Figure 10 Galvanic Corrosion at Iron-Copper Pipe Junction

Figure 10 shows the junction of iron and copper pipes containing a solution of a copper salt. The oxidation potential of iron is sufficiently greater than that of copper so that iron is capable of reducing Cu" ions to copper metal. In this case, iron corrodes near the junction, and additional copper builds up on the copper pipe near the junction.

The solution to which the metal junction is exposed need not contain a salt of one of the metals for galvanic corrosion to occur. If the iron-copper junction were exposed to water without Cu⁺² ions, the reduction reaction would be as shown in Equation (2-4).

Again, iron would corrode near the junction, but in this case hydrogen would be formed on the surface of the copper.

Prevention of Galvanic Corrosion

A method called cathodic protection, discussed previously in this module, is often used to retard or eliminate galvanic corrosion. One of several ways of accomplishing this is to attach a third metal to the metals to be protected. This metal must have an oxidation potential even greater than that of the metal to be protected. The most active metal then tends to corrode in place of the protected metal. The metal that corrodes to protect another metal is called a sacrificial anode. This method is applied in the original design of structural materials. Zinc is a common sacrificial anode and is often used in cooling water systems that contain seawater.

Galvanic corrosion can also be limited by: 1) using only metals that are close on the activity series (discussed in the chapter on Corrosion Theory), 2) electrical insulation of dissimilar metals, and 3) using poorly-conducting electrolytes (very pure water).

The relative surface areas of the two metals are also important. A much larger surface area of the non-active metal, compared to the active metal, will accelerate the attack. It has been determined that the relative surface area is the determining factor in the corrosion rates.

The required electrical current for galvanic corrosion will be stopped if the dissimilar metals are:

separated by a non-conducting junction, separated from a conductive environment, and located in a poorly conducting electrolyte (pure water).

Summary

The important information of this chapter is summarized below.

Crud and Galvanic Corrosion Summary

Crud is corrosion products in the form of finely divided, insoluble oxide particles suspended in the reactor coolant or loosely adhered to metal surfaces or activated corrosion and wear products.

Scale is the deposition on the surfaces of the piping from the formation of insoluble compounds from normally soluble salts. Most common are calcium or magnesium carbonates.

Galvanic corrosion is the corrosion that results when two dissimilar metals with different potentials are placed in electrical contact in an electrolyte.

The problems of crud in reactor plants are:

Fouling of coolant flow paths Fouling of heat transfer surfaces High general background (ambient) radiation levels Radiation hot spots

Radioactive waste disposal

The causes of a crud burst in the reactor coolant are:

Increased oxygen concentration Reduced (or significantly changed) pH Large temperature change

Physical shock (for example, starting and stopping pumps, changing speeds of pumps, reactor scram, or relief valve lift)

Galvanic corrosion functions on the principle of the electrochemical cell, and occurs when two electrochemically dissimilar metals are joined together in a conducting medium. The two dissimilar metals generate an electrical potential, and this electrical potential serves as the driving force for the electrical current flow through the corrodant or electrolyte. The less resistant metal, called the active metal, becomes anodic. The other metal, called the noble metal, becomes cathodic.

The two locations susceptible to galvanic corrosion are piping transitions between two dissimilar metals and at sacrificial anodes.

Measures used to control galvanic corrosion include:

Cathodic protection by introducing a third metal (sacrificial anode, normally zinc) to the metals being protected or using only metals that are close on the activity series.

Choosing relative surface areas such that the material to be protected has a larger surface area than the active metal.

Separating dissimilar metals with a non-conducting material

Separating the metals from a conductive environment Use of poorly conducting electrolytes (pure water)