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By Yun Chung
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The Korean government scientist who insisted that the white material was not aluminum corrosion products may not know the term, filiform corrosion. That is the form of corrosion responsible for the white material on the aluminum parts.
The Korean Navy launched the patrol combat corvette, christened Cheonan in 1989. She broke in two halves and sank, killing 46 seamen, in the Yellow Sea in March 2010. The government formed a military-civilian joint investigation group (JIG) to determine the cause of this tragedy. Six months later, it published a 313-page report, "On the Attack against the ROK [Republic of Korea] Ship Cheonan."
Based on visual examinations of the two halves of the Cheonan, salvaged from the seabed, the JIG decided a proximity explosion underwater was responsible for her sinking. Then, the JIG searched for and found torpedo parts in the seabed 50 days after the incident. Only five days later, the JIG presented them as the "conclusive evidence" of a North Korean submarine attack, insisting that the white material on the torpedo parts was proof.
To support this claim, the JIG presented energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis data of the white material. JIG's scientists believed, albeit erroneously, these data alone were sufficient to establish that the white material consisted of amorphous (that is, non-crystalline) aluminum oxide particles.
According to the JIG, the aluminum metal powder in the North Korean torpedo warhead would change during an explosion to amorphous aluminum oxide particles, which would subsequently somehow adhere or stick to the torpedo parts. No aluminum oxide particles were found to have adhered, however, to the torpedo parts as elaborated below.
Several professors contradicted the JIG interpretation of the EDS/XRD data. These professors tried to account for a significant sulfur peak in the EDS spectrum of the white material that JIG's scientists ignored. Prof. Jeong Gi-young (mineralogy) of Andong National University in Korea identified the white material as amorphous aluminum sulfate hydroxide hydrate, which grew as a precipitate in-situ, not as adhered particles of amorphous aluminum oxide.
Dr. Yang Pan-seok (geological science) of the University of Manitoba in Canada said that it was basaluminite, which is also a sulfate, similar to what Jeong said. Prof. Lee Seung-hun (physics) of the University of Virginia in the U.S. thought JIG's scientists made an erroneous interpretation of their EDS data. He thought the spectrum peaks were more like those of aluminum hydroxide than aluminum oxide.
Corrosion science established long ago that the most common species of aluminum corrosion products in water is aluminum hydroxide. "Handbook of Corrosion Engineering" by Pierre R. Roberge lists several minor species of corrosion products of aluminum, including some sulfate, which can come from sulfates in seawater or even from sulfurous pollutants in the air.
If the white material were aluminum oxide particles that came from a torpedo explosion, the torpedo parts, both aluminum and steel, should have been covered all over with these white particles. However, the white material was found only with the aluminum parts. The steel parts were rusty but without any white material. This observation alone can disprove the JIG scientist who insisted the white material came from a torpedo explosion and was not aluminum corrosion products.
The aluminum parts such as propellers had telltale signs of filiform or under-film corrosion, easily recognizable to corrosion engineers. Aluminum corrosion products are white and voluminous, about 30 times the volume of the metal that corrodes. This is why the white material on the aluminum torpedo parts has to be aluminum corrosion products, which are aluminum hydroxide mixed with some sulfates. The white material cannot be the amorphous aluminum oxide particles from a North Korean torpedo explosive.
Both the steel and aluminum parts of the torpedo had (polymeric) coating, which should have protected the substrate metal from corrosion. After only 50 days in cold seawater at 3 degrees Celsius, according to the JIG data, the torpedo parts with black top coating should have looked pristine (that is, un-corroded) when salvaged.
Instead, the torpedo parts were anything but pristine. Both the steel and aluminum parts of the torpedo lost much of the black coating and both corroded heavily. The aluminum propellers/cover plates of the salvaged torpedo parts had a profuse amount of white corrosion products over about 80 percent to 100 percent of the surface area, both inside and outside, where the coating deteriorated.
To have the coating deteriorate and to form this much corrosion products, these torpedo parts must have been in seawater for two years or more, not just 50 days. This estimate, however rough it may be, is consistent with the advanced degree of deterioration of the black coating of the steel parts and the subsequent buildup of heavy corrosion products.
The JIG stated that the torpedo parts were a smoking gun in the Cheonan incident or the "conclusive evidence" of a North Korean torpedo attack. Nothing could be further from the truth. The JIG cannot use the white material on the salvaged torpedo parts as a link to a North Korean torpedo explosion. There might have been no torpedo explosion after all.
The JIG report includes a fracture analysis. This was very simplistic, skimpy, and flawed. Several fracture patterns on the Cheonan are so complex that they are clearly incompatible with forces attributable to an external underwater explosion. A better diagnosis of the fracture than in the JIG report is of paramount importance. A correct fracture analysis would be a true smoking gun that can point to the cause of the Cheonan sinking. The JIG report failed to prove or convince the public that a North Korean torpedo sank the Cheonan.
Yun Chung (yunchung2@gmail.com), a retired materials engineer, specializes in materials selection for various industrial plants, including a seawater treating plant, and failure analyses, involving metal corrosion and fractures. The views expressed in the above article are the author's own and do not reflect the editorial direction of The Korea Times
