WO1999002760A1

WO1999002760A1 - Use of anionic inhibitors to reduce corrosion in anodes used in electrochemical applications

Info

Publication number: WO1999002760A1
Application number: PCT/US1998/014285
Authority: WO
Inventors: James B. Bushman
Original assignee: Bushman James B
Priority date: 1997-07-09
Filing date: 1998-07-09
Publication date: 1999-01-21

Abstract

A method for improving corrosion resistance of anodes used in impressed current electrochemical reactions calls for situating an anode in a predetermined environment. An electrical potential is applied between the anode and at least one other metal in contact with the environment to create an electrical potential. An anionic inhibitor is incorporated into the environment adjacent the anode. Anions from the anionic inhibitor are attracted to the anode to reduce the rate at which the anode corrodes.

Description

USE OF ANIONIC INHIBITORS TO REDUCE CORROSION IN ANODES USED IN ELECTROCHEMICAL APPLICATIONS

Background of the Invention

The present invention is directed to the art of metal anodes, and more particularly to anodes used in promoting impressed direct current electrochemical reactions. The present invention is particularly directed to reducing corrosion in iron-containing anodes, such as high-alloy super stainless steel anodes, as well as anodes formed from materials containing a relatively high content of cast iron and approximately 5 - 25% silicon.

Impressed current anodes are used in facilitating electrochemical reactions. They are used in a variety of environments for numerous applications including cathodic protection, electrowinning of base metals, electroplating, electrogalvanizing of steel, and the production and treatment of foils. In the case of cathodic protection, the anodes are used to inhibit corrosion of other metal objects. For example, impressed current anodes are used to cathodically protect steel reinforcements found in concrete structures such as bridges and parking decks. The steel reinforcements are subject to corrosion, especially in marine environments or in areas where de-icing salts are spread on winter roadways. The salts cause the reinforcing steel to oxidize, and the resulting rust can extend to cause cracks in, and eventual destruction of, the

concrete.

Reinforcing steel is protected by embedding a cathodic protection anode between the reinforcement and the surface of the concrete. An electric potential is supplied to the anode such that the reinforcing steel becomes cathodic. Valve metal substrates coated with a precious metal or precious metal oxide are commonly used anode materials suited for this application. One such commonly used anode material is platinum-coated titanium. The platinum jacketing serves as the anodic current carrier and virtually all the current is transferred between the platinum and the surrounding electrolyte at surfaces where the coating is intact. If the coating is scraped off and the titanium substrate is exposed to the environment, the substrate will passivate or form an oxide film, thus directing the current to flow where the platinum is located. Unfortunately, the applicable coatings required to produce the capability of anodic current discharge and low consumption rates include

only those in the platinum metal and metal oxide families, all of which are very expensive and must be applied under expensive and controlled conditions.

Iron-based anodes such as those made from high alloy or superalloy stainless steels provide economical alternatives to prior art anode materials in cathodic reactions. They also provide improved corrosion resistance over other iron-containing anodes of the prior art. However, the iron

content in the anodes renders them subject to the corrosive effects brought by salts, water and other oxidizing situations.

In U.S. Patent No. 5,667,649, the present inventor describes that high-alloy super stainless steel anodes offer improved corrosion resistance over

prior art cathodic protection anodes comprised of high levels of iron (i.e., over

70%). However, it is still desirable to continue to improve upon the corrosion resistance of all iron based anodes including super stainless steel anodes, stainless steel anodes, and other iron-based anodes.

Because iron-based anodes are used in a variety of electrochemical reactions, and in environments that are ripe for corrosion, it is

desirable that improved corrosion resistance be made available for such anodes.

Such an improvement would increase the life of the anode, thus requiring fewer replacements.. The present invention contemplates a new method for improving the corrosion resistance of iron-based anodes used in electrochemical reactions.

Brief Description of the Invention In accordance with the present invention there is provided a method for improving corrosion resistance of iron based anodes used in electrochemical reactions.

In accordance with a more limited aspect of the invention, a method for improving corrosion resistance of anodes used in impressed direct current electrochemical reactions calls for situating an anode in a predetermined electrolyte environment for conducting an electrochemical reaction. An anionic

inhibitor is incorporated into the environment adjacent the anode. An electrical potential is applied between the anode and other metallic components in the

environment. Anions from the anionic inhibitor are attracted to the anode to reduce corrosion attack of the anode metal.

