US3132975A

US3132975A - Process for pickling and passivating enclosed structures

Info

Publication number: US3132975A
Application number: US32551A
Authority: US
Inventors: Freud Herbert Manfred
Original assignee: FRAMALITE SOC
Current assignee: FRAMALITE SOC; SOCIETY FRAMALITE
Priority date: 1959-06-04
Filing date: 1960-05-31
Publication date: 1964-05-12
Anticipated expiration: 1981-05-12
Also published as: BE591583A; DE1155652C2; DE1155652B; NL131125C; CH394760A; FR1236531A; NL252277A; GB949184A; LU38770A1

Description

y 1964 HERBERT MANFRED FREUD 3,132,975

SURNAMED JEAN FRASCH PROCESS FOR PICKLING AND PASSIVATING ENCLOSED STRUCTURES Filed May 31, 1960 PlC'KLlNG- i' pifikl l SOLD-1.01! introduced at bottom to displace air while filling the hollow structure l introduced at top to displace the pickling solution R i G while the hollow structure remains Filled with liquid i/complezmg Salts in water solution introduced at the a top to displace rinsing COMPLEXlNG water while the hollow structure remains filled with liquid I /Wafer introduced at top to displace the complaxing salts while hollow struc- RINSIN G ture remains filled with liquid Pa ssivatin agents In 501,11 1D". introduced PASSlVATl M6 at top to dis lace the water while hollow structure remains Filled with liquid DRYING m DESSICATING 0xygen Gas FILLING United States Patent ()fiice 3,132,975 PROCESS FOR PICKLWG AND PASIVATENG ENCLOSED STRUCTURES Herbert Manfred Freud, sin-named Jean Frasch, Nanterre,

France, assignor to Society Framalite, Paris, France, a

corporation of France Filed May 31, 19st), Ser. No. 32,551 Claims priority, application France June 4, 1959 8 Claims. (Cl. Mil-6.14)

This invention is addressed to pickling and passivating surfaces of ferrous metals which are not exposed for treatment in a conventional manner, and it relates more particularly to a new and improved process for pickling and passivating concealed surfaces of iron base alloys and ferrous metals such as the internal surfaces of enclosed structures of substantial dimension or capacity, as represented by heat exchangers, boilers, gas generators, liquid storage tanks, and other process or storage equipment.

Numerous processes have been developed and used commercially for the pickling and passivating of surfaces of steel, iron, and iron base alloys which are readily accessible for treatment and for observation. Such exposed surfaces are available for cleaning either by mechanical means, such as sandblasting, shot peening and the like, or by chemical means, such as solution of the oxides in pickling acids and the like. When use is made of pickling solutions, application for removal of the oxides from the surfaces can be made by brush-coating or by spraycoating, or the surfaces can be wet by the pickling liquids upon immersion of the article in the pickling bath.

Removal of the oxides by the pickling acids can be followed immediately by washing with aqueous medium for the removal of the pickling acids from the cleaned surfaces of the metal. Rinsing to remove the pickling solutions can be followed almost immediately either by passivating or by drying to avoid reoxidation of the treated ferrous surface.

In the event that the pickling acids are allowed to remain on the ferrous surface, because of inadequate rinsing, it

, has been found that the residual acids remaining on the surface are capable of reacting even after drying, to cause undesirable corrosion or rusting. In the event that drying to remove moisture after pickling is incomplete or too slow, it has been found that the moisture present on the surface will begin to react almost immediately in combination with the oxygen in the air, to cause undesirable rust formation.

Such rinsing, passivating and drying operations can be easily and quickly carried out when concerned with metallic surfaces measurable in a few square meters or even as many as 10 square meters. The problem becomes quite different when one has to process surfaces of several hundreds or thousands of square meters, and especially when such surfaces are not readily accessible, such as when they form the internal surfaces of enclosures which may have a capacity of thousands of cubic meters and when such concealed and inaccessible surfaces have to be substantially completely pickled and passivated.

By way of example, a heat exchanger for use in nuclear generation of electrical power, wherein such heat exchanger may consist of 24 ferrules 30 meters high, having a volume of 350 cubic meters and an internal surface measurable as 30,000 square meters, all of which is combined in a compact structure connected with access piping of 300 mm. diameter. Because of the size, it is impractical to immerse such structure into a pickling bath. In the alternative, the pickling solutions might be introduced into the interior of the exchanger in amounts to fill the exchanger for reaction until the oxides (rust, scale, soldering flux and the like) have been dissolved or stripped from the metal Walls.

