Singleton Salt Spray Chamber

When we talk about salt spray test, we will think of Singleton Salt Spray Chamber, which can simulate the corrosive environment and test the corrosion resistance and reliability of products.

Salt spray test is mainly divided into 4 types:
● Neutral salt spray test (NSS);
● Acetic acid salt spray test (AASS);
● Copper accelerated acetic acid salt spray test (CASS);
● Alternating salt spray test.

And Artificial simulated salt spray test includes:
Neutral salt spray test (NSS), acetate spray test (ASS), copper salt accelerated acetate spray test (CASS), alternating salt spray test.

Most corrosion of metal materials occurs in the atmospheric environment, which contains corrosive components and factors such as oxygen, humidity, temperature changes and pollutants.

Salt spray corrosion is a common and most destructive atmospheric corrosion.

The corrosion of metal materials by salt spray is mainly caused by the electrochemical reaction of conductive salt solution penetrating into the metal, forming a "low potential metal-electrolyte solution-high potential impurity" micro-battery system, electron transfer occurs, and the metal as the anode dissolves to form a new compound, i.e., the corrosive product.
The main role in the salt spray corrosion process is played by chloride ions, which have strong penetrating ability and can easily penetrate the metal oxide layer into the metal, destroying the passive state of the metal. At the same time, chloride ions have very small hydration energy and are easily adsorbed on the metal surface, replacing the oxygen in the oxide layer that protects the metal, causing damage to the metal.

Salt spray test is an accelerated corrosion resistance evaluation method in artificial atmosphere. It is to atomize salt water of a certain concentration; then spray it in a closed constant temperature box, and reflect the corrosion resistance of the tested sample by observing the changes of the tested sample after being placed in the box for a period of time. It is an accelerated test method. The salt concentration of chloride in the salt spray environment can be several times or dozens of times that of the salt spray content in the natural environment, which greatly increases the corrosion rate. The time to obtain the results of the salt spray test on the product is also greatly shortened.

Testing a product sample in a natural environment may take a year or even several years to corrode, but testing in an artificial simulated salt spray environment only takes a few days or even hours to get similar results.

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 and 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 increases.
The International Electrotechnical Commission points 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 neutral salt spray tests, it is generally believed that the test temperature is more appropriate at 35°C.

2. Solution concentration
● When the concentration is below 5%, the corrosion rate of steel, nickel, and brass increases with the increase in concentration;
● When the concentration is greater than 5%, the corrosion rate of these metals decreases with the increase in concentration.
This is because, in the low concentration range, the oxygen content increases with the increase in 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 depolarization ability of oxygen also decreases, that is, the corrosion effect weakens.
For metals such as zinc, cadmium, and copper, the corrosion rate always increases with the increase of the concentration of the salt solution.

3. Sample placement angle
The direction of salt spray settling is close to the vertical direction. When the sample is placed horizontally, its projection area is the largest, and the amount of salt spray on the sample surface is the largest, 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 250g, and when the steel plate plane is parallel to the vertical line, the corrosion weight loss is 140g per square meter.
GB/T 2423.17-93 standard stipulates: The placement method of flat samples should make the test surface at a 30-degree angle to the vertical direction.

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

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

Salt spray tests are 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 uses a test equipment with a certain volume space - the Singleton Salt Spray Chamber, to create a salt spray environment in its volume space by artificial methods to assess the salt spray corrosion resistance of the product.
Compared with the natural environment, the salt concentration of chloride in its salt spray environment can be several times or dozens of times that of the general natural environment salt spray content, which greatly increases the corrosion rate and greatly shortens the time to obtain the results of the product salt spray test.
For example, if a product sample is tested in a natural exposure environment, it may take 1 year for it to corrode, but in an artificial simulated salt spray environment, it only takes 24 hours to get similar results.
Artificial simulated salt spray tests include: neutral salt spray test, acetate spray test, copper salt accelerated acetate spray test, and alternating salt spray test.

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

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

(3) Copper salt accelerated acetic acid salt spray test (CASS test)
It is a rapid salt spray corrosion test recently developed abroad. The test temperature is 50°C. A small amount of copper salt-copper chloride is added to the salt solution to strongly induce corrosion. Its corrosion rate is about 8 times that of the NSS test.

In summary, the specific time conversion is:

Neutral salt spray test 24h ⇌ natural environment 1 year

Acetic acid salt spray test 24h ⇌ natural environment 3 years

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

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