Galvanized steel Pipe is
surface finished pipe that has been coated with a thin layer of zinc to help prevent corrosion. Zinc, a natural, healthy, and abundant element was first used in construction in 79 AD. Zinc metal has a number of characteristics that make it a well-suited corrosion protective coating for iron and steel products. Zinc’s excellent corrosion resistance in most environments accounts for its successful use as a protective coating on a variety of products and in many exposure conditions.
This proven corrosion resistance is a result of zinc’s ability to form dense, adherent corrosion by products, which leads to a rate of corrosion considerably lower than ferrous materials – 10 to 100 times slower. These corrosion products, known as the zinc patina, act as an additional barrier between the steel and the environment.
In the early 20th century, galvanized pipe replaced cast iron. Typically, galvanized piping rusts from the inside out, causing both water pressure problems and eventual pipe failure. These plaques can flake off, leading to visible impurities in water and a slight metallic taste. The life expectancy of such piping is about 70 years, but it may vary by regions.
Galvanization is the process of applying zinc coating to steel or iron. Although galvanization can be done with electrochemical and electrode position processes, the most common method in current use is hot-dip galvanization. In industry, the term GI stands for galvanized iron, referring to common galvanized steel used in many applications.
A number of different methods of applying zinc coatings to steel are commercially available, each of which has unique characteristics. Often all of these coatings are incorrectly labeled “galvanizing,” but it is important to understand the differences. The products produced by each of these processes have different uses depending on their applicability, relative economics and expected service life. This section will explore each of these coatings in more detail.
The service life of zinc coatings is a linear function of the zinc coating thickness. However, zinc coating thickness alone can be deceiving when evaluating zinc applied by different processes. In addition to thickness, the amount of available zinc per unit volume, or density, is also important. While coating densities for some types of zinc coatings are nearly identical, others differ considerably. The coating densities, in terms of thickness required to equal 1 oz of zinc per square foot of surface, are:
| Coating |
Thickness to reach 1 oz/ft2 |
| Hot-dip galvanizing (batch or continuous), electroplating, zinc plating |
1.7 mils (43 µm) |
| Metallizing (zinc spraying) |
1.9 mils (48 µm) |
| Mechanical plating |
2.2 mils (55 µm) |
| Zinc-rich paint |
3-6 mils (75-100 µm) |
Each of these thicknesses, representing the same weight per unit area of zinc, would be expected to provide equivalent service life; i.e. 1.7 mils of hot-dip galvanized would give about the same service life as 2.2 mils of mechanical plating or 3-6 mils, depending on the paint formulation, of zinc-rich paint, assuming bond strength and edge protection are not factors.
