Rust and grease must be completely removed from the sample substrate for test. Whether the substrate is sandblasted, polished or phosphated, be careful not to be exposed to humid air, so as to prevent the formation of water film on the surface of the substrate, causing re rusting or thus reducing the adhesion between the coating and the substrate. It is especially emphasized that it is strictly forbidden to touch the effective parts of the substrate with fingers, because the grease and sweat on the fingers will stain the board surface, resulting in local blistering and rusting of the coating.
The key of salt spray test is to prepare the concentration of electrolyte solution. The solutes of various components should be weighed strictly according to the proportion to ensure the accuracy of pH value. Otherwise, it will directly affect the test results.
The samples and parts after coating should be sealed with paint and the exposed parts of the substrate should be covered. Otherwise, rust marks will flow and pollute the board surface, which will bring difficulties to the grading work.
When checking the board and parts regularly, the board surface should be kept in a moist state, and the exposure time of the board surface in the air should be shortened as far as possible.
After the test, the panel surface should be evaluated objectively immediately, including blistering, discoloration, rusting and falling off. It can also increase the detection and evaluation of adhesion and scratch unilateral corrosion distance according to customer requirements.
If necessary, the surface should be scratched once. Do not repeat knife application to avoid coating flanging at scratch and widening unilateral corrosion distance. According to experience, the scratch on the plate surface is usually a cross x, while the cylindrical workpiece can be drawn into a parallel line II. However, the distance between the scratch and the edge of the plate should be greater than 20 mm, and the single edge cutter should be used according to the method recommended in GB / T 9286-1998 standard.
It should be noted that the method of measuring the unilateral corrosion distance at the scratch. According to the author’s working experience for many years, during the test, the original corrosion state should be kept in the periodic inspection, and the evaluation results of single grade should be recorded. When the comprehensive grade evaluation is carried out after the test, the most seriously corroded part on one side of the scratch shall be selected for measurement, and then the rust spot shall be peeled off carefully with a tool, and the coating shall not be damaged as far as possible. The corrosion distance shall be measured after flushing with water
① Because the coating blisters along the scratch edge, the substrate is corroded, and the coating is incomplete, and the unilateral corrosion distance is greater than the specified requirements. In this case, coating adhesion is not ideal, resulting in blistering and corrosion;
② When measuring, it seems that the rust spot is large. After rust removal, the coating is complete, the adhesion is good, and there is no corrosion spread on the substrate. The unilateral corrosion distance measured after rust removal shall prevail. The rust spot is large, which may be caused by the local accumulation of rust due to the wide scratch;
③ After rust removal, the coating peeled off from the substrate, and the substrate was corroded in a large area. This kind of situation often occurs in the thick film of multi-layer coating, which indicates that the coating has poor wettability and adhesion.
It must be noted that the plate in the salt spray box should be placed regularly, and the salt spray settlement should be measured regularly to ensure that the salt spray is evenly distributed in all parts of the box.