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 correspondingly increased. For this reason, the newly issued 2000 national standard GB/T5237.1-5237.5 "Aluminum alloy construction "Profile" in the 1993 version of GB/T5237 on the basis of a major revision of the original metallurgical standard YS/T100-1997 "electrophoretic painting aluminum alloy building profiles" and YS/T407-1997 "static powder electrostatic spraying aluminum alloy building profiles 》Commonly unified into 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 an alkali resistance test by electromotive force; the other is an alkali drop test method (alkali Spot test), which is the instrumental measurement method used by some domestic inspectors for the drop alkali test method. ].
1 Experimental principle
China's aluminum alloy construction profile national standards and Japanese industrial standards do not describe the principles of the drop-alkaline test, and in order to better grasp the operation method of the drop-alkaline test, it is necessary to understand the principle of the drop-alkaline test method. 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.
Observations of 2 Visual Observation Methods and Discussion of Main Influencing Factors
The key considerations for the dripping test are the control of the test temperature and how accurately the time that the oxide film was 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.
3 Method description of instrument measurement
The instrument measurement method is not described in the national standard for its specific operation method, but the Japanese Industrial Standard JISH8601 stipulates that according to JISH8681-1:1999 "Aluminum and Aluminum Alloy Anodized Film Corrosion Resistance Test Method - Part 1: Alkali Resistance" The "test" was performed and the instrument measurement method was described in detail in JISH8681-1. In order to have a clearer understanding of the instrument measurement method for personnel engaged in quality inspection work in China, this paper introduces the operating points of the alkali drop test method (instrument measurement method) specified in the Japanese Industrial Standard JISH8681-1. The main test instruments of this method are: drip instruments (capable of continuously dropping test liquids at a set time interval according to a set amount of test liquid), thermostatic instruments, and resistance measuring instruments. The test requirements are as follows:
Project test requirements
Test air temperature 35°C±1°C
Test solution temperature 35°C±1°C
The test area for each test point is approximately 28mm2 (diameter of 6mm)
The test volume for each test point is approximately 16mg
Test drop time interval 5s
Concentration of sodium hydroxide solution 100g/L
Before testing, the surface of the sample should be cleaned with a soft cloth impregnated with a suitable solvent (such as acetone, ethanol, etc., which does not corrode the sample). Then use an alkali-resistive ink or other ink to draw a certain distance on the surface of the sample and a circle with an inner diameter of about 6 mm or a synthetic resin tape with holes with a diameter of 6 mm sticks to the sample surface, and place the sample at the temperature Maintain a constant temperature of 35 °C ± 1 °C for at least 30 min in a constant sample temperature of 35 °C ± 1 °C. Then, the test solution was continuously dripped into the marked circle on the sample using a drip instrument. When the number of corrosive blisters increases to half of the total number of test points, immediately place the sample in the rinse water, and gently scrub and dry with a cotton ball or the like on the test surface. Record the time from the initial drop or the drop later to the time the sample was thrown into the water. Measuring the conductivity of each test point with a measurable resistance instrument requires 3 measurements per point. When the reading of the instrument reaches 5000 Ω or less, the test point is considered conductive and the oxide film at this test point is considered to have been dissolved. . Calculate the interval between the last conductive test point and the time the sample was put into the water. This time can be used to evaluate the alkali corrosion resistance of the sample. However, the author believes that the operation of the instrument measurement method does not necessarily require a constant change. Based on the test principle of this method, proper modification of the operation steps is still possible. For example, some domestic inspectors changed the operation of automatic dropping of the test solution to manual dropping of the test solution. This should be considered a good change, because this change makes the instrument measurement method more widely applicable. General laboratories are Can be used without the need to purchase a dedicated test drop instrument.
4 Conclusion
(1) The instrumental measurement method is used to judge the penetration of the oxide film by measuring the conductivity of the film. Compared with the visual observation method, the operation is simple and easy, reducing the influence of human factors and reproducible results. better.
(2) Compared with the visual observation method, the instrument measurement method has less requirements on the experience of the inspectors, which is beneficial to the rapid inquisition of the inspectors who are newly exposed to the test.