What Is Salt Spray Test？

author: YITECH

2025-02-22

Salt spray test is an environmental test that mainly uses artificial simulated salt spray environment conditions created by salt spray test equipment to assess the corrosion resistance of products or metal materials.

What is a salt spray test? Salt spray test is an environmental test that mainly uses artificial simulated salt spray environment conditions created by salt spray test equipment to assess the corrosion resistance of products or metal materials.

It is mainly divided into two categories, one is natural environment exposure test, and the other is artificial accelerated simulated salt spray environment test. The former is time-consuming and labor-intensive, so such natural environment exposure test is rarely carried out. It is mainly artificial simulated salt spray environment test. Artificial simulated salt spray environment test is an accelerated corrosion resistance evaluation method of 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 after the tested sample is 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 general 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.

1. Principle of salt spray test

Most of the corrosion of metal materials occurs in the atmospheric environment, which contains corrosive components and corrosion 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 penetration of conductive salt solution into the metal to produce electrochemical reactions, forming a "low potential metal-electrolyte solution-high potential impurity" micro-battery system, electron transfer, and the metal as the anode dissolving to form new compounds, namely corrosive products. The main role in the salt spray corrosion damage process is played by chloride ions, which have strong penetrating ability and can easily penetrate the metal oxide layer into the metal and destroy 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 the metal to be damaged.

2. Classification of salt spray tests

Salt spray tests are mainly divided into 4 types: neutral salt spray test (NSS), acetic acid salt spray test (AASS), copper accelerated acetic acid salt spray test (CASS), and alternating salt spray test.

Neutral salt spray test (NSS test)

Neutral salt spray test is the earliest and most widely used accelerated corrosion test method. Generally, it uses a 5% sodium chloride salt solution with a pH value adjusted to a neutral range (6.5-7.2) 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.
Neutral salt spray test is applicable to: metals and their alloys, metal coatings (anodic or cathodic), conversion films, anodic oxide films, and organic coatings on metal substrates.

Acetic acid salt spray test (ASS test)

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 collected liquid in the salt spray box is 3.1-3.3, the solution becomes acidic, and the salt spray formed finally becomes acidic. Its corrosion rate is about 3 times faster than that of the NSS test. If the pH value of the initially prepared solution is 3.0-3.1, the pH value of the collected liquid is generally in the range of 3.1-3.3. The pH value should be measured at 25℃±2℃ with an acidity meter, or it can be tested daily with precision pH test paper with a measurement accuracy of no more than 0.1. The pH value of the solution can be adjusted with glacial acetic acid or sodium hydroxide.
The acetic acid salt spray test is applicable to: copper + nickel + chromium or nickel + chromium decorative plating, and also to aluminum anodized film.

Copper accelerated acetic acid salt spray test (CASS test)

In the copper accelerated acetic acid salt spray test (CASS test), copper chloride (CuCl2·2H2O) is added to the prepared salt solution, and its concentration is 0.26 g/L±0.02 g/L (i.e. 0.205g/L±0.015 g/L anhydrous copper chloride), which strongly induces corrosion, and its corrosion rate is about 8 times that of the NSS test.
The copper accelerated acetic acid salt spray test is applicable to: copper + nickel + chromium or nickel + chromium decorative plating, and also to aluminum anodized film.

Alternating salt spray test

Alternating salt spray test is a comprehensive salt spray test, which is actually a neutral salt spray test plus a constant humidity and heat test. It is mainly used for cavity-type complete products. Through the penetration of the humid environment, salt spray corrosion is not only generated on the surface of the product, but also inside the product. It is to alternately switch the product under the two environmental conditions of salt spray and humidity and heat, and finally assess whether the electrical and mechanical properties of the complete product have changed.

3. Salt spray test judgment

The judgment methods of the salt spray test results mainly include rating judgment method, weighing judgment method, corrosion product appearance judgment method and corrosion data statistical analysis method.

Rating judgment method

The rating judgment method is to divide the percentage of the ratio of the corrosion area to the total area into several levels according to a certain method, and use a certain level as the basis for qualified judgment. It is suitable for flat sample evaluation.

Weighing judgment method

The weighing judgment method is to weigh the weight of the sample before and after the corrosion test, and calculate the weight lost by corrosion to judge the corrosion resistance of the sample. It is particularly suitable for assessing the corrosion resistance of a certain metal.

