Title:
ELECTROLYTIC METHOD OF MARINE FOULING CONTROL
United States Patent 3661742
Abstract:
An improved method of inhibiting the sustained attachment of marine organisms to metallic surfaces while preventing corrosion of the metallic surface by cathodic protection. Inhibition of marine organism attachment takes place when toxic ions are forced into solution by reversing and increasing the current density in the cathodic protection system at periodic intervals for a short period of time.


Inventors:
Osborn, Oliver (Lake Jackson, TX)
Prows, Bernard L. (Lake Jackson, TX)
Application Number:
05/048523
Publication Date:
05/09/1972
Filing Date:
06/22/1970
Assignee:
The Dow Chemical Company (Midland, MI)
Primary Class:
Other Classes:
114/222, 422/6
International Classes:
C23F13/02; (IPC1-7): C23F13/00
Field of Search:
204/147,148,198,197 114
View Patent Images:
US Patent References:
Primary Examiner:
Tung T.
Claims:
We claim

1. In an electrolytic method for the cathodic protection of a metallic surface in contact with seawater wherein said surface is covered with a metal whose ions are toxic to marine organisms and wherein a current is applied between said surface as the cathode and an anode the improvement comprising reversing said current once each hour for between about 0.5 minute to about 2 minutes at a density of about 10 ma/ft2 to about 1,000 ma/ft2 or once each day for between about 0.5 minute to about 1.5 minutes at a density of between about 1,000 ma/ft2 to about 2,500 ma/ft2, which reverse current is greater than that utilized for said cathodic protection, thereby forcing said toxic ions into solution which inhibit the formation of said marine organisms on said metallic surfaces without causing an appreciable amount of corrosion thereto.

2. The improved method of claim 1 wherein the toxic metal covering is selected from the group consisting of copper, mercury, silver, tin, arsenic and cadmium.

3. The improved method of claim 1 wherein the current is reversed each hour for between about 1.0 minute to about 1.5 minutes at a density of between about 50 ma/ft2 to about 200 ma/ft2.

4. The improved method of claim 1 wherein the current is reversed each day for between about 1.0 minute to about 1.5 minutes at a density of between about 1,200 ma/ft2 to about 2,000 ma/ft2.

5. In an electrolytic method for the cathodic protection of a metallic surface in contact with seawater wherein said surface is covered with a metal whose ions are toxic to marine organisms and wherein a current is applied between said surface as the cathode and an anode, the improvement comprising reversing said current once each hour for between about 0.5 minute to about 2 minutes at a density of between about 10 ma/ft2 to about 1,000 ma/ft2, thereby forcing said toxic ions into solution which inhibit the formation of said marine organisms on said metallic surfaces without causing an appreciable amount of corrosion thereto.

6. The improved method of claim 5 wherein the toxic metal covering is selected from the group consisting of copper, mercury, silver, tin, arsenic and cadmium.

7. The improved method of claim 5 wherein the current is reversed each hour for between about 1.0 minute to about 1.5 minutes at a density of between about 50 ma/ft2 to about 200 ma.

Description:
BACKGROUND OF THE INVENTION

There are many effects on metallic surfaces when they are exposed to seawater, for example, corrosion and marine fouling. Various efforts have been made to protect metallic surfaces from the growth of marine life thereon, i.e. anti-fouling paints and electrolytic systems. The use of cathodic protection systems for the protection of metallic surfaces against corrosion is likewise widely recognized. In such protection systems the metallic surface to be protected acts as the cathode and another metal in a sacrificial capacity acts as the anode.

There have also been devised dual systems for controlling both corrosion and marine fouling of metallic surfaces in the presence of seawater. One such system is disclosed in U.S. Pat. No. 3,303,118. This patent utilizes a system of electrodes whereby a ship's hull can be protected from corrosion by cathodic protection and protected from marine fouling by the decomposition of seawater which creates decomposition products which are toxic to marine organisms. U.S. Pat. No. 3,497,434 discloses a continuous protection system to inhibit growth of marine life on a metal surface while at rest.

The conventional processes, both electrolytic and non-electrolytic, have experienced various problems which make them less than satisfactory. Anti-fouling paints have been found to have a short life span of protection. Previous electrolytic processes experience the problem of having no protection against corrosion when the metallic surface is protected against marine fouling and vice versa.

It is the principal object of the present invention to provide an improved electrolytic system which protects against both corrosion and marine fouling of metallic surfaces.

Another object of the present invention is to provide an improved electrolytic system which protects metallic surfaces from marine fouling by forcing toxic ions into solution thereby inhibiting marine life growth thereon.

A further object of the present invention is to provide an improved electrolytic system which protects against marine fouling by forcing toxic ions into solution at the metal-solution interface which inhibit marine life attachment thereto.

These and other objects and advantages of the present invention will become obvious from a reading of the following detailed description.

It has now been discovered that, in an improved electrolytic system for the cathodic protection of a metallic surface in contact with seawater wherein the metallic surface is connected in electrical circuit with an impressed current anode by covering the metallic surface to be protected with a metal whose ions are toxic to marine organisms and at periodic intervals applying a reverse current to the cathodic protection system, said reverse current having a higher density than the current of the cathodic protection system, said toxic ions are forced into solution at the metal-seawater interface which inhibit the formation of the marine organisms on the metallic surface without causing an appreciable amount of corrosion to the metallic surface.

