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Aluminum (Al) alloys are the most common non-ferrous metals used (approximately 25 million tons per year) and the second most commonly used metal alloy after steel1. Some of the properties of Al alloys that attribute to their worldwide use include lightness, thermal conductivity, electrical conductivity, suitability for surface treatments, and corrosion resistance. Al alloys are also combined with other metals/materials to achieve desired properties for specific applications. Al alloys can be joined to other materials with ease to enhance their combined properties with the following techniques: welding, bolting, riveting, clinching, adhesive bonding, and brazing1.
Aluminum (Al) alloys are the most commonly used non-ferrous metals (approximately 25 million tons per year) for various technical applications and the second most commonly used metal alloy after steel. Al alloys are also combined with other metals/materials to get specific desirable properties for particular applications. However, a system of dissimilar materials can lead to potential corrosion problems such as galvanic and/or crevice corrosion. In this work, atmospheric galvanic corrosion of aluminum (Al) alloy was predicted by combining electrochemical techniques and accelerated laboratory corrosion tests. Three different galvanic couples were analyzed where 6061-T6 Al alloy (UNS A96061) was coupled with a passivating metal (304 stainless steel or UNS S30400), noble metal (copper or UNS C11000) and a conductive polymer matrix composite (PMC) reinforced with carbon fiber. The galvanic current flowing between the anode and cathode was measured using the zero-resistance ammeter (ZRA) technique in the humidity-chamber setup. Electrolytes with varying amounts of chloride contents were used to imitate different atmospheric conditions. An equation based on Faraday’s law was developed to calculate the corrosion rate in grams per meter square per day (gmd) by relating the time of wetness (TOW) from the field exposure to the galvanic current measured from the accelerated laboratory experiments. The total exposure time was divided into wet periods (Twet) and dry periods (Tdry). Cyclic corrosion test chamber following a modified GM9540P cycle and outdoor exposure experiments are planned for the future.
Fundamental aspects in the design and application of corrosion sensors, as well as challenges that may lead to the reporting of inaccurate or misleading corrosion data, has been discussed and illustrated by the case of under-deposit corrosion monitoring using a multi-electrode array.
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Aircraft and similar structures are exposed to many different types of environmental conditions over their operational life and this very much determines the type of damage they experience. For example sea based aircraft experience a much more aggressive environment than land based aircraft. Therefore it is critical to know the environmental exposure of an aircraft over the course of its lifetime to make meaningful predictions of accumulated corrosion damage.Computer simulation models have been demonstrated to be able to predict the rate of corrosion and the incidence of pitting but they require information on the electrolyte thickness conductivity/corrosivity and the polarization properties of the material. Therefore to use a computational model to predict the corrosion rate for an aircraft at a particular point of time during its service we would have to define the model parameters at that time as well as the exposure history.In this paper a service life model concept is developed based on environmental data collected using sensors combined with a galvanic corrosion simulation model. The galvanic simulation model input data is identified from the sensor data to predict the accumulation of corrosion damage. Applications of the model are presented and discussed.The approach is applicable not only to aircraft but to a wide range of structures.
Various austenitic stainless steels such as UNS S30409, S31609, S32109 and 34709 are widely used in complex refinery or chemical plants at temperature ranges between 550°C and 950°C. However, Stress Relaxation Cracking (SRC) in welded joints or cold deformed parts has been a serious problem during fabrication or operation. Several researches were conducted to construct SRC test methods. This included the evaluation of SRC susceptibilities among various austenitic stainless steels and to determine SRC mechanism within TNO Science and Industry or JIP1-4. It was concluded that SRC was caused by the accommodation of strain due to both carbide/nitride precipitation hardening inhibiting dislocation movement and the formation of precipitation free zone along the M23C6 carbide at grain boundary during stress relaxation process of welding residual stresses at temperatures between 550°C and 750°C.