Salt Spray Test Related Knowledge

The salt spray test chamber (salt spray testing machine) is mainly used to simulate salt spray environmental conditions and evaluate the corrosion resistance of related products or materials based on the corrosion of the sample in the salt spray environment. It is one of the "three defenses" (humid heat, salt spray, mold) test equipment for artificial climate environments. It is an important test equipment for studying the adaptability and reliability of various environments in machinery, light industrial electronics, instrumentation and other industries.

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Does the salt spray test use ordinary salt water? How much time is equivalent to 1 hour of salt spray test in natural environment?
What is salt spray test?
Salt spray testing is an accelerated corrosion resistance evaluation method for artificial atmospheres. It atomizes salt water of a certain concentration and then sprays it into a closed thermostatic chamber. It reflects the corrosion resistance of the tested sample by observing the changes of the tested sample after being placed in the chamber for a period of time. It is an accelerated test. According to this method, the salt concentration of chloride in the salt spray environment is several or dozens of times that of the general salt spray in the natural environment, which greatly increases the corrosion rate. The salt spray test is conducted on the product, and the time to obtain the results is also greatly shortened.

The principle of salt spray corrosion
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 conductive salt solution penetrating into the metal and causing an electrochemical reaction, forming a "low potential metal-electrolyte solution-high potential impurity" micro-battery system, electron transfer occurs, and the metal used as the anode dissolves. New compounds are formed that are corrosives. The main role in the salt spray corrosion damage process is chloride ions. It has a strong penetrating ability and can easily penetrate the metal oxide layer and enter the interior of the metal, destroying the passivity of the metal. At the same time, chloride ions have very small hydration energy. It is easily adsorbed on the metal surface and replaces the oxygen in the oxide layer that protects the metal, causing the metal to be damaged.

Classification of salt spray test
Salt spray tests are mainly divided into four types: neutral salt spray test (NSS); acetic salt spray test (AASS); copper accelerated acetic salt spray test (CASS); alternating salt spray test.
1. Neutral salt spray test (NSS test) is the earliest accelerated corrosion test method with the widest application field. It uses 5% sodium chloride salt solution, and the pH value of the solution is adjusted to the neutral range (6 to 7) as the solution for spraying. The test temperature is all 35℃, and the sedimentation rate of salt spray is required to be between 1~2ml/80cm².h.
The neutral salt spray test is suitable for: metals and their alloys, metal coatings (anodic or cathodic), conversion coatings, anodic oxidation films, organic coatings on metal substrates.
2. The acetic acid salt spray test (AASS test) is to add an appropriate amount of glacial acetic acid to the prepared salt solution to ensure that the pH value of the collection liquid in the salt spray chamber is 3.1 to 3.3. The solution becomes acidic and the final salt spray is formed. becomes acidic. Its corrosion rate is about 3 times faster than the NSS test. If the pH value of the initially prepared solution is 3.0 to 3.1, the pH value of the collection solution is generally in the range of 3.1 to 3.3. The pH value should be measured with an acidimeter at 25°C ± 2°C. Precision pH test paper with a measurement accuracy of no more than 0.1 can also be used for daily testing. The pH value of the solution can be adjusted with glacial acetic acid or sodium hydroxide.
The acetic acid salt spray test is suitable for copper + nickel + chromium or nickel + chromium decorative coatings, and is also suitable for aluminum anodized films.
3. Copper Accelerated Acetic Salt Spray Test (CASS test) In the prepared salt solution, add copper chloride (CuCl2·2H2O) with a concentration of 0.26 g/L±0.02 g/L (i.e. 0.205g/L±0.015 g/L anhydrous copper chloride), strongly induces corrosion, and its corrosion rate is about 8 times that of the NSS test. The copper accelerated acetate salt spray test is suitable for copper + nickel + chromium or nickel + chromium decorative coatings, and is also suitable for aluminum anodized films.
4. The alternating salt spray test is a comprehensive salt spray test. It is actually a neutral salt spray test plus a constant humidity and heat test. It is mainly used for cavity-type complete machine products. Through the penetration of the moist environment, salt spray corrosion occurs not only on the surface of the product, but also inside the product. It alternately converts the product under two environmental conditions: salt spray and humid heat, and finally assesses whether there are any changes in the electrical and mechanical properties of the complete product.

How to prepare the salt water for the salt spray experiment? Is it ordinary salt water?
Preparation of saline solution for neutral salt spray test. Dissolve analytically pure sodium chloride in purified water at a concentration of (50±10) g/L. Use pH test paper to measure the pH value of the test solution at 25°C to be 6.5 to 7.5. When it exceeds the range, analytical grade hydrochloric acid or sodium hydroxide solution can be added for adjustment. The prepared solution must be filtered before use.
Salt, from a chemical point of view, refers to a compound in which a metal ion or ammonium ion (NH4) is combined with an acid ion or a non-metal ion. It is also an indispensable substance in our daily life. To be more specific, there are two types of salt, one is industrial salt and the other is edible salt.
Industrial salt: It is widely used in industry and is one of the most basic raw materials in the chemical industry. Its main component is sodium chloride, which is called the "mother of the chemical industry". Among the main products of the basic chemical industry, hydrochloric acid, caustic soda, soda ash, sodium chloride, chlorine, etc. are mainly produced with industrial salt as raw materials, and are also an essential chemical substance for the operation of salt spray chambers.
Edible salt refers to processed edible salt with sodium chloride as the main component obtained from seawater, underground rock (mine) salt deposits, natural brine (salty) water, etc. Edible salt is one of the most commonly used seasonings in cooking. First, it is also an indispensable substance for normal physiological activities of the human body. Due to the important role of edible salt in our daily diet, our country has strict regulations on the quality of edible salt.
To put it simply, the standards for industrial salt are relatively simple, while the standards for edible salt not only stipulate the content of sodium chloride, but also limit the content of other ingredients that affect human health, thus ensuring human health. Therefore, this also determines that although the main component of industrial salt is sodium chloride, it cannot be directly used as edible salt. On the contrary, edible salt cannot be used in salt spray test chamber tests, otherwise the test results will be affected.
For example, the table salt we usually eat contains potassium iodate. The presence of impurities such as potassium iodate will weaken the accuracy of product testing, and it will also clog the nozzle, which will reduce the service life of the equipment. Be sure to choose a professional one. "Salt" to test. Only in this way can the test accuracy meet the requirements.

How much time is equivalent to 1 hour of salt spray test in natural environment?
When testing a product sample in a natural environment, the corrosion time may take one year or even several years. However, when testing under artificial salt spray environment conditions, it only takes days or even hours to obtain similar results. So what is the approximate relationship between the salt spray test and the corrosion in the natural environment? How much time is equivalent to 1 hour in the salt spray test in the natural environment?

The approximate time conversion relationship between them is: (for reference only)
Neutral salt spray test 24h ⇌ natural environment 1 year
Acetate salt spray test 24h ⇌ natural environment 3 years
Copper salt accelerated acetate salt spray test 24h ⇌ natural environment 8 years