Metallographic Investigations of the Premature Corrosion Failure of Steel Seam-Welded Galvanized Cold Water Pipes
A premature corrosion failure in seam-welded galvanized steel cold water pipes was reported to occur after just 18–24 months in service. The aim of the present work was to investigate the causes of this premature corrosion of pipes, through metallographic investigations. This work comprised chemical analyses, visual examination of internal surfaces of pipes and microstructural examinations of the corrosion perforation regions (pitting corrosion areas) using optical and scanning electron microscopy with energy-dispersive x-ray analysis. The investigations showed that failures resulted from incorrect installation—pipes were installed with yellow brass ball valves. When mixed with yellow brass, galvanized steel triggers dezincification and results in electrolytic action when combined with nonferrous metals, such as copper and brass.
KeywordsGalvanized steel pipes Premature corrosion failure Water distribution system
Iron and steel pipes have been used in water distribution systems for over five centuries . Galvanized (zinc-coated) steel was the dominant pipe material for most of the twentieth century and is still installed in buildings today. Economically, modern zinc-coated steel pipes are inexpensive and recyclable. With a long life span, such material is very attractive. Zinc coatings protect steel by providing a physical barrier as well as cathodic protection for the underlying steel. In this process, zinc, as a more ignoble metal, acts as a sacrificial anode, while steel (as a cathode) is not attacked. However, there are many reports of galvanized steel water pipe failures due to corrosion after several months or few years of service life [2, 3]. Accelerated corrosion occurs under certain conditions of flow water chemistry. The internal wall of the steel pipe, exposed to aqueous environment (flowing water), usually suffers more severe corrosion (so-called internal corrosion) than the external wall exposed to indoor conditions . Many water distribution systems begin to experience pinhole leaks caused by pitting corrosion on the internal surfaces of the piping. Zinc corrosion in flowing water is a complex process controlled largely by water chemistry and temperature. Relatively small differences in water chemistry can produce relatively substantial changes in corrosion products and rate. A coating of zinc protects galvanized pipe, but when corrosion occurs, it deposits high levels of zinc and iron into the tap water. A zinc coating on galvanized pipe may contain lead, copper, cadmium, chromium, aluminum, barium and other impurities. As a result of these impurities, corrosion of galvanized pipe may result in the release of trace metal concentrations [5, 6, 7, 8, 9, 10, 11]. Galvanized pipes are suitable only when the pH of the natural water is about 7 or moderately basic (between 8 and 12) .
According to the European Standards [12, 13], there are conditions under which galvanized steel pipes will sustain corrosion damage. One of the main factors influencing corrosion of zinc-coated pipes is water composition. The influence of chloride, nitrate and sulfate ions on pitting corrosion is determined by the so-called concentration ratio S1, the sum of chloride, nitrate and sulfate ion concentrations divided by the hydrogen carbonate ion concentration. According to these standards, pitting corrosion of zinc-coated steel pipes is extremely unlikely with S1 below 0.5 and very likely with S1 above 3.
Contemporary regulations in many countries even prohibit the use of galvanized steel pipes in drinking water systems and do not recommend them for hot water circuits. Today, almost all installations in Denmark are built with stainless steel and polymeric materials pipes. In many other European countries, copper pipes are still widely used, and the use of hot-dip galvanized steel pipes is decreasing. In some countries, including Poland, galvanized steel piping is still popular, and this paper presents an investigation of cold water galvanized steel premature piping failure. Corrosion failure of seam-welded galvanized steel cold water pipes was reported to occur just after 18–24 months in service, and, according to the information from the installer, S1 values determined in long-term measurements were below 0.5.
However, this paper presents an unusual case when only cold water galvanized steel pipes corroded in the same internal water supply system, while hot water pipes made of the same galvanized steel showed no signs of corrosion.
Chemical analysis of the investigated pipes, and EN 10255 standard, in wt.%
EN 10255 standard
The bright, silvery gray areas present in the vicinity of the pits (Figs. 1, 2) on the inner surface of galvanized steel pipes were also observed by other authors , but were apparently not thoroughly examined. In , only macro-images of such areas were shown.
As shown in Fig. 2, pitting in the examined cold water galvanized steel pipes occurred only in close proximity to the screw connection pipe valve. According to information obtained from the installation user, the examined cold water pipes were installed with yellow, in contrast to the hot water galvanized steel pipes where the nickel-plated brass ball valves were installed.
The use of yellow brass valves in direct electric contact with the galvanized steel pipes is a significant installation error, and it was the cause of the premature corrosion failure of the cold water zinc-coated steel pipes. The phenomenon called contact corrosion (bimetallic corrosion) occurs when two different metals are attached to one another (in this case, galvanized steel and yellow brass) and contact an electrolyte (flowing water). This results in different electrode potentials in the two metals with respect to the electrolyte. The base metal (zinc layer on the steel) is transformed into an anode, and this leads to the accelerated corrosion of that metal and inhibits the corrosion of the other, more noble metal (yellow brass), which is transformed into a cathode. This is what happened in the cold water galvanized steel pipes in the failed water supply system. When mixed with brass, galvanized steel triggers dezincification, and it results in electrolytic action [9, 10].
Corrosion failure of two seam-welded galvanized steel cold water pipes, occurring just after 18–24 months in service, was investigated. According to the installation user, S1 values determined in long-term measurements were below 0.5 (results not reported here) which justified the application of galvanized steel pipes in this water supply system. Microscopic and microanalytical examinations showed that the cold water pipe failures were associated with the use of yellow brass valves in direct electric contact with the galvanized steel pipes. This conclusion is additionally supported by the lack of signs of corrosion in the same galvanized steel pipes in hot water installation, where nickel-plated ball valves were used.
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