Black iron pipe corrosion rate field observation

Physical-object demonstration. Tom produces black iron pipe to teach that the magnetite (Fe3O4) scale formed at hot-rolling temperatures adheres to steel because its CTE matches, in contrast to hematite rust which spalls off.

This is mechanics of attack, and the eight forms that Fontana and Greene break corrosion into are sort of the chemistry of attack. So they're a little different. There's uniform attack — in the area of steel we call that rust. We have localized attack, which can be pitting or gouges with crevice corrosion. We have erosion, cavitation where you have bubbles collapse and great pressures that break off the protective surface oxide. One of the problems is that most steel does not have a protective surface oxide. Now there is a lot of steel that does. [Tom produces a piece of black iron pipe.] Here's black iron pipe — if you go around the basement of most buildings or a boiler room you'll see this stuff, it's unpainted. Black iron pipe, when it's hot coming off the rolling mill, will form at high temperatures Fe3O4, which is called magnetite, a magnetic form of iron.

Tom holds up a Home Depot pipe nipple as a physical-object demonstration. Magnetite (Fe₃O₄) coating from mill heat treatment provides practical nobility. Teaching numbers: 0.002–0.010 inches/year corrosion rate; ~25-year shipboard piping lifetime — the historical naval ship life until NAVSEA mandated 50-year lifetime, forcing the move to cupro-nickel, Monel, and titanium.

The second point from yesterday: metals may possess immunity, or we called it nobility, or they can have practical nobility, which means they've got a coating of some sort of protective oxide or something else. To give you an example of something that has a protective oxide — anybody know what this is? [Tom holds up a length of pipe.] Some of you must be mechanical engineers. You should call this a pipe nipple. It's threaded on both ends. Bought it at Home Depot, cost me like a buck. Three-quarter inch by six, black nipple. Why is it a black nipple? Because it's got a coating of Fe₃O₄, magnetite, on the surface. When you heat up steel to above about five or six hundred degrees Fahrenheit — and this thing gets to temperatures of eight or nine hundred degrees when they're forming it in the steel mill — it will form, not Fe₂O₃ which you know as rust, but Fe₃O₄ which is called magnetite. It's slightly magnetic. Ever play that little game with the magnets where you put the mustaches and the beards on the little figures? That's magnetite. That's another use for magnetite.