It is widely recognized that the concentration of sodium chloride plays a crucial role in determining the outcome of metal corrosion during neutral salt spray testing. In a salt spray test chamber operating under controlled temperature conditions, the rate of corrosion is primarily influenced by two key factors: the concentration of the salt solution and the level of dissolved oxygen within it. When the oxygen content in the solution is sufficient to support electrochemical reactions, the corrosion rate is mainly dictated by the salt concentration. However, when the salt concentration exceeds 5%, the dissolved oxygen begins to decrease as the concentration increases, which may no longer be enough to sustain the necessary electrochemical processes. In such cases, the corrosion rate becomes limited by the available oxygen. The maximum corrosion rate typically occurs at around a 5% sodium chloride concentration, which is why a standard 5% aqueous solution is commonly used in such tests.
As mentioned, the concentration of the salt solution in the salt spray test chamber is a major factor affecting the accuracy of the test results. To minimize its impact, experts recommend regularly measuring the solution's concentration. One effective method for doing so is using the density measurement technique. At 25°C, the density of a 5% sodium chloride solution generally ranges between 1.029 and 1.041 g/mL. A hydrometer can quickly and accurately measure this value, allowing for efficient monitoring of the solution’s consistency.
Note: The test samples discussed here are primarily anti-corrosion coatings, such as paint films. Other types of samples should be considered for reference only.
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