An advantage of the present invention is that the anode lasts much longer than one that is left untreated.

Another advantage of the invention is that a lesser amount of anode material is required for use in an environment where a greater amount of

untreated anode was previously used. Still other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description.

Detailed Description of the Invention

Corrosion inhibitors are used to delay or prevent corrosion of metals in environments in which corrosion typically occurs. Anionic inhibitors are typically used to passivate or produce a large positive shift in a metal's corrosion potential. They are commonly used to control galvanic corrosion. Galvanic corrosion can occur between two dissimilar metals that are in contact with one another such as iron and copper. The corrosion can also occur on a single metal where separate grains of metal are different, and one becomes the cathode and the other the anode. A third type of galvanic corrosion concerns oxygen concentration corrosion. This involves a single metal, where part of the metal is starved of oxygen and the other part is in an oxygen rich environment.

These two parts form an anode/cathode couple.

Anodes used in connection with electrochemical or cathodic

reactions are preferably inherently corrosion resistant. However, there is room to improve the corrosion resistance of such anodes. Anionic inhibitors situated

in the environment surrounding iron-based anodes enhance the self protection

capabilities of these anodes.

Anionic corrosion inhibitors, namely calcium or sodium chromate, silicate, nitrite, benzoate, phosphate, dibasic phosphate, hydrogen carbonate and others are used to enhance the corrosion resistance of iron based anodes. The anionic inhibitor is incorporated into the environment surrounding an iron-based anode used for cathodic protection. The anionic inhibitor that is selected for a particular situation is compatible with the environment in which it

is incorporated. It is preferably applied by admixing it into the environment, painting it to a surface of the surrounding environment, or possibly painting it directly to the anode itself. In addition, the inhibitor can be electrochemically deposited on the anode surface prior to using it as a part of a cathodic reaction.

The anionic inhibitors contain negatively charged ions that tend to passivate, reduce and retard the corrosion rate of iron materials such as steel. Iron-containing anodes are used in a variety of cathodic reactions. The corrosion resistance of these anodes is improved using anionic inhibitors in the surrounding environment. The various applications where this improvement is realized include those involving anodes in cathodic protection, electrowinning of base metals, plating, electrochemical applications, de- watering, electrogalvanation of steel, the production and treatment of foils, and

other applications.

There are a number of specific situations where the anionic inhibitors can be used in cathodic protection applications. These are listed by way of example only and are in no way limiting. One such situation involves

the case of anodes used for cathodic protection of reinforcing steel in concrete.

Here, calcium nitrite is admixed into the concrete environment that surrounds

the anode. In a second situation, anionic inhibitors are admixed in the backfill around anodes placed in the soil in underground applications, (e.g. to cathodically protect buried gas pipelines). Here, the anionic inhibitor is added to the sand or to the coke breeze or other backfill material that surrounds the anode. A third situation where the anionic inhibitor can be used to enhance the corrosion resistance of an iron-based anode used in cathodic protection involves

the water box portion of a closed loop heat exchanger. The anionic inhibitor is placed directly in the water where the anode is situated.

Although it is known to use anionic inhibitors such as nitrites for passivating galvanic corrosion reactions, anionic inhibitors have never before been used in applications involving cathodic protection or other impressed current electrolytic reactions to reduce the corrosion rate of separately installed anodes. By placing an anionic inhibitor closely proximate or in contact with the anode material, the current flow is reversed. The anionic inhibitor is drawn to the surface of the anode material. Positive ions (cations) flow toward the metal object being protected (such as reinforcing steel, steel water boxes or metal underground pipes or the like), and negative ions flow toward the installed anode. As a result, the anionic inhibitor is drawn to and reacts with the iron in the anode to protect it from corrosion. At the same time, the reinforcing steel or

other steel structure is protected from rapid corrosion by operation of this

cathodic protection system.

