1Definitions, Purpose and Significance
The salt spray resistance of the paint film refers to the resistance of the paint film to the erosion of salt spray. Because the air in coastal and offshore areas is rich in diffuse tiny droplet-shaped salt spray, in addition to the high relative humidity, the specific gravity of the salt-containing air is also larger than that of air, and it is easy to settle on various objects, and the non-carbides in the salt spray are very corrosive, and have a strong corrosive effect on metal materials and protective coatings. As one of the general tests of corrosion resistance, the salt resistance test has been widely used to evaluate and compare the corrosion resistance of substrate pretreatment, coating systems or their combinations, and has become a very useful means in the identification procedures of many industrial products, mining, underground engineering, and defense projects.
2. Relevant standards
GB/Tl771-1991 Determination of the resistance of color rattan and varnish to neutral salts
IS072531l996 Pigment and varnish – Determination of neutral salt spray resistance
ASTMB117-2003 Salt spray test
ASTMB287-74 acetic acid salt spray test
ASTMB368-97 Copper Accelerated Acetate Salt Spray Test (CASS Test)
3. Introduction to inspection methods
3.1 Test principle
Although outdoor salt spray corrosion (specific test environment) can more truly reflect the influence of the environmental conditions on the product at a certain stage, due to the consideration of uncontrollable outdoor uncertainties and the need for short test time, many industrial product standards stipulate that the salt spray resistance test is used to evaluate the salt spray corrosion resistance of paint film. The test is based on the results of a large number of outdoor salt spray corrosion tests, and the relationship between the outdoor environment and the damage of the paint film is derived, the purpose is to simulate the salt spray corrosion environment in nature in the laboratory, and to evaluate the salt spray corrosion resistance of the paint film through the observation of the sample during the test and after the test is completed.
3.2 Test equipment
At present, the test equipment commonly used at home and abroad is the salt spray corrosion Test Chamber. The device is mainly composed of a box (spray chamber), a salt solution storage tank, a liquid level controller, a compressed air system that has been processed by Tongdang, a central spray tower of one or more atomizing nozzles, an adjustable baffle, a model support, an air saturator, a box immersion heater and the necessary temperature and humidity controllers. When the salt solution is exported from the solution storage tank and flows through the liquid level controller into the bottom of the spray calculation tower, under the action of the air flow (pressure difference) of a certain pressure, it is sucked in by the self-priming nozzle and polarized to form a dense salt spray, which is evenly settled on the test model in the fog shouting chamber after being guided by the baffle at the upper part of the spray tower.
3.3 Key points of operation·
3.3.1 Temperature control
The temperature in the salt spray chamber used for neutral salt spray resistance test and acetic acid/salt spray test should be kept at (35±2) °C, and the temperature of the air saturator should be higher than the temperature in the chamber (5-10) °C. The temperature in the salt spray chamber used for the acid/copper acceleration-salt spray test should be maintained at (49±1)°C
3.3,2 Preparation of salt spray test solution
3,3.2.l Neutral salt resistance test solution preparation
The chemically pure grade sodium chloride in accordance with the conditions of GB1266 is fully dissolved in the tertiary water in accordance with GB6082, and the concentration is (50:t10)g/L. The pH of the solution measured with a precision pH meter (accuracy 0,1 pH) at 25 °C should be in the range of (6,5-7.2). When the range is exceeded, pure hydrochloric acid or sodium hydroxide solution can be added for adjustment, and the prepared solution can be used only after depletion.
3.3.2.2 Preparation of cool acid-salt spray solution
First, the chemically pure grade sodium chloride that meets the conditions of GB1266 is fully dissolved in the tertiary water that meets the conditions of GB6682 with a concentration of (50:t10) g/L. Then use glacial acetic acid to adjust the pH value of the solution, so that the pH value of acetic acid monosalt solution is in the range of (3.1-3.3), and the prepared solution can be used only after filtration.
