US3718499A - Process for protecting ferrous base metals against corrosion,and articles produced thereby - Google Patents

Abstract

IRON, STEEL AND OTHER FERROUS BASE METAL ARTICLES ARE PROTECTED AGAINST CORROSION, SUCH AS WATER AND STEAM CORROSION, BY A PROCESS IN WHICH SUCH ARTICLES ARE TREATED WITH A PYROCHLORE-MICROLITE MINERAL, CONTAINING GROUP V METAL CONSTITUENTS, IN AN ALKALINE WATER MEDIUM.

Description

nited States Patent Oflice 3,718,499 Patented Feb. 27, 1973 US. Cl. 117-127 13 Claims ABSTRACT OF THE DISCLOSURE Iron, steel and other ferrous base metal articles are protected against corrosion, such as water and steam corrosion, by a process in which such articles are treated with a pyrochlore-microlite mineral, containing Group V metal constituents, in an alkaline water medium.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a process for protecting ferrous base metal articles, such as iron, steel, and ferrous base alloys, against corrosion, and more specifically to a process for protecting against corrosion that occurs in the presence of hot water and steam at temperatures and/or pressures exceeding ambient conditions. The invention also relates to the corrosion resistant ferrous base metal article prepared by the process.

During recent years, much attention has been directed to the problem of developing corrosion resistant ferrous base metals. The need for metals that will withstand hot water and steam corrosion has become highly important in the power industry, and especially in the nuclear power generators.

The corrosion problems have become so difiicult that resort has been made to costly stainless steel constructions in order to attempt to fabricate power systems that will withstand the ravages of the boiling water and that can be used in pressurized water reactors employing temperatures up to about 650 F. Nevertheless, there have been many difliculties, and numerous costly incidents of corrosion, resulting from stainless steel deterioration in this kind of service. For example, stainless steel has produced problems of stress corrosion in areas sensitized by heat, such as the welded joints.

Corrosion in the power plants has been identified in some instances with the oxygenation of the boiler water upon cooling and then use of the oxygenated water at elevated temperatures. Removal of the oxygen has not provided, however, a complete solution to all the corrosion problems.

One difficulty encountered in the pressurized water reactors is known as the crud problem which is caused by the formation of iron hydroxide on unprotected steel. The iron hydroxide or crud has a tendency to create many problems, for example, to foul the turbines in such systems. Crud formed in the reactor system tends to carry over into and form detrimental deposits on the generator blades. It has been observed that the deposit of crud causes the formation of concentration cells on ferrous metal structural members which, in turn, leads to the creation of pit cavities on such members, and may cause eventually structural failures.

It is well known that silicate coatings, typically used to protect ferrous base metals are adequately insoluble in water, only up to about boiling temperatures. At temperatures substantially above 212 F., however, none of the conventional coatings has been found suitable for industrial use.

Existing techniques for controlling corrosion in boilers involve the introduction of alkaline substances or bases, such as morpholine, hydrazine, and ammonia, into boiler water; and although such treatments reduce some of the corrosion, these alkaline treatments have nonetheless proved unsuccessful in preventing the severe corrosion problems, including the structural failures currently plaguing the power generating industry.

SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide a surface barrier for ferrous base metal articles against corrosion, and, specially, for example, against water and steam corrosion.

It is another object of the present invention to protect ferrous base metal articles against corrosion from water and steam at temperatures above 212 F., such as are commonly encountered in the boiling water and steam reactors used in the power industry.

It is another object of the present invention to pro- ;vide a water insoluble coating for protecting ferrous base metal articles.

It is still another object of the present inventon to provide a coating for ferrous base metal articles that reduces the formation of iron hydroxide in boiling water and steam reactors.

'It is still a further object of the present invention to provide a surface barrier for ferrous base metal articles sufficient to reduce instances of corrosion.

It is still yet a further object of the present invention to provide ferrous base metal articles which are suitable for service under corrosion conditions.

According to this invention, ferrous base metal articles are protected against corrosion by treating such articles with a quantity of p'yrochlore-microlite mineral, containing Group V metal constituents, in a liquid medium.

