Salt spray testing is a widely used accelerated corrosion testing method that evaluates the corrosion resistance of protective layers such as powder coatings by simulating marine or salty humid atmospheres. Salt spray chambers provide controllable test conditions to accelerate the appearance of coating defects or weaknesses, providing critical data for coating formulation optimization, process improvement, and quality monitoring. This evaluation focuses on powder coating, which is electrostatically sprayed and cured to form a continuous film layer, which is commonly used for metal product protection.
Test principle
Salt spray tests are mainly based on the principle of electrochemical corrosion. When salt spray droplets settle on the surface of the coating, the sodium chloride electrolyte can penetrate the micropores or defects of the coating, forming corrosive galvanic cells on the surface of the metal substrate and accelerating the corrosion of the substrate. The test chamber atomizes the sodium chloride solution through the nozzle to keep the temperature, humidity and sedimentation rate in the chamber stable. Commonly used international standards include ISO 9227, ASTM B117, and domestic standards such as GB/T 10125, which specify core parameters such as solution concentration, pH value, box temperature, and sedimentation rate of the collection solution. For example, the neutral salt spray test (NSS) solution is 5%±1% sodium chloride, pH value 6.5~7.2, the chamber temperature is usually maintained at 35°C±2°C, and the sedimentation rate should be controlled at 1.0~2.0 mL/(80cm²·h).
Operation process
The test chamber needs to ensure uniform and stable parameters to ensure reproducibility of results. The core parameters include temperature control accuracy, atomization system stability, settlement rate uniformity, and corrosion resistance of the box. The operation process usually covers the steps of specimen preparation, box parameter setting, specimen placement, continuous spraying, and regular observation. The specimen should be free of oil stains and scratches, and the edges should be properly sealed. The placement angle affects the salt spray settlement, and it is usually recommended to be 15°~30° from the vertical direction. The test period is set according to the life expectancy of the coating, which is commonly 500 hours, 1000 hours, etc.
Evaluation methodology
The evaluation of powder coating protection performance is mainly based on the time, scope and degree of corrosion phenomenon. Common evaluation indicators include: blister level, rust level, corrosion spread width at scratch and adhesion change. Blistering and rust ratings can be rated according to the ISO 4628 series of standards. Scratch tests (e.g., the fork method) assess the coating's filamentous corrosion resistance, and the corrosion spread width (in mm) measurement formula can be expressed as:
W = (Wt - W0)/2
Among themWis the unilateral average spread width,Wtis the total corrosion width of the scratch after the test,W0is the original scratch width. The adhesion can be tested by the pull method or the grid method, and the changes before and after the test can be compared.
Influencing factors
Powder coating performance is affected by resin type, pigment, curing degree and film thickness. For example, epoxy coatings have better salt spray resistance but are easy to powder, while polyester coatings have better outdoor weather resistance. The uniformity of film thickness directly affects the protective effect, and the area with insufficient film thickness is prone to early corrosion. The interpretation of the results should be combined with the application environment: the salt spray test can only predict the relative performance of the coating in the salty and humid environment, and cannot directly estimate the actual service life. After the test, the specimen should be thoroughly washed and dried to avoid continued corrosion of residual salt.
Notes:
As an acceleration test, the corrosion mechanism of the salt spray test is different from the actual outdoor exposure, such as the lack of dry-wet cycle, ultraviolet and other synergistic effects. Therefore, the test results should complement each other with cyclic corrosion test and outdoor exposure data. During operation, it is necessary to regularly calibrate the sedimentation rate, check for nozzle blockage, and monitor solution consumption to ensure consistent test conditions. Specimen evaluation should be carried out independently by multiple inspectors to reduce subjective bias.
Summary
Salt spray chambers provide a standardized acceleration for evaluating the protective properties of powder coatings. By strictly controlling the test parameters and systematically evaluating the corrosion indexes, the corrosion resistance differences of different coating systems can be effectively compared, providing a basis for product development and quality control. However, the limitations of this method should be recognized and combined with other test methods for comprehensive judgment to more fully predict the long-term behavior of the coating in the actual environment.
Cited Literature
ISO 9227:2017, Corrosion test of man-made atmospheres - Salt spray test
ASTM B117-19, Standard Specification for Operation of Salt Spray Test Equipment
GB/T 10125-2021, Corrosion test of artificial atmosphere - salt spray test
ISO 4628-2:2016, Coating Corrosion Assessment - Foaming Degree Assessment