Salt spray corrosion principle
Most of the corrosion of metal materials occurs in the atmospheric environment, which contains corrosion components and corrosion factors such as oxygen, humidity, temperature change and pollutants. Salt spray corrosion is one of the most common and destructive atmospheric corrosion. The corrosion of metal materials by salt spray is mainly due to the penetration of conductive salt solution into the metal to produce electrochemical reaction, forming a "low potential metal electrolyte solution high potential impurity" micro battery system, electron transfer, dissolution of metal as anode and formation of new compounds, namely corrosives. Chloride ion plays a major role in the process of salt spray corrosion damage. It has a strong penetration ability. It is easy to 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 easy to be adsorbed on the metal surface to replace the oxygen in the oxide layer protecting the metal, so that the metal is damaged.
Salt spray corrosion test method and classification
Salt spray test is an accelerated corrosion resistance evaluation method of artificial atmosphere. It atomizes a certain concentration of brine; Then spray it into a closed constant temperature box to reflect the corrosion resistance of the tested sample by observing the change 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 fog environment is several or dozens of times that in the general natural environment, which greatly improves the corrosion rate. Carry out the salt fog test on the product, The time to get the results is also greatly reduced.
When a product sample is tested in natural environment, the corrosion time may take one or even several years, while when tested in artificial salt spray environment, similar results can be obtained as long as a few days or even hours.
Salt spray test is mainly divided into four types: neutral salt spray test (NSS); Acetic acid salt spray test (AASs); Copper accