Organic coatings are the primary means of protecting structures from atmospheric corrosion in harsh environments. Environmental compliance and a desire for increased performance continue to drive the development of new coatings; however, this process may take many years. Corrosion tests for coatings development, qualification, and selection often provide poor correlation to service performance. Conventional tests and characterization procedures may not assess the most relevant degradation mechanisms of loss of barrier properties, localized corrosion, and galvanic attack. Furthermore, these tests do not yield information on interactions between specific environmental parameters and material degradation or the kinetics of failure. To improve coating evaluations, electrochemical sensors and data acquisition systems have been developed to measure coating performance and corrosion continuously during atmospheric tests. The sensing elements are incorporated in multisensor test panels that can be cleaned, coated, and scribed like conventional coating test panels. The multisensor panels support measurement of coating barrier properties, free corrosion, and galvanic corrosion. The barrier properties and free corrosion are measured using impedance-based methods, and galvanic corrosion is quantified using a zero resistance ammeter. Local measurements of panel surface temperature, air temperature, and relative humidity are used to correlate degradation rates and barrier properties to environmental conditions. The simultaneous environmental measurements also provide records that can be used to ensure the validity of performance comparisons between separate tests and within a single test. A description of the sensors, electrochemical measurements, and methods for coating testing are reported.

Although a great deal of success in structural integrity management has been achieved, it may be realized by excessively conservative assumptions, high inspection burdens, and aggressive maintenance and repair strategies. Corrosion has been found to reduce fatigue life, but methods to account for it in fatigue modeling are still limited. There is a recognized need to improve structural integrity calculations through the inclusion of the effect of existing corrosion damage on fatigue. The feasibility of a corrosion modified fatigue analysis process that uses finite element analysis and linear elastic fracture mechanics techniques to predict fatigue life of a corroded aircraft component based on the corrosion damage location is demonstrated in this work. A corrosion modified equivalent flaw size was successfully used to predict fatigue crack growth from AA7075-T651 specimens with two different notch geometries and corrosion damage at the notch center.

Atmospheric corrosion represents an annual multi-billion dollar cost burden for the aerospace and defense sectors. For many aircraft, particularly those operating in marine environments, up to ninety percent of corrosion is due to galvanic interactions at dissimilar metal couples. As new materials are introduced with the acquisition of more advanced aircraft, galvanic corrosion is likely to remain a concern. The ability to model galvanic corrosion accurately holds the promise of being able to both predict the performance of new material combinations to guide material selection and predict corrosion damage for maintenance planning. Such models often utilize data collected under immersion test conditions that are not representative of the thin-film electrolytes that are relevant to atmospheric corrosion and may diminish model accuracy and utility. In this work, an atmospheric cell is presented that allows for measurements of corrosion kinetics using thin-film electrolytes. It is observed that the limiting oxygen reduction current density on various alloys is increased several orders of magnitude over immersion results. A segmented, galvanic sensor is presented that enables the experimental quantification of spatial distributions of galvanic current under thin film conditions that is compared to model predictions for verification of the suitability of immersion and thin-film electrolyte polarization data inputs.

The most effective means to control atmospheric corrosion of aircraft is through the use of protective coatings. In addition to combating corrosion, which represents a risk to the safe operation of an asset, there are strong economic and environmental drivers to extend the service life of aerospace coatings. Repair and replacement of exterior coating systems that no longer meet protective requirements generate a significant volume of environmentally hazardous waste, which includes the coating material, media used for coating removal, as well as the waste materials generated in surface preparation and reapplication of the coating system. Development and use of the most durable coatings systems has often been limited by the ability to predict service performance in accelerated tests. Existing accelerated test techniques do not adequately employ the chemical, thermal, or mechanical stressors that produce relevant damage mechanisms, such as cracking at structural discontinuities in coated airframes. Additionally, single coating layers may be qualified individually rather than as part of a representative multilayer stack-up. As a result, current test methods cannot be used for accurate quantification of coating performance and service life. In this work, test methodologies previously described that employ combined environmental and mechanical loading modes are utilized to excite relevant failure modes of a multilayer system, such as coating cracking at sealant-filled lap joints. The mechanisms and kinetics of damage progression are quantified throughout static and dynamic atmospheric tests using in situ measurements of coating system properties. It is observed that the coating barrier properties and resistance to cracking at a lap joint are dependent upon both the individual effects of stress, temperature, and humidity as well as the combined interaction effects of these stressors.

