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ABSTRACT 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/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. INTRODUCTION The annual cost of corrosion for Air Force aircraft and missiles is estimated to be $3.6 billion with corrosion accounting for 22.2% of the maintenance budget. Organic coating systems with corrosion inhibitors are the primary means of protecting aircraft structures. Historically, organic coatings for corrosion protection have contained toxic inhibitors (e.g., hexavalent chromium), volatile organic compounds (VOCs), and hazardous air pollutants (HAPs). The need for improved performance and environmental compliance continues to drive coating development. Many material evaluation and qualification methods for pretreatments, primers, and full coating systems rely on salt fog testing of scribe panels according to ASTM International(ASTM) B117 and ASTM D1654. The salt fog test is a relatively simple chamber test standardized in the 1950s where components or samples are exposed to constant conditions of humidity, temperature, and salt spray. It is accepted that salt fog testing is most appropriately used for quality assurance purposes, and significant efforts have been made to establish more sophisticated cyclic testing to achieve corrosion processes that are similar to those that occur in operational environments. These efforts have been conducted to yield better performance data for automotive products such as GMW14872, and to better simulate acidic or industrial environments ASTM G85. The cyclic corrosion tests are performed in programmable automated test chambers available from several manufacturers.
- Overview (0.34)
- Research Report (0.31)
- Materials (1.00)
- Government > Regional Government > North America Government > United States Government (1.00)
- Government > Military (1.00)
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- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)
ABSTRACT Sensitization of 5xxx series aluminum alloys is a significant concern for the U.S. Navy as the alloys have found use in a variety of ship structures for improved strength-to-weight ratios and enhanced corrosion resistance. Sensitization occurs when precipitates of ร-phase magnesium aluminide form at grain boundaries when 5xxx alloys are exposed to elevated temperatures for prolonged periods. These ร-phase precipitates are anodic relative to the aluminum matrix and corrode rapidly in the presence of an aggressive electrolyte. This corrosion can result in large crack formation under applied stress. Galvanic primers have been found to be effective at reducing this crack formation by effectively polarizing the sensitized structure into a passive potential range. This paper will review primers that Luna has developed and explore a variety of commercially available ones. Performance test results will be presented including those obtained using a unique atmospheric corrosion test method that measures time to crack initiation on coated/bare notched and sensitized 5xxx alloy samples. The method allows for mechanical load and test environment variability and is low-cost to enable rapid sample throughput. The top coating candidates are presented, based on a variety of performance testing, including the aforementioned atmospheric corrosion tests. The galvanic coatings have been found to drastically increase the lifetime of coupons by reducing the time to crack initiation and crack growth rate in accelerated atmospheric testing. The protective coatings may find use in extending the service life of existing structures and protecting new ones from sensitization effects. INTRODUCTION The Navy uses 5xxx series aluminum alloys in ship structures for improved strength-to-weight ratios and enhanced corrosion resistance. Specifically, UNS A95456 (AA5456) is the primary material of construction of the Ticonderoga (CG-47) class cruiser superstructures. The superstructure of the LCS Freedom class and the hull and superstructure of the LCS Independence class are constructed from UNS A95083 (AA5083). Magnesium is a primary alloying element of these alloys (>3.5% Mg) which renders them susceptible to sensitization. Sensitization occurs when precipitates of ร-phase magnesium aluminide form at grain boundaries when 5xxx alloys are exposed to elevated temperatures for prolonged periods. These ร-phase precipitates are anodic relative to the aluminum matrix and corrode rapidly in the presence of an aggressive electrolyte. Figure 1 shows an example of an as received 5xxx alloy microstructure compared to a sensitized one. Table 1 shows typical Mg wt% and yield strength for five different 5xxx alloys. Sensitization of 5xxx alloys thus promotes pitting, intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC). In order to mitigate this corrosion risk, a series of galvanic coatings have been characterized, that when applied to sensitized 5xxx aluminum alloys, retard pitting, IGC, and IGSCC initiation. Custom aluminum-rich primers were developed and characterized along with several commercially available zinc and magnesium-rich primers.
- Materials > Metals & Mining > Aluminum (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.34)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)
ABSTRACT 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. INTRODUCTION As is the case for many metal systems, aluminum alloys are susceptible to environment-assisted cracking (EAC) under the right combinations of stress (applied or residual), environment (such as those containing chlorides), and microstructure. EAC represents a significant risk to vehicles, structures, and equipment operated in corrosive atmospheres. Many studies of EAC in aluminum alloys have been carried out over the past decades, with the majority of investigations being conducted under immersion conditions. These studies have highlighted the importance of both anodic dissolution and hydrogen to EAC processes. However, the EAC mechanism is still not fully understood, particularly under atmospheric conditions.
- Materials > Metals & Mining (0.97)
- Energy > Oil & Gas > Upstream (0.71)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)
Combined Mechanical Stress and Environmental Exposure Accelerated Coating Testing
Kramer, Patrick (Luna Innovations Inc.) | Grumbach, Christina (The Boeing Company) | Williams, Kristen S. (The Boeing Company) | Feickert, Aaron J. (North Dakota State University) | Friedersdorf, Fritz (Luna Innovations Inc.) | Pennell, Sean (The Boeing Company) | Schultz, Karen A. (The Boeing Company) | Croll, Stuart G. (North Dakota State University)
ABSTRACT 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. INTRODUCTION A significant volume of environmentally hazardous waste is generated during repair and replacement of many coating systems such as those applied to the exterior of aircraft. Hazardous air pollutants (HAPs) and volatile organic compounds (VOCs) are needed, often in large quantities, for most remediation activities. The waste streams are associated with not only the chemical strippers and dusts generated during removal but also with substrate pretreatment chemicals that may contain hexavalent chromium, a known carcinogen. The amount of physical blast media and chemical stripper needed for stripping a single aircraft can be on the order of tens-of-thousands of pounds and several hundred gallons, respectively. Current standardized coating testing methodologies have not been suitable for accurately predicting the lifetime and performance of conventional and advanced coating systems on aircraft structures. During service, aircraft coating systems are subject to a combination of dynamic chemical, thermal, mechanical, and radiative stressors. The failure modes driven by the combined influence of all of these stressors are often different than the failure modes induced in standardized test methods. This is because standard methods do not account for mechanical stress and may only consider a single variable at a time. In some cases, components of a coating stack-up are qualified individually and not at the system level where incompatible material properties developed during service conditions lead to cracks and checks in the coatings. The result of the lack of understanding of causes of in-service failures and the inability to replicate them afforded by current test methods is the qualification of sub- optimal material systems that require shortened repair/replacement cycles. New representative coating evaluation protocols are needed to realistically assess the performance of complex coating material combinations, there-by enabling more accurate service life predictions, optimized maintenance intervals, accelerated development of high performance coatings, and ultimately significant reduction in hazardous waste generation.
- Aerospace & Defense > Aircraft (0.88)
- Energy > Oil & Gas > Upstream (0.46)
- Materials > Chemicals > Commodity Chemicals > Petrochemicals (0.34)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Health, Safety, Environment & Sustainability (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)