3,132,975 Patented May 12, 1964 Removal of the acids by rinsing is impractical with structures of such dimension because a number of hours would be required for draining the pickling solution, and an equal amount of time would be required for replacement with water. Further, replacement with water would operate merely to dilute the acids remaining on the surfaces, such that several washings would be required before adequate removal of the acids could be achieved. During each such emptying and filling cycle, the treated surfaces, especially in the upper portions of the exchanger, would be exposed for substantial amounts of time to oxygen in the air while acid is still present on the surface, whereby rusting would take place before protection by the rinsing water.

On the other hand, rinsing by continuous flow of water through the exchanger would also require a number of hours (such as 8-10 hours) before the water rising from the outlet is free of acids, as evidenced by a neutral pH. Even then, ferrous ions present evenin low concentration in the rinsing water will flocculate as Fe(OH) when the rinsing water is at a pH of about 5. These light flocs of Fe(OH) will deposit on the metallic surfaces and, after drying, will give rise to dust, thereby to interfere with the desirable operation of the apparatus.

It has been found that even when the apparatus has been perfectly rinsed and all acid products in the pickling bath have been completely removed, it is necessary to drain off the water before passivating the surfaces. Between the time that the water is drained off and the structure is filled with the passivating solution, portions of the treated surface will be unprotected for as much as 4 or more hours, which is sufiicient to give rise to rusting.

Finally, when the surfaces have been passivated, it be comes necessary to dry the surfaces quickly, preferably without the use of heat, because water vapor produced by such heat will not find easy escape and thus cause rusting of the apparatus. The passage of air through the appara-tus to accelerate drying is also objectionable since it will introduce an important quantity of carbon dioxide which will be capable of forming carbonic acid upon solution in water which, in combination with the oxygen in the air, will react almost immediately to cause rusting or corrosion.

It is an object of this invention to provide a process for pickling and passivating large concealed surfaces on the interior of a housing, and it is a related object to provide a method and means for achieving effective pickling and passivating of ferrous surfaces of substantial area, even such area is concealed from View as the interior surface of a housing.

FIG. 1 is a flow sheet illustrating the procedures of this invention.

The process embodying the features of this invention embodies the following combination of steps:

(1) The picklingsolution is introduced into the interior of the structure from an inlet in the bottom side until the structure is filled to overflowing. As the pickling solution rises through the structure, air is displaced from the structure in a manner to eliminate the formation of any air pockets, thereby to enable the liquid to come into full contact with all of the surfaces. For this purpose,

' an overflow pipe is provided in the uppermost portion of acids, but it is preferred to make use of a solution of the type described in the French Patent No. 966,785, issued June 12, 1946, to Herbert Manfred Freud, also called Jean Frasch, comprising a mild pickling agent. Such mild agents are preferred over strong acids such as hydrochloric acid or sulfuric acid, which tend to liberate hydrogen and induce embrittlement of the steel while also possibly creating gas pockets whereby the pickling solution is expelled from contact with the surface of the metal.

The time that the pickling solution is allowed to remain in reactive contact with the ferrous surface depends upon the pickling solution employed and the amount of rust or scale to be removed. The end point can be determined by titration of the solution at predetermined intervals to determine the increase in the amount of iron in the solution. When the increase in iron dissolved over a 6-hour period is less than 0.2 gram per gallon, pickling can be considered to be complete. With a pickling solution of the aforementioned patent, this will usually take about 24-48 hours.

(2) Upon completion of the pickling operation, the surfaces are rinsed without exposure of the pickled surfaces to air. For this purpose, the rinsing water is circulated through the housing from the top to the bottom so that the housing will always remain filled with liquid, thereby to avoid the formation of voids or gas pockets. To maintain such condition, it is preferred to introduce the rinsing water at a rate greater than the rate of removal of liquid from the bottom of the housing. This described balance is established by the use of an overflow pipe branched above the rinsing water inlet and operated either by means of a trap or by an expansion vessel.

The incoming rinsing water displaces the pickling solution towards the bottom of the housing, with some dilution possibly taking place. Rinsing is thus continued until the liquid issuing from the bottom side of the housing is practically neutral or at least above a pH of 6. While this will indicate substantially complete removal of acid, the presence of Fe(OH) in the wash water will be indicated by the appearance of a blue coloration due to the formation of Prussian blue upon the addition of potassium ferrocyanide in the Prussian blue test.