In addition, it has been discovered that a fraction of the pyrochlore-Inineral in a liquid medium beneficiates ferrous base metal articles, especially when water serves as the liquid medium.

DESCRIPTION OF THE PREFERRED EMBODIMENT The ferrous base metal articles that are to be protected against corrosion, may be constructed from iron, steel, or ferrous base alloys. The articles may he formed of carbon steels (having a carbon content from about 0.10% by weight to about 0.40% by weight), and particularly the Weldable carbon steels, because of their ready availability, comparatively advantageous economic position in the industry, and their otherwise convenient maintainability. The articles may be formed of the stainless steels, such as the 300 series, that are presently being widely adopted in the nuclear power generators, and the invention is especially applicable to the sensitized parts that are ordinarily most susceptible to corrosion. The articles may be formed of the cast irons, such as gray iron, malleable iron, or the ductile iron. The articles most desirably are resistant to corrosion under basic conditions in pH ranges as high as 12. Other metals having corrosion resistance in that basic pH range may be used in some circumstances, although the ferrous base metals are the most economical for most purposes.

The surface of the ferrous base metal articles to be protected sometimes has an adherent oxide layer, usually of the sub-oxides, such as FeO and Fe O Such oxide films typically occur by mere exposure of the metal article to air at temperatures below the point at which the oxidation layer matures.

Although formation of the adherent oxide layer is not required in practice of this invention, it has been observed that typically such oxide layers form, if for no other reason, than by the inherent oxidation produced by exposure of ferrous base metals to dry air or water at ambient temperatures.

It is a discovery of the present invention that ferrous base metal articles, ordinarily after formation of the adherent oxide layer on the surface thereof, are provided with improved resistance to corrosion by teatment with at least a fraction of pyrochlore-microlite mineral in a liquid medium, for example, water. The pH of the said mineral in the liquid medium is desirably maintained in the range of above about 6, and usually above 7. The pH at the upper end of the range ordinarily does not exceed about 12, and is usually maintained around 10.

In particular, when a pyrochlore-microlite mineral is admixed With water, a fraction of the mineral is solubilized and has the effect of raising the pH of the water. It has been observed that basic pH conditions contribute to the corrosion reducing effect of the invention. In addition, the fraction of the pyrochlore-microlite mineral which is not rendered soluble in water, tends to form a cloudy colloidallike suspension or dispersion in water which combines with the basic properties of the mixture to improve the resistance to corrosion.

Heretofore only 1 to 2% by weight of the pyrochloremicrolite mineral has been thought to be soluble in water. However, by increasing the surface area of the mineral, such as by comminuting or ball-milling, the solubility may be increased. It is believed that the sodium constituent which is exposed on the surface of the mineral to water assists in the solubilizing and/or dispersion of the Group V metal constituents, such as the columbium metal constituent. It is preferred to have the effective portion of the mineral in a particle size that will pass a 400 mesh screen, and for some purposes in a particle size classified by levigation in appropriate fluids.

Pyrochlore-microlite minerals are well known columbium and tantalum containing minerals, classed as multiple oxides and made up of a mixture of oxides of more than one metallic element. One suggested chemical composition of those oxide minerals containing columbium or tantalum as a major constituent can be expressed by the general chemical formula:

in which A contains Na and one or more additional elements, selected from the group consisting of U, Ca, Th, Fe Mn, Zr, K, Mg, Ce, Di, Er, Y, and La; and in which E contains at least one Group V metal selected from the class consisting of Nb and Ta, and at least one additional element selected from the group consisting of Ti, Sn, W, Zr, and Fe The ratio of m to n is between 1:1 and 1:2.

It should be understood that the pyrochlore-microlite minerals are made up of an entire series of minerals. Pyrochlore, on the other hand, is the columbium rich end member of the pyrochlore-microlite minerals and typically occurs associated with alkalic rocks, in pegmatites, nepheline syenite, various alkalic dike rocks, carbonatites associated with alkalic intrusions, extrusive alkalic rocks, greiesen and in decomposition products of these rocks. In fact, the alkaline character of the pyrochlore containing minerals is believed to account for the rapid increase in pH that is observed when pyrochlore is placed in a liquid medium such as water.

