In a piping system there are a number of factors working alone or in combination that can effect the rate of corrosion occurring in pipe. Depending on the degree of these factors a new piping system can show signs of corrosive wear in as little as two years after installation.

The corrosive effect the source water can have on the piping system does not mean poor water quality is being delivered. In fact in the Pacific Northwest region of the United States and Canada, the source water quality has been rated as some of the best drinking water in the world. Yet, that water has a very corrosive effect on piping systems.

The rate of corrosion on a piping system is related to basically these factors:

the pH of the water
the amount of oxygen in the water
the chemical make up of the water
the amount of galvanic corrosion from the use of dissimilar metals contained in or in contact with the piping system
the temperature of the water
the velocity/pressure of the water in the pipe

THE pH OF THE WATER
Acidity of a water sample is measured on a pH scale. The pH scale ranges from zero (maximum acidity) to 14 (maximum alkalinity); the middle of the scale, 7, represents the neutral point. Acidity increases from neutral toward 0. The pH scale is based on logarithmic progression like the commonly used “Richter” scale for earthquake measurement. A difference of one pH unit represents a tenfold change in acidity. Normal rain has a pH of 5.6 – slightly acidic because of the carbon dioxide picked up in the earth’s atmosphere by the rain. Acid rain with a pH of 3.6 has 100 times the acidity of normal rain with a pH of 5.6!

In copper pipe systems when the pH is more than 8, a copper oxide film is usually formed on the pipe walls. This film acts as a barrier that slows the effects of corrosion. However, when the pH in the water supply is lower than 8 in the water supply the copper oxide film (barrier) is dissolved leaving no protective barrier and the pipe is subject to the corrosive action of the water.

THE AMOUNT OF OXYGEN IN THE WATER

A domestic water system is an “open” system where the water in use is always being replenished with fresh oxygenated water. Dissolved air in water consists of about 30% oxygen and the rest is mostly nitrogen, which is non-corrosive. Oxygen degrades metals through an electro-chemical process of internal oxidation. The result is that metal gradually gets converted to an oxide (rust), becoming thinner and weaker in the process. As the pipe corrodes the impurities are deposited in the water lines. Encrusted build up is the direct result of the oxidation process.

While oxygen content decreases under higher water temperature and higher pressure it is these higher temperatures and higher pressure conditions that speed up the oxidation process. Experience shows that corrosion is more pronounced in hot water lines.

THE CHEMICAL MAKE UP OF WATER

Dissolved minerals in the water and the basic chemical composition of the water may have differing effects on the corrosive forces at play. For example a moderate to high level of calcium would help form a protective coating on the pipe which could slow down the corrosive effects. However, high levels of calcium may cause a calcium build up in the pipe.

DISSIMILAR METALS - GALVANIC CORROSION

Galvanic corrosion, also known as electrolysis, occurs when different metals come into contact with each other. When conflicting metals are touching each other one of the metals has a tendency to give up electrons. Basically at the point of contact the metal giving up the electrons dissolves over time. The most frequent cases of this happening are when galvanized pipe and copper are connected; copper pipes touch steel studs, or steel pipe hangers. Contrary to common belief, the effects of galvanic corrosion are limited to the immediate area of contact. The use of dielectric fittings helps stop the problem but does not repair the resulting thin walled and damaged pipe.

WATER TEMPERATURE

The higher the water temperature the faster the rate of oxidation. Experience shows that corrosion is more pronounced in hot water lines.

WATER VELOCITY

Water velocity problems are usually associated with a “closed” loop piping system where the need to pump or circulate the water is required.

Erosion corrosion occurs at locations where water turbulence develops. Turbulence can be caused by excessive velocity, sudden changes in direction (sharp turns, elbows) and through “flow” obstacles such as burrs and solder excess.

In the case of copper pipe, a copper oxide layer is established under the right pH conditions on the inside surface of the pipe and this acts as a protective barrier. However, when the water velocities are above 4 ft/sec the copper oxide layer is destroyed or cannot form in the first place. Without this oxide layer the metal will tend to deteriorate at a more rapid rate.

The major contributing factors to this type of erosion corrosion include:

water velocities exceed 4 ft/sec
oversized circulation pumps
installation of undersized distribution lines
multiple or abrupt changes in the direction of the pipe
burrs on the inside of the pipe
improper soldered joints
improper balanced system

HEALTH ISSUES
The internal corrosion of piping systems raises health concerns. These health concerns include the negative health effects associated with the leaching of lead, copper and other harmful metals from water pipes into the drinking water supply.