Understand the Basics of Salt Spray Chambers and Salt Spray Tests

The working principle of the salt spray chamber:
Corrosion is the damage or deterioration caused by the action of the environment on a material or its properties. Most corrosion 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 destructive atmospheric corrosion. The salt fog here refers to the atmosphere of chloride. Its main corrosive component is the chloride salt in the sea, namely sodium chloride, which mainly comes from the sea and inland saline and alkaline areas.

The corrosion of metal materials by salt spray is caused by the electrochemical reaction between the chloride ions that penetrate the oxide layer and protective layer of the metal surface and the internal metal. At the same time, chloride ions contain a certain amount of water, which is easily adsorbed on the pores and cracks on the metal surface to push out and replace oxygen in the chloride layer, transforming insoluble oxides into soluble chlorides, and turning passive surfaces into active surfaces. Causing extremely adverse reactions to the product.

Salt spray corrosion can damage the metal protective layer, causing it to lose its decorative properties and reduce its mechanical strength; Some electronic components and electrical circuits may experience power line interruptions due to corrosion, especially in environments with vibrations; When salt mist falls on the surface of the insulator, it will reduce the surface resistance; After the insulator absorbs the salt solution, its volume resistance will decrease by four orders of magnitude; The active parts of mechanical or moving parts increase friction due to the generation of corrosive substances, which can cause the moving parts to become stuck.

Main structure of salt spray test chamber: spray system, heating system, electrical control system and other components.
1. Spray system
A spray system is installed inside the salt spray test chamber, and the flow plate disperser is used according to the Bernoulli principle. There is a glass nozzle inside, and the nozzle is made of quartz glass fired, with a smooth and precise angle inside. The instrument evenly sprays salt mist and can naturally settle and distribute evenly in the laboratory. Adjust the spray amount and spray direction angle by adjusting the baffle diffuser.

2. Heating system
The heating system of the test chamber adopts an electric heating tube for heating, which can avoid liquid deposition and generate conductivity. The electric heating tube is installed in the test chamber and can work continuously at a constant temperature. In order to ensure uniform temperature inside the box, a fan is installed at the back of the box, and the fan is stirred to evenly disperse the heat from the electric heating pipe throughout the entire working room. The motor shaft can be sealed. To avoid spreading to the outside of the box, the use of platinum resistor PT100 as a temperature control accessory has the characteristics of high sensitivity and precise temperature control.

3. Electrical control system
The electrical control of the salt spray test chamber is composed of the temperature control system of the saturation tank, the spray control system, the alarm system and the temperature control system. The temperature control system uses PT100 to detect the temperature of the salt spray chamber and the saturation tank, and the detection data is reflected to the corresponding temperature control instrument. The temperature control instrument is compared with the set target temperature, and after the PID runs, the output shifts to trigger the conduction angle of the thyristor, thereby adjusting the voltage at both ends of the electric heating tube and changing the output power of the electric heating tube. By doing so, the temperature control is close to the target temperature value.

How to do salt spray test?
Classification of salt spray tests:
Laboratory simulated salt spray can be divided into three categories: neutral salt spray test, acetate spray test, and copper salt accelerated acetate spray test.

1. The neutral salt spray test (NSS test) is the earliest and currently the most widely used accelerated corrosion test method. It uses 5% sodium chloride saline solution, and the solution PH value is adjusted in the neutral range (6.5~7.2) as the solution for spray. The test temperature is set at 35 ℃, and the required settling rate of salt spray is 1-2ml/80cm/h.

2. The acetate spray test (ASS test) was developed based on the neutral salt spray test. It involves adding some glacial acetic acid to a 5% sodium chloride solution, reducing the pH value of the solution to around 3, making the solution acidic. The resulting salt mist also changes from neutral to acidic. Its corrosion rate is about three times faster than the NSS test.

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

4. Test time: 16H (minimum)
5. Factors affecting salt spray testing
The main factors affecting the results of salt spray test include: test temperature and humidity, concentration of salt solution, sample placement angle, pH value of salt solution, salt spray sedimentation and spray mode.

