In general, 300 series alloys are the most corrosion-resistant and are nonmagnetic. This is normally not a problem in hydronic heating/cooling systems or domestic-water systems, but a large chilled-water-storage system could have oxygen levels become low enough to have issues with microbially influenced corrosion (known as MIC). SS requires some oxygen to build an adhering oxide layer, like aluminum car wheels. There is one potential problem with this if done incorrectly (see, “Mixing materials may equal trouble”). In the past decade, we have seen thinner SS being proposed as an alternative to galvanized steel pipe and larger-diameter copper pipe, primarily for domestic potable-water piping. 316 and 316L add molybdenum to reduce susceptibility to chlorides. SS with the L designation is recommended for all SS that will be welded and might have corrosion issues, like fume exhaust and some pipe systems. 304L has reduced carbon content to minimize the tendency for SS to corrode at welds. The most common alloy is 304 SS, which adds 18% chromium and 8% nickel to steel. Anaerobic and chloride corrosion can affect SS. This is true in many circumstances, but not all. Stainless steel is widely considered to be resistant to all corrosion. Drain, waste, and vent (DWV) piping is thinner (Type M). In the HVAC industry, most copper is Type L (medium thickness) hard (tempered) copper, although underground soft (annealed) copper is often Type K (thick). Also, copper is generally more noble and corrosion-resistant than steel or galvanized steel. Copper is an expensive material but has the advantage of weighing less than steel and may require fewer employees to install, depending on weight and union restrictions. However, some contractors propose replacing galvanized steel domestic-water pipe with copper up to 6-in. (More information is available via the American Galvanizers Association.)Ĭopper pipe often is used in both hydronic and domestic applications, especially for 2-in. If the interior needs a continuous galvanized layer, consider mechanical couplings. Repairing galvanizing on the inside of the pipe is difficult or impossible. If galvanized pipe is being welded, the welder needs to be careful to grind down to the raw steel. It can fail in environments with high sodium (e.g., softened water that started out very hard) because the sodium makes the adherent oxide film detach and react more like steel pipe where the oxide flakes off. Galvanizing works almost perfectly in applications where it is wetted and dried periodically (e.g., road signs and guard rails). Galvanized steel pipe has all the advantages of steel pipe, plus improved corrosion resistance in most environments, although at a slightly higher cost. It provides a sacrificial anode (zinc) to receive corrosion instead of the steel corroding.It coats the surface like paint, and under most circumstances it forms a very adherent oxide layer like aluminum and SS.Galvanizing has two methods of corrosion reduction: Galvanized steel pipe is steel pipe that is dipped into a pool of zinc (see Figure 1). However, the high rigidity of steel means that although it expands less, it exerts very high forces on anchors. Consult ANSI/MSS SP-58: Pipe Hangers and Supports – Materials, Design, Manufacture, Selection, Application, and Installation for details about hangers and hanger spacing.Ī low coefficient of thermal expansion minimizes the need for expansion loops and expansion joints. More flexible materials, however, may require hangers on as close as 4-ft centers or even continuously. Steel pipe is manufactured in 21-ft lengths, and the hangers can be spaced that widely for large-diameter pipe. Rigidity is important because it determines the distance between hangers. Some building owners do not want these chemicals in steam that may be used for humidification because of health concerns however, not using these amines might require a change to stainless steel (SS) piping or adding a separate “clean steam” system for humidification and for sterilization of medical instruments. If amines (commonly cyclohexylamine, morpholine, or diethylethanolamine (DEAE) are fed properly to neutralize condensate pipe pH, condensate pipes can last the life of the building. ![]() However, corrosion is an issue in steam-condensate pipes, and many engineers specify schedule 80 steel pipe simply because it takes about twice as long to rust through as schedule 40 pipe. It also is a good choice for steam and steam-condensate systems because it handles high temperatures and pressures well, and corrosion is normally not an issue in steam pipes. ![]() Steel often is used for closed hydronic systems because it is inexpensive, especially when compared with other materials in systems with high pressures, and corrosion is relatively easily controlled in these systems.
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