CORROSION FAILURES

Exposure of unprotected metal components to the environment will cause them to corrode.  The degree of corrosion depends upon the severity of the environment as well as the metal’s inherent resistance to corrosion.  A seacoast environment, with its moist, salt-laden air, is much more corrosive than the dry air in the desert.

Coating systems are often used to protect metal components from the environment.  Their degree of success depends upon the type of coating used and its overall thickness.

Shown below are typical examples of common corrosion failures. 

1. Fencing

This fence is located on a western-facing hillside a few miles from the coast.  As a result, it came in direct contact with the moist, salt-laden, onshore breezes.  The coating system applied to the fence was not robust enough for this environment.  The corroded areas were cut out and replaced.  The fence was stripped of its original coating and re-coated with a much thicker coating consisting of an epoxy primer followed by a urethane topcoat.

2. Buried Pipe

Two examples of water main failures are shown. The first water main developed a leak after 8 years of service.  Although primed and tape-wrapped for corrosion protection, the pipe surface showed extensive pitting and corrosion damage.  In contrast, the inside of the pipe was undamaged.  The resistivity of the soil was measured.  Based upon the readings, the soil was found to be corrosive.  It was recommended that the water main be cathodically protected to prevent future failures. The second water main was in place since the 1920s.  It was bare cast iron, buried beneath the foundation of a home.  When the pipe began to leak it caused the surrounding soil to settle and the foundation of the home above it to crack.  In this instance, the hole in the water main started on the inside surface.

3. Copper Water Lines

If the soil is corrosive enough it can even attack copper water lines as these examples show.  Typically, the surface of the water line at the bottom of the trench experiences the most corrosion.  The difference in oxygen concentration between the soil which was dug up to create the trench and then replaced (thus high oxygen content) and the undisturbed soil at the bottom of the trench (low oxygen content) creates a corrosion cell.  This cell, in addition to the soil’s general corrosiveness, causes the copper tube at the bottom of the trench to corrode.

4. Components
   1. Fire Protection Sprinkler

   Corrosion of a fire protection sprinkler caused it to activate inside a home while the owners were on vacation.  The cause of the failure was dezincification of the brass water restraint mechanism.  Dezincification is the selective leaching of zinc from a brass alloy.  It typically occurs in brasses with zinc contents greater than 15%.  The resulting metal is so porous and weak it can no longer withstand the applied stresses.

   2. Sink

   Pressed steel sinks are prone to corrosion.  Corrosion begins in the seams of the sink between the spotwelds.  As the corrosion products grow in size, they cause failure of the spotwelds due to the pressure they exert within the seam.  The sink then begins to leak.

   3. Bolt

   Outdoor exposure to metal components such as bolts can cause them to severely corrode, especially near the ocean.  The bolt and nut shown were left unprotected in a seacoast environment.

   4. Propellers

   Failures of brass or bronze propellers are not uncommon.  Two examples are shown.  In both cases, failure initiated as a result of dezincification followed by fatigue crack propagation through the propeller blade.  The dark area is where the zinc leached from the brass due to dezincification.  A fatigue crack then initiated in this weakened area.

   5. Electronic Equipment

   The corroded circuit board shown in the photo came from a fast food restaurant headset.  It had been in service approximately 3 to 4 months.  Energy dispersive x-ray analysis of the corrosion products found zinc, chloride, and phosphorus.  The most like source of these elements is the flux used during soldering.  Incomplete rinsing after soldering left a flux residue which corroded the tin terminals.