Last Tuesday morning I was the guest lecturer at a course in the Department of Materials Science & Engineering over at
. I spoke about corrosion to 18 students in Professor Mike Hurley’s class on Materials Selection. One of my goals was to shake the young people from the usual college attitude that problems come out of a textbook, and the answers also can be found inside. Boise State University
Half my lecture was on fire sprinklers. I began by pointing at the sprinkler heads protruding from the ceiling of the room. Sprinklers are “firefighters in the ceiling.” They operate properly about 93% of the time (and would likely work another 4% of the time if people just would remember to turn them back on after doing maintenance).
Sprinklers heads have to work where we live and work. They can fail year round due to a variety of problems that should be considered during design. One set of problems involves leaks due to corrosion. Another involves mechanical damage. Students play hall sports in dorms and throw things like basketballs and footballs at them. In hotel rooms we hang things from them.
There also are seasonal problems. In the winter, sprinkler piping can burst from water freezing inside. In the summer if the ceiling temperature gets too high, then the fusible solder link on the head shown above can creep and break without a fire and melting.
The traditional and very reliable design for the seal on a sprinkler head is like the cap on a beer or soda bottle. About a decade ago Central Sprinkler Company tried another seal design, like a cork in a wine bottle. They used a brass plug with a rubber donut in a groove, called an O-ring seal, on two sprinkler head designs called the Omega and the GB. Unfortunately the “cork” sometimes got stuck in the bottle. Tests on heads removed from systems showed that a quarter to a third of them would not operate properly.
The Consumer Product Safety Commission (CPSC) threatened to sue Central Sprinkler. Central’s lawyers said look, a sprinkler is not a consumer product, so you should go away. CPSC replied that we will act to protect public safety, so you can lead, follow, or get out of our way. CPSC recalled 8 million Omega sprinklers in 1998, and another 35 million GB sprinklers in 2001. Those 43 million recalled sprinklers were more than the 39 million installed annually in the
During discussions before the recalls Central suggested that some of the sticking was from the rubber O-rings swelling due to contact with cutting oil used during threading of steel pipes. Designers should have included that oil in the service environments they considered before selecting materials. Central also suggested that some of the sticking may have been due to use of “stop leak” products like those used on car radiators. Mineral deposits from hard water or corrosion are more likely culprits.
Most sprinkler heads are made of brass, an alloy consisting mostly of copper and zinc. Brass can fail by a form of dealloying corrosion called dezincification. The zinc corrodes away and dissolves in the water, leaving a porous copper layer. The component still has the original dimensions though. When the porous layer goes all the way through the component, it leaks.
With some tap waters there is a pathological version called meringue dezincification. Here the zinc precipitates on inside surface, and a fluffy white deposit plugs the opening. This can be a serious problem in some areas. Currently there is a $90 million class action lawsuit involving brass plumbing fittings.
At the end of my lecture I pointed them to some articles about materials selection in the ASM Handbook. I also mentioned foreign sources of information like the Nickel Institute in