Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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r~he prese~t invention rela~es to corrosion protection
of metals and, more par-ticularly, -to methods o~ anodic
protection from corrosion of ai~`~erent objects in contact
with aggressive conducti~g media, which objects are made
~rom materials passivating in such media.
'~he invention is the mos~ advantageous ~or the corrosion
protection o~ equipment in the chemical, ~ood, textile, and
pulp and paper industries.
'~here are other methods of corrosion protection, but
the method o~ anodic electrochemical protection proves to be
the mos~ e~ective in a number o~ cases.
'l~he essence o~` this me~hod ls as ~ollo~s.
Upon comln~ lnto contac~ with an aggressive medium, a
metal is corroded and acquires a potential which is called
a steady-state potentlal. I~ the metal's potential is some-
how altered towards the positive side, the ra~e o~ corrosion
slows down substantially at certain values o~ the potential~
~his phenomenon is re~erred to as passlvation o~ metals, while
the proce~s of bringi~g a metal's potential within a passiva-
tion range is called a passivation process. ~h0 range of
potentials within which metals are passive is called the
passive range. 'l~he ex-tent o~ this range depends upon the
metal and ~he aggressive medium which is in contact with
the metal. By shi~ting ~he electrical pote~tial o~ a metal
into the passive ran~e, the metal can be protectod ~rom
corrosion.
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rL'he electrical potential o~ a metal liable to corrosion
can be brought i~to the passive ra~ge with a view o-P reducing
the rate o~ corrosion by appiyin~ electric curre~t, which
thls case is re~errea to as polarizing current; between
the metal to be protected from corrosion~ whlch in ~his
case is an a~ode, and a~ auxiiiar~ electrode which is a
cathode. ~`he same e~`ect can be produced by adding into the
corrodin$ solution ~r agents ~hm~ ~re called passiva-tors.
Passivation o~ a metal by means o~ polarizing curre~t is re-
~erred to as anodic corrosion protection; the bringing o~
the metal's electric po-tential into the passive ran~e is
called anodic polarization.
~ ne impleme~tion of the ~oregoing method in lndustrlal
conditions calls ~or some ~ to reliably maintain the
passive state of objects ~o be protected ~rom corrosion.
l~ormall~, the passivation of ~etals is e~fected a~d
their passive s-tate i.s maintained by electrical potential
adjus-tment devices which cha~ge either the ma~nitude o~ the
current applied between an object ~ be ~xotected ~rom
corrosion ~the anode) and ~n auxiliar~ electrode (the
cathode), or bhe t1me lnter~als Detween whlch ~aid current
is applied. ~he same ef~ct a~be a-ttained by using the
current of a ~oltaic couple made up by the object being
protected and an auxiliary elec-trode (the protector) which
is in con~act with an aggressive medium.
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There is known an anodic corrosion protection method
carried out with the aid of a periodic-action potential
regulator (cf. US Patent No. 3,442,779, Cl. 204-1~7). According
to this method, use is made of a cathode and a reference :
electrode which makes it possible to check the potential of .
an article being protected from corrosion. Said article,
cathode and reference electrode are in contact with a corroding
medium. The range within which the article's potential is ..
altered is set by a setting device of the potential regulator
and is found within the passive potential range.
According to the method under review, polarizing
current is applied when the potential of -the object being
protected from corrosion drops to the lower limit of the
predetermined potential adjustment range.
A positive shift in the potential of the object
takes place under the action of the polarizing current. As soon
as the upper limit of the potential adjustment range has been
reached, the supply of polarizing current is discontinued. Then,
the potential of the object being protected from corrosion
again gradually drops to reach the lower limit of the potential
adjustment range, which necessitates a repeated application of
polarizing current. The time interval between individual
applications of polarizing current is determined by the rate
of a drop of the potential from the upper to the lower limit - :~ .
of the potential adjustment range, as well as by the properties
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of the metal from which the object which is to be protected
from corrosion is made, the type of the aggressive medium, and
changes in the temperature and level of the conducting medium.