(3) It is desirable to eifect removal of the Fe(OH) which otherwise would adhere to the walls of the housing, and give rise to dusting, as previously described. For this purpose, the surface is treated with a compound capable of complexing reaction with the contained Fe(OH) to form a soluble compound which can be removed in subsequent rinses to effect substantially complete cleaning of the surfaces. Such complexing compounds as citrates, tartrates, malates, and so on, can be introduced in solution in aqueous medium in the latter stages of the rinsing operation, or immediately thereafter as a separate rinse. To conserve on the amount of complexing compound utilized, it is desirable to separate the rinsing cycle from the complexing or solubilizing cycle so as to enable recycling of the complexing solution. The circulation of the complexing solution from the top to the bottom of the housing for displacement of rinsing water without the formation of voids or air pockets is continued until the Prussian blue test with potassium ferrocyanide is negative.

(4) The complexing cycle is followed by a rinsing cycle to eliminate complexing compounds from the system. This is again accomplished by the circulation of clean Water through the housing, as previously described, in a manner to keep the housing filled with water until the pH of the water issuing from the housing is the same as that of the water introduced.

(5) The surfaces are then passivated by introduction of a passivating solution into the housing to replace the rinse water, preferably by introduction of the passivating solution through the inlet at the top and drainage of the water from the outlet at the bottom, always keeping the housing filled with aqueous medium. Again, to conserve on the amount of material employed, it is desirable continuously to recirculate the passivating solution through the housing until the reaction is completed. In most instances, this can be achieved in from l-24 hours.

For passivation, use can be made of any of the well known or commercial passivating agents for iron, such as an oxidizing agent in solution, but it is preferred to make use of a dilute solution of an amine or amide acid bichromate. When passivation has been completed and the surfaces are stabilized against rapid rusting or corrosion, the liquids are drained from the housing for the first time to empty the housing since pickling.

(6) It will be found that the Walls of the housing will still be Wet with large amounts of water present as a thin film of a few microns in thickness. It is desirable to effect rapid removal of the water for drying the surfaces before rusting can again take place. In accordance with the practice of this invention, the opening or openings at the top of the housing are sealed and a suction is drawn from the inlet at the bottom. This operates to draw down liquid. For most efficient use, it is desirable to repeat the suction steps at frequent intervals to provide a pulsating effect whereby as much as of the retained moisture can be drained from the housing in a matter of a few hours.

(7) Suction drying is followed by complete drying by the circulation of dry oxygen or ozonized oxygen from the bottom of the housing to the top, using a vacuum pump at the top to draw off oxygen to maintain subatmospheric conditions within the housing. Thus the evacuation rate should be in excess of the rate of introduction of the oxygen so as to maintain vacuum conditions within the housing, thereby to permit the oxygen completely to penetrate the interior of the housing for contact with the remaining moisture whereby the tnoisture evaporates to saturate the oxygen for removal.

Desiccation with an oxygen stream is continued until the absence of moisture in the exhaust stream is indicated, as by means of absorbent pellets (such as blue silica gel, which turns red when wet).

(8) When the walls of the housing have been dried, the access openings are closed and the apparatus is filled with the fluid with which it is adapted to be filled in use.

It will be understood that the description of the invention by reference to a heat exchanger is made by way of illustration but not by way of limitation, and that the described steps of pickling and passivating massive and concealed surfaces of ferrous metals may be equally applied to boilers, cisterns, storage tanks, or other apparatus and structures of ferrous metal.

It will be further understood that while best results will be secured by the combination of successive steps described, simple modifications and alterations in the process and in the conditions thereof can be made without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. The process of pickling and passivating internal ferrous surfaces of massive hollow structures comprising introducing a pickling solution into the structure in an amount completely to fill the structure and in a manner to effect substantially complete removal of air thereby to provide for complete wetting of the interior surfaces of the structure with pickling solution, maintaining the pickling solution in contact with the interior surfaces for a time sufficient to remove rust and corrosion from said surfaces, introducing rinsing water into the structure while simultaneously removing pickling solution, said rinsing water being introduced at a rate at least as great as the rate of removal of the pickling solution whereby the interchange is effected without exposure of the surfaces to air, continuing the introduction of rinsing water until the water issuing from the structure is substantially neutral, introducing a solution containing an agent capable of forming a soluble complex with iron hydroxides remaining within the structure to remove the iron hydroxides from the structure by the introduction of rinsing water for displacement of the complexing solutions, introducing a passivating agent in solution into the structure for replacement of the rinsing water, and maintain ing the passivating solution in contact with the pickled surface until passivation is completed, each of the foregomg steps, with the exception of the first filling of the structure, being carried out without exposure of the surfaces to air, draining the passivating solution from the structure and then drying the pickled and passivated surfaces.

