CA1230924A - Corrosion rate probe - Google Patents

Abstract

Abstract of the Disclosure A system, method and probe for monitoring the corrosion rate of different metals in identical corrosive fluids and under identical conditions, such metals being employed as electrodes other than reference electrodes in a probe having also a reference electrode. The preferred probe comprises a probe body holding at least four elec-trodes, where one is placed in the center and the others around it in a circle. The center electrode is preferably used as a permanent reference electrode. Each electrode on the circle can then be connected in turn to a channel on a multistation PAIR meter as test and auxiliary electrodes, whereby the corrosion rate on each of them can be monitored in turn. This arrangement greatly enhances the utility of the multistation PAIR meter and with only two entry ports in a system, all channels can serve to answer specific questions on specific electrodes or metals. The novel probe can also be used with a one channel PAIR meter employed with a multi-station manual switch.

In an alternative electrode arrangement, the center electrode is used as a permanent auxiliary electrode while the system itself is used as a reference electrode and a plurality of test electrodes are employed.

Description

~;3092A~

This application is a division of Canadian Patent applicatlon No. 431,186 filed June 24, 1983.

tion This invention relates to measurlng corrosion and lt relates more partlcularly to instrument~ and tech-nlques used in the study o~ corrosion processes, and most particularly to-the monitorlng Or corrosion rates on a number o~ dl~erent metals and ror variou~ galvanic ~itua-tions, prererably under actual proce~s conditions.

Industrial plants, as well aQ oll rleld e~uip-ment, are built ~rom many dl~eren~ structural and runctional metals, and often a multitude Or these are in contact with the aggressi~e ~luld~. It has been assumed ln the past that the corrosion rates of these dir~erent metals tin particular, the dirrerent steels) can be adequately represented by a slngle test speclmen such as AISI-101~ steel. This assump-tlon is incorrect~or several reasons:

a) It has been obser~ed that pitting rates on these di~erent steels are entirely di~ferent.

b) The response to lnhibitors o~ dlfrerent metals is orten qul~e di~erent. For instance, ~t has been observed that the corroæion o~ I
SA-533A s~eel increases much ~aster at mar- :
ginal corrosion inhibitor concentrations than the corroslon rate ror AISI-1018 or AS~'M 285C.

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c) Different metals respond differently to process upsets (in chemical cleaning studies it has been found, for instance, that ASTM~533A alloy corrosion is accel-erated much less in the presence of oxygen or under oxidizing conditions than AISI-101 carbon steel.) d) Galvanic coupling of metals is unavoidable and different metals again respond differently to such effects.
It is therefore, essential to monitor eorrosion rates on a number of different metals and ~or various galvanie situations as well as under aetual proeess eonditions. Such monitoriny may be eoncerned with general corrosion rates, pitting corrosion, adequate protection by inhibitors or for the purpose of determining process upsets. The purpose of such monitoring, of course, is aimed at failure prediction and proper maintenance outage scheduling.
Corrosion rates can be conveniently monitored by the linear polarization technique sueh as is carried out with the PAIR instruments and probes available from the Petreco Division of the Petrolite Corporation. A PAIR instrument is described and claimed in U.S. Patent No. 3,406,101, issued October 15, 1968 to James W. Kilpatrick. The PAIR
technique employs three electrodes on an electrode holder, of which one is the test electrode (cathode), another one ~Z3~92~

used as auxlllary electrode (anode) and the third a~
re~erence electrode. This technlque, thererore, requires one entry port in order to attaln one data polnt. Further-more, it has been assumed ln ~he pas~ that the linear polarization technique ls only adequate ~or general corro-sion rate monitorlng while a zero resistance amm~ter is necessary to monitor galvanic current sltuatlons. There~ore the current state o~ the art requires several instruments and a multltude Or entry ports into an actual system in order to monitor corrosion on dir~erent metals and ~or gal-vanic situ~tions.

The ob~ects of this lnvention are to provlde instrumentation, circuitry and a method to enable this complex monltoring task to be simplir$ed.

Other oblects o~ the invention will be apparent from the followlng descriptlon o~ the~invention.

Summary o~ the Invention The above and other ob~ects o~ the invention may be accomplished by pro~ldlng a ~ovel multi-electrode probe for use in measuring corrosion rates, clrcuit means for connecting such probe to a corrosion rate measuring instrument, and a method ror monitoring ~he corrosion ra~e o~ di~rerent metals in identical corroslve ~luids and under ldentical condltlons, employing such probe and circuit means.