Normally, the intervals at which polarizing current is applied
: vary from a few seconds to many minutes. :.
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The method under review ensures corrosion protection .:
in any technological conditions, yet the relatively high switching .
frequency reduces the service life of the potential regulators
and thus affects the reliability of corrosion protection.
; 10 Besides, this method does not work in cases of power failure.
There is further known a method of anodic corrosion
protection, whereby an article liable to corrosion (the anode)
is passivated and malntained in the state of passivation by
the current of a voltaic couple produced by bringing into con-tact
the article to be protected from corrosion and a cathodic
protector (cf. N.D. Tomashov, G.P. Chernova, "Passivnovst i . :
zashchita metallov ot korrozii" / "Passivation and Corrosion
Protection of Metals" /, Moscow, 1965. p. 153; M. Stern, H.
Wissenbery, J. Electrochem. Soc., 1959, 106, 751). The protector
is made of noble metals (plat:inum, palladium, gold), corrosion-
resistant stainless steels, metal oxides, or graphite.
The operation of the voltaic couple and, consequently,
the reliability of corrosion protection, are largely determined
by the area ratio of the protector and the object to be
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protected from corrosion.
This method of corrosion protection is highly reliable,
easy to implement and independen L of external power supply
sources. On the other hand, the protectors' area must be
sufficiently large both to maintain the passive state of an
article to be protected from corrosion and ensure passivation
in the initial period and if the stable passive state of the
article is for some reason disturbed. Successive utilization
of cathodic protectors is possible only when their area is com-
parable with, and even in excess of, the area of an object to be
protected from corrosion. The latter consideration casts doubt
upon technical feasibility and practical expediency of using
cathodic protectors for anodic protection. This a].so accounts
~or the fact that cathodic pro-tectors have not found extensive
application in the field of anodic corrosion protection.
It is an extremely complicated and arduous task to bring
the equipment to be protected from corrosion into the passive
state. This task also makes it necessary to disconnect the
equipment from the production line. The above factors make
it clear that successful implementation of the anodic protection
method is only possible if there are reliable ways to maintain
the passive state of an object to be protected from corrosion.
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The fore~oing methods of eorxosion protection are -
either not reliable enough to justif~ thelr extensive application,
or are impractable ~rom the technollgical point of view.
In view of the above considerations, it is an object
of the present inventlon to pxovide a highly reliable method
of corrosion protection.
It is another object of the invention to provide a
corrosion protection method which would malce it possible to
prolong the serviee life of the potential adjustment means. ~ -
10Aceordingly, the present invention provides a method
of proteeting an object in contact with a condueting medium
from corrosion by means of anodic polarization of said object,
eomprising periodieally passing polarizing eurrent ~rom an
external souree bekween an auxiliary eleetrode, i.e. a eathode,
and said objeet when the latter~s eleetrieal potential lies
outside preseribed limits to bring said objeet into a passive
state and eleetrically shortLng said auxiliary electrode to said
objeet externally of said eondueting medium, during~ ~me intervals
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when said polarizing~is not being passed from said extexnal
souree between said auxiliary electrode and said object to
~orm a voltaie eouple and so maintain said passive sta-te under
normal operating eonditions,
The funetion of the auxiliary eleetrode, with whieh
the objeet to be proteeted from corrosion comes into eontact,
ean be performed by the eathode and other electrodes whieh
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are in contact with the aggressive conducting medium and can
operate as cathodic protectors.
The corrosion protection method does not affect the
possibility of changing within a broad range the magnitude
or time of application of polarizing current, which is the major
advantage of polarizing corrosion protect:ion devices. Besides,
the method is highly reliable.
The possibility of changing within broad limits the
magnitude or time of application of polarizing current is of
utmost importance during the passivation of the metal of objects
to be protected from corrosion, or in cases of sharp changes
in the parameters of a technological process, which changes
can bring the objects being protected from corrosion into the
active state. When the technologlcal proces9 goes on normally,
the method reliably maintains the passive state of objects
being protected from corrosion.
