Rust Busters: Corrosion Monitoring Systems and Anti-Corrosion Coatings

Mara Shedd
Senior Materials Research Engineer
Intelligent Systems Group

With an estimated cost of $22.9 billion annually, corrosion of defense structures continues to be one of the top maintenance cost drivers for the Department of Defense.  This month, the Intelligent Systems and Materials groups at Luna will present corrosion control and monitoring strategies for naval applications at the Mega Rust Conference, being held June 24-26, in Newport News, Virginia.  We will present several technologies, including an embedded system for measuring sensitization, a chrome-free coating for mitigating stress-corrosion cracking, and an embedded system for monitoring localized corrosion and barrier coating condition on marine shafts. 

Sensitization of Aluminum Alloys

Recent incidents with the aluminum alloys used in commercial ferries and patrol boats as well as in Navy ships have shown that the 5XXX series of aluminum alloys are prone to elemental migration and phase transformations at only slightly elevated temperatures (i.e., sensitization), leading to localized intergranular corrosion and stress-corrosion cracking. Sensitization occurs when an intermetallic compound β (Al3Mg)precipitates at the grain boundaries of the super-saturated alloy.  The β phase preferentially dissolves, leading to intergranular corrosion and stress-corrosion cracking when a tensile stress is applied.

The only means of quantifying the degree of sensitization, currently, is a destructive nitric acid mass-loss test, ASTM G-67.  For this test, a sample is immersed in concentrated nitric acid for twenty-four hours, after which the mass loss is measured.  A sample is considered sensitized with a mass loss greater than 25 mg/cm2, unsensitized with a mass loss less than 15 mg/cm2, and indeterminately sensitized with a mass loss between 15-25 mg/cm2.  Due to the destructive nature and safety concerns associated with this procedure, the Navy is interested in a non-destructive procedure to quantitatively show the difference between low, medium, and high DoS levels. 

An unsensitized AA5XXX sample after phosphoric acid etching reveals no β at the grain boundaries
A sensitized AA5XXX sample after phosphoric acid etching reveals extensive β coverage at the grain boundaries, making the alloy extremely prone to intergranular corrosion
The Embedded Aluminum Sensitization Monitoring System we are developing at Luna uses an eddy current technique for obtaining sufficient resolution for classification and trending of DoS, non-destructively.  In the future, this information will allow for improved condition monitoring and damage state awareness.  To control corrosion and cracking of these 5XXX aluminum alloys, we are also developing a chrome-free coating that acts to mitigate stress-corrosion cracking.  The zinc-rich primer functions as a sacrificial coating to galvanically protect the aluminum alloy and has been shown to drastically improve stress-corrosion cracking resistance.


Shaft Life Monitoring

The final technology that we will present at the Mega Rust Conference is being developed to meet a need for in situ assessment of shaft and barrier coating condition.  Currently, fleet-wide maintenance requirements dictate drydocking ships at ten year intervals for full shaft removal and inspection to determine the coating condition and assess any pitting/fatigue damage.  The sensing elements and instrumentation of our technology would allow for in situ measurements of shaft health at more frequent intervals and would reduce the risk of catastrophic shaft failure between inspection cycles, were the covering materials to fail prematurely.

The technology has been designed to reside permanently encased in the coatings and composite shaft cover.  A wireless power and signal transfer system would allow for underwater communication to the sensors.  The two sensors include a means to detect the degradation of the protection system by observing changes in local conductivity and eddy current sensors for an indication of pitting or cracking damage to the shaft.

Stay tuned for more updates on these exciting technologies!

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