Salt spray test test method-can be used for aluminum corrosion test(3)

Influencing factors of salt spray test

1. Test temperature and humidity

   The critical relative humidity for metal corrosion is about 70%. When the relative humidity reaches or exceeds this critical humidity, the salt will deliquesce to form an electrolyte with good conductivity. When the relative humidity decreases, the concentration of the salt solution will increase until crystalline salt is precipitated, and the corrosion rate will decrease accordingly. As the temperature rises, the molecular motion intensifies, and the corrosion rate of high salt spray is faster. The International Electrotechnical Commission pointed out that for every 10°C increase in temperature, the corrosion rate increases by 2 to 3 times, and the conductivity of the electrolyte increases by 10 to 20%. For the neutral salt spray test, it is generally considered that the test temperature is selected at 35°C.

2. Concentration of solution

   When the concentration is below 5%, the corrosion rate of steel, nickel, and brass increases with the increase of the concentration; when the concentration is greater than 5%, the corrosion rate of these metals decreases with the increase of the concentration. This is because in the low concentration range, the oxygen content increases with the increase of the salt concentration; when the salt concentration increases to 5%, the oxygen content reaches relative saturation, and if the salt concentration continues to increase, the oxygen content decreases accordingly. As the oxygen content decreases, the depolarizing ability of oxygen also decreases, that is, the corrosive effect is weakened. For metals such as zinc, cadmium, and copper, the corrosion rate always increases with the increase of the salt solution concentration.

3. The placement angle of the sample

   The sedimentation direction of the salt spray is close to the vertical direction. When the sample is placed horizontally, its projected area is the largest, and the surface of the sample bears the largest amount of salt spray, so the corrosion is the most serious. The research results show that when the steel plate is at a 45-degree angle to the horizontal line, the corrosion weight loss per square meter is 250 g, and when the steel plate plane is parallel to the vertical line, the corrosion weight loss is 140 g per square meter. The GB/T 2423.17-93 standard stipulates that the method of placing flat samples should be such that the test surface is at an angle of 30 degrees to the vertical.

4. pH value

   The lower the pH, the higher the concentration of hydrogen ions in the solution, the stronger the acidity and the stronger the corrosiveness. The neutral salt spray test (NSS) pH value is 6.5~7.2. Due to the influence of environmental factors, the pH value of the salt solution will change. For this reason, the salt spray test standards at home and abroad have stipulated the pH range of the salt solution, and proposed methods to stabilize the pH value of the salt solution during the test to improve the reproducibility of the salt spray test results.

5. Salt fog precipitation and spray method

   The finer the salt spray particles are, the larger the surface area is formed, the more oxygen is adsorbed, and the more corrosive. Traditional spray methods include air pressure spray method and spray tower method. The most obvious disadvantage is that the uniformity of salt spray deposition is poor and the diameter of salt spray particles is large. Different spray methods will also affect the pH of the salt solution.

Standards related to salt spray test

How long is one hour of salt spray test equivalent to the natural environment?

The salt spray test is divided into two categories, one is the natural environment exposure test, and the other is the artificial accelerated simulated salt spray environment test. The artificial simulated salt spray environment test is to use a kind of test equipment with a certain volume space-the salt spray test box, in its volume space, artificial methods are used to create a salt spray environment to assess the quality of the salt spray corrosion resistance of the product . Compared with the natural environment, the salt concentration of chloride in the salt spray environment can be several or tens of times the salt spray content of the general natural environment, which greatly increases the corrosion rate. The salt spray test is performed on the product and the result is obtained. The time is also greatly shortened. For example, if a product sample is tested in a natural exposure environment, it may take 1 year for its corrosion, while the test under artificially simulated salt spray environment conditions requires only 24 hours to obtain similar results.

The artificial simulated salt spray test also includes neutral salt spray test, acetic acid salt spray test, copper salt accelerated acetic acid salt spray test, and alternating salt spray test.

(1) The neutral salt spray test (NSS test) is one of the earliest accelerated corrosion test methods with the widest application field. It uses 5% sodium chloride salt aqueous solution, the pH value of the solution is adjusted in the neutral range (6-7) as the spray solution. The test temperature is 35℃, and the sedimentation rate of the salt spray is required to be between 1~2ml/80cm².h.

(2) The acetic acid salt spray test (ASS test) is developed on the basis of the neutral salt spray test. It is to add some glacial acetic acid to 5% sodium chloride solution to reduce the pH value of the solution to about 3, the solution becomes acidic, and the salt mist formed in the end also changes from neutral salt mist to acid. Its corrosion rate is about 3 times faster than the NSS test.

(3) The copper salt accelerated acetic acid salt spray test (CASS test) is a kind of rapid salt spray corrosion test recently developed abroad. The test temperature is 50℃, and a small amount of copper salt-copper chloride is added to the salt solution to strongly induce corrosion. . Its corrosion rate is approximately 8 times that of the NSS test.

The specific time is converted into:

Neutral salt spray test 24h ⇌ natural environment 1 year

Acetate spray test 24h ⇌ natural environment 3 years

Copper salt accelerated acetic acid salt spray test 24h ⇌ 8 years in natural environment