According to the corrosion protection method, the
auxiliary electrodes (the cathodic protectors) are only used
to maintain the passive state or reduce the rate of a drop
in the potential of objects being protected from corrosion
in the absence of polarizing current; hence, the area of the
auxiliary electrodes may be 50 to 100 times less than that o~
objects being protected from corrosion. This factor makes
it possible to employ cathodic protectors for corrosion
protection.
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According to the method, the potential regulator .
operates only during the initial period, or when the parameters
of technological processes deviate from prescribed values and when
the cathodic protectors are no longer able -to maintain the
passive state of objects being protected f:rom corrosion. This
makes the opera-ting conditions of the periodic-action potential
regulators less severe and prolongs their service life.
According to the method, corrosion protection is effect-
ed by the cathodic protector current, so the method of the
present invention is little affected by power failures.
Other objects and advantages of the present invention
will become more apparent from the :Eollowi.ng detailed description
of a preEerred embodiment -thereoE to be read in conjunction with
the accomapnying draw:incJ which is a block d:iagram of the device
for effecting the proposed method.
Referring now to this drawing, in an object 1, which
is to be protected from corrosion and is ~-illed with a conducting
: medium 2 (for example, sulphur:ic acid or ammonia solution), there
is placed a reference electrode 3 which is electrically coupled
to a regulator 4 determining the electrical potential of the
object 1. The ~otential regul~tor ~ comprises a se-tting device
(not shown) and differential amplifier S. The reference
electrode 3 is connected to one of the inputs of the amplifier 5, ~
whereas ;
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an auxiliary electrode, or cathode, 6 which is also immersed
in the conducting medium 2 is connected to its other input.
The output of the amplifier 5 is connected to the input of a
relay 7 which is a final control element. The relay 7 has make
contacts 8 which are connected into the clrcuit between
rectifier 9 and the auxiliary electrode 6. The relay also
has break contacts 10 which are connected into the circuit
arranged between the electrode 6 and the object 1.
The reference electrode 3 may be a ealomel, ehlorine-
silver, or mereury-sulphate eleetrode.
The auxiliary electrode 6 may be made from a material
which can be used as a cathodic protector, for example, platiniæed
metal, or graphite.
If at a given instant, the measured value oE the
eleetrieal potential of the objeet 1, i.e. of the anode, is
below the lower limit of the adjustment range set by the setting
deviee (not shown), the error signal (a signal of the difference
between the preseribed and aetual potential values), amplified
by the amplifier 5, switehes on the relay 7. Said relay 7
through its make eontaets 8 connects the rectifier 9 to the
eathode 6 and anode 1. Polarizing current in the anode-cathode
eireuit ehanges the potential oE the objeet 1 to a positive
value. ~s the potential reaches the upper limit of the potential
adjustment range, the relay 7 discontinues the supply of
polarizing eurrent to the reetifier 9 and eonneets the eathode
6 through the contaets 10 to the
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anode 1, whereby a voltaic couple is ~ormed. ~l'he current
o~ voltaic couple (-the anode-cathode protector) slows down
the rate o~ the reduc~ion o~ the po~e~tial of the object 1,
~hich is conducive to a stable passive sta-te o~ said object 1
In the course of the passivation o~ the object 19 intervals
betwee~ applications o~ polarizing current are increased
until the current o~ the voltaic couple is suff'icient to
preserve the passi~e state o~ the object 1 being protected
~rom corrosio~. When this state is achieved, the potential
regulator 4 remains idle~ ~his operating mode o~ the device
can be maintained ~ , until -the passi~e state
of the object 1 is disturbed ~or some reason.
1~' the physico-chemical proce~ses u~derway in the techno-
logical system, or some other factors disturb tha passive
state of the object 1, ~he current of the voltaic couple is
insu~icient to restore the passi~e state o~ said ar~icle 1
whose reduced potential again brings into pla~ -the potential
regulator 4 which restores the passive state o~ the object 1.
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