Cyclic loading of mechanical components promotes the formation of dislocation dipoles in metals, which can serve as precursors to crack nucleation and ultimately lead to failure. In the laboratory setting, an acoustic nonlinearity parameter has been assessed as an effective indicator for characterizing the progression of fatigue damage precursors. However, the need to use monochromatic waves of medium-to-high acoustic energy has presented a constraint, making it problematic for use in field applications. This paper presents a potential approach for field measurement of acoustic nonlinearity by using general purpose ultrasonic pulser-receivers. Nonlinear ultrasonic measurements during fatigue testing were analyzed by the using contact and immersion pulse-through method. A novel cross-correlation filtering technique was developed to extract the fundamental and higher harmonic waves from the signals. As in the case of the classic harmonic generation, the nonlinearity parameters of the second and third harmonics indicate a strong correlation with fatigue cycles. Consideration was given to potential nonlinearities in the measurement system, and tests have confirmed that measured second harmonic signals exhibit a linear dependence on the input signal strength, further affirming the conclusion that this parameter relates to damage precursor formation from cyclic loading.

Repair and replacement of exterior coating systems that no longer meet aesthetic or protective requirements generate a significant volume of environmentally hazardous waste, which includes the coating material combined with solvents and/or media used to remove the coatings, as well as the waste materials generated in surface preparation and reapplication of the coating system. There are strong economic and environmental drivers to extend the service life of aerospace coatings. However, development, selection, and use of the most durable coatings systems have often been limited by the ability to predict service performance in accelerated tests. Current accelerated test methods do not adequately employ the chemical, thermal, mechanical, or radiative stressors that produce relevant damage mechanisms in coated structures that can be used for accurate quantification of coating performance and service life. Test methodologies are being developed that employ combined environmental and mechanical loading modes to overcome this issue. The mechanisms and kinetics of damage progression are quantified continuously throughout a test using in situ measurements of coating system properties and substrate corrosion. Mechanical test fixtures and simulated structural components are being used to apply stresses to coating systems in accelerated atmospheric test chambers. The combined mechanical and environmental tests are expected to produce failure modes not achieved using traditional atmospheric test chambers. An overview is given of the test methods, in situ measurement systems, coating characterization, and combined effects atmospheric exposure testing.

Organic coating systems with corrosion inhibitors are the primary means of protecting structures from atmospheric corrosion in harsh environments. Environmental compliance and a desire for increased performance continue to drive coating development and new product introductions. Current corrosion tests and measurement methods for qualification and selection of aerospace coatings often provide poor correlation to service environment performance and do not assess the highest risk failure modes of localized corrosion, galvanic attack, and environment assisted cracking. Furthermore, traditional coating characterization methods do not quantify material and environmental interactions needed to establish relative coating performance in accelerated tests, outdoor exposures, or service environments. New coating qualification typically includes accelerated corrosion tests, outdoor exposures, and aircraft trials; however, these product introductions may take 10 – 15 years. There is an important need for improved measurement and monitoring techniques that can be used to accelerate new coating introductions and monitor coating performance in outdoor service environments. A recently developed corrosion and coating monitoring system includes an extensible network of measurement systems each with multimodal sensors for comprehensive evaluation of the capacity of a corrosion protection system to control alloy free corrosion and galvanic corrosion, maintain barrier properties, and resist environment assisted cracking. The sensors are compliant with the ANSI(1)/NACE TM0416-2016 for monitoring atmospheric corrosion. The system is compatible with existing accelerated test chambers and suitable for use in outdoor service environments. An overview of the atmospheric corrosion and coating degradation sensors and measurement techniques to support comparative testing, materials selection, and site monitoring will be presented.

