The afflicted vessel is USS Independence, the second in the sailing branch's fleet of fast, reconfigurable Littoral Combat Ships. Eventually, these ships are supposed to be the "workhorses" of tomorrow's Navy.As Bloomberg reported, the Navy has discovered "aggressive" corrosion aroundIndependence's engines. The problem is so bad that the barely year-old ship will have to be laid up in a San Diego drydock so workers can replace whole chunks of her hull.In contrast to the first LCS, the steel-hulled USS Freedom, Independence is made mostly of aluminum. And that's one root of the ship's ailment.Corrosion is a $23-billion-a-year problem in the equipment-heavy U.S. military. ButIndependence's decay isn't a case of mere oxidation, which can usually be prevented by careful maintenance and cleaning. No, the 418-foot-long warship is basically dissolving, due to one whopper of a design flaw.There are technical terms for this kind of disintegration. Austal USA, Independence's Alabama-based builder, calls it "galvanic corrosion." Civilian scientists know it as "electrolysis."It's what occurs when "two dissimilar metals, after being in electrical contact with one another, corrode at different rates," Austal explained in a statement."That suggests to me the metal is completely gone, not rusted," naval analyst Raymond Pritchett wrote of Independence's problem.Independence's corrosion is concentrated in her water jets - basically, shipboard versions of airplane engines - where steel "impeller housings" come in contact with the surrounding aluminum structure. Electrical charges possibly originating in the ship's combat systems apparently sparked the electrolysis.It's not clear why Austal and the Navy didn't see this coming. Austal has built hundreds of aluminum ferries for civilian customers. The Navy, for its part, has operated mixed aluminum-and-steel warships in the past.
Thursday, June 23, 2011
Navy ship disintegrating
Galvanic corrosion is nasty. [Link]
2 comments:
My late husband's father had a little 20' pleasure boat, which had zinc strips on the bottom that needed to be replaced periodically as they corroded away. They were there to keep the aluminum parts from corroding. Presumably, the zinc was deflecting electrolysis away from the aluminum, I dunno. My point is, if pleasure craft makers know this, shouldn't the shipyards? Seems like kind of a major item to overlook.
The article mentions this, as the builder has made many aluminum hulled ships and that they were puzzled by this.
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