Corrosion prevention system

A method to reduce corrosion of a metal conduit in an aqueous electrolyte conduit system including a nonmetallic conduit system physically attached to the metal conduit comprising impressing an electric potential between an electrode positioned at least partially within the nonmetallic conduit and a ground, the potential being at least about the difference between the electrochemical reaction potentials of reations occurring at the electrode and at the metal conduit.

BACKGROUND OF THE INVENTION 
This invention relates to corrosion of metals. More in particular the 
invention concerns the reduction of corrosion of metal members in systems 
containing both metallic and nonmetallic components. 
Most metals, and especially those containing major amounts of iron, are 
known to corrode, or rust, when exposed to salt water or other 
environments capable of conducting an electric current and an electric 
potential. To retard corrosion of metals, various coatings and anodic or 
cathodic protection techniques have been developed. Such techniques are 
exemplified in U.S. Pat. Nos. 3,313,721 and 3,477,930; Morgan, Cathodic 
Protection 253-265 (1959); and Fontana et al., Corrosion Engineering 
205-214 (1967). 
In certain equipment an electrically conductive liquid solution, i.e. an 
electrolyte, flows from electrically charged members through nonmetallic 
conduits and directly into, and through, metallic conduits attached to the 
nonmetallic conduit. The electrolyte can flow from the metallic conduit to 
a suitable receiving container for storage or disposal. Oftentimes stray 
electric current will flow through the electrolyte and cause the metallic 
conduit to corrode even though this conduit is not in physical contact 
with the electrically charged member. Corrosion of the metal portion of 
this system has previously been reduced by inserting a graphite electrode 
through the nonmetallic conduit, and into the electrolyte, and connecting 
this electrode to a ground. A portion of the stray current flowing through 
the electrolyte was removed, and corrosion of the metallic conduit 
reduced, by means of such an electrode, but corrosion resulting from stray 
electric current in such an electrode protected system is still excessive. 
An apparatus and method is desired to minimize corrosion of metallic 
conduits in a system including a source of stray electric current, a 
nonmetallic conduit and an electrolyte. 
SUMMARY OF THE INVENTION 
It has been found that corrosion of a metal conduit in an aqueous 
electrolyte conduit system, which includes a nonmetallic conduit 
physically attached to the metal conduit, can be reduced. Such reduction 
in corrosion is achieved by impressing a predetermined electric potential 
(volts) between an electrode positioned at least partially within the 
nonmetallic conduit and a ground. The electric potential is at least about 
the difference between the electrochemical reaction potentials (volts), at 
the reaction temperature, of the reactions occurring at the electrode and 
at the metal conduit. 
The present invention includes a system to reduce the corrosion of a metal 
conduit through which an aqueous electrolyte flows. The system comprises, 
in combination, a source of stray electric current with a conduit, having 
at least a nonmetallic inner surface, attached thereto. A metal conduit is 
attached to the nonmetallic conduit. The nonmetallic and metal conduits 
are adapted to contain the electrolyte as such electrolyte flows to, or 
from, the stray electric current source, such as an electrolytic cell, 
from a suitable feedstock container or to predetermined location for 
disposal. To remove stray electric current flowing through the electrolyte 
from, for example, the electrolytic cell, an electrode with a higher 
oxidation potential than the metal conduit is suitably attached to the 
nonmetallic conduit to be in electrical contact with the electrolyte 
within such conduit. A means to impress an electric potential between the 
electrode and an electrical ground is in combination with the electrode. 
Such potential impressing means is of sufficient size to provide an 
electric potential at least equal to about the difference between the 
electrochemical reaction potentials (volts), at the reaction temperature, 
of the reactions occuring at the electrode and at the metal conduit. 
Removing stray electric current from the conduit system in the herein 
described manner reduces the corrosion rate of the metal conduit whether 
the exterior surface of the metal is buried in the earth, submerged in 
water or entirely exposed to air. Thus, the possibility of the metal 
conduit, such as a pipe, corroding sufficiently from stray electrical 
current to permit undesired leakage of the electrolyte is minimized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In one embodiment, a nonmetallic, electric nonconductive pipe is attached 
to an electrolytic cell for producing gaseous chlorine from an aqueous 
solution of sodium chloride. Suitable nonmetallic materials for the pipe 
are, for example, polytetrafluoroethylene, polyethylene, vinyl esters, 
acrylonitrile-butadiene-styrene copolymers and the like. The nonmetallic 
pipe is physically, axially, attached to a metal pipe, generally an iron 
alloy containing at least 50 weight percent iron, by well-known means to 
permit the sodium chloride solution, or brine, to flow through both the 
nonmetallic and iron pipes. The nonmetallic pipe is affixed directly to 
the electrolytic cell to electrically insulate the cell from metal bodies 
in close proximity thereto. Such insulation is, however, not completely 
effective in preventing stray electric current from flowing from the cell 
through the electrolyte and to metal bodies, such as the iron pipe. Stray 
electric current from the cell hastens the corrosion, or rusting of, for 
example, iron in the metal pipe. 
