US3477931A - Method and apparatus for automatic electric corrosion-proofing - Google Patents

Description

Nov. 11, 1969 K'ENJI UEDA ET AL 3,477,931

METHOD AND APPARATUS FOR AUTOMATIC ELECTRIC CORROSION-PROOFING Filed March 28. 1966 3 Sheets-Sheet 1 FIG. 1

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METHOD AND APPARATUS FOR AUTOMATIC ELECTRIC CORROSION-PROOFING Filed March 28. 1966 3 Sheets-Sheet 2 FIG. 6

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Nov. 11, 1969 KENJI UEDA ET AL 3,477,931

METHOD AND APPARATUS FOR AUTOMATIC ELECTRIC CORROSION-PROOFING Filed March 28, 1966 5 Sheets-Sheet 3 FIG.4

AC 100V FIG.5

FIG. 9

RESET INPUT 1 POTENTIAL OF MILDv STEEL (ON THE BASIS OF SATURATED OALOMEL ELECTRODE) (V) I I I Y o TlME zsec) 3O 4O INPUT INVENT R5 KENJ! UE H H/ROSHI OGHWH WWMd/W United States Patent 3,477,931 METHOD AND APPARATUS FOR AUTOMATIC ELECTRIC CORROSION-PROOFING Kenji Ueda and Hiroshi Ogawa, Nagasaki-ski, Japan, assignors to Mitsubishi Jukogyo Kabushiki Kaisha, Chiyoda-ku, Tokyo, Japan Filed Mar. 28, 1966, Ser. No. 537,845 Claims priority, application Japan, Mar. 30, 1965, 40/ 18,525; May 31, 1965, 40/132,093 Int. Cl. C23f 13/00; B01k 3/00 US. Cl. 204-147 11 Claims ABSTRACT OF THE DISCLOSURE In a method and apparatus for corrosion proofing metallic structures immersed in an electrolyte, such as sea water, the metallic structure is used as a cathode and an anode is placed in spaced relation thereto, the usual third or standard electrode being omitted. A potential is applied bctween the anode and the metallic structure to cause a relatively small current to flow from the anode to the metallic structure, the latter acting as a cathode. The current is maintained at a value such that the potential of the metallic structure or cathode is maintained at a value at which the metallic structure is corrosion-proof. The potential drop between the metallic structure and the anode, and the resistance drop through the electrolyte or sea water, between the anode and the metallic structure, are continuously measured and the difference between the two measured values is maintained at a constant value equal to the work potential necessary to corrosion-proof the work. In a variation, the current flow may be maintained within relatively small values either side of the required potential value for the metallic structure.

This invention relates to the corrosion-proofing of metallic structures and, more particularly, to a novel method of and apparatus for automatically electrically corrosion-proofing metallic structures by detecting the potential difference between the work to be protected and a. non-consumable anode and, responsive thereto, maintaining the corrosion-proofing electrolytic potential for the work at a predetermined value.

For the electrochemical corrosion-proofing of work pieces, which are usually metallic structuresimmersed in an electrolyte or electrolyte solution, the. so-called corrosion-proofing process is employed. In this process, the work is a cathode having current flowing thereinto from an anode immersed in the solution in spaced relation to the cathode whose potential is thereby polarized to a corrosion-proofing potential. To efrectperfect protection against corrosion of the work, it is essential to maintain the potential of the work at the corrosion-proofing value throughout the processing.

A known expedient designed to maintain the potential of the work at the proofing potential throughout the course of processing is to position a third electrode in relatively close opposition to the work. This third electrode may be a standard electrode of silver chloride, saturated ealomel, or similar material. A potential is applied between the first-mentioned anode and the work, and a second potential is applied between the third or standard electrode and the work, and has a polarity 31,477,931 Patented Nov. 11, 1969 A conventional arrangement of this type has the disadvantage that the proofing potential of the work is controlled only in the vicinity of the standard electrode,

and hence the current density on the surface of the work;

is not uniform. Because of the characteristic of the standard electrode, no current can'pass therethrough, which necessitates that the control arrangement beconstructed as an expensive high-input resistance type device. An additional disadvantage is that the standard electrode isf consumed, to some extent, and has a relatively short An object of the present invention is to provide a simplified electro-corrosion-proofing method whereby the work can be maintained accurately at the proofing potential without the use of a standard electrode.

Another object of the invention is to provide a simplified automatic apparatus for electro-corrosion-proofing and not requiring the use of a standard electrode to maintain the work at the proofing potential.

