The salt spray settling rate in a salt spray test chamber refers to the amount of salt deposited on a unit area over a specific period. This parameter is crucial in salt spray testing, as it reflects the density and uniformity of the salt mist, which directly influences the test's accuracy and reliability. In addition to chloride ions, which are the primary cause of corrosion, oxygen diffusion within the liquid film on the metal surface also plays a significant role in the corrosion process.
Smaller salt particles have a larger surface area, allowing more oxygen to be adsorbed, which increases their corrosive potential. In nature, over 90% of salt spray particles have a diameter of 1 µm or less. Research shows that for particles of this size, the amount of oxygen adsorbed on the surface balances with the dissolved oxygen inside the particle. Beyond this point, further reduction in particle size does not significantly increase oxygen adsorption.
However, if the oxygen in the liquid film remains stagnant, the metal will corrode rapidly as the oxygen is quickly consumed, slowing down the reaction. On the other hand, if the liquid film is continuously refreshed, the corrosion process can continue. The renewal rate of the liquid film increases with the salt spray settling rate. If the settling rate is too low, the liquid film may not be replenished effectively, affecting the corrosion process.
When the settling rate is below 0.3 mL/80 cm²·h, the corrosion rate increases as the rate rises. This is because the liquid film becomes thinner, allowing oxygen to reach the cathode surface more easily, leading to a gradual rise in corrosion. However, when the settling rate exceeds 1.2 mL/80 cm²·h, the liquid film thickens, and oxygen diffusion becomes limited due to the increased distance. As a result, the corrosion rate no longer accelerates.
Therefore, maintaining a moderate salt spray settling rate ensures stable corrosion rates and consistent test results. It is generally recommended to control the settling rate between 1 and 2 mL/80 cm²·h to ensure accurate and reproducible outcomes in salt spray testing.
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