Aluminum Sensitization and Non-destructive Testing
Aluminum alloy ship hulls are utilized on many commercial and military ship platforms. Over their lifetime, they often require non-destructive and destructive testing. Among the common hulls that require such examination include CG 47 Class Cruisers, Littoral Combat Ships (LCS) – both Freedom and Independence variants – as well as Amphibious Assault Ships, Expeditionary Fast Transport, Landing Craft Air Cushions, Ship-to-Shore Connectors, and both CVN 68 Nimitz class and CVN 78 Ford class aircraft carriers. 5XXX series aluminum alloys are commonly used for these assets. They are required to undergo non-destructive testing (NDT) during the material forming, material joining, and in-service. These alloys have the propensity to become sensitized at high temperatures over time, thus driving the need for in-service NDT. While a high Degree of Sensitization (DoS) may not be harmful by itself, high DoS levels make the material more susceptible to intergranular cracking (IGC) and / or stress corrosion cracking (SCC) when exposed to the aggressive environments. Friction Stir Welds (FSW) and Gas Metal Arc Welds (GMAW) may be inspected with phased array ultrasound (PAUT) and full matrix capture / total focus method (FMC/TFM) for welding process and in-service defects. This article discusses some in-service non-destructive testing and destructive testing used for the detection of sensitization, intergranular cracking, and stress corrosion cracking of aluminum ship hulls.
Figure 1: Aluminum sensitization and associated failure mechanisms: Intergranular cracking (IGC) and / or stress corrosion cracking (SCC) may be detected with high frequency phased array ultrasound and non-linear ultrasonic testing.
Figure 2: Sensitized aluminum may evaluated destructively or non-destructively.
Figure 3: SCC mutual dependence on sensitization, environment and tensile strength
Ultrasonic Testing of Aluminum Ship Hull Welds
During the alloying process, magnesium is diffused evenly through the aluminum matrix for the desired corrosion and material properties . Sensitization of aluminum 5XXX alloys often occurs when structures are exposed to elevated temperatures for extended periods of time. This being the case especially if the alloys are at or close to exhaust areas. During this sensitization stage, a beta phase Mg2Al3 forms and precipitates along the grain boundaries. Sensitization is the pre-cursor to IGC, followed by SCC and IGC. If measured accurately, this could be used as a predictive tool. Magnesium is less stable and kinetically more active than elemental aluminum . As a result, magnesium is more likely to dissolve in low and neutral pH environments. Similarly, the beta-phase Mg2Al3 along the grain boundary can dissolve rapidly in salt water climates. This combination of the beta-phase and the sea water environment leads to IGC. SCC is typically downstream of IGC. First, the material must be sensitized enough to for the beta-phase to form along the grain boundaries. Additionally, these grain boundaries must be exposed to a salt water environment. Lastly, sufficient tensile stress must be applied to initiate cracking. In short, SCC may occur if these three conditions are met. Early stages of IGC and SCC may be detected using non-linear ultrasonic testing in tandem with high frequency phased array ultrasonic testing (PAUT).
Destructive Testing for Aluminum Sensitization
Sensitization is most commonly measured using the destructive testing known as nitric mass acid loss test (NAMLT) as described by ASTM G-67 . This test requires that 4” x 4” or comparable sized specimens be cut out from the ship hull for further machining. After said materials are cut out, they will be destructively tested. The underlying goal of the test is to quantify sensitization by measuring the mass loss per unit area (mg/cm2) during immersion in 70% nitric acid for 24 hours. The mass loss can range from 0 to 90 mg/cm2. This is referred to as the Degree of Sensitization (DoS), which used for maintenance decisions. Under this test, a sample with a measured mass loss greater than 25 mg/cm2 is considered to be susceptible to intergranular forms of corrosion . It is also worth noting that there is correlation between DoS and SCC growth rates  and weldability of 5XXX alloys .
Phased Array Testing for Aluminum Sensitization
Phased array testing of the friction stir and GMAW welds may be performed with PAUT probes in the 5 to 10 MHz range with 16- or 32-element apertures. Typically, aluminum plate thickness that are tested non-destructively linger in the 6 to 13 mm thickness range. The 5 MHz PAUT platform is most useful for detecting in-service cracking, fatigue cracks developed from IGC and SCC, and welding process welding defects. The 10 MHz PAUT platform is most useful for detecting downstream failure mechanisms caused by aluminum sensitization. This includes both early and mid-stage IGC and SCC.
Figure 2: Phased array testing of aluminum ship hull GMAW weld.
Nonlinear Ultrasonic Testing (NLU) for Aluminum Sensitization
NLU is a promising technique for microstructural characterization. These methods are based on measuring the nonlinearity (i.e., amplitude-dependence) of materials elastic parameters. Experimental observations indicate that nonlinear elastic response of solids carry specific information about microstructural features related to the presence of discontinuities, microcracks, dislocations, and precipitates. The aforementioned presences are manifestations of damage and imperfection. The elastic properties of most intact materials at low strains are linear (i.e., strain invariant). The imperfections introduce relatively soft bonds within the stiffer, surrounding medium at microscopic scales. The locally increased compliance of imperfect interfaces and inter-grain bonds result in elastic nonlinearity at macro-scale. If an initially (almost) linear medium such as an aluminum alloy is fatigue damaged, thermally damaged, or sensitized; it will exhibit nonlinear elastic behavior. This being the case even at low strains caused by high-power ultrasonic testing. Finite amplitude ultrasonic waves mobilize the nonlinearity of the damaged material. The nonlinearity of the medium results in distinct distortions in the measured ultrasonic signals. E.G. The response will contain (i) higher harmonics of the input frequency and/or (ii) time-of-flight increases and/or (iii) amplitude decreases with increasing input voltage.
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