A further object of the invention is to provide a method of and apparatus for corrosion-proofing work whereby the work can be kept at a potential in excess of the proofing potential merely by supplying the work with a constant current but intermittently.

Yet another object of the invention is to provide a method of and apparatus for corrosion-proofing 'work whereby satisfactory corrosion-proofing can be obtained by supplying, in alternation, two constant currents having respectively different values.

Yet a further object of the invention is to provide an improved method of and apparatus for electro-corrosionproofing to prevent corrosion of a metallic structure immersed in an electrolyte solution and in which the potential difierence between the work to be treated and a nonconsumable anode is detected and utilized to control the proofing potential of the work to a predetermined value.

For an understanding of the principles of the invention, reference is made to the following description of typical embodiments thereof as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 is an explanatory schematic and circuit diagram illustrating a conventional arrangement for electrocorrosion-proofing;

FIG. 2 is a view similar to FIG. 1, illustrating an arrangement in accordance with the present invention;

FIG. 3 is a potential-current density diagram with respect to non-consumable metallic anodes, explanatory of the principles of the invention;

FIG. 4 is a schematic wiring diagram of one form of apparatus embodying the invention;

FIG. 5 is a set of curves illustrating the change of potential with time immediately after the interruption of the proofing current supplied to a bare mild steel plate immersed in an electrolyte solution;

FIG. 6 is a set of curves of potential changes with respect to time immediately following interruption of the proofing current supplied to the work, with respect to non-consumable anodes maintained at a predetermined potential in flowing sea water;

FIG. 7 is a schematic wiring diagram of another embodiment of apparatus for practicing the invention, and related to the curves of FIGS. 5 and 6;

FIG. 8 is a schematic and circuit diagram of a contactless meter relay for gating silicon controlled rectifiers used in practicing the invention; and

FIG.' 9 is a schematic wiring diagram illustrating the operation of the relay of FIG. 8 in gating a silicon controlled rectifier.

In the conventional prior art arrangement illustrated in FIG. 1,. a corrosion-proofing potentialis applied between the work piece and an anode 4. A standard electrode 5 of silver chloride, saturated calomel, or the like is disposed in close opposition to work 3, with all three elements 3, 4 and 5 being immersed in an electrolytic solution, such as, for example, sea water. The potential between work 3 and anode 4 isapplied from a source voltage controller 1 through a rectifier 2.

Between standard electrode 5 and work 3 there is interposed a standard power source 6 for setting the potential of work 3, source 6 having a polarity reversed with respect to the polarity of the potential applied from rectifier 2. Source 6 is supplied through a miniature amplifier 7 connected to source voltage controller 1.

With the arrangement shown in FIG. 1, if the potential of work 3 is equal to the potential set by standard source 6, the input to amplifier 7 is zero. However, if the potential of work 3 deviates from a predetermined potential, as due to a change in the proofing conditions, an input corresponding to such diiference is applied to amplifier 7. As a result, the amplified output from amplifier 7 controls the controller 1 to adjust the voltage from source E and thereby adjust the proofing current in a correcting direction, such as to maintain the proofing potential of work 3 at a constant value.

In the arrangement of the invention shown in FIG. 2, an anode A and a work piece C to be corrosion-proofed, and both of which are metals, are immersed in a uniform electrolyte solution W and a voltage E is applied between anode A and cathode or work C from a DC. power source E having an internal resistance R If the potentials of anode A and cathode C are, respectively, PA and PC, both of which represent the sum of the natural potential and the polarized potential, the current supplied is I, the voltage between terminals a and c is V and the sum of the resistance of the conductors and the liquid in the circuit aAC-c is R then the voltage V between terminals a and c can be expressed by the equation:

Vac=EIR :P P +IR (1 Hence, the potential (P of the work (C) is Pc VaCP IRs In Equation 2, the resistance R is a constant which depends on the configurations, dimensions, and relative positions of anode A and cathode C, and also on the specific resistance of the electrolyte solution. This means, therefore, that the cathode potential P can be controlled through control of the voltage Vac between the anode and cathode, the anode potential P and the current supplied I or, in other words, through the control of the voltage drop IR between the anode and cathode.

It has been experimentally confirmed by the inventors that, when non-consumable electrodes of precious metals, such as platinum and platinum alloys, magnetic iron oxide, lead-silver alloy, platinum plated titanium plate, and the like are employed as the material of anode A, the anode potentials P can be kept at constant values, in each case, as shown in FIG. 3. To wit, with respect to a standard electrode ofsaturated calomel immersed in sea water, the potential of a magnetic iron oxide anode is 1.5 v., that of a platinum plated titanium "plate anode is 1.20 v., and that of a lead-silver anode is 1.23 v. In consideration of the current range which is necessary for corrosion-proofing of the work C, a constant anode potential P can be obtained by choosing a suitable surface area for the non-consumable electrode, as mentioned above.

