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Analysis of Salt Fog Corrosion Test on Surface Anti-corrosion Ability of Lighting Products

by:GESTER Instruments     2021-04-29
1. Corrosion mechanism The working environment of lighting products is atmospheric and indoor environment, and the harsh conditions are in the ocean climate or the seabed. Atmospheric corrosion is divided into three categories: dry atmospheric corrosion; wet atmospheric corrosion; wet atmospheric corrosion. 1. Characteristics of atmospheric corrosion Atmospheric corrosion is characterized by the corrosion process in which the metal surface is under a thin layer of electrolyte, and its corrosion mechanism conforms to the law of electrochemical corrosion: when a continuous thin layer of electrolyte is formed on the metal surface, the cathodic process of atmospheric corrosion is :O2+2H2O+4e→4OH- The anode process is: Me-ne→Men+; when all Fe and Zn are immersed in reducing acid solution: the cathode is hydrogen depolarization; under the acidic water film formed by urban polluted atmosphere: the cathode is Oxygen depolarization corrosion. Under a thin liquid film: Oxygen can easily reach the metal surface to form an oxide film, anodic passivation, and anodic polarization; the thickening of the liquid film (wet atmosphere) requires a diffusion process for oxygen to reach the metal surface, so corrosion Controlled slowdown. Corrosion mechanism: The rust layer of atmospheric corrosion is under humid conditions, and the rust layer acts as a strong oxidant. The anode in the rust layer occurs on the Fe3O4 interface of the metal: Fe-2e→Fe2+; the cathode occurs on the interface of Fe3O4 and FeOOH: 6FeOOH+2e → 2Fe3O4+2H2O+2OH-, the rust layer participates in the cathode process. In the industrial atmosphere, SO2, NO2, H2S and NH3 all increase atmospheric corrosion. When the humidity is greater than 70%, a water film is formed, electrochemical corrosion occurs, and the corrosion rate increases sharply; when the humidity is less than 70%, the electrochemical corrosion rate of sulfur dioxide is very small. In particular, metals such as iron, zinc, cadmium, and nickel that are not resistant to sulfuric acid are extremely susceptible to corrosion in the atmosphere. It is generally believed that the corrosion mechanism of SO2 is the autocatalytic process of sulfate cavities. Once the rust layer generates sulfate, the rust layer has no protective ability. Therefore, carbon steel must be protected under outdoor atmospheric conditions. Methods include anti-rust paint, Zn, AI, etc. or adding atmospheric corrosion resistant alloy elements, such as Cu, P, etc., so that the rust layer has a good protective effect. 2. Seawater corrosion characteristics Seawater is about 3.5% NaCI solution, (NaCI, MgCI2, CaSO4, K2SO4, CaCO3, MgBr2) with an average conductivity of 4×10-2S.CM-1, which is much larger than river water and rainwater. When the salt concentration and temperature increase, the solubility of oxygen in seawater decreases. Seawater is a typical electrolyte solution, which is a characteristic of electrochemical corrosion. Except for the active Mg and its alloys, the seawater corrosion process is an oxygen depolarization process, and the corrosion rate is determined by Oxygen diffusion process control. For most metals such as iron, steel, zinc, etc., due to the high concentration of CI-, the retardation effect of the anode process is very small, and it is not easy to reduce it. Only by increasing the alloying element molybdenum to inhibit the destructive effect of CI- on the passivation film, it can be improved. Corrosion resistance. Due to the high conductivity of seawater, the low resistance resistance, and the macroscopic and microscopic battery activity of seawater corrosion, the galvanic corrosion caused by the contact of dissimilar metals in the seawater has a considerable destructive effect. For example, ships and bronze propellers can cause corrosion of steel hulls tens of meters away. 2. Corrosion resistance test and determination of corrosion resistance 1. Weather resistance test method Neutral salt spray test: 5% NaCI solution, PHu003d6.5-7.2 Acetic acid salt spray test: 5% NaCI solution, add appropriate amount of glacial acid , PHu003d3.1-3.3 copper accelerated acetic acid salt spray test: 5% NaCI solution, adding 2g/L±0.2g/L CuCI2.2H2O, PHu003d3.1-3.32, metal material and protective layer weather resistance simulation test standard A ) Test equipment requirements: The test chamber material is not corrosive and does not affect the test results; the salt spray cannot be sprayed directly on the sample surface; the condensed water anywhere in the working space cannot drip onto the sample; the temperature fluctuation of the test chamber is ±0.5℃, which is uniform The degree is ±2℃. B) Selection of test conditions: test solution acidity: PHu003d7±0.2 test solution composition can have four choices according to needs: ①, 3% NaCI solution; ②, 5% NaCI solution; ③, 20% NaCI solution; ④ , Artificial sea water; 27g/L NaCI+6g/LMgCI2+1g/LKCI+1g/LCaCI2C) Test temperature: 35±2℃ D) Salt spray precipitation: 1.0-2.0ml/80cm2.hE) The test cycle can be as required Choice: ①, spray continuously for 24 hours, and observe the time when corrosion begins. ② Continuous spraying for 8 hours, stopping for 16 hours, 24 hours is a cycle, and the number of test cycles is specified according to the effects of different products. ③, intermittent spraying for 8 hours (the fountain stops for 5 minutes and stops for 45 minutes), stops for 16 hours, and a cycle of 24 hours. The number of test cycles is specified according to the effects of different products. 3. The salt spray resistance test method of paint film is to make paint board according to GB1765; test solution: 3.5% NaCI solution, PHu003d6.1-7.5; working temperature of test chamber: 40±2℃; spray cycle: spray 15 minutes every 45 minutes. 4. Evaluation method of corrosion resistance test. The usual evaluation method of corrosion test results is: appearance and appearance after removal of surface corrosion products; the distribution and quantity of pitting corrosion, cracks, and bubbles are evaluated according to the nature of the base material and the coating. The substrate is the cathode. The surface of steel and aluminum alloy die-castings with Ni-Cr or Cu-Ni-Cr is graded according to GB6461; the substrate is graded according to GB12335 for anodes such as steel galvanized and copper tinned; the start of corrosion time is the most commonly used corrosion resistance judgment Method: The results of corrosion resistance test can also be judged by weight change, microscope observation and mechanical property change. The salt spray resistance of the paint film is rated according to GB1740.
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