一、Salt spray test process
Different standards provide for a slightly different test process, this article to GJB 150.11A-2009 “military equipment laboratory environmental test methods Part 11: salt spray test” as an example, explain the salt spray test test process, including specific:
1. Salt spray test standard: GJB 150.11A-2009
2. Test piece pretreatment: remove contaminants, such as oil, grease, dust, pretreatment should be as little as possible.
3. Initial test: visual inspection, if necessary, electrical and mechanical performance testing, recording baseline data.
4. Test steps:
a. Adjust the temperature of the test chamber to 35° C and keep the specimen for at least 2 hours;
b. Spray for 24 hours or as specified;
c. Dry the specimens at a temperature of 15° C to 35° C and a relative humidity of not more than 50% for 24 hours or for a specified period of time;
d. Repeat the salt spray and drying procedure once to complete both cycles.
5.Recovery: Gently rinse the specimens with running water.
6.Final Test: Visual inspection, physical and electrical performance tests if necessary, and recording of test results.
7.Results analysis: Analyse the test results from three aspects: physical, electrical and corrosion.
二、Factors affecting the salt spray test
The main factors affecting the results of the salt spray test include: the test temperature and humidity, the concentration of the salt solution, the angle of placement of the sample, the pH value of the salt solution, the amount of salt spray deposition and the spray method.
1）Test temperature and humidity
Salt spray corrosion fundamentally stems from a material’s electrochemical responses, where temperature and humidity play pivotal roles in modulating the pace of this reaction. An upsurge in temperature typically catalyzes a more rapid progression of salt spray corrosion. The International Electrotechnical Commission (IEC) has illuminated this phenomenon through studies on accelerated atmospheric corrosion testing, noting that a 10°C increase can potentially amplify the corrosion rate by a factor of two to three, while also boosting the electrolyte’s conductivity by 10 to 20%.
Yet, it’s not merely a linear escalation; the actual corrosion rate doesn’t always correspond with the temperature rise in a straightforward manner. If the experimental temperature climbs too high, a disparity between the salt spray corrosion mechanism and real-world conditions may arise, calling into question the reliability of the results.
The story is different with humidity. Metal corrosion has a critical relative humidity point, approximately 70%, beyond which the salt begins to dissolve, creating a conductive electrolyte. Conversely, as humidity levels drop, the salt solution’s concentration surges until crystalline salt precipitation occurs, leading to a subsequent slowdown in corrosion rates. It’s a delicate dance between temperature and humidity, each influencing the other in complex ways, to determine the pace at which corrosion marches forward.
2）The pH of the salt solution
The pH of the salt solution is one of the key factors in determining the results of a salt spray test. When the pH is lower than 7.0, the concentration of hydrogen ions in the solution increases as the pH decreases and the acidity increases, thus increasing the corrosiveness.
3）Sample placement angle
When the salt spray falls almost vertically, the projected area of the sample increases if the sample is in a horizontal position, resulting in the most intense erosion of the sample surface by the salt spray, and thus increasing the degree of corrosion.
4）Concentration of the salt solution
How the concentration of a salt solution affects the rate of corrosion depends on the type of material and its surface covering. When the concentration does not exceed 5 per cent, we observe that the corrosion rate of steel, nickel and brass increases as the concentration of the solution rises; conversely, when the concentration exceeds 5 per cent, the corrosion rate of these metals shows a tendency to corrode inversely proportional to the increase in concentration. However, for metals such as zinc, cadmium and copper, the corrosion rate is always positively correlated with the concentration of the salt solution, i.e., the higher the concentration, the faster the corrosion rate.
In addition to this, factors affecting the results of the salt spray test include: interruption of the test, treatment of the test specimen, spraying method, spraying time, and so on.
Post time: Mar-02-2024