WHAT IS PINHOLE LEAK?
A pinhole leak is a final breakthrough event of the progressive attack of pitting corrosion on copper water plumbing. A copper water plumbing system can be in a condition of having significant damage by pitting corrosion, but not have pinhole leaks. The challenge is how to discover pitting corrosion before pinhole leaks develop. Discovery of such latent damage would require removal and inspection of the internal surfaces of sample plumbing in the system.
Pinhole leaks in copper pipes are a widespread plumbing problem and are often expensive to identify, locate and repair. Re-piping and other methods of preventing this leaky pipe problem can cost homeowners and businesses thousands of dollars.

WHAT CAUSES PINHOLES IN COPPER PIPES?
Although no one knows the exact cause of pinhole leaks in every situation, most researchers/experts attribute their occurrence to some form of electrolytic corrosion, which is an electrochemical phenomenon in which a metal reacts with its environment causing it to deteriorate.

"The Washington Suburban Sanitary Commission (WSSC) experts believe that pinhole leaks are not the result of corrosive water but of dissimilar metals interacting and starting a battery of reactions that just consume the pipe. Otherwise, dissolving of the copper would be a widely spread problem." (Washington Post, July 2000)

Copper pipe, though primarily comprised of copper particles, is not homogeneous and contains multiple other metallic particles including alloying metals (for strengthening the copper) and unintentional impurities. The presence of water (an electrolyte) in a copper pipe causes electric currents to flow naturally between these dissimilar metal particles which, in turn, leads to corrosion.

The cell which triggers this corrosion process has three essential components:

• anode (the metal that is corroding)
• cathode (a second metal in contact with the anode)
• electrolyte (such as water, which provides the corrosive medium)

At the anode, the corroding metal passes into the electrolyte as positively charged ions, releasing electrons which participate in the cathodic reaction. Hence, the corrosion current between the anode and the cathode consists of electrons flowing within the metal and ions flowing within the electrolyte.
The surface of one component of the pipe may act as the anode and the surface of another component (in contact with it) may act as the cathode. Usually, corrosion cells will be smaller and numerous, occurring at different points on the surface of the pipe. Anodes and cathodes may arise from differences in the constituent phases of the metal itself, from variations in surface deposits or coatings on the metal, or from variations in the electrolyte (water).

The ability of metals to resist corrosion is to some extent dependent upon their position in the Galvanic Series. The "nobler" or less reactive metals, tend to act as cathodes and remain intact, while the more reactive metal particles act as anodes and corrode. A more noble metal will not generate a flow of positive ions. The reverse of this is the least noble metal, which will generate an electrical current. The farther two metals are separated from one another in the Galvanic Series, the more powerful is the electric current and corrosion produced by their contact in the presence of an electrolyte.

The illustration below shows a typical corrosion process in a copper pipe. In the presence of water, metals in the interior of a copper pipe (such as Copper (Cu), Aluminum (Al) and Iron (Fe)) interact with one another which naturally causes electric currents to flow between them. This current causes a series of electrochemical reactions, which leads to the creation of "corrosion cells" and ultimately to the dissolution of the materials in the pipe.

In some cases, the copper in the pipe is less noble with respect to the surrounding metals, and acts as the anode and corrodes. In other cases, it is more noble, acts as the cathode and the other metals corrode. Regardless of which metal acts as the anode or cathode, the fact is that the metals in the pipe are reacting, creating corrosion cells and slowly destroying the integrity of the pipe.