[China Aluminum Network] With the continuous improvement and improvement of the surface treatment technology of aluminum alloy profiles for construction, the requirements for product quality should also be increased accordingly. For this reason, the newly issued 2000 national standard GB/T5237.1 -5237.5 "aluminum alloy building profiles" in the 1993 edition of GB/T5237 on the basis of a large-scale revision, the original metallurgical standard YS/T100-1997 "electrophoretic painting aluminum alloy building profiles" and YS/T407-1997 "powder "Electrostatic spray aluminum alloy building profiles" come together in the standard, while increasing the content of fluorocarbon paint spray profiles. In the revision process, a large number of references adopted advanced foreign standards, in which GB/T 5237.2-2000 "Aluminum Alloy Building Profiles Part 2 Anodizing, Coloring Profiles" performance indicators in the dripping test with the Japanese Industrial Standard JISH8601 " Aluminium and aluminum alloy anodic oxide film" is consistent with the alkalinity-resistant performance specification. However, there are still some differences in the test methods. The dripping test method specified in Chinese Standard GB/T 5237.2 is visual observation or instrument measurement, while the alkali resistance test method in Japanese Industrial Standard JISH8601 only specifies Instrumental measurement method [Note: The alkali resistance test method in Japanese Industrial Standard JISH8601 specifies two kinds of instrument measurement methods, one is the voltage test method (alkalire sistance test by electro motive force); the other is the alkali drop test method ( Alkalis pottest), which is the instrumental measurement method used by some domestic inspectors for the drop alkali test method. ].
1 Principles of the experiment The national standard for aluminum alloy building profiles and the Japanese industrial standard have not described the principles and methods of the drop-alkaline test. To better grasp the operation method of the drop-alkaline test, it is necessary to understand the principle of the drop-alkaline test. of. The alkali drop test is mainly used to examine the alkali corrosion resistance of the anodized film. For the anodized film, its alkali corrosion resistance is relatively poor, when a certain concentration of sodium hydroxide solution is dropped on the surface of the anodized film, the anodized film will be eroded quickly if the plugging is poor or the oxide film is loose When the anodic oxidation film is poor in alkali corrosion resistance due to other reasons, the erosion rate will be faster. Therefore, the calculation of the penetration time of the anodic oxide film can be used to evaluate the alkali corrosion resistance of the anodic oxidation film. However, since the etching rate of the oxide film by the sodium hydroxide solution is high, it is difficult to evaluate the alkali corrosion resistance of the oxide film. At present, there are two main test methods for dripping tests, one is visual observation and the other is instrumental measurement. The visual observation method is based on that when the sodium hydroxide solution is dropped on the surface of the oxide film, the oxide film will slowly dissolve. The chemical reaction equation is as follows:
Al2O3·χH2O+2NaOH=2NaAlO2+(χ+1)H2O
During the dissolution process of the oxide film, the sodium hydroxide solution continues to erode inside the oxide film. After the sodium hydroxide solution erodes to the surface of the substrate metal, the metal aluminum and the sodium hydroxide solution undergo substitution reaction, and hydrogen gas will be precipitated during the reaction process. Corrosion bubbling occurs. The chemical reaction equation is as follows:
2Al+2NaOH+2H2O=2NaAlO2+3H2
The instrument measurement method is based on the electrical insulating properties of the anodic oxide film. The aluminum substrate is a good conductor of electricity, and the aluminum anodic oxide film is a high-resistance insulating film whose insulation is related to the thickness of the oxide film in the oxide film. In the process of being dissolved by the sodium hydroxide solution, the resistance will gradually decrease as the thickness of the oxide film decreases, and when the resistance is reduced to a certain value, it can be regarded as conductive, ie, the oxide film is considered to be dissolved.
2 Observations of the visual observation method and the discussion of the main influencing factors The key points to consider in the dripping test are the control of the test temperature and how to accurately determine the time when the oxide film just penetrated. The test method for dripping alkali in China GB/T 5237.2-2000 stipulates: "At 35 °C ± 1 °C, about 10mg, 100g / LNaOH solution was dropped to the surface of the profile sample, visually observed droplets until the Corrosion bubbling, calculating the time the oxide film was penetrated.Also can be used to measure the time of oxide film penetration." That is to say, the national standard has approved two kinds of dripping alkali test methods, namely visual observation method and instrument measurement method. For the visual observation observation method, the description of the national standard is relatively simple, some considerations in the test operation and its influencing factors are not described. In order to ensure the accuracy of the test results, attention should be paid to the influencing factors during the operation to minimize or avoid the influence of these factors. The following points should be noted in this method: (1) Control of the sample The test surface of the sample must be kept intact and no damage such as scratches or scratches is allowed. The surface to be inspected must be clean and dirt, oil, and other dirt must not be covered on the surface to be inspected. Therefore, it is generally used before testing. The organic solvent that does not damage the oxide film gently wipes the surface of the sample; (2) The concentration of the test solution must be strictly controlled to 100 g/L. The concentration is too low or too high, which will directly lead to large test results. (3) Test temperature control, not only to ensure that the test environment temperature is controlled at 35 °C ± 1 °C, and the test solution and sample must also be controlled at 35 °C ± 1 °C, for which it should be First, the test solution and the sample are placed in a constant temperature instrument for a period of time. Only when the test solution and the sample are constant at 35°C±1°C can the test be performed; the fourth is the selection of the thermostatic instrument, and the constant temperature instrument. Selection in this method is a very important part, because the selected constant temperature instrument should not only play a role in constant temperature, but also must consider the ease of observing the change of the sample in the instrument. If none of the selected constant temperature instruments can be clearly identified, Observing the observation port of the change of the sample in the instrument, it is impossible to accurately determine when the sample begins to corrode. In addition, the visual observation method was also influenced by the experience of the test personnel. In the actual inspection work, it was found that there was not a clear process from the dissolution of the anodized film to the penetration of the oxide film (the sample began to corrode and bubbling). The change brings great difficulty to the determination of the oxide film penetration time. This places high requirements on the test personnel. The test personnel must have very rich practical experience and can accurately determine when the oxide film is Penetrate and begin to corrode.
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