Microlite, on the other hand, is the tantalum rich end member of the pyrochlore-microlite minerals. Intermediate members of the pyrochlore-microlite minerals include pyrrhite, koppite, hatchettolite, chalcolamprite, endeiolite, marigacite, ellsworthite, neotantalite, and metasimpsonite, One particularly useful ore, which is known to contain pyrochlore-microlite minerals, is araxa ore.

A typical analysis of an effective fraction of pyrochlore, reciting the principal metal constituents of the mineral in the form of oxides, is as follows:

4 TABLE 1 Compound: Percent by weight Nb O 58 C210 14.5

NaO 4 FeO 5 SiO 0.5 TiO 4 Rare earth metal oxides 4.5

Pb0 6 ThO 1.5

Other fractions of pyrochlore may be effective, and one or more of the Group V metal constituents, in combination with one or more other of the constituents in the pyrochlore, as for example the alkali or alkaline earth metal constituents such as the sodium constituents, are in part responsible for the corrosion resistant effect on the ferrous base metal surfaces treated by the process of this invention.

It should be understood that the treatment of ferrous base metal articles, with pyrochlore-microlite minerals in a liquid medium, such as water, inherently involves the interaction of a number of Group V metals or metal oxides and rare earth, alkali metal, or alkaline earth metal oxides, at a pH in the range from about 6 to 14. It may be desirable under some conditions to utilize other minerals containing Group V metals or metal oxides and combining them with various other metals or metal oxides and introducing the mixture into a liquid medium under basic conditions. Also, under certain conditions it may be desirable to compound the effective constituents of pyrochloremicrolite mineral ore or the pyrochlore concentrate of the ore from readily available sources of the individual ore components.

Other alkali columbium and/or tantalum containing mineral ores which have the ability to increase the pH of water when dispersed therein may be useful under some circumstances or in the practice of this invention.

It is believed that the active corrosion inhibiting agents in the pyrochlore-microlite mineral ore are the metal salts of niobium and tantalum acids, especially the alkali and alkaline salts, and particularly the alkali metal salts of such acids, for example, the alkali metal niobates and tantalates, such as NaNbO KNbO Na Ta O NaTaO The niobic and tantalic acid radicals are known for their tendency to form complex salts and their insolubility in water. It is believed radicals of niobium and/or tantalum acids, for example, the radicals of the niobic and tantalic acids, react with the surface of the ferrous base metal article to form a complex salt therewith that is insoluble in water and thereby provides a surface barrier against Water and steam corrosion.

In one method of practicing this invention, a pyrochlore-microlite mineral, or the pyrochlore concentrate alone, is first introduced in a liquid carrier, the latter of which is preferably an electrolyte, such as water, for contact with the metal surface to be protected. The exact mechanism by which the ore is carried by the liquid medium to the metal surface is not known at this time, and is not necessary for the practice of this invention. It is believed that an electrolytic system is created in the liquid medium between the metal surface to be protected and the mineral. A portion of the mineral ore may be soluble in the liquid medium, a portion may be dispersed as a colloidal suspension in which the dispersed mineral ore particles are so small that they do not form a separate phase, but are not so small that they cannot be said to form true solutions, or a combination of a solution or colloidal dispersion may occur.

A practical method is to add an excess of the mineral ore to the liquid medium. A portion of the added ore is dispersed and/or solubilized, while the other portion, which is insoluble, can be removed by filtering or allowed to circulate in the system. Addition of a pyrochlore concentrate to water raises the pH to about 10.

As a general rule, and in the latter example, in particular, as in all good boiler practice, it is desirable to limit the access of oxygen to the water. Excessive oxygen can cause the formation of adherent black spots of iron, vanadates, columbates, or tantalates which is undesirable from the standpoint of economical use of such expensive ores. Accordingly, de-oxygenation of the water is usually effected prior to addition of the ore to the medium.

Typically, it is desirable to control the pH of the pyrochlore-microlite mineral ore dispersed in the water to within the range of about 6 to 14, usually from 8 to 12.