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The novel probe o~ thi~ invention comprises electrode holding means; a flrst rod-shaped metallic elec-trode held by the holdlng means and extending outwardly from and centrally positioned with respect thereto; at least three other rod-shaped metallic electrodes o~ dl~rer-ing composition also held by and extending ou~wardly ~rom the holding means and arranged symmetrically in a clrcle around the first electrode, all the elec~rodes being para-11~1 to each o~her; means for in~ulating and sealing the electrodes ln a rluid tight manner rrom the holding means;
electrical termlnal means on the holdlng means; and conduc-tive means in the holding means ~or connecting each o~ th-e electrodes to a respectlve terminal means. The electrodes are preferably cylindrical.

The cent~dlty positloned electrode is pre~erably used as a ref'erence elec.trode, with e~ach Or the other elec-trodes being adapted ~or us~ either as a test or an auxlliary electrode~ Alternatively~ ~he centrally posltioned electrode may serve.as a permanen~ auxillary electrode, with the other el~ctrodes being test electrodes. In that embodlment, the system is used as a re~erence electrode, e.g., the holding means being a~apted ~or such purpose.

At least one o~ the te~t or test~auxiliary elec-trodes may be a galvanlcally coupled electrode comprising two dimensionally xubstantlal~y identical metallic rods, d~f'er-lng ln composltion, which are electrlcally connected by conductlve means which also serves to hold the two rods i.n ~Z3~929L

end to end relationship, fluid tight sealing means being positioned between the rods and at the end of the electrode opposite the holding means. Such galvanically coupled electrodes are disclosed and claimed in our copending application No. 431,185 filed June 2~, 1983.
The invention also includes circuit means for connecting a probe of the preferred type, i.e., with a center reference electrode, as above described, to a corrosion rate measuring instrument having one or more channels, each channel being provided with terminal means, each terminal means including terminals for connection to electrodes serving as r~erence, test an~ auxiliary electrodes, respectively. With a multi-channel instrument, such circuit means comprises: conductive means including parallel branches connecting the reference electrode with the reference electrode terminal in each terminal means; and conductive means including parallel branches connecting each electrode other than the reference electrode to a test electrode terminal in one terminal means and an auxiliary electrode terminal in a different terminal means, each terminal being connected to only a single electrode.
Where the instrument is of the single channel variety, the connecting circuit comprises: first and second multiple pole switching means, each having a plurality of stationary poles and a common terminal mounting a movable contactor; conductive means for connecting the common ~ ~ 3 09 ~ ~

terminals of the first and second swltch~ng me~ns with the tes~ and auxiliary ele trode termlnals respectively, o~ the measuring inætrument; conductive means includlng parallel branches ~or connecting each electrode other than the reference electrode ~o a pole of each Or the first a~d second switchlng means g each pole belng connected to only a single electrode; and conductlve means ror mechan-ically interlocking said flrst and second switching means so that the two contactors move synchronously in a rixed sequence.

The invention ~urther includes a system and method ~or monitoring the corroslon rate Or dlfferent metals in identlcal corrosive fluids and under identical conditions, such metals belng employed as electrodes other than rererence electrodes in a probe ~vlng also a reference electrode.
The system comprises ln ~omblnation, ~ corrosion rate probe havi~g a re~erence electrode and a plurallty Or electrodes other than re~erence electrodes; a corrosion rate measuring ins~rument havlng one or more channels, each provided w1th terminal means including terminals ror connection to elec-trodes ser~lng as re~erence, test and auxiliary e~ectrodes, respectively; means ror connecting each elec~rode other than the re~erence elec~rode ln sequence to a channel ln the measurlng lnskrument and, where a dedicated auxillary electrode is not employed, rirs~ as a test ele~trode and then as an auxiliary electrode; and means ror connecting the re~erence electrode with the re~erence electrode ~erminal o~ the meas-uring instrument. Where the measuring instrument has .~ ~3(~ 4 multiple channel~, each ehannel i5 adapted to be actlva~ed ln turn to monl~or a di~ferent elec~rode serving as a test electrode. Where a single channel lnstrumel~t is used, the ~ystem includes swltching means ~or accompl:lshing the connec-tion sequence.