Thus, if control is achieved so that the difference between the voltage Vac between the anode and cathode and the voltage drop IR due to theresistance R between'the electrodes, that is (Vac-IR can be-kept constant, it is possible to keep the potential P of the cathode C or work to be corrosion-proofed, at a constant level, that is, at the proofing potential. To carry out such :contrOl in practice, it is necessary only to detect the potential dilference between the anode and cathode and to regulate accordingly the current to be supplied to the work by the anode.

One form of apparatus for practicing the invention is illustrated in FIG. 4. Referring to FIG. 4, the apparatus includes a slide rheostat-type transformer 11 for regulating the source voltage, and a rectifying means 12 incorporating silicon controlled rectifiers, referred to as SCRs. The work to be corrosion-proofed, and which is immersed in an electrolyte solution such as sea water, is indicated at 13, and a non-consumable anode 14 is positioned in opposition to work13 to supply current to the work, and is also immersed in the electrolyte solution.

A relatively small transformer 15 is operatively connected with the proofing current circuit to generate a voltage IR proportional tothe proofing current I, and is connected across a rectifying means 16 incorporating silicon controlled rectifiers. Also connected across the rectifier means 16 is a potentiometer 17 providing the Voltage IR for compensating for the resistance R between work 13 and anode 14 with respect to the current supplied to the input of the rectifying means. A voltage setting means is illustrated at 18 as including a source of constant voltage and a variable resistor for providing a selected constant voltage. The apparatus shown in FIG. 4 further includes a miniature voltage amplifier 19 and gating pulse generator 20 which supplies gating pulses to the silicon controlled rectifiers SCR in accordance with the intensity of the DC. voltage as amplified by the miniature voltage amplifier 19.

The voltage between the anode and work is indicated by a volt meter V which indicates a voltage V and the voltage generated to compensate for the voltage drop IR of the resistance R between the work and the anode is indicated by a volt meter V In the. circuit construction shown, the voltage of the voltage setting means 18 is set to the ditference between the potential of the anode (on the basis of a saturated calomel electrode) and the proofing potential of the work. For example, if mild steel is corrosion proofed with the use of a platinum plated titanium plate as the anode, it is clear from FIG. 3 that the anode potential P is 1.2 v. (on the basis of a saturated calomel electrode) and the proofing potential of the work is -0.8 v. (on the basis of a saturated calomel electrode), and hence the voltage set on means 18 is 1.2- (-0.8)=2. 0 v. Next, a switch SW for generating a voltage IR which compensates for a voltage drop due to solution resistance R between work 13 and anode 14, is closed, and the proofing current I is measured by an ammeter A, the voltage between the work and anode by voltmeter V and the voltage generated by the potential circuit by voltmeter V Then the variable resistance of the potential circuit is adjusted so that the relation V IR =-2.0 may hold within the expected proofing current range. The value (V 2.0)/I represents the resistants (R between the work and anode.

By the foregoing procedure, the set positions of the potential circuit and the voltage setting means 18 are dedetermined. Thus, the input voltage to the voltage amplifier 19 is This means that the proofing current I is adjusted so that As can be kept at zero and, because the work is kept at a constant proofing potential, its corrosion can be prevented.

With the apparatus of FIG. 4, corrosion proofing tests were conducted in a laboratory with a small aluminum boat, as follows:

(1) A corrosion proofing test was carried out on a bare mild steel plate having a surface area of 0.6 m? and immersed in 70 l. of a 3% aqueous solution of sodium chloride, with platinum as the anode. Regardless of whether the work was kept still or moving, the average potential of the mild steel was kept at -0.8 v., with respect to a standard electrode of sea water-silver chloride.

(2) A small aluminum boat with a total wetted surface area of 156 m. was subjected to a corrosion proofing test with a platinum plated titanium plate having a surface area of 0.71 dm. attached as an anode abaft'With respect to a standard electrode of seat water-silver chloride, the stern could be kept at 0.88 v., the midship at -0.84 v., and the stem at 0.83 v.

Although the above embodiment has been described as a case wherein silicon controlled rectifiers are applied, it is also possible to apply other means such as transistors, saturable reactors, electromagnetic relays, and the like which those skilled in the art can readily employ without departing from the spirit of the present invention.

