9622 Corrosion Characterization Procedures

Presently, in order to characterize corrosion susceptibility of aircraft aluminum alloys, accelerated laboratory corrosion tests as well as natural corrosion tests are used. The tests include exposure of the specimen into a certain corrosive environment as defined in the respective specification and then determination of weight loss as well as type and depth of corrosion attack. Some companies involve in their internal corrosion test evaluation procedures measurement of pitting density as well. The specifications for atmospheric corrosion tests include the determination of tensile properties after exposure of the specimens to atmospheric corrosion for certain time intervals. The most widely used laboratory corrosion tests will be shortly reviewed in this chapter. Yet, with the exception of the atmospheric corrosion test, any correlation of the above tests to the real service environment of aircraft components is relatively arbitrary. Most characteristic is the case of the widely used exfoliation corrosion test. It has been originally specified to evaluate the quality of a certain heat treatment procedure and not to simulate in the laboratory any operational corrosive environment. Even the salt spray test, which obviously relies on exposure close to the sea, should be interpreted very carefully. Presently, no models exist to correlate the accelerated corrosion attack of a specimen at laboratory conditions to the long-term gradually accumulated corrosion in the operating conditions of a structure. In addition, the mentioned corrosion susceptibility evaluation does not provide any information concerning the possible influence of progressing corrosion attack on the material's mechanical properties.

One should consider that current design specifications of aircraft components do not account for corrosion-induced structural degradation in service. Even in calculations of the residual strength of aged components, the material properties employed refer to the virgin material. In case studies discussed recently in the open literature [39], the presence of corrosion in an aged aluminum aircraft member is correlated to an increase of the stress gradients applied on the member; it is assumed that the corrosion-attacked surface layer of the material is not capable of carrying a load. Thus, the thickness of the material may be reduced essentially and the stress gradients increase, respectively. This approach accounts for the corrosion-induced decrease of yield and ultimate tensile stress. Yet, in several recent publications corrosion has been related to a dramatic material embrittlement as well [41, 42, 60, 61]; the phenomenon has been attributed to corrosion-induced hydrogen evolution [60], which is trapped at different states in the material [61]. The assumption that the corroded material surface layer is not capable of carrying a load does not account for corrosion-induced hydrogen embrittlement.

The remarks made above indicate that characterization of corrosion susceptibility should involve information on the residual mechanical properties of a structural material following exposure to a corrosive environment. In addition, there is a need to advance models capable of correlating the in-service expected long-term corrosion induced material property decrease to the decrease determined at accelerated corrosion attack conditions in the laboratory. This will allow as well to specify accelerated laboratory corrosion tests which simulate the inservice expected corrosion attack. The corrosion processes used to precorrode the specimens are described below; they refer to the most commonly used corrosion tests in today praxis.

Exfoliation Corrosion (EXCO) Test The EXCO test was conducted according to ASTM specification G34-90 [62]. Following the G34-90 specification the recommended duration of specimen exposure in the corrosive solution is 48 and 96 h, respectively. In order to determine with even greater precision the behavior of the 2024-T3 alloy for the entire test duration of 96 h and to locate the exact time for the initiation of pitting and exfoliation corrosion, additional specimens were used as well; they correspond to intermediate durations of exposure. For the corrosive solution, the following chemicals were diluted in 1 L distilled water to the indicated concentrations: NaCl(4.0 M), KNO3 (0.5 M), and HNO3 (0.1 M). The initial pH was 0.3. The required amount of the solution was 10 mL/cm2 according to the specification G34-90. The specimens were exposed individually. The solution temperature was maintained constant at 25 ± 3°C during the entire experiment by conducting the experiment in a controlled temperature chamber. During the specimen exposure, regular pH measurements of the solution were taken.

To determine weight loss, specimens were weighed before and after exposure. Specimen cleaning after removal from the corrosive solution was conducted by rinsing in distilled water, soaking in concentrated nitric acid, rinsing in distilled water, and thoroughly drying in a hot-air stream. Particular attention was payed to the removal of hydroxide deposits (white film) from the surface of the exposed coupons.

Alternate Immersion Test The alternate immersion (AI) test was conducted according to ASTM specification G44-94 [67]. As for the EXCO test, additional periods of exposure were selected in order to record the development of corrosion and to determine the exact appearance time of pitting. A 3.5-wt% NaCl solution in distilled water was used. The amount of solution according to ASTM G44-94 was 32 mL/cm2. All specimens were exposed simultaneously to the solution. The total amount of solution was 29 L. The solution temperature was maintained constant at 25 ± 3° C during the entire experiment. The solution pH was in the 6.4-7.2 range.