The method comprises: (a) immersing the electrodes in the corroslve ~luld; ~b~ allowlng them to remain ln the fluld for a period Or undisturbed corrosion;
and ~c~ connectln~ each electrode other than the reference electrode in turn to a channel in a corrosion rate measuring ins~rument, and, ~here a dedlcated auxlllary electrode is not employed, first as a test electrode and ~hen as an aux-illary electrod~. The corroslon rate measurin~ lnstrument may have multiple channels with each channel being activated in turn to monltor a a~erent electrode serving as a test slectrode; or may be a single channel~lnstrument, the connec-tlon s~quence being accomplished by switching means.

Descrlption of the Drawings ~ig. 1 ls a bottom (~luid-side~ view of the probe assembly of this invention.

Fig. 2 is a view, partly in section, taken along line 2-2 of the probe assembly shown in Figo 1~

Fig. 3 is a circult diagram illustrating ~he connection o~ seven electrodes from a probe assembly as shown ln Figs. 1 and 2 to a mul~ichannel corrosion rate mon.toring instrument intended ror use with three electrode probes.

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Fig~ 4 ls a circuit diagram lllustrating the connection o~ seven electrodes from a probe assembly as shown ia Figs. 1 and 2 to a single channel corrosion rate monltoring lnstrument intended ~or use with three eiectrode probes.

Detailed Descrl~t_on of the Inventlon .
Rererrlng to Flgs. 1 and 2, there ls shown a probe assembly whlch comprises a metallic body 1, on whlch are carried replaceable cyllndrlcal electrodes 2, 3, ll~ 5, 6, 7 and 8, Or which electrode 8 is a re~erence electrode and the remalning electrodes are Or varying compositlon and serve ln turn as te~t and auxiliary electrode~. These elec-trodes extend outwardly ~rom probe body 1 and are parallel to each other. Re~eren~e electrode 8 is centrally positioned with respect to probe body 1 and the other electrodes are arranged symmetrically in a clrcle around the re~erence electrode. Fig. 2 shows three of t~ese electrodes, namely electrodes 2, ~ and 8. These electrodes are secured by threaded or other means, not shown, to metallic pin~ 9, 12 and 15, respectively, which ln ~urn are connecbed withln the body 1 by lnsulated electrlcally conductive means 22, 23 and 24, respectively, to the respective terminal lugs, 16, 17 and 18 o~ a multi-connector elec~rlcal fi~ting 20 carried on ~op Or body 1. Lugs 16, 17 and 18 are electrically common to respective plns, not shown, o~ multlpln fitting 20 3 secured to the top o~ probe body 1 by screws 25. Electrical connec-tion to a multl-channel corroslon rate moni~oring ins~r~ent ~LZ309;~4 such as the Petrolite Corporation's Model 1010 ten-channel PAIR meter is by a cable connector 21, carrying conductors connected respectively to the pins of fitting 20. Such conductors may be as shown in U.S. Patent No. 3,639,876, issued February 1, 1972 to Homer M. Wilson. However, any insulated conductive means may be used for this purpose, including Conax (trade mark) connectors with special insulating glands, which are particularly useful for high temperature and high pressure applications. El~ctrodes 3, 4, 6 and 7, not shown in Fig. 2, are similarly secured and connected with conductors in cable connector 32. The conductors are shown schematically ir~ the circuit diagram o~ Fig. 3 as 72, 73, 74, 75, 76, 77 and 59.
Probe body 1 carries external threads 24 which are adapted to engage with interior threads of a pipe fitting which forms part of a piping system carrying fluids whose corrosive effect it is desired to determine, as shown in U.S. Patent No. 3,639,876. A cavity 41, preferably cylindrical, as shown, is provided from the top of the body 1 to a region adjacent the lower cylindrical portion carrying threads 42. The lower portion of the body 1 is provided with a plurality of passageways 26, 27, 28, 29, 3~, 31 and 32. These passageways are threaded, at least in part, to receive insulating members 33, 34, 35, 36, 37, 38 and 39 through which pass metallic pins, of which three, namely 9, 12 and 15, passing through members 33, 36 and 39, respect-ively, are shown in Fig. 2. Insulating members 33, 34, 35, 31 Z309~4 36, 37, 38 and 39, which include fluid tight sealing means, may be of the construction shown in U.S. Patent No. 3,639,876.
The ends of the pins 9, 12 and 15 extend within the cavity 41 and are provided with threads or other attaching surfaces, not shown, onto which electrical connections may be made. More particularly, electrical conductors 9, 10, 11, 12, 13, 14 and 15 are secured to the attaching surfaces of the pins by terminal clips or equivalent means, not shown.
; Further details wlth respect to the electrodes, pins, insulating and sealing means, etc., are like those shown in U.S. Patent No. 3,639,876.
~ lthough the probe as described above has the reference electrode in the center and the other electrodes arranged around it in a circle, alternative arrangements are, of course, possible~ e.g., where the center electrode could be used as a permanent auxiliary electrode and the system itself is used as a reference electrode.
Moreover, one or more of the test or test/auxiliary electrodes may be a galvanically coupled electrode comprising two dimensionally substantially identical metallic rods, differing in composition, which are electrically connected to conductive means which also serves to hold the two rods in end to end relationship, fluid tight sealing means being positioned between the rods and at the end of the electrode opposite the holding means, as described and claimed in our aforementioned copending application No.
431,185.

