How it works:
Salt spray chambers evaluate the corrosion resistance of coatings and other materials by simulating and accelerating corrosive environments in the ocean or salty and humid atmospheres. Its core principle is to atomize a certain concentration of sodium chloride solution (usually 5%±1%) through compressed air to form uniform salt spray settling in a closed incubated chamber. The chamber usually maintains a temperature of 35°C±2°C and ensures that the salt spray sedimentation rate is between 1.0 and 2.0 mL/(80cm²·h). This process can be performed with neutral salt spray tests according to standards (e.g., GB/T 10125, ISO 9227, ASTM B117) or by adjusting the pH value by adding acetic acid (ASS test) or copper salts with acetic acid (CASS test) to simulate more severe corrosion conditions.
Evaluation indicators
After the coating was tested by the salt spray test chamber, its corrosion resistance was mainly evaluated by the following quantitative indicators. These indicators need to be observed and measured in conjunction with the test period specified in the standard (such as 240 hours, 500 hours, 1000 hours).
The main evaluation indicators include:
1. Foaming degree: Evaluate the size and density class of the blister according to a standard spectrum (e.g., ISO 4628-2).
2. Rust grade: Evaluate the percentage of corroded area on the surface of the metal surface of the matrix, usually classified as 0 (no rust spots) to 5 (corrosion area>50%).
3. Corrosion spread width at scratch site
For specimens with artificial scratches (e.g., "X" lines), measuring the width of corrosion spread on one side of the scratch is key to evaluating the coating's impermeability resistance. Its spread widthWIt can be measured under a microscope and averaged over multiple points. The smaller the value, the better the adhesion and shielding of the coating.
The evaluation of corrosion spread can be quantitatively compared with the following formula:
S = (W₁ + W₂ + ... + Wₙ) / n
Among them,Srepresents the average corrosion spread width,WₙRepresentative NonThe width of the measurement point,nis the total number of measurement points.
Test parameters
In order to ensure the reproducibility and comparability of the test results, the operating parameters of the salt spray test chamber must be strictly controlled. The key parameters and their impact are shown in the table below.
| Parameter Name | Control requirements and influences |
| Sodium chloride solution concentration | Usually 5% ± 1%. Too high a concentration will accelerate corrosion and deviate from the real environment. |
| Temperature inside the box | It is usually set to 35°C±2°C. Temperature stability affects corrosion rates. |
| Salt spray sedimentation rate | 1.0 to 2.0 mL/(80 cm²·h). Uneven settlement will lead to uneven erosion of the sample. |
| Spray pressure | Stability is required to ensure that the atomized particles are uniform and fine. |
| pH | Neutral salt spray (NSS) is 6.5-7.2. ASS or CASS tests are subject to standard adjustments. |
| Template placement angle | It is usually 15°-30° from the vertical direction to ensure that the salt spray settles freely on the surface. |
Test process
Standard testing procedures include sample preparation (cleaning, scratching), box placement, setting parameters, continuous or cyclic spraying, periodic inspections, and post-test evaluation. Interpretation of results It should be noted that the salt spray test is an accelerated corrosion test, and the results mainly reflect the relative corrosion resistance ranking of the coating, rather than accurately corresponding to the actual service life. Different coating systems (such as epoxy zinc-rich primer and polyurethane topcoat) will exhibit different failure modes under the same test conditions, which need to be comprehensively analyzed in combination with adhesion, thickness and other data.
Conclusion
Salt spray test chambers are an effective and widely recognized accelerated testing tool for evaluating the corrosion resistance of coatings. By strictly following standards, controlling test parameters, and systematically evaluating indicators such as blistering, rust, and scratch corrosion, valuable data support can be provided for coating research and development, quality control, and process improvement. It should be recognized that this test is one of the means of material screening and comparison, and should be combined with other environmental aging test data for comprehensive judgment in practical application.
References
GB/T 10125-2021, Corrosion test of artificial atmospheres, salt spray test.
ISO 9227:2022， Corrosion tests in artificial atmospheres – Salt spray tests。
ASTM B117-19， Standard Practice for Operating Salt Spray (Fog) Apparatus。
ISO 4628-2:2016， Paints and varnishes – Evaluation of degradation of coatings – Designation of quantity and size of defects, and of intensity of uniform changes in appearance – Part 2: Assessment of degree of blistering。