Copper or Galvanized? A Look at Corrosion

The plumbing in my house has sprung several pin-hole leaks, so corrosion is on my mind, and I thought we could look at it today.
The world around us is not static – it is constantly undergoing physical and chemical changes. A nail exposed to the elements will ultimately rust. It has undergone a chemical change, converting the nail – mostly iron – into rust – a compound composed of iron and oxygen. Where did the oxygen come from? The atmosphere is about 20% oxygen, so we’ve got lots of that handy. But if we placed the nail in the air with no moisture, the chemical change to produce rust would not occur. This is a key reason why “moth-balled” aircraft are stored in desert areas like Mojave: no moisture, no rust. Water is a necessary “facilitator” in the chemical reaction of rusting.
Well, there’s no shortage of moisture in a water pipe. But how does the oxygen get in there? Like the carbon dioxide that we explored last week, gaseous oxygen will dissolve in water, up to about 10 milligrams per liter (or parts per million). Tap water almost always has a small amount of oxygen present. So now we have the three ingredients needed for pipe corrosion: a metal, oxygen, and water.
But not all metals are created equal. Some corrode readily, while others are much slower to corrode. We use stainless steel and copper pipes to minimize corrosion – but they’re pretty expensive, so our water mains are not made of these materials. The most common piping metal is iron. It’s strong enough for our needs, and relatively inexpensive, but iron will rust.
So how can we reduce the risk of corrosion of our iron pipes? One way is to coat it with another metal, zinc: a process called “galvanizing.” Zinc actually corrodes faster than iron, but will form a layer of non-corroding scale on the interior surface of the pipe, and thus place a barrier between the water and the pipe metal. Remember, if the water doesn’t come in contact with the metal, corrosion will not occur.
So why do galvanized pipes corrode? This coating process is not always uniform, so localized areas of the pipe may not be fully protected, and small areas of corrosion – my pin-hole leaks – will form. A few years ago, I had a contractor apply an epoxy coating to the interior of my household plumbing, and this process was supposed to provide the corrosion barrier in a more complete manner than galvanizing. Unfortunately, this barrier has failed, too.
My mission over the next couple of weeks is to replace the leaking galvanized iron pipe in my house with copper pipe. As we noted, copper is a metal that is much more difficult to corrode; however, it is more expensive. Ouch!

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