i~3~9~4 It wlll be apparenk that the multi-eleckrode probe of this inYention ls not restric~ed ~o the detalls Or any particular probe holder or assembly, such a~ that above described, but ex~ends to any holder suitable rOr holdlng three or more electrodes in a circular arrangemen~
around a central electrode, wlth any sultable lnsulatlon and seallng means. Moreover,.as above lndlcated, the probe may be one using the body Or the probe ltself, l.e., the holder, as the r~erence electrode and havlng a plurallty of test electrodes which can be used in turn, along with a permanent auxiliary ~lectrode whlch may be centrally posl-tioned.

The present multi-electrode may, by employlng a multlstation manual switch, be used wlth a slngle channel corrosion rate measuri~ instrument, pre~erabty one based on llnear polarlzation and using a three electrode probe, such as the Petrollte Model M 103 PAIR meter. However, the multl-electrode probe is particularly use~ul with, and greatly enhances the utility Or, a multl-channel meter such as the Petrolite Corporation Model 1010 ten channel meter, when used in association with it. With the use of such an arrangement and wlth only two entry parts in a system, all channels can serve to answer specific questions on speclfic electrodes or m~tals~

The clrcuit diagram o~ Flg. 3 illustrates the circuit contemplated for connecting the probe shown in Figs.
1 and 2 with a multichannel corroslon rate monitoring .

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lnstrument~ preferably one based on linear polarlzatlon and intended for a three electrode probe, such as the . . .
Petrolite Corporation's Model 1010 PAIR me~er, des~gnated generally as 51. Ik wlll be seen that six channels Or the PAIR meter are utlllzed, each provlded with terminal means designated 52, 53, 54, 5~, 56 and 57, respectively. Each termlnal means lncludes terminals ror connection to elec-trodes em~loyed as reference, test and auxlliary electrode, labelled R, T and A, respectively. The rererence electrode 8 Or the probe ls connected by mean~ o~ conductors 59 and 60 and branchlng conductcr~ 61, 62, 63, 64, 65 and 66 to th~ ter-minals 52R, 53R, 54R and 57R, respectively. Electrodes

2, 3, 49 5, 6 and 7 are connected by means Or conductors 72, 73, 74, 75, 76 and 77, respecti~ely, to terminals 52T, 53T, 54T, 55T, 56T and 57T respectively. Branching conductors 82, 83, 84, 85, 86 and 87 are connected with conductors 72, 73, 74, 75, 76 and 77, respectively, and are connected at thelr other ends with termlnals 53A, 54A, 55A, 57A, and 52A, , respectlvely. Thus, each o~ the non-re~erence electrodes 2~ 3, 4, 5, 6 and 7 may be used in sequence as a test, then auxiliary electrode, with the result that the corro~ion rate on each o~ these electrodes can be mon~tored ln turn.