Example

A steel-reinforced concrete structure in need of surface repair

was scarified or scabbled (i.e., roughened up or blasted) and the loose concrete was removed to expose more concrete. A super stainless steel mesh anode was placed over the exposed concrete and fastened into place with Christmas tree fasteners. Next, a concrete mixture admixed with calcium nitrite was poured over the anode and exposed concrete at a thickness of about one to two inches. The concrete set and the anode was encapsulated. Concrete is hygroscopic in that it absorbs moisture and holds it in place around the anode mesh. Because the anode is sensitive to current density, it is operated at low current density in the range of about 0.05 to 3.0 milliamperes/square foot of anode material. Higher current densities may cause accelerated corrosion of the anode. The anode resists corroding at low current densities, and thus a large surface area for the anode is preferred. For example, in the case of where a titanium based anode material may call for about 0.2 square feet of mesh per square foot of concrete surface. Super stainless steel anodes may call for twice that amount (about 0.4 square feet) of mesh per square foot of concrete surface. In operation, the anionic inhibitor is drawn to the surface of the anode material.

The positive ions flow from the anode to the reinforcing steel rod, and the negative ions flow from the reinforcing rod to the anode., As a result, the anionic inhibitor is drawn to and reacts with the iron in the stainless steel anode to protect it from corrosion. The anode can be operated at higher current

densities than before. Also, a smaller amount of anode material is required to accomplish the same result.

The invention has been described with reference to the preferred embodiment. Obviously modifications and alterations will occur to

others upon a reading and understanding of this specification. It is intended to

include all such modifications and alterations insofar as they come within the

scope of the appended claims or the equivalent thereof.

Claims

I claim:

1. A method for improving corrosion resistance of anodes used in impressed direct current electrochemical reactions, comprising the steps of: situating an anode in a predetermined environment for conducting an electrolytic reaction; incorporating an anionic inhibitor to the environment adjacent the anode; applying an electrical potential between the anode and at least one other metal in contact with the environment; and attracting anions from the anionic inhibitor to the anode to inhibit corrosion of the anode.

2. A method for improving corrosion resistance of anodes, according to claim 1, wherein the anode is comprised substantially of an iron alloy.

3. A method for improving corrosion resistance of anodes, according to claim 1, wherein the anionic inhibitor is compatible with the

environment and selected from the group consisting of calcium or sodium chromate, silicate, nitrite, benzoate, phosphate, dibasic phosphate and hydrogen

carbonate.

4. A method for improving corrosion resistance of anodes, according to claim 1, further comprising the steps of: admixing a calcium nitrite anionic inhibitor in concrete; encapsulating an anode in the admixed concrete; attracting anions from the calcium nitrite to the anode to inhibit corrosion of the anode.

5. A method for improving corrosion resistance of anodes, according to claim 4, wherein the anode that is encapsulated in the admixed concrete contains iron.

6. A method for improving corrosion resistance of iron- based anodes used in electrochemical cathodic reactions, comprising the steps of: situating an iron-based anode in a predetermined environment

for conducting a cathodic reaction; incorporating an anionic inhibitor to the environment adjacent

the iron-based anode; and attracting anions from the anionic inhibitor to the iron-based

anode to inhibit corrosion of the anode.

7. A method for improving corrosion resistance of anodes, according to claim 6, comprising the additional step of:

applying an electrical potential across the iron-based anode.

8. A method for improving corrosion resistance of iron based anodes, according to claim 6, wherein the step of incorporating includes admixing.

9. A method for improving corrosion resistance of iron- based anodes, according to claim 6, wherein the anionic inhibitor is compatible with the environment and selected from the group consisting of calcium or sodium chromate, silicate, nitrite, benzoate, phosphate, dibasic phosphate and hydrogen carbonate.

10. A method for improving corrosion resistance of anodes used in impressed current reactions, comprising the steps of:

admixing a nitrite anionic inhibitor in a concrete environment; encapsulating an iron-based anode in the admixed concrete;

applying an electrical potential between the anode and at least one other metal in contact with the concrete environment; and attracting anions from the nitrite anionic inhibitor to the anode to

reduce corrosion of the anode.

11. A corrosion inhibiting system, comprising:

a power source; an anode electrically connected to the power source for use in a cathodic protection system; an electrolyte adjacent the iron-based anode containing an anionic inhibitor and a surrounding environment, the anionic inhibitor being compatible with the surrounding environment.

12. The corrosion inhibiting system of claim 11 wherein the

anionic inhibitor is selected from the group consisting of: calcium or sodium chromate, silicate, nitrite, benzoate, phosphate, dibasic phosphate and hydrogen carbonate.

13. The corrosion inhibiting system of claim 11 wherein the aggregate comprises concrete and a nitrite anionic inhibitor.

14. The corrosion inhibiting system of claim 11 wherein the

anode contains iron.