The arrangement of FIG. 4 provides a highly practical corrosion proofing method and apparatus whereby standard electrodes, which are inconvenient with respect to maintenance and handling, can be omitted and corrosion of metals can be prevented by a simplified circuit arrangement. The arrangement shown in FIG. 4 is intended to keep the value V -IR constant, but the same objective can be obtained by simplified means such as will be described hereinafter with the embodiment of the invention shown in FIGS. 7, 8 and 9.

In this other embodiment of the invention, two limits are established for the current necessary to provide corrosion proofing of a Work or structure.

First, the upper limit I and lower limit I for the range of proofing current for carrying out corrosion proofing of a work or structure C to be proofed in a stabilized manner, are set. The lower current limit I represents a constant current value below the proofing current applicable to structure C while the latter is kept still in the solution. The upper limit I represents a constant current value above the proofing current applicable to structure C when the latter is in the solution having a most vigorous flowing condition. The anode A is assumed to be a non-consumable metallic body capable of maintaining a constant anode potential P in the current range 1 -1 If therefore the proofing potential of structure C is P then the proofing potential, at a certain moment when the current value I is maintained, will show an upper limit P -l-u of proofiing potential necessary for accomplishing stabilized corrosion proofing, and the terminal voltage value V'ac between the anode and cathode at that time will be A"- 'c+ 1 s Also, the proofing potential at a certain movement when the current value I is maintained will Show a lower limit value P' a of the proofing potential necessary for stabilized corrosion proofing. Assuming that the terminal voltage between the anode and cathode then will be (V"ac), then where or represents a value, OL O.

Thus, the terminal voltage values Vac and V"ac between the anode and cathode, at the upper and lower limit values of the proofing potential which are necessary for accomplishing corrosion proofing in a stabilized manner, can be obtained. Now, therefore, if the terminal voltage Vac between the anode and cathode drops below the lower limit of the terminal voltage Vac while a current of the constant value I is being supplied from the anode A to the structure C to be corrosion proofed in an electrolyte solution W, a current of constant value at the upper limit I is supplied from the anode A to the structure C to be proofed thereby to lower the proofing potential. When the terminal voltage Vac between the anode and cathode has been brought by the current value 1;; to the upper limit value Vac, the current I, is again supplied to raise the proofing potential, and these steps are repeated. Consequently, the cathode potential P can be stabilized and kept within a range defined by the upper limit P' -l-a and the lower limit P -d of the proofing potential.

In the procedure above described, the lower limit current value I is supplied when the terminal voltage Vac, between the anode and cathode, reaches the upper limit V"ac. It is possible, in this case, to shut olf the power source E instead of supplying the current value I thereby to keep the current at 1:0 for some time. The inventors thus tie-energized a structure C of bare mild steel plate, to be corrosion proofed, kept at a constant proofing potential in a 3% aqueous solution of sodium chloride, and also deenergized a nonconsumable metallic anode A kept at a constant potential in an inching flow of natural sea water, and actually determined the changes of potential with time immediately after the de-energizing. The results obtained were as shown in FIGS. 5 and 6, respectively.

As can be seen from FIGS. 5 and 6, both the anode and cathode did not immediately attain the natural potentials but showed some time delay. Taking advantage of this phenomenon, therefore, the potential P of the work or structure to be proofed can be detected by the terminal voltage value Vac between the anode and structure with a current 1:0, so as to be maintained at the proofing potential, in the same way as above described. Letting the anode potential at a certain moment during the period in which the current I=0 be P -fl and the potential of the structure C be P -i-a, the terminal voltage value Vac between the anode and cathode is 'AB)( 'c+) wherein P is an anode potential at the current density necessary for keeping the anode potential constant, and [3 is a value 3 0. Now if a current of constant value 1 flows from an anode A to a structure 0 to be corrosion proofed, in an electrolyte solution W, and if the terminal voltage (Vac) between the anode and cathode has reached the upper limit value V"ac, the anode and cathode are both tie-energized to a current 1:0. When the terminal voltage Vac has dropped below the value Vac, after some time with current 1:0, the constant current value I is again supplied from anode A to structure C. When the terminal voltage Vac has reached the value Vac on supply of the current (I the anode and cathode are both de-energized again, and it is only necessary to repeat the above sequence.

