Degree of Sensitization (DoS) Probe
ElectraWatch DoS Probe:
To maximize the stability and, in some cases, speed of Naval vessels, Aluminum-Magnesium alloys are often used to construct ship superstructures. The primary metal, aluminum, offers the advantage of being lightweight; adding magnesium to the aluminum creates an alloy that is both lightweight and strong. In the tempers commonly used, H116 and H321, the aluminum alloys are supersaturated with magnesium to maximize strength. However, this condition is thermodynamically metastable and exposure to elevated temperatures (above 60 degrees C) for extended periods of time causes the magnesium, which should be evenly dispersed within the aluminum matrix, to form beta-phase (Mg2Al3) precipitates. These precipitates are strongly anodic to the Al-Mg solid solution. If they form along the grain boundaries of the material as a connected network, galvanic corrosion can occur. The formation of this precipitate is called sensitization. Sensitized material can experience intergranular corrosion, exfoliation, and stress corrosion cracking (SCC) in marine atmospheric conditions.
Sensitization is a serious issue for the Navy and a sensor system or fieldable probe to assess the degree of sensitization (DoS) is needed to assist in maintenance decisions and guide predictions of structural integrity or health in service. The conventional laboratory approach is ASTM G67 which involves immersion of the alloy in nitric acid for 24 hours and measuring the mass loss resulting from intergranular corrosion. If the mass loss is greater than 25 mg/cm^2, the material is sensitized and fails the G67 test. If the mass loss is less than 15 mg/cm^2, the material is not sensitized and passes the test.
The problems with the G67 test for ship inspection are that it is destructive, requires laboratory testing of coupons, is expensive, and is labor and time intensive. The ElectraWatch DoS Probe solves these problems and offers a field portable, nondestructive probe capable of minimizing ship repair times, increasing fleet readiness, and reducing maintenance costs.