Re~err~ng to Fig. 4, there is illustrated a circuit ror connecting the probe shown in Fig~. 1 and 2 with a slngle channel corroslon rate monitoring instrument, pre~erably one based on linear polarizatlon and intended ~or a three electrode probe, such as Petrolite Corporation's Model M 103 PAIR meter, designated generally as 90 and -12~

3~924 ~aving termlnals for connec~lon to electrodes employed as rererence, test and auxiliary electrode, lablelled R, T and A, respectively. Electrodes 2, 3, 4, 5, 6 and 7 o~ the probe are connected by means o~ conductors 92, 93, g4, 95, 96 and 97, re~pectively, to poles 102, 103, 104, 105, ld6 and 107 Or a rirst multlpole rotary sw~tch deslgnated generally a 100.
Branchlng conductors 112~ 113, 114~ 115, 116 and 117 are con-ducted with conductors 92, 93, 94, 95, 96 and 97, respectively, and are connected at their other ends wlth pole~ 122~ 123, 124, 125, 126 and 127 of a second multipole rotary switch designated generally aa 120. The common termlnal 101 Or the switch 100 15 connected by means o~ conductor 91 to the auxiliary elec~rode terminal A of the monitoring instrument 90. S~m~larly, the common termlnal 121 o~ swltch 120 ls connected by means of con-ductor 111 to the test electrode terminal T of the monitoring instrument 90~ Switches 100 and 120 are mechanlcally lnter-locked by conventional means shown sc~ematically by dash line 130 so that the swltch contactors 108 and 128, mounted on common termlnals 101 and 121, respectlvely, move synchronously in a flxed sequence. Reference electrode 8 of the probe ls connec-ted via conductlve means 98, which may include termlnals 132 and 133 associated with the swltching means, with the reference electrode terminal R o~ the monitorlng instrumen~ 90. Thus (as 1n the case of the circult shown in Fig. 3), each of the non-re~erence electrodes 2, 3D 4, 5J 6 and 7 may be used ln sequence as a test, then auxiliary electrode, with the result that the corrosion rate on each o~ these electrodes can be monitored ln turn. I

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In a ~ystem and process ror mon.ltorlng the corrosion rate of di~reren~ metals ln ident:ical corrosive ~luld and under identlcal condltion~, where a probe havlng three electrodes is employed o~ whlch one ls a rererence electrode, the electrodes should be employed ln a trlan-gular configuration, rather than ln a linear con~iguratlon~
to avoid erroneous results. However~ a linear conrlguration of three electrodes may be success~ully employed where the center electrode i3 a dedicated auxillary electrode, the body o~ the probe ls employed as the re~erence electrode, and the other two electrodes are te~t electrQdes.

Variou~ modi~icatlons can be made ln the appa ratus, circuitry and procedure by those skilled ln the ar~
in l~ght Or the above descriptlon without departing ~rom the spirit o~ the inventlon as defined ln the appended claims.

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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In combination, (a) a corrosion rate probe having a reference electrode and a plurality of electrodes other than reference electrodes;
(b) a corrosion rate measuring instrument having one or more channels, each channel being provided with terminal means including terminals for connection to electrodes serving as reference, test and auxiliary electrodes respectively;
(c) means for connecting each electrode other than said reference electrode in sequence to a channel of said corrosion rate measuring instrument such that each of said other electrodes is connected to said measuring instrument first as a test electrode and then as an auxiliary electrode; and (d) means for connecting said reference electrode with the reference electrode terminal of said measuring instrument.

2. The combination of claim 1 wherein said corrosion rate measuring instrument has multiple channels and each channel is adapted to be activated in turn to monitor a different electrode serving as a test electrode.

3. The combination of claim 1 wherein said channel is the sole channel in said corrosion rate measuring instrument, said combination including switching means for accomplishing the connection sequence.

4. A method of monitoring the corrosion rate of different metals in identical corrosive fluids and under identical conditions, said metals being employed as electrodes other than reference electrodes in a probe having also a reference electrode, said reference electrode being connected to the reference electrode terminal of a channel of a corrosion rate measuring instrument, said channel having terminals for connection to electrodes serving as reference, test and auxiliary electrodes, respectively;
(a) immersing the electrodes of said probe in said corrosive fluid;
(b) allowing said electrodes to remain in said corrosive fluid for a period of undisturbed corrosion;
and (c) connecting each electrode other than the reference electrode in sequence to said channel of said corrosion rate measuring instrument, each electrode other than the reference electrode being connected in sequence to said channel of said corrosion rate measuring instrument first as a test electrode and then as an auxiliary electrode.

5. The method of claim 4 wherein said corrosion rate measuring instrument has multiple channels and each channel is activated in turn to monitor a different elec-trode serving as a test electrode.

6. The method of claim 4 wherein said channel is the sole channel in said corrosion rate measuring instrument and the connection sequence is accomplished by switch-ing means.