3.3, 2.3 copper accelerated acetic acid a salt spray solution preparation
In the first step, the chemically pure group sodium chloride that meets the conditions of GB1266 is fully dissolved in the third-grade water that meets the conditions of GB6682, and the law is (50±10) g/L. The second step is to add human reagent-grade copper dichloride (CuCl,2H,0) 0_25g per liter of saline, and dissolve and mix thoroughlyThe third step is to adjust the pH value of the solution with glacial acetic acid at 25 °C to make it in the range of (3.l-3.3), and the prepared solution can be used only after filtration.
3.3.3 Control of salt spray sedimentation
There should be at least two clean salt spray collectors made of glass or other temperamental materials in the salt spray exposure zone, one of which should be installed near the outlet and one far from the outlet of the spray, in such a way that only the settling salt spray is collected and not the liquid dripping from the specimen or other parts of the chamber. Salt spray sedimentation: After a minimum of 24 h cycles, the collected solution is calculated, and each collector per unit area of 80 c㎡ should collect (1-2) ml of salt solution per hour, with a liquid degree of (50±10) g/L and a pH value of (6,5-7.2). The collected sodium chloride solution should not be reused.
3.3,4 The preparation, maintenance and pretreatment of the test sample The selection, treatment, preparation method and curing time of the substrate of the salt spray test sample are generally required in the relevant product standards, and can also be prepared and cured in accordance with the provisions of GB/T1765-89 "Paint Film Preparation Method for Determining Humidity and Heat Resistance, Salt Resistance and Weather Resistance (Artificial Acceleration)", or agreed between the entrusted test party and the inspection agency. The back and perimeter of the plate to be tested can be applied with the sample under test or with a coating system known to have better corrosion resistance. If it is necessary to do the scribing (cutting through the line, scratching, scratching and scratching) test, a scribing tool with a tungsten carbide (cemented carbide) tip can be used (such as the single-edged cutter in GB9286 recommended by GB/Tl771-91 and the ANSIB94 in ASTMD1654-2005, type E in 50) but similar tools such as blades, needles, cutting knives, scissors and other similar tools are not allowed. A clear V-shaped line with all coatings on the substrate is carved on the surface of the sample without burrs. If it is necessary to do the test of cutting through the metal plating, the degree of marking shall be agreed between the commissioning test party and the inspection agency. If necessary, several lines can be drawn as agreed, and the quality of the lines can be observed with the help of a low-magnification glass. Common forms of scribing: two intersecting lines, one straight line parallel to the long side, two straight lines parallel to the long side, two vertical lines, V-shaped, etc., as shown in Figure 40:
3.3.5 test sample testing, testing, inspection and evaluation
When the curing period of the test plate is over, except for one of the standard plates, the remaining three pieces should be immediately put into the testing machine
The test sample should be placed on the sample bracket in the box with the surface facing up, and no contact is allowed between the samples and between the samples and the box, and the samples are not allowed to be stacked. In addition, the sample should be changed at regular intervals in time on the test plate rack, such as the exchange of the front, middle and rear rows.
During the test, cocoa shall refer to the provisions of the relevant product standards, or entrust the test party and the inspection agency to determine a damage index as the end point of the test, such as: the width of one-way rust, the level of blistering, rusting and falling off in the unmarked area, etc. The common test time is a multiple of 24 hours, such as the recommended exposure period in the American ASTMB117-1997 standard is 24, 49, 96, 200, :500, 720 hours.
When the sample is inspected, the test sample can be taken out of the box and compared with the standard board in sufficient light or under light. The evaluation can be carried out with reference to the requirements of Xiangmei product standards, or with reference to the provisions of GB/T1740-89 "Determination of Damp and Heat Resistance of Vessel Film" and ASTMD1654-92 "Evaluation of Samples Coated with Pigment or Paint after Subjecting to Corrosive Environment", etc., to inspect and grade the degree of unidirectional expansion and spread of the sample marking and unmarked area, as well as blistering, rusting, falling off, discoloration, etc. There is no assessment within 5mm around the sample and the hole, and the final result is subject to the two pieces that are consistent with the intermediate level of the three tests.
