The coating corrosion resistance testing methods include outdoor exposure corrosion test and artificial accelerated corrosion test. The outdoor exposure test is particularly useful for qualifying the performance of coatings and plating processes for outdoor use. The test results can usually be used as the basis for formulating coating thickness standards. The artificial accelerated corrosion test is mainly to quickly identify the quality of the electroplating layer. However, any accelerated corrosion test cannot characterize and replace the actual corrosion environment and corrosion state of the coating. The test results can only provide a relative basis.
Artificial accelerated corrosion test methods include: neutral salt spray test; acetic acid salt spray test; copper salt accelerated acetic acid salt spray test; corrosion paste test; Acetate amine corrosion test; hydrogen sulfide test; humidity test, etc.
Outdoor exposure test (ISO4542-1981)
1. Exposure conditions
According to the environment of the area where the exposure site is located, the atmospheric conditions are generally divided into four categories:
(1) Industrial atmosphere
In industrial areas where factories are concentrated, the atmosphere is seriously polluted by industrial media (such as SO 2 , H 2 S, NH 3 and coal ash, etc.) .
(2) Oceanic atmosphere
Areas within 200m of the seaside are vulnerable to salt spray pollution.
(3) Rural atmosphere
The air is clean, and the atmosphere is basically not polluted by industrial media and salt spray.
(4) Suburban Atmosphere
In urban fringe areas, the atmosphere is slightly polluted by industrial media.
2. Exposure method
According to the purpose of the test, the sample can be exposed in the following ways:
(1) Open exposure
Open exposed specimens are placed directly on the frame. The frame is made of materials that can withstand corrosion under specified conditions. The height of the lowest edge of the frame from the ground is not less than 0.5m. The shelf is at an angle of 45° to the horizontal and faces south. The height of plants near the shelf should not be greater than 0.2m.
(2) Block exposure
The test is carried out in a sheltered exposure booth, which can be covered with an umbrella roof using common roofing materials. The roof is sloped to allow rainwater to flow down, but it must completely prevent rainwater from leaking from the roof and completely or partially shield the specimen from direct sunlight. The roof height should not be less than 3m.
(3) Closed exposure
The shutter box is used as a closed exposure shed. When designing, it should prevent atmospheric subsidence, sunlight radiation and direct strong wind. However, it should be kept in circulation with the air from the outside. The roof should be impermeable and properly sloped, with cornices and rain gutters . The shutter box should be a type of movable shutter box, the atmosphere inside and outside the box can exchange air, and rain and snow will not enter the box . Select the internal dimensions of the shutter according to the number of specimens to be placed on the rack. The shutter box should be placed in the open space of the test site
superior. If more than two shutter boxes are placed on the same test station, the minimum distance between the boxes should be equal to twice the height of the boxes .
3. Requirements and placement of samples
(1) Requirements for samples
① In order to minimize edge effects and obtain representative corrosion, the surface area of each specimen should be as large as possible. In any case not less than 0.5dm 2 . The sheet sample uses a 50mm×100mm steel plate (or other metal plate) as the base metal. There are no restrictions on the specifications of the component samples.
②Each sample should have a mark that is not easy to disappear, such as a steel letter or a plastic plate with lettering.
③Before the sample is tested, there should be a special record card to record the source, number, quantity, thickness, basic properties of the coating . And write the test outline (including test purpose, requirements, inspection cycle, etc.). 1~3 pieces of each sample should be kept in a desiccator for inspection and comparison during the test process.
④ In any batch of tests, the selection of the number of specimens shall be based on the type of specimens, the number and the expected number to be removed for inspection during the exposure test. For a specified evaluation, the number of each sample used shall not be less than 3, and the surface area shall be at least 0.5dm 2 .
⑤ After the sample starts to be exposed to the sun outdoors, check and record every ten days within the first month. Check records every month thereafter . After more than 2 to 3 years of exposure, check the records every 6 months.
(2) Placement of the sample
① Do not make any contact between the specimens, or between the specimens and any other materials that may affect the corrosion of the specimens. Generally, non-metallic materials that are resistant to atmospheric corrosion and will not corrode the sample can be used to make fixtures or hooks, fix the sample on the frame , and make the contact area between the sample and the fixture as small as possible.
② Corrosion products and rainwater containing corrosion products shall not fall from the surface of one sample to the surface of another sample . The samples shall not cause each other to cover each other, nor shall they be covered by other objects.
③Easy access to the surface and easy removal of the sample.
④ Prevent the sample from falling or being damaged.
⑤ The samples are exposed to the same conditions, evenly exposed to the air from all sides.
⑥ Raindrops splashed back from the ground shall not reach the surface of the sample.
⑦ For outdoor exposure, the surface of the sample should face south; generally, the surface of the sample is inclined, and it should not be shaded by nearby plants or other objects.
⑧For the test exposed under the umbrella canopy or in the shutter box, unless otherwise specified, the sample is generally inclined at an angle of 45°.
The evaluation of the test results is usually evaluated according to "the rating of the electroplating sample after the cathodic coating is exposed to the atmosphere" unless it has been clearly stipulated .