An electrode is inserted through a wall portion of the nonmetallic pipe, at 
a location spaced apart from the metal pipe, and attached to such pipe by 
well-known means, such as bolting. At least a portion of the electrode is 
in physical contact with the sodium chloride solution passing through the 
pipe. When the electrode is electrically connected to an electrical 
ground, a portion of the stray current flowing from the cell through the 
sodium chloride solution will pass through the electrode and to the 
ground. However, the iron pipe will still corrode, since the remaining 
portion of the stray current will continue flowing through the nonmetallic 
pipe and will enter the iron pipe. 
When the electrolyte is a solution of sodium chloride and water, the 
reaction occurring at the electrode positioned in the nonmetallic pipe, 
when an electric potential is applied thereto, is: 
EQU 2Cl.sup.- = Cl.sub.2 (gas) + 2 electrons (e) 
The electrochemical reaction potential for this reaction at 25.degree. C is 
about (-) 1.3 volts (V). An electrochemical reaction potential of about 
0.4 V results when the reaction at the metal pipe is: 
EQU Fe .fwdarw. Fe.sup.++ + 2e 
To remove substantially all of the stray electric current from the 
nonmetallic pipe before such current reaches an iron pipe in physical 
contact with an aqueous sodium chloride solution, a direct current "bias" 
of at least 1.7 V, i.e. the difference between 1.3 V and -0.4 V, is 
applied to the electrode. The use of such a bias minimizes and preferably 
substantially eliminates corrosion of the iron pipe caused by stray 
electrical current. A bias within the range of from about 0.9 V to about 
2.0 V will reduce corrosion of the iron exposed to the sodium chloride 
solution. 
The specific bias applied to the electrode depends upon the compositions of 
the electrolyte and the metal pipe and the reactions which occur at the 
electrode and metal pipe. For example, following are representative of 
anode reactions occurring when various salts are dissolved in the 
electrolyte. 
______________________________________ 
Approximate Poten- 
Salt Reaction tial at 25.degree. C (volts) 
______________________________________ 
Na.sub.2 SO.sub.4 
2H.sub.2 O.fwdarw.O.sub.2 + 4H.sup.+ + 4e 
-0.8 
Na.sub.2 Br 
2Br.sup.- .fwdarw.Br.sub.2 +2e 
-1.1 
NaI 2I.sup.- .fwdarw.I.sub.2 + 2e 
-0.5 
______________________________________ 
An electric bias about equal to the difference between the potentials of 
the reactions occurring at the electrode and at the metal pipe is 
preferred. 
Electrodes suitable for use in the present invention include, for example, 
graphite, titanium and platinum. Titanium electrodes can be coated with an 
electrode activating layer of ruthenium and titanium oxide or cobalt 
oxide. 
The following examples will further illustrate the invention. 
EXAMPLE 1 
An aqueous sodium chloride solution containing a minor amount of impurities 
was passed to an electrolytic cell through a corrodable iron alloy pipe 
and thereafter through a substantially nonconductive, corrosion resistant 
organic plastic pipe. Electric current leakage of 0.57 amperes from the 
cell through the solution in the pipe caused undesirable corrosion of the 
iron pipe. A corrosion resistant metal electrode with a greater oxidation 
potential than iron was inserted into the plastic pipe at a position 
spaced apart from both the iron pipe and the electrolytic cell. This 
electrode was suitably electrically attached to an electrical ground. A 
potential difference of -4.4 volts between the electrode and the ground 
was measured with a voltmeter. 
Attachment of the grounded electrode to the plastic pipe resulted in 
removal of about 0.4 amperes of current, but did not eliminate corrosion 
of the iron pipe. At the approximate temperature at which the solution 
flows through the pipes (about 25.degree. C), the electrochemical reaction 
potentials occurring at the electrode and iron pipe are about (-) 1.3 
volts and 0.4 volts, respectively. When a potential of 1.7 volts was 
impressed between the electrode and the ground by means of a rectifier 
about 0.57 amperes, or substantially all, of the current leakage from the 
electrolytic cell was removed from the system. Corrosion of the iron pipe 
was minimized when such impressed potential was applied to the system. 
EXAMPLE 2 
Substantially as in Example 1, a potential of 2.0 volts was impressed 
between the electrode and the ground. About 0.58 amperes was removed from 
the system to satisfactorily reduce and minimize corrosion of the iron 
pipe. 
EXAMPLE 3 
Substantially as in Example 1, a potential of 1.5 volts was impressed 
between the electrode and the ground. About 0.54 amperes of the current 
leakage from the electrolytic cell was removed from the system. Corrosion 
of the metal pipe, through which the aqueous solution was passing, was 
reduced.