The common methods for evaluation at the marking of the test sample are: air blowing method, rinsing and scraping method, and new division method of tape.
The air puffing method involves rinsing each specimen with a slow stream of water at a temperature of less than l10°F (45°C) after the exposure cycle is completed. The hand-held nozzle is sprayed along the entire streak at an angle of about 45°, with an air pressure of 550kpa and a nozzle diameter of 3mm. The air nozzle is used to mechanically disturb the adjacent scribing surface to ensure the passage of air jets. The air injection is completed within 5 minutes of removing the specimen from the blast chamber. If the air injection cannot be completed within the specified time, the test plate should be immersed in room temperature water or stored in a plastic bag to avoid any murmuring caused by dry manipulation.
The rinse and scraping method means that after the dew cycle is completed, each specimen is drenched with a slow stream of water at a temperature of 1 l0 °F (45 °C) or less. Use a scraper that conforms to the squeegee described in ASTMD1654-924.5 and scrape the specimen under a slow drenching water. Holding the scraper, the scraper surface is perpendicular to the sample face and parallel to the scribing line, scraping back and forth across the scribing line to remove the corroded normal layer of the substrate and the coating that has lost its adhesion, but should not scrape off the coating that still has adhesion (adhesion Measurement Instrument: adhesion Tester). The scraping operation is completed within 15 minutes of removing the specimen from the exposure box. If the scraping operation cannot be completed within the specified time, the specimen is immersed in room temperature water or stored in a plastic bag to avoid any effects of dry manipulation.
After the exposure cycle of the tape removal method is completed, the test board is washed with clean water and allowed to dry at (23+2)°C, and a piece of tape with a size of about 25mmx150m"n is glued to the cutting line, and then quickly pulled away in the direction perpendicular to the test plate.
It is worth noting that; Whether it is air blowing, rinsing and scraping or tape stripping, it is generally only suitable for the final result evaluationIt is not applicable to the evaluation of the test results in the manual continuous test, that is to say, the above treatment cannot be repeated for the same group of test samples, but the evaluation of test results at different test stages can be satisfied by increasing the number of test plates and taking out the evaluation at different stages. If necessary, the manufacturer and the testing institute may also agree to adopt other applicable assessment methods.
After the above treatment, the amount is taken from the width of the corrosion touch or paint layer damage of the marking and straightening, and the one-way rust is evaluated. For the measurement of one-way corrosion, the representative maximum, minimum and average values should be taken, and generally the average value of the representative "erosion distance" should be taken unless otherwise specified.
The assessment of the opponent's unmarked area can be recorded and rated by the most damaged area, or ASTMD7l4 can be used to evaluate. If it is necessary to assess the formation of corrosion under the paint film. A neutral paint stripper can be used to carefully remove the coating.
3.4 Factors influencing the results of salt spray test
3.4.1 The salt box and all parts in contact with the salt solution or salt spray should be made of emotional, non-breathable materials, otherwise the synchronous corrosion of the parts will affect the inspection results of the test plate.
3.4.2 The angle of the baffle of the spray tower should be adjusted to the appropriate position to avoid the test effect of free sedimentation of salt base on the surface of the shadow mouth due to the direct alignment of the nozzle with the direction of the nozzle.
3.4.3 The emptying pipe of the salt spray box should have enough size to reduce the back pressure in the box, and the end of the emptying pipe should also have a shielding wrench, otherwise the pressure or vacuum fluctuation in the box will have a certain impact on the inspection results. 3.4.4 The compressed air that provides spray action for the salt spray chamber should be degreased, dewatered and dusted, otherwise it is easy to plug the nozzle or directly affect the test results.
(It is recommended that you use a paint Film Thickness Gauge for the north and south tides)