In Equations 4 and 5 above, the anode potential P is a constant value P and, if the rates of potential changes due to de-energizing of the potentials of anode A and structure C are the same, then afi and therefore Equation 5 can be rewritten as Vac=.P -P 2a. Accordingly, the potential P of the structure C to be proofed likewise can be kept in the range of the proofing potential 'c+) c 'c The just mentioned embodiment of the invention will now be described in some detail with particular reference to the schematic wiring diagram of FIG. 7. In FIG. 7, a volt AC. power source E is in operative connection with an anode A of non-consumable metal and which is disposed in opposition to a structure C to be corrosionproofed in an electrolyte solution W. Source E is connected, through a switch S with a source voltage regulator T composed of an autotransformer T and a transformer T and with a full wave rectifier R; including selenium rectifiers, a DC. voltage regulator r including a first normally open contact R of an electromagnetic relay R, and an ammeter A Anode A and structure C are connected to the input terminals of a meter relay M whose power input terminals are connected through switch S to source E Meter relay Mr is designed so that it indicates the voltage between the anode A and structure C to be proofed, and so that, when the pointer moves beyond the higher limit value H or the lower limit value L, the relaying is changed over to the highest limit relay H or the lower limit relay L, respectively. Lower limit relay L closes its lower contact L as shown, when the meter indication drops below lower limit value L, and closes its upper contact L when the indication rises above the lower limit value L. Similarly, the higher limit relay H closes its lower contact H as shown, when the meter indication drops below the higher limit value H, and closes its upper contact H when the indication rises above the higher limit value H. The lower contact H' of higher limit relay H, and the change-over contact L of the lower limit relay L are connected to the positive pole of a DC power source E and the negative pole of the power source E is connected to the lower contact L of the lower limit relay L through a switch Sy, coupled to the switch S and electromagnetic relay R. Contact L' may be connected to change-over contact H' of higher limit relay H'through the second normally open contact R" of relay R. In the device of the above construction, the lower limit value L of meter relay Mr is set to and the higher limit value H is set to P' P +I R +a. Next, switches S and S' are closed, relay R is energized, and the contacts R and R are closed.

In this condition, a current value I greater than that of a proofing current with the electrolyte in the most vigorous flow condition, is set by the power source voltage regulator T Then contact R is opened and a current value I greater than the minimum current necessary for keeping the anode potential P at a constant value P is set by DC voltage regulator r.

The apparatus of FIG. 7 now functions as follows. First, switches S and S close to start the corrosion proofing. At the start of proofing, the potential P of the work C is higher than the predetermined proofing poten tial P' and therefore the voltage Vac between work C and anode A corresponds to the lower limit value L on meter relay Mr. In other words, the meter relay reads a voltage value less than P' P' I R a. Accordingly, the change-over contactL of lower limit relay L of meter relay Mr closes the lower contact L as shown, whereby relay R is energized and contacts R and R are closed. Upon closure of contact R, the change-over contact H of higher limit relay H closes lower contact H' so that relay R can hold. Upon closure of the contact R, a constant voltage is applied between anode A and work C. In this case the resistance (R between the two electrodes A and C can be regarded as constant in the same electrolyte solution, and therefore a substantially constant current 1;, flows betwe'en the electrodes A and C.

Thus, the terminal voltage Vac between the electrodes A and C, at this time, is Vac=P P +L R In this equation, the anode potential P and the voltage drop I R between electrodes A and C are constant, whereas the cathode potential P decreases gradually, with the result that the terminal voltage increases steadily. When limit value L of relay Mr, contact L of relay L changes over from contact L to contact L However, because contact H of higher limit relay H is closed in this state, relay R holds until the terminal voltage Vac further rises to the higher limit value H at which contact H l of relay H changes over from contact H 1 to contact H' Thus, relay R holds until the terminal voltage Vac reaches the higher limit value H, and the current I flows during this period. ,7

The supply of current I is interrupted the moment the terminal voltage Vac reaches the higher limit value H; However, as the higher limit value H is the potential P of work C, immediately after interruption of current I corresponds to the lower limit value of the proofing potential, P' a. When the terminal voltage Vac reaches the higher limit value H, contact H of relay H is changed over to contact H and relay R is de-energized. Thereby, contact R transfers and the current supply is changed over from I -to 1,. At this time theterminal voltage is Vac=P -P +I R -|-a. .From then on, by the current I the potential P of anodeA is maintained at P whereas the potential P of work C: increases gradually because the latter cannot be kept polarized. With current I now being supplied, the terminal voltage Vac drops until its value reaches That is, the potential of Work C rises to the higher limit value P +a of the proofing potential. Thereupon, contact L of relay L of meter relay Mr closes its lower contact L and changes over the supply of current from I to I As the current I; fiows, the potential of work C again drops as above described, and the terminal voltage Vac begins increasing. Above the lower limit value L, the self-sustaining circuit operates, and the current 1 continues to flow until the higher limit value H is attained. By the repetition of the above operation, the potential'P of work C can be kept within the range of proofing potential (P' a) P (P' -|ot).

