This invention relates to corrosion protection systems, and more particularly to corrosion protection systems for joined dissimilar metals which are employed in an aqueous environment.
Potable water, or similar type electrolyte normally contains certain corrosion causing components such as dissolved oxygen. Other oxidizing components of many varieties can be present in this medium, depending on the particular sample of water, the location from where the water was obtained as a geological function, certain water treatments such as softening, and the many other variable conditions to which water can be subjected.
It is common for potable water and other types of water systems to contain iron or iron bearing alloys which can form a galvanic corrosion cell between the iron bearing material and each oxidizing atom or molecule involved in a like reaction. These reactions can reach factors of 10.sup.(x) per square inch. Therefore, it has not been possible to render each separate corrosion cell inactive by direct action to each oxidizing atom or molecule.
Cathodic protection systems have been developed to reduce corrosion caused by oxidizing components found distributed in the potable or similar type water containing the vulnerable metal. In a cathodic protection system, the vulnerable metal is electrically charged by a variable or constant voltage in order to cause the formation of a monatomic hydrogen layer. This hydrogen layer, when undisturbed, forms an interface between the oxidizing components and the vulnerable metal. In a passive environment where little or no turbulence exists, as well as other factors which would have an effect upon the integrity of the monatomic hydrogen layer, this form of protection can be useful in the reduction of corrosion cell formation from the waterborne corrosion inducing components. Cathodic protection is even more effective when the base metal is coated, such as with glasslining, cement or epoxy, reducing the contact area between the vulnerable metal and the aqueous medium.
However, impressed cathodic protection is not as effective and can actually accelerate corrosion in certain aggressive environments. Examples are an environment where there is turbulent water flow against the vulnerable metal removing the hydrogen layer and leaving a charged surface, or an environment characterized by elevated temperature, pH extremes, or contact of the vulnerable metal with a more noble dissimilar metal.
For dissimilar metal junctions, it can be shown that impressed cathodic protection of the vulnerable metal will accelerate the corrosion of the vulnerable metal by 20% to 45% or more when any significant turbulence is present. Naturally occurring cathodic protection that occures between joined dissimilar metals will cause only slight corrosion rate reductions in a low temperature environment, about 23.degree. Centigrade where very little to no turbulence is present, and the water sample is non-aggressive, tap water, untreated at point of use.
The major water heater manufacturers have long recognized the shortfall of cathodic protection against dissimilar metal junctions and consequently avoid the use of dissimilar metal junctions in commercial units. Many water heater tanks have di-electric inlet and outlet factory attachments. Thermostat copper and brass shanks, as well as relief valve probes, are plastic coated. Drain valves are plastic, instead of copper or brass whenever possible. Copper electric heating elements are tin coated. Also, the new class of heating elements are now commonly produced. These heating elements are comprised of stainless steel or an iron alloy called Incolloy which further reduces dissimilar metal corrosion. Plumbing codes almost always specify that di-electric unions must be used when copper pipe is run to and from a hot water heater.
Various methods have been proposed to prevent corrosion. For example, L. Applegate, Cathodic Protection, McGraw Hill, February 1960, pp 1-28, discloses a method to stop the reaction between separate bars of copper and iron which involves application of an impressed current to the dissimilar metals. However, this method as disclosed by Applegate only has a laboratory application. The metals are never directly joined. Furthermore, the positive current electrode is connected directly to the more noble copper bar which causes the copper to become very active and disintegrate. Moreover, Applegate discloses that the accepted conclusion is that dissimilar metals should not be used due to the unpredictable nature and widespread potential for system corrosion.