The alternate immersion apparatus consisted of two Plexiglas tanks and two transfer pumps suitable for corrosive solutions. The operation of the apparatus was controlled automatically. The specimens, suspended by polyvinyl chloride (PVC) beams, were submerged in the first tank. After 10 min exposure, the solution in the first tank was pumped into the second tank, where it remained for 50 min, before it was pumped back into the first tank. The time cycle (1 h) was constant for all experiments. As an example, the 30 day exposure corresponded to a total of 720 cycles. In order to determine and record weight loss, the specimens were weighed before and after the experiment. Specimen cleaning was conducted according to ASTM specification G1-90 [68].

Intergranular Corrosion The intergranular corrosion test was conducted according to ASTM G110-92 specification [69]. Prior to immersion in the test solution, each specimen was immersed for 1 min in an etching cleaner at 93°C. The etching cleaner was prepared by adding 50 mL of nitric acid, HNO3, (70%) and 5 mL of hydrofluoric acid, HF (48%), to 945 mL of distilled water. The specimens were first rinsed in the reagent water and immersed in concentrated nitric acid (70%) for 1 min. The specimens were rinsed again in distilled water and dried into moving air. Then they were immersed into the test solution. The test solution consisted of 57 g of sodium chloride (NaCl) and 10 mL of hydrogen peroxide (H2O2), prepared just diluted to 1 L with reagent water. The test solution volume per exposure area was 8 mL/cm2 of specimen surface area. The solution temperature was maintained at 30 ± 3°C. The exposure time was 6 h. After exposure, each specimen was rinsed with reagent water and allowed to dry. Examination of specimens was made following the specification ASTM G110-92.

Salt Spray Test The salt spray test was conducted according to ASTM B117-94 specification [70]. The salt solution was prepared by dissolving 5 parts by mass of sodium chloride in 95 parts of distilled water. The pH of the salt solution was such that when atomized at 35°C the collected solution was in the pH range from 6.5 to 7.2. The pH measurement was made at 25°C. The temperature at the exposure zone of the salt spray chamber was maintained at 35 + 1.1-1.7°C. The test duration was 30 days. After the exposure the specimens were washed in clean running water to remove salt deposits from their surface and then immediately dried.

Cyclic Acidified Salt Fog Test The cyclic acidified salt fog test was performed according to ASTM G85-94 specification, Annex A2 [71]. The salt solution was prepared by dissolving 5 parts by weight of sodium chloride, NaCl, in 95 parts of distilled water. The pH value of the solution was adjusted to range between 2.8 and 3.0 by the addition of the acetic acid. The temperature in the saturation tower was 57 ± 1°C. The temperature in the exposure zone of the salt spray chamber was maintained at 49 + 1.2-1.7°C. The specimens were subjected to 6 h repetitive corrosion cycles; they included: 45 min spray, 120 min dry-air purge, and 195 min soak at high relative humidity. The test duration was 30 days.

Atmospheric Corrosion Test The tests were made in accordance to ASTM G50-76 specification [72]. Exposure racks and framers were prepared according to the requirements of the above specification. A specimen exposure angle of 30° from the horizontal, facing south, was selected. Before exposure, corrosion specimens were weighed to the nearest 1 x 10-4 g. Prior to the exposure all data recommended in ASTM G33-88 specification [73] were recorded. Atmospheric factors were recorded continuously. Specimens were removed every 3 months starting from the third month of exposure. In the present work, the results cover an exposure period of 21 months; tests of longer exposure duration are ongoing. After the exposure, weight loss, depth of pits at skyward, and groundward surface were measured; pitting density was also determined using an image analysis facility.

9.6.2.3 Mechanical Testing

Following exposure in the corrosive environments described above, the corroded tensile specimens were subjected to tensile testing. All tensile tests carried out are summarized in Tables 9.33- 9.36. The test series performed included: (i) tensile tests on uncorroded specimens to derive the reference tensile behavior of the material, (ii) tensile tests on specimens subjected to accelerated corrosion tests, (iii) tensile tests on specimens exposed to the corrosive environment for different exposure times to determine the gradual tensile property degradation during corrosion exposure in accelerated laboratory tests as well as in atmospheric

TABLE 9.33 Tensile Tests Performed on Alloy 2024-T351
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