While the above description has been directed to the case in which the electrolyte solution W is kept still, the same applies to the case where the electrolyte solution W is flowing. When the electrolyte solution W is 'kept still, the potential P of the work is kept at a proofing potential within a range of P i-a, but when the solution is flowing, the rate at which the work C is depolarized is increased. Thus, the potential of the'work, after the terminal voltage Vac attains the upper limit H resulting in the change-over of current supply to the current I reaches the lower limit value L within a shorter period of time than when the electrolyte solution W is kept still. For this reason, in a flowing electrolyte,the time during which the current I flows is shortened, while the percentage of the time during which the current I flows is increased, and more current flows than when the solution is kept still, while the potential of the subject C 'being' corrosion proofed is kept within the range of P' ia.

Usually, corrosion proofing of metallic structures such as work C can be accomplished by maintaining thepoten tial of the structure at a value 0.2 to 0.3 v. lower than the natural potential P of the same structure in anele ctrolyte solution W. If therefore the proofing potential P for the structure C to be corrosion proofed by the apparatus of FIG. 7 is P 0.25 and a is less than 0.05, then the potential P of the structure C will be controlled within the range of (P O.3) P (P O-.Z).

In Table I, there are shown the results of experim ni if conducted, with a device of the type just described, upon test pieces of bare mild steel plate, 0.6 m? in size, dipped in 70 l. of an aqueous solution containing 3% of sodium chloride, with a view to finding out the effectsof flow conditions on the current and on the potential in a point of the bare mild steel plate.

The potential of mild steel in the 3% aqueous solution of sodium chloride was about 0.6 v. (on the basis of saturated calomel electrode). The potential of live platinum anode was 1.2 v. (on the basis of saturated calomel electrode) and the working current was I =0.l a, or :053 a. In this experiment, the voltage Vac between anode A and the work C was so set that the lower limit Vac=2.60 v., set upper limit Vac was 3.08 v. and resistance R between anode A and work C was 1.8 ohms. The point at which the potential was determined was the point on the mild steel plate which showed an average potential. The flowing condition was obtained by circulating the solution at a constant flow rate with a centrifugal pump. The measured values are the averages in the stabilized state after the test pieces, both in the static and flowing conditions, were kept as they were for about an hour.

As will be clear from Table I, the time required for polarization of the work C to be corrosion proofed (mild steel) was not aflected by the flow condition but the rate at which it was depolarized in the flowing state was very high as compared with that in the static state. Under the experimental conditions, the current I flowed for 14.4% of the unit time in the static state and for 51.9% in the flowing state. Potentials differed slightly with the state, whether static or flowing. This was because potential distribution on the mild steel varied with the 'flow condition, the potential being concentrated close to the anode in the flowing state. Thus, it was confirmed by the experimental results as given in Table I that the apparatus of FIG. 7 is fully capable of performing automatic electro-corrosion proofing.

While the apparatus of FIG. 7 is of a contact type which uses a small electromagnetic relay in the changeover mechanism of meter relay Mr and an electromagnetic relay R for switching the proofing current between values, it is also possible to employ a contactless device as shown, for example, in FIG. 8. FIG. 8 shows a contactless meter relay circuit which actuates silicon controlled rectifiers replacing the contact R in FIG. 7. This circuit includes a phototransistor Ptr, and the rays of light from a light projector are interrupted by a sub-index or shutter K attached to a moving pointer K. Thereby, a minute current flows through the phototransistor circuit which is kept at equilibrium, and this current is amplified by a transistor amplifier Tr. In this way, a suflicient current or voltage to turn the SCR on or off is obtained.

Thus, an automatic electro-corrosion proofing device of the contactless type can be provided byincorporating a contactless meter relay, having two set points, as above, in lieu of the meter relay Mr, electromagnetic relay R, and DC power ED of the apparatus shown in FIG. 7, by replacing contact R of the relay R with the SCR shown in FIG. 9, and by connecting the output obtained as the pointer passes through the lower limit set point of the contactless meter relay to the input terminal of the SCR and connecting the output obtained as the pointer passes through the upper limit set point of the relay to the reset input terminal. The operation of this contactless corrosion proofing apparatus, and the effects thereby attainable, are exactly the same as those of the apparatus shown in FIG. 7. As a control means for the corrosion proofing apparatus, the meter relay is most suitable because it is inexpensive and is capable of giving indication simultaneously with control. However, it is not objectionable to use other control means if it functions as a relay at the two set voltage points.