Corrosion appearance determination method

The corrosion appearance determination method is a qualitative determination method. It determines whether the product produces corrosion after the salt spray corrosion test. This method is mostly used in general product standards.

Statistical analysis method of corrosion data

The statistical analysis method of corrosion data provides a method for designing corrosion tests, analyzing corrosion data, and determining the confidence level of corrosion data. It is mainly used to analyze and count corrosion conditions, rather than for the quality determination of a specific product.

4. Salt spray test process

Specimen pretreatment

The contaminated specimen surface should be pretreated to ensure that there are no pollutants such as oil, grease or dirt (dust) on the surface. The pretreatment of the specimen should be as little as possible.

Initial inspection

All specimens should be inspected under standard atmospheric conditions before the test. The inspection is mainly visual inspection. If necessary, electrical and mechanical performance inspections should be carried out according to relevant standards to obtain baseline data and record the inspection results.

Test process

According to the inspection standards of the corresponding salt spray test, adjust the temperature and humidity of the test chamber and test within the specified test period and cycle.

Recovery

Gently rinse the specimen with running water under standard atmospheric conditions.
Final inspection Visual inspection of the specimen, physical and electrical performance inspections when necessary, and record the test results.

Result analysis

Analyze the test results from three aspects: physical, electrical, and corrosion.

5. Factors affecting salt spray test

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 deposition, and spray method, etc.

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. The oxygen content in the salt solution is related to the concentration of 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. If the salt concentration continues to increase, the oxygen content decreases accordingly. When the oxygen content decreases, the depolarization ability of oxygen also decreases, that is, the corrosion effect is weakened. However, for metals such as zinc, cadmium, and copper, the corrosion rate always increases with the increase of salt solution concentration.

Sample placement angle

The sample placement angle has a significant effect on the results of the salt spray test. The direction of salt spray settling is close to the vertical direction. When the sample is placed horizontally, its projection area is large, and the amount of salt spray on the sample surface is also large, so the corrosion is serious. The research results show that when the steel plate is at a 45-degree angle to the horizontal line, the weight loss due to corrosion is 250 g per square meter, and when the plane of the steel plate is parallel to the vertical line, the weight loss due to corrosion is 140 g per square meter. GB/T2423.17-93 standard stipulates that "the placement method of flat-plate samples should make the test surface at a 30-degree angle to the vertical direction."

Test temperature and humidity values

Temperature and relative humidity affect the corrosion 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 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.

The higher the test temperature, the faster the salt spray corrosion rate. International Electrotechnical Commission IEC60355:1971 "AN APPRAISAL OF THE PROBLEMS OF ACCELERATED TESTING FOR ATMOSPHERIC The "Natural Salt Spray Test" standard states: "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%." This is because the temperature increases, the molecular motion intensifies, and the chemical reaction speed accelerates. For the neutral salt spray test, most scholars believe that the test temperature is more appropriate at 35°C. If the test temperature is too high, the salt spray corrosion mechanism is quite different from the actual situation.

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 more hydrogen ions in the solution The higher the concentration, the stronger the acidity and the stronger the corrosion. Salt spray tests on Fe/Zn, Fe/Cd, Fe/Cu/Ni/Cr and other electroplated parts show that the corrosion of the acetic acid salt spray test (ASS) with a pH value of 3.0 is 1.5 to 2.0 times more severe than the neutral salt spray test (NSS) with a pH value of 6.5 to 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 the salt solution, and proposed a stable test process for the salt solution pH value method to improve the reproducibility of salt spray test results.

Factors affecting salt spray test results include:

1. Test interruption

2. Test sample cleaning

3. Test sample processing

4. Test sample placement

5. Salt spray solution configuration

6. Salt spray solution concentration

7. Spray method

8. Salt spray test temperature

9. Spray time

10. Pressure balance, etc.

Salt spray test is an environmental test that mainly uses artificial simulated salt spray environment conditions created by salt spray test equipment to assess product and corrosion resistance. The test results are related to many factors such as solution concentration, pH value, test temperature and humidity, and sample placement.

DongGuan YiTai Electronic Technologies Co.,Ltd

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