The metal to be protected from marine fouling is corrosion-protected by any well-known cathodic protection system in which the metal itself is the cathode and an external power source which performs the anodic function. Also, the method of the present invention can advantageously be applied to metallic surfaces which are already provided with special metallic paints for corrosion protection.

The covering of the metallic surface to be protected can be accomplished by a variety of means, for example, by plating the surface with a metal or an alloy or painting the surface with an electrically conductive metallic paint. An electrically conductive metallic paint is one whose metallic pigment particles are so concentrated so that each one is in contact with its neighbors to permit electrical flow. No matter how the protective covering is applied, it should contain metals whose ions are toxic to marine life, i.e. copper, mercury, silver, tin, arsenic, and cadmium. Some of the more commonly employed marine, anti-fouling paints contain cuprous oxide as the toxicant.

The current reversal or "slugging" action of the present invention can be accomplished by utilizing a timer and a double pole-single relay system, but any arrangement of electrical apparatus capable of obtaining the desired result of current flow reversal at a sufficiently increased rate is acceptable. It should be noted that the "slugging" action of the present invention can be accomplished manually as well as through the employment of an automatic timer system.

The length of time, defined previously as periodic intervals, between current reversals is determined by many various factors. The growth rate of the marine organisms, which corresponds to their strength of attachment, must be considered in the determination. The temperature of the seawater in which the metal to be protected is submerged has an effect on the rate of maturity of the marine life therein and consequently their holding power, thus this is another factor to be considered. Further, the current density level which is applied at current reversal is a significant factor in determining, in conjunction with the previous factors, the interval of time between current reversals.

At the moment of current flow reversal, the cathode of the system becomes the anode and vice versa, and the current density is increased from a constant cathodic protection rate to a rate of between about 10 milliamperes/ft 2 to about 1,000 milliamperes/ft 2 for a period of between about 0.5 minutes/hour to about 2 minutes/hour, or a rate of between about 1,000 ma/ft 2 to about 2,500 ma/ft2 for a period of between about 0.5 minutes/day to about 2.0 minutes/day.

Although the novel method of this invention is operable in the above stated ranges, to remain within the limits of economic feasibility, it is preferred, at the moment of current flow reversal, to increase the current density from a constant cathodic protection system rate to a rate of between about 50 milliamperes/ft2 to about 200 milliamperes/ft2 for a period of between about 1 minute/hour to about 1.5 minutes/hour or a rate of between 1,200 ma/ft2 to about 2,000 ma/ft2 for a period of between about 1 minute/day to about 1.5 minutes/day so as to control the release of metallic ions in such a manner as to give maximum efficiency of utilization. Under these conditions, any marine life attachments which may have begun to form on the metallic surface would be killed or forced to lose their "grip" at regular intervals.

The following example is provided to more clearly illustrate the invention, however, and is not to be construed as limiting to the scope thereof.

EXAMPLE

The apparatus constructed for determining the effect of various toxic metals and current levels on attached marine organisms consisted of a timer-relay control panel, housed in a weather-tight box. The necessary variable potentiometers and fixed resistors were included in the circuitry.

Various types of metal coupons were attached by means of insulators to a 4 feet × 8 feet × 14 inches framework of wooden 2 × 4 inch planks. Electric wire leads from each coupon and the anodes led to the control panel where various currents were supplied by means of a 6-volt car battery, continuously recharged. The timer mechanism was operated with 110 volt alternating current.

The apparatus was placed in seawater with a salinity at 3 1/2 percent and a constant current flow of about 4.0 ft/sec. A constant cathodic protection of 3.0 ma/ft2 was supplied to each of the 6 × 12 inch iron, zinc and copper coupons. Three coupons of each type of metal were used.

Cathodic protection was terminated 1 minute out of each hour, during which time, the current flow was reversed and increased to values of 10, 100, and 1,000 milliamperes (ma)/ft2 on the first, second, and third coupons, respectively.

Marine life attachments were inhibited on all of the copper plates. The results were moderate on the coupon with 10 ma reverse current, strong inhibition on the 100 ma coupon, and complete on the 1,000 ma coupon. Little or no effect, however, was noticeable on the iron or zinc plates. After the period of current reversal, it was noticeable that very little corrosion had been caused to the plates by the reversing of the current.

In a similar manner, cathodic protection to the sample coupons is terminated 1 minute out of each day, during which time, the current flow is reversed and increased to values of one thousand fifteen hundred and two thousand five hundred milliamperes (ma)/ft2 on the first, second and third coupons, respectively.

Marine life attachment is inhibited on all of the copper plates. Moderate results are obtained on the coupon with 1,000 ma reverse current, strong inhibition on the 1,500 ma coupon, and complete on the 2,500 ma coupon. Very little corrosion is caused to the plates by the reversing of the current. Little or no effect is noticed on the iron or zinc plates.