2. The process as claimed in claim 1 which includes continuously alternating a step of circulating the pickling solution through the structure for dynamic reaction with a step of allowing the pickling solution to remain static within the structure for static reaction.

3. The process as clahned in claim 1 which includes maintaining the pickling solution in contact with the surface until the amount of additional iron dissolved in the pickling solution corresponds to less than 0.2 gram per gallon over a 6-hour period.

4. The process as claimed in claim 1 in which the step of drying the interior surfaces of the structure after the passivating solution has been drained therefrom comprises creating a vacuum within the structure for the displacement of moisture from the walls of the structure.

5. The process as claimed in claim 4 in which the suction is generated at frequent intervals to provide a pulsating action for drawing moisture from the surface.

6. The process as claimed in claim 1 in which oxygen is circulated through the structure after drainage of the passivating solution to take up moisture remaining on the surface for removal from the structure.

7. The process as claimed in claim 6 which includes withdrawing the gases from the structure at a rate greater than the introduction of oxygen to maintain vacuum conditions within the structure whereby greater access between the oxygen and the wet surfaces is available.

8. The process as claimed in claim 1 in which the introduction of a complexing solution is continued until the efiiuent from the structure is free from discoloration by the potassium ferrocyanide test.

References Cited in the file of this patent UNITED STATES PATENTS 2,428,364 Frager Oct. 7, 1947 2,524,757 Brines et al. Oct. 10, 1950 2,773,623 Schuster et al Dec. 11, 1956 2,907,689 Kidder et a1. Oct. 6, 1959 3,063,866 Mayer et a1 Nov. 13, 1962

Claims

1. THE PROCESS OF PICKLING AND PASSIVATING INTERNAL FERROUS SURFACES OF MASSIVE HOLLOW STRUCTURES COMPRISING INTRODUCING A PICKLING SOLUTION INTO THE STRUCTURE IN AN AMOUNT COMPLETELY TO FILL THE STRUCTURE AND IN A MANNER TO EFFECT SUBSTANTIALLY COMPLETE REMOVAL OF AIR THEREBY TO PROVIDE FOR COMPLETE WETTING OF THE INTERIOR SURFACES OF THE STRUCTURE WITH PICKLING SOLUTION, MAINTAINING THE PICKLING SOLUTION IN CONTACT WITH THE INTERIOR SURFACES FOR A TIME SUFFICIENT TO REMOVE RUST AND CORROSION FROM SAID SURFACES, INTRODUCING RINSING WATER INTO THE STRUCTURE WHILE SIMULTANEOUSLY REMOVING PICKLING SOLUTION, SAID RINSING WATER BEING INTRODUCED AT A RATE AT LEAST AS GREAT AS THE RATE OF REMOVAL OF THE PICKLING SOLUTION WHEREBY THE INTERCHANGE IS EFFECTED WITHOUT EXPOSURE OF THE SURFACES TO AIR, CONTINUING THE INTRODUCTION OF RINSING WATER UNTIL THE WATER ISSUING FROM THE STRUCTURE IS SUBSTANTIALLY NEUTRAL, INTRODUCING A SOLUTION CONTAINING AN AGENT CAPABLE OF FORMING A SOLUBLE COMPLEX WITH IRON, HYDROXIDES REMAINING WITHIN THE STRUCTURE TO REMOVE THE IRON HYDROXIDES FROM THE STRUCTURE BY THE INTRODUCTION OF RINSING WATER FOR DISPLACEMENT OF THE COMPLEXING SOLUTIONS, INTRODUC ING A PASSIVATING AGENT IN SOLUTION INTO THE STRUCTURE FOR REPLACEMENT OF THE RINSING WATER, AND MAINTAINING THE PASSIVATING SOLUTION IN CONTACT WITH THE PICKLED SURFACE UNTIL PASSIVATION IS COMPLETED, ECH OF THE FOREGOING STEPS, WITH THE EXCEPTION OF THE FIRST FILLING OF THE STRUCTURE, BEING CARRIED OUT WITHOUT EXPOSURE OF THE SURFACES TO AIR, DRAINING THE PASSIVATING SOLUTION FROM THE STRUCTURE AND THEN DRYING THE PICKLED AND PASSIVATED SURFACES.