Environment-assisted cracking (EAC) of aluminum alloys in corrosive atmospheres is a significant maintenance and safety issue for aerospace and naval structures. EAC is influenced by the interaction of stress, environment, and alloy microstructure. Atmospheric environmental conditions and corrosion kinetics are dynamic due to diurnal cycles and changing operating conditions, where temperature, relative humidity, and surface contaminants interact to control thin film electrolyte properties. In the case of EAC and other localized corrosion processes, such as crevice corrosion, separation of the anode and cathode may occur due to variation of chemical composition, oxygen availability, and pH differences between the crack tip, mouth, and boldly exposed surfaces. Conventional electrochemical immersion testing is not well suited to study factors and interactions leading to EAC in corrosive atmospheres. The bulk electrolyte conditions for electrochemical immersion testing are vastly different than the thin film properties that are operative in atmospheric corrosion. For instance, the dynamic temporal and spatial variations and effects of cyclic relative humidity on the salt concentration, film thickness, and oxygen diffusion cannot be captured. Additionally, standard three electrode, immersion test cell measurements are not well suited to directly investigate the variation and distribution of cathodic and anodic currents that develop over a sample surface during EAC or crevice corrosion under atmospheric conditions. Thin film electrolyte electrochemical tests have been conducted using a segmented, multi electrode sensor with an artificial crevice to quantify the interaction of crack tip and crack mouth during cyclic atmospheric corrosion tests. These tests are compared to EAC measurements under similar conditions to inform a better understanding of the processes that are significant to EAC of aluminum alloys. Maximum crack velocities are observed when high cathodic current is measured at the tip of artificial crevices suggesting hydrogen embrittlement.

Environment-assisted cracking (EAC) of aluminum alloys in corrosive atmospheres is a significant maintenance and safety issue. EAC is influenced by the interaction of stress, environment, and microstructure. Atmospheric conditions and corrosion kinetics are dynamic due to diurnal cycles and changing operating conditions. Temperature, relative humidity, and surface contaminants interact to control thin-film electrolyte properties. Within a crack, the separation of the anode and cathode may occur due to concentration gradients between the crack tip, mouth, and external surface. Conventional immersion testing is not well suited to study factors and interactions leading to atmospheric EAC because the bulk electrolyte conditions for immersion testing are different from the thin-film properties. Additionally, standard three-electrode immersion measurements are not well suited to directly investigate the variation and distribution of cathodic and anodic currents on a sample surface and within an EAC crack. In this work, atmospheric electrochemical tests have been conducted using a segmented, multielectrode sensor with an artificial crevice to quantify local, dynamic anodic and cathodic current distributions. These tests are compared to EAC growth rate measurements. Maximum EAC growth rates are observed when high cathodic current is measured at the tip of artificial crevices, suggesting a hydrogen embrittlement mechanism.

Environmentally assisted cracking (EAC) of aluminum alloys in corrosive atmospheres is an important maintenance and safety issue for U.S. Department of Defense assets. EAC initiation and propagation of cracks is influenced by the complex interactions of load, environment, and alloy properties. Traditional environmental fracture testing conducted under immersion or constant humidity conditions may produce results that are different than measurements collected under thin electrolyte layers or droplets formed during atmospheric exposure. In addition, most standard methods do not provide instantaneous measures of crack velocity that can be used to identify specific environmental conditions that promote cracking. Improved assessment of EAC susceptibility and the conditions that promote cracking of aluminum alloys has been accomplished with an autonomous, in situ measurement system that can be used in accelerated corrosion test chambers and outdoor exposure sites. Continuous measurements of crack length throughout a corrosion study can be obtained using a tensile loaded notched specimen, compact load frame, and a force sensor to track load shedding with crack propagation. These measurements can be used to compare alloy performance, determine environmental conditions that promote EAC, and evaluate the effectiveness of corrosion control coatings and methods. Aluminum alloy testing with varying environmental and mechanical parameters (e.g., relative humidity, salt composition, degree of sensitization, and stress intensity) has demonstrated a strong dependence of crack velocity on cyclic relative humidity (RH). Specifically, in a number of tests, crack velocity increased to a maximum during drying (decreasing RH) at intermediate humidity. This result may be important to understanding the processes that promote EAC and indicates that high humidity and salt loading may not always be the most aggressive conditions for evaluating EAC susceptibility. Results of AA5083 alloy testing in cyclic accelerated corrosion tests, including ASTM B117, ASTM G85 A5, and GM9540P are reported. A subset of results for AA7075-T651 is also presented to demonstrate applicability of the test method for a different alloy that was not as highly sensitized to EAC.