After addition to the liquid medium, the mineral ore may be allowed to contact the ferrous base metal for a sufiicient time to render the metal resistant to corrosion in the contemplated system. For example, in a power generating system, the ore may be allowed to circulate in the water at ambient temperatures and pressures for a time sufficient to build up a corrosion resistance barrier before the system is raised to operating conditions.

The corrosion inhibiting method of this invention is readily acceptable for use in various boilers in power industry installations. For example, small amounts of mineral ore can be added to boiler water to retard boiler tube fouling. Similarly, the pyrochlore concentrate of the pyrochlore-microlite ore can be added to any process water system in which'iron, steel, or ferrous base metal pipe lines are used and corrosion and fouling are serious problems. In some large installations, automatic systems can be employed to continuously replenish the supply of ore or pyrochlore concentrate in the water.

It may be desirable to maintain the liquid medium at minimum temperatures in the range of 180 to 200 F. during the operation of this process, since upon cooling such liquid mediums typically absorb oxygen and there fore increase the likelihood of fouling. Alternatively, the system may be not allowed to cool until adequate protective films have formed on the surfaces of the system to guard against oxidation.

It is customary in the nuclear power industry to demineralize thewater employed in the generators by ion exchange techniques to concentrations in the range from 2 to 10 parts solids per one billion parts water.

The concentration of pyrochlore-microlite ore or pyrochlore concentrate in water, therefore, for beneficiation may be as low as 2 to 10 parts solids per one billion parts water. Higher concentrations of mineral ore or pyrochlore concentrate ordinarily will be used in the practice of this invention, the upper limit being set by the practical limits of suspension and/ or solubilization of the material in water and/ or economic considerations. One suitable technique is to add the ore after treating with the demineralizer in order to allow it tocirculate throughout the system before returning to the demineralizer.

Other additives may be used in combination with the mineral ore or the pyrochlore concentrate. The use of the ammonia, nitrides, or morpholine in conjunction therewith may he desirable in many instances.

In addition, it should be noted that the pyrochloremicrolite mineral ore, or pyrochlore concentrate thereof, can be placed in a liquid carrier such as water and applied to a ferrous base metal article by brushing, painting or the like to render the article corrosion resistant.

There are many unusual advantages resulting from the protection of ferrous base metal articles in accordance with the treatment process of this invention. For example, the inclusion of small quantities of selected mineral ores, such as araxa, pyrrhite, koppite, hatchettolite, chalcolamprite, etc., in conventional boiler water is sufiicient to obviate the long existing problem of boiler tube fouling.

In addition, the process of this invention lends itself to many applications. The ferrous base metal article may first be fabricated to the desired shape, such as a turbine blade, reaction vessel, or die, and then subjected to the treatment process described hereinabove, in order to prevent corrosion conditions in use. On the other hand, existing ferrous base metal installations can made corrosion resistant by means of the treatment process by merely adding controlled amounts of pyrochlore-microlite ore to any liquid medium, flowing through the installation.

Although the exact nature of the protective mechanism is not fully understood, it is believed that the inclusion of the pyrochlore-microlite ore or pyrochlore concentrate in a liquid medium under conditions of controlled pH, e.g., 6 to 14, causes a succession of complex oxide compounds to attach to the surface of the ferrous base metal article exposed to the mineral constituents during treatment. A protective, corrosion resistant barrier thereby is formed on the surface of the ferrous base metal article.

At the same time, it is also believed that the ore when used in the water contained in pipes, boiler tubes or the like, enters into an electrolytic action between the surfaces of the ferrous base metal pipes or tubes and the ore which, in turn, reduces the corrosion.

In any event, however, the process of this invention is in no way limited to the theory of its operation. Other modes of applying the principles of the invention can be employed. For example, the sub-fractions of pyrochloremicrolite ore most effective for the beneficiating treatment may be selected. Various techniques for forming protective layers on the surfaces of the ferrous base metal may be employed. These and other changes can be made according to the principles of the invention, provided, however, that the features stated in any of the claims, or equivalent of such, are employed.

The following examples serve to illustrate the invention:

EXAMPLE 1 An excess of pyrochlore-microlite mineral ore was added to and admixed with demineralized water having a pH of about 6.2. A portion of the mineral formed a murky solution and/or suspension. Another portion settled at the bottom of the vessel and was permitted to remain there. The pH of the admixture rose to approximately 10.