A. Test temperature and humidity
Temperature and relative humidity affect the corrosive effect of salt spray. The Critical relative humidity for metal corrosion is about 70%. When the relative humidity reaches or exceeds this critical humidity, the salt will deliquescence and form an electrolyte with good conductivity. When the relative humidity decreases, the concentration of the salt solution will increase until crystalline salts are precipitated, and the corrosion rate will correspondingly decrease.

The higher the test temperature, the faster the salt spray corrosion rate. The International Electrotechnical Commission IEC60355:1971 AN APPRAISAL OF THE PROBLEMS OF ACCELERATED TESTING FOR ATMOSPHERIC CORROSION standard points out that “when the temperature increases by 10 ℃, the corrosion rate increases by 2-3 times, and the conductivity of electrolyte increases by 10-20%”.

This is due to the increase in temperature, the intensification of molecular motion, and the acceleration of chemical reaction speed. For neutral salt spray testing, most scholars believe that selecting the test temperature at 35 ℃ is more appropriate. If the test temperature is too high, the salt spray corrosion mechanism differs significantly from the actual situation.

B. Concentration of salt solution
The effect of salt solution concentration on corrosion rate is related to the type of material and coating. When the concentration is below 5%, the corrosion rate of steel, nickel, and brass increases with the increase of concentration; When the concentration is greater than 5%, the corrosion rate of these metals decreases with the increase of concentration. The above phenomenon can be explained by the oxygen content in the salt solution, which is related to the concentration of the salt. In the low concentration range, the oxygen content increases with the increase of salt concentration,

However, when the salt concentration increases to 5%, the oxygen content reaches relative saturation, and if the salt concentration continues to increase, the oxygen content correspondingly decreases. When the oxygen content decreases, the oxygen Depolarization capacity also decreases, that is, the corrosion effect weakens. However, for metals such as zinc, cadmium, and copper, the corrosion rate always increases with the increase of salt solution concentration.

C. Sample placement angle
The placement angle of the sample has a significant impact on the results of the salt spray test. The settling direction of salt mist is close to the vertical direction. When the sample is placed horizontally, its projection area is the largest, and the surface of the sample also bears the most salt mist, thus causing the most severe corrosion.

The research results indicate that when the steel plate is at a 45 degree angle to the horizontal line, the corrosion loss weight per square meter is 250 g, and when the steel plate plane is parallel to the vertical line, the corrosion loss weight is 140 g per square meter. The GB/T2423.17-93 standard stipulates that the placement method for flat samples should be such that the test surface is at a 30 degree angle from the vertical direction

D. PH value of salt solution
The pH value of the salt solution is one of the main factors affecting the results of the salt spray test. The lower the pH value, the higher the concentration of hydrogen ions in the solution, and the stronger the acidity and corrosiveness. The salt spray test on electroplated parts such as Fe/Zn, Fe/Cd, Fe/Cu/Ni/Cr showed that the corrosion of the acetate spray test (ASS) with a pH of 3.0 in the salt solution was 1.5-2.0 times more severe than that of the neutral salt spray test (NSS) with a pH of 6.5-7.2.

E. Due to environmental factors, the pH value of the salt solution may change. Therefore, domestic and international salt spray test standards have specified the pH range of salt solutions and proposed methods to stabilize the pH value of salt solutions during the test process, in order to improve the reproducibility of salt spray test results.
F. The duration of the experiment.

Salt Spray Test Chamber | Salt Fog Test | ASTM B117 Salt Spray ChamberSalt Spray Test Chamber is applicable to the salt spray corrosive test for the protection layer of components, parts, electronic and electrical parts and metal materials and industrial products. The salt fog test chamber meets the following standards: IEC60068-2-11 (GB/T2423.17), GB/T10125, GB/T1771, ISO9227, ASTM-B117, GB/T2423-18, QBT3826, QBT3827, IEC 60068-2-52, ASTM-B368, MIL-STD-202, EIA-364-26, GJB150, DIN50021-75, ISO3768, 3769, 3770; CNS 3627, 3885, 4159, 7669 etc.

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