[serial] paint inspector, I want to tell you (4) - do you know about the operation of coating scratch in corrosion test?

1 Scratch purpose at present, many international and domestic standards related to corrosion test stipulate that scratch should be made before the test, and then the relevant corrosion test should be carried out The main purpose of coating scratch before the corrosion test is as follows: (1) simulate and study the performance of specific types of damage during exposure; (2) Obtain the acceleration effect during corrosion, such as quality control during production; (3) predict the general corrosion resistance of coating by observing the corrosion rate after exposure 2 The scratch shape generally requires to penetrate the coating to the metal substrate, using different scratch tools to produce a representative scratch effect of different shapes This difference will greatly affect the results of corrosion tests, because the purpose of scratches on the coating system is to enable oxygen and electrolyte to contact the active metal surface exposed to the test environment The tool used to scratch is determined according to the coating type and coating thickness The scratch method and repeated re scratch under the same condition have a great influence on the corrosion process The common shapes of scratches are X-shaped, T-shaped or a single line, as shown in Figure 1 The length of the scratch should be significantly greater than the expected spread width after the corrosion test Fig 1 the width of the scratch shape on the painted test plate also has an effect on the corrosion test results The narrower the scratch, the stronger the passivation tendency of the exposed metal surface due to the accumulation of a small amount of corrosion product solution Therefore, it is very important to select a suitable tool so as to draw a scratch of suitable width for corrosion test The direction of scratch has a great influence on the results of corrosion test For example, in the brine spray test, the horizontal scratch will accumulate more salt solution on the template placed in a smaller angle along the vertical direction The distribution of salt solution on the horizontal scratches is also relatively uniform, while the diagonal or vertical scratches are prone to generate gradient distribution The solution along the scratches increases gradually, and the increase of salt solution will lead to the acceleration of corrosion rate, but on the painted steel plate, the high concentration of salt solution may inhibit corrosion, because the corrosion products formed by it are very dense The scratch cross section along the entire length of the scratch shall be as uniform as possible Cut the coating smoothly in the direction of the scratch According to the different scratch tools selected, the cross-section shape of the scratch can be V-shaped or U-shaped, as shown in Fig 2 The shape, depth and uniformity of the scratch depends on the manual scratch tool, and is also related to the scratch operator Figure 2 cross section shape of scratch through coating 3 Several standards related to scratch 3.1 common standards related to scratch ▲ GB / T 1771-2007 / ISO 7253:1996 determination of neutral salt spray resistance of paints and varnishes ▲ ISO 20340:2009 performance requirements of protective coating systems for paints and varnishes for offshore buildings and related structures ▲ ASTM D1654-2008 Standard Test Method for evaluation of painted specimens subjected to corrosive environment ▲ GB / T 30786-2014 / ISO 17872:2007 paints and varnishes - Guidelines for marking scratches on metallic plate coatings for corrosion testing - 3.2 description related to scratches ▲ GB / T 1771-2007 / ISO 7253:1996: "each scratch shall be at least 25 mm away from each edge and scratch of the test sample"; "the cross-section of the scribed line shall be parallel to both sides or widened at the upper part, with a certain width (0.3 mm-1.0 mm)"; ▲ ISO 20340:2009: "a scratch shall be 50 mm long and 2 mm wide, Parallel to the short side and 25 mm away from the adjacent long side and short side; ▲ ASTM d1654-2008 stipulates: "if there is no special agreement, generally scribe all organic coatings on the base material to the metal matrix to produce one or several uniform bright lines without burrs In addition, it is also specified that the scoring form is V-shaped notch "; ▲ ASTM d1654-2008 and GB / T 1771-2007 / ISO 7253:1996 both stipulate that" the degree of marking through metal coating shall be determined by relevant parties "; ▲ GB / T 30786-2014 specifies the scratch form, length, distance, etc 4 Scratch tools and auxiliary tools 4.1 most of scratch tools are made of tungsten carbide, a few of them are made of carbon steel, which is hard Scratch tools specified in other common standards: ▲ ASTM d1654-2008 recommends the use of some scratch tools listed in the appendix of ANSI B 94.50 type E ▲ GB / T 30786-2014 note: it is not allowed to use tools such as blades, needles, scalpels for scratches 4.2 auxiliary tool ▲ metal ruler, used to draw scratches of specified length ▲ microscope, magnifier 25 times ▲ magnifying glass, magnifier 5-10 times, used to observe whether the scratches are uniform and to assist in removing debris generated during the scratch 5 The scratch operation steps fix the test sample plate to ensure that the scratches are uniform and avoid sliding of the test plate or tool Use the scratch tool to scratch along the metal ruler, and draw the scratch penetrating the coating to the substrate The scratch action is continuous and the speed is consistent Note: do not damage the surrounding coating Scratch through the coating to the substrate with the minimum number of times Use a magnifying glass with a magnification of 5 to 10 times to check whether the scratch is uniform and to assist in removing debris generated during the scratch [guide] paint inspector, I want to say to you (5) - do you have a deep understanding of the content test of harmful heavy metal elements in paint? 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