The apparatus of FIG. 7 can be used also in the case where electrodes A and C are de-energized, and the working current reduced to zero, as the terminal voltage reaches the value V" with a constant current I flowing. However, in this case, the DC. voltage regulator r is omitted and the connections are made in such a manner that the output current of rectifier R, can be switched on and oil directly by contact R. Thus, the connections are made in such a manner that the current can be completely interrupted when contact R of relay R is open. If the apparatus shown in FIG. 7 is modified in this man ner, it will function as follows: I

First, the lower limit value L of meter relay Mr is set to P' P' -2a and the higher limit value H is set to Pf -P' +I R +a. Current I is set to a constant value above the proofing current value in the most vigorous flowing condition of the electrolyte. As corrosion proofing is started by closing switches S and S' the voltage Vac between work C and anode A is indicated at a point below the lower limit value L of meter relay Mr because, in this state, the potential P of work C is higher than the predetermined proofing potential P and the potential P A of the anode A is lower than the potential P A obtained on initiation of current flow. Accordingly, lower limit relay L of the meter relay closes contact L' and energizes relay R the relay remains transferred and a constant voltage is impressed between the electrode A and work C. The terminal voltage Vac between the electrodes A and C, at this time, is Vac=P' -P +I R As the terminal voltage Vac reaches the value contact H' of higher limit relay H is changed over to the contact H This de-energizes relay R with the resuit that the circuit is opened and the current I; is reduced to zero. The potential P of work C immediately before reduction of the current l to zero is P -a, and the terminal voltage Vac immediately after the reduction of the current I to Zero is Vac=P -P +a.

With no current flowing, the potential P of anode A drops while the potential P of work C increases, and the terminal voltage Vac declines to P P' --2a. In other words, the potential of the work C increases until it attains substantially the value P -i-a. As the terminal voltage Vac drops to P -P' 2oz, lower limit relay L of meter relay Mr, set to the above voltage value, closes the contact L' and energizes the relay R, thereby resuming the supply of current I The potential P of work C now begins dropping and the terminal voltage Vac begins rising. Upon arrival of the terminal voltage at the value Vac=P' P' +I R +a, the current I is again reduced to zero by the transfer of higher limit relay H.

By the repetition of the above sequence of operations, the potential P of work C is maintained substantially within the range of (P' +a) P (P' -ot). When the electrolyte solution W is in a flowing state, the voltage drop of work C caused by the current I takes more time than when electrolyte W is in a static state, and hence the time required for terminal voltage Vac to increase to the higher limit value P -P +I R +a is prolonged. When the terminal voltage Vac has reached the upper limit and no more current flows, the polarization of anode A and work C is lost more quickly than when electrolyte W is in the stationary or still state, and the lower limit is attained in a shorter period of time. This means that the working time of current I is increased and its off time is shortened as compared with the case where the solution is kept still. While more current is supplied in the flowing state of electrolyte W, the current flow is responsive to the higher and lower limits of the terminal voltage Vac in the same manner as in the static state of electrolyte W. Thus, regardless of the changes in the electrolyte flow conditions, the potential P of work C is 'maintainedsubstanti'ally within the range of (P +ot) P (P' a). T able H shows the'results of corrosion proofing tests conducted on test pieces of bare mild steelplate, 0.6 m? in size, dipped in 70 l. of an aqueous solution containing 3%- of sodium chloride, with the apparatus as just described, using a platinum anode. The results respresent the effects of flow conditions upon the current andthe potential in a point of bare mild steel plate.

TABLE II Potential in a point of bare mild steel plate (on the basis of saturated calomel electrode) I=0.7 a I working I= time Min. pot. Max. pot. Flow condition time (see) '(sec. (v.) (v.)

-In the above experiment, the voltage Vac between anode A and work C had a lower limit value Vac of 1.97 v. and an upper limit value V"ac of 3.35.v., and the current I was 0.70 a. The resistance R between anode A and work C was 1.8 ohms. The measuring points for the voltages were the points where average potentials for the mild steel plate were obtained. In Table II, there is some discrepancy between the potentials in the two different states, flowing and static. This is because potential distribution differs with the flow condition, and the potential tends to be concentrated close to the anode A in the flowing state, and further because the respective rates at which the anode A and mild steel plate C lose their polarization are not the same.

