium hydroxide. Higher concentrations are not harmful.

We have also discovered that compounds containing zinc will prevent embrittlement, and these compounds are particularly useful at boiler temperatures above 200 C. Any soluble zinc compound, such as sodium or potassium zincate may be employed, but the zinc concentration in the boiler must be maintained equivalent to or greater than approximately 3%, and preferably 5%, of the sodium hydroxide present. Higher concentrations are not harmful and, indeed, are desirable at boiler temperatures above 300 C.

In the use of the various materials and compounds described above' for preventing embrittlement or intercrystalline cracking, the materials or compounds may advantageously be introduced continuously into 'the feedwater. Of course, in connection with soluble chromates, as described above, periodic treatments may suflice,

but with the other materials continuous treatment is recommended for best results.

During our extensive work on the subject, we have discovered that intercrystalline cracking results from electrolytic action between the grain boundaries and the crystal faces. Iron tends to go into solution at the grain boundary and hydrogen or other positive ion deposits at the crystal face.v While this electrolytic action may be effectively controlled by the use of chemical inhibitors in the boiler water, we have discovered that the intercrystalline cracking may also be prevented without the use of chemical inhibitors. In particular, we have found that it is possible to prevent the loss of iron from the grain boundaries to the solution by making the metal of the boiler stated, the metal of the boiler is made the-negal tive pole of the electric current and the electrode arranged in each drum of the boiler is made the positive pole. The electrodes should be selected to provide for as uniform a current distribution as possible. This current need only be a few milliamperes. A larger current creates no difliculty so long as excessive amounts of hydrogen gas are not released, i. e. amounts of hydrogen which might produce an explosive mixture with the oxygen in the steam.

Where the term parts is used herein, parts by weight are meant.

It is to be understood that the term boiler as used herein is employed in a generic sense to include all devices wherein water is heated to its boiling point, such as steam boilers, digesters, autoclaves and the like. It is also to be understood that in the temperature-concentration tables appearing in the subjoined claims, linear interpolation may be used to determine the concentrations for intermediate temperatures.

In the subjoin dfizlaims, the term (lignin material is employe in a broadsenseto embrace not only lignin itself, either in its crude or purifiedcondition, but also any material, solution, compound or extract containing lignin, or

any decomposition, polymerization or other derivative product thereof, whether produced by thermal, mechanical or chemical means. Also, where reference is made to maintaining any such lignin material in solution in boiler water, it is to be understood that the material may be present either in the form of a true solution, a. hydrotropic solution, a colloidal solution, or an emulsion.

While We have described in detail several preferred embodiments of our'invention, it is to be understood that the details of procedure may be variously--modified without departing from the spirit of the invention or the scope of the subjoined claims.

We claim:

1. The .process of protecting boiler metal against embrittlement which? comprises introducing a phlobatannin into the boiler water in an amount adequate fo1""retarding the embrittlement producing tendency of other substances therein. 0

.2. The process of protecting boiler metal against embrittlement by boiler water containing caustic soda which comprises maintaining concentrations of cutch in the boiler water available for retarding the embrittlement producing tendency of the caustic soda in accordance with the following table:

Minimum ratio by w ight of cutch to caustic soda Boiler temperature 3. The process of protecting boiler metal against embrittlement by boiler water containing caustic soda which comprises maintaining.

Minimurn ratio by weight of quebracho to caustic soda Boiler temperature 4. The process of protecting boiler metal against embrittlement by boiler water containing caustic soda which comprises maintaining concentrations of depside tannins in the boiler water available for retarding the embrittlement producing tendency of'the caustic soda in accordance with the following table:

Boiler temperature- '5. The process of protecting boiler metal against embrittlement by boiler water contain ing caustic soda, which comprises adding and maintaining in solution in the boiler water a concentration of lignin approximately 3% by weight of the caustic soda present in the boiler water.

6. The process of protecting boiler 'metal against embrittlement by boiler water containing caustic soda, which comprises adding and maintaining in solution in the boiler water a concentration of lignin at least 3% by weight of the caustic soda present in the boiler water.

7. The process of protecting boiler metal against embrittlement by boiler water containing caustic sodawhich comprises adding and maintaining concentrations of ligning in solution in the boiler water in accordance with the following table:

Boiler temperature 3t0100 3150100 4to 8to 100 8. The process of protecting boiler metal,

against embrittlement, which comprises adding to the boiler water waste liquor from the digestion of wood in wood pulp manufacture in an amount adequate to maintain in solution in the boiler water a concentration of lignin at least 3% of the caustic soda present in the boiler water.

9. In a process of reducing the embrittling action of boiler water for use in boilers operated at boiler temperatures not substantially exceeding 225 C., the improvement which comprises heating lignin material at a temperature between approximately 225 and 350 C., and introducing the resulting product into the boiler in an amount adequate to maintain in solution in the boiler water a concentration of'lignin at least 3% by weight of the caustic soda present.

10. The process of protecting boiler metal against embrittlement which comprises adding and maintaining in solution in the boiler water decomposition products of lignin-containing material selected from the group consisting of eugenol, vanillin, guaiacol, and their derivatives in an amount adequate for retarding the embrittlement producing tendency of other substances therein.

11. The process 0! protecting boiler metal against embrittlement which comprises adding and maintaining in solution in the boiler water decomposition products of tannin-containing material selected from the group consisting of pyrogallol and its derivatives in an amount adequate for retarding the embrittlement-producing tendency of other substances therein.

12. The process of protecting boiler metal against embrittlement cracking, which comprises,

adding and maintaining available in solution in the boiler water at temperaturesabove 200 C. zinc in an amount exceeding that which is useful for treating the boiler water for prevention of scaling and pitting by an amount adequate for retarding the embrittlement-producing tendency oi! substances in the boiler water.

13. The process of protecting boiler metal against embrittlement cracking by boiler water containing caustic soda, which comprises adding and maintaining available in solution in the boil er water for retarding the embrittlement-producing tendency of other substances therein a concentration of zinc at least as great as three parts by weight to 100 parts by weight of the caustic soda present.

14. The process of protecting boiler metal against embrittlement by boiler water containing caustic soda, which comprises adding and maintaining available in solution in the boiler water for retarding the embrittlement-producing tendency of the caustic soda a concentration of an oxidizing agent, selected from the group consisting of soluble nitrates, in a concentration at least as great as 2 parts by weight of the oxidizing agent to 100 parts by weight of the caustic soda.

WILBURN C. SCHROEDER. ABRAHAM A. BERK.