EXAMPLE 2 Carbon steel plate was introduced into a vessel containing the mineral-water admixture prepared as in Example 1. A control was prepared by introducing a carbon steel plate into demineralized water. The two samples were permitted to stand for three days at ambient temperatures, and then compared. The carbon steel plate in the water containing the pyrochlore-microlite mineral showed little evidence of formation of iron hydroxide. By contrast, the carbon steel plate of the control was covered with iron hydroxide flocculant.

EXAMPLE 3 Two vessels, one containing the mineral-water admixture as prepared in Example 1, and a second containing demineralized water were heated to boiling for a period of time to expel all of the oxygen. The two vessels were then allowed to cool to ambient temperatures and a carbon steel plate introduced into each. The carbon steel plates were observed after one week. The carbon steel plate in demineralized water was etched and pitted. The carbon steel plate in the mineral-water admixture of Example 1, evidenced substantially reduced corrosion.

EXAMPLE 4 A vessel containing the admixture as prepared in Example 1, and a second control vessel containing demineralized water were first heated to boiling. Thereafter, carbon steel plates were introduced into each vessel at boiling temperatures and the samples continued in at boiling temperatures for approximately one month. Thereafter the water in both vessels was allowed to cool to ambient temperatures and permitted to stand for a few days. The

carbon steel plates were removed and compared. The carbon steel plate in the mineral-water mixture according to Example 1 showed inhibition to corrosion, whereas the carbon steel plate in the water sample was pitted or etched.

EXAMPLE 5 300 series sensitized stainless steel plates were tested following the procedures of Example 3. The stainless steel plate in the mineral-water mixture showed reduced corrosion, while the sensitized stainless steel plate in the water sample was clearly subjected to corrosion.

I claim:

1. In a proces for protecting a ferrous base metal article against corrosion, and which is adapted to reduce the corrosion of said article in the presence of steam or water, the steps comprising, treating said article with at least a fraction of a pyrochloro-microlite mineral ore contained in a water medium in an amount and for a time sufficient to provide said article with improved resistance to corrosion, said water medium containing said ore having a pH in the range of above about 6.

2. A process according to claim 1 wherein said mixture has a basic pH.

3. The process according to claim 1 wherein said water medium comprises water having a pH in the range from about 8 to 12.

4. The process according to claim 1 wherein said water medium comprises water having a pH at about 10.

5. In a process for protecting a ferrous base metal article, the steps comprising treating said metal article with an admixture of at least a fraction of the pyrochlore concentrate of pyrochlore-microlite ore in water, said admixture having a pH above about 6.

6. A process according to claim 5 wherein said pyrochloro-microlite ore is selected from the group consisting of: pyrrhite, koppite, hatchettolite, chalcolamprite, endeiolite, neotantalite, metasimpsonite and araxa.

7. A process according to claim 6 wherein said pyrochlore-rhicrolite ore is araxa.

8. In a process for protecting a ferrous base metal article and which is adapted to reduce the corrosion of said article in steam or water environments, the steps including, treating said article with an inhibition agent comprising the columbium metal constituents of a product derived from pyrochloremicrolite mineral ore in a liquid water medium in amounts and for a time sufiicient to provide said article with improved resistance to corrosion in such steam or water environments, said liquid water medium having a basic pH.

9. A process according to claim 8 in which said pH is in the range to about 14.

10. A process according to claim 8 in which said pH is in the range from about 8 to 12.

11. A process according to claim 8 in which said inhibition agent is alkaline in water.

12. The article produced by the process of claim 1.

13. The article produced by the process of claim 8.

References Cited UNITED STATES PATENTS 2,416,064 2/ 1947 Patterson et al 10614 2,824,829 2/1958 Quaely 117-127 X 3,035,926 5/1962 Larrieu 10614 3,048,495 8/1962 Petkus et a1. 117-127 X 3,359,181 12/1967 Peterson 10614 3,366,510 1/1968 Daendliker 117-127 X FOREIGN PATENTS 714,484 7/1965 Canada 106-14 ALFRED L. LEAVITT, Primary Examiner I. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.