The potential of live platinum anode A was 1.2 v. (on the basis of saturated calomel electrode). Since the corrosion proofing potential of the mild steel plate C is about 0.8 to 0.9 v. (on the basis of saturated calomel electrode), the potentials given in Table II are well within the above range. As will be also clear from the table, the time required for polarization is not affected by the flow condition but the rate at which the polarization is lost in the flowing state is materially affected. In the flowing state, the current supplied is about five times as much as that supplied in the stationary or still state. In the above experiment, the flowing state was realized by circulating the solution by means of a centrifugal pump, and was a state in which a constant flow rate was attained. The measured values represent the average values after the attainment of constant and stabilized states. From the results given in Table II it was ascertained that the modified apparatus also can serve adequately as an automatic electrocorrosion proofing device.

As described hereinbefore, the present invention provides a simplified automatic electro-corrosion proofing method which comprises determining the potential dif- 'ference between a non-consumable metallic body, used as an anode, and a structure to be corrosion proofed which is disposed opposite to the anode in an electrolyte anode to the structure in such a way that the above potential difference reaches a predetermined value corresponding to a range of a proofing potential for the structure and that the proofing potential is kept within the predetermined range, whereby the potential of the structure being corrosion proofed can be positively maintained in the range of proofing potential throughout and thus the corrosion proofing can be accomplished perfectly without the use of a standard electrode or the like.

What is claimed is:

1. A method for preventing corrosion of metallic struc-. tures immersed in an electrolyte solution, comprising the steps of: immersing an anode of non-consumable'material in the electrolyte solution in spaced relation to the structure acting as a cathode; effecting a unidirectional current flow from the anode through the electrolyte solution to the structure; and regulating the current flow to maintain the difference between the potential, applied between the anode and the structure, and the resistance voltage drop, including the solution resistance voltage drop between the anode and the structure, Within a predetermined limited range corresponding tothe potentialof the structure. having a value at which the structure is corrosion-proof. 2. A -method asclaimedin claim 1, inwhich the current flow is maintained at a constant value.

3. A methodas in claim 2,' including the step of determining thepotential difference between'the anode and the'structure; and,ffesponsive to any drop'of'said potential difference'below a predetermined value, supplying said current at said constant value.

41A method as claimed in claim 3, including the step of, responsive to any increase of said potential difference above a-second predeterminedvalue, supplying said'current at 'a different and smaller constant value. l

5. A method'as claimedin claim 3,- including the step of, responsive to any increase of said potential difference above a second predetermined value, interrupting the flow of-current between the anodeand the structure.

6.'A method as claimed in claim 5, including the step of, responsive to any dropof said potential difference below a' first predetermined value, initiating the flow of current at a first constant value; responsive to an increase of said potential difference to said first predetermined value, interrupting the supply of current at said first constant value; and, responsive to an increase of said potential difference above said first predetermined value and above a second and higher predetermined value, initiating a fiowof current from said anode to the structure at a constant value less than said first constant value.

7. A method as claimed in claim 1, including the step of, responsive to a variation of said ditference'and varying the current in a direction to restore said difference to a preselected value.

' 8. A method as claimed in claim 1, including the step of determining the potential difference between said anode and the structure; and, 'when said potential difference varies from a determined potential value, varying the current fiow in a direction to restore the potential difference to such determined value.

9. Apparatus for preventing corrosion of a metalli structure immersed in an electrolyte solution comprising, in combination, a source of unidirectional potential; means connecting the'structure to the negative terminal of said source; an anode'of non-consumable material immersed in the electrolyte solution in spaced relation to the structure and connected to the positive terminal of said source; measuring means operable to measure the potential applied, between said anode and the structure, and the resistance voltage drop, including the solution resistance voltage drop,-between said anode and the structure; and means operable, responsive to the potential difference between said measured applied potential and the resistance voltage drop to control the flow of current from said anode to the structure to maintain the potential difference between said applied potential and said resistance voltage drop within'a predetermined limited range corresponding to the potential of the structure having a value at'whic the structure is corrosion-proof. Y

10. Apparatus "as claimed in claim 9, including means operable by said measuring means, responsive to any drop of said potential difference below a first predetermined value, to initiate said current flow at a first constant value and, when said potential difference attains said firstpre}, determined value, to interrupt the flow of current of said first constant value.'. I

11.. Apparatus as claimed in claim 10, including means operable by-said measuring means, responsive toj said potential difference increasing above said first-predetermined value and above a' second and higher predetera mined value, to initiate a current flow from said anode to the structure at a second constant value less than said first constant value.

References Cited UNITED STATES PATENTS Sabins 204-196 Fry 204196 Husock 204-496 McNulty 204197 JOHN H. MACK, Primary Examiner 5 T. TUNG, Assistant Examiner U.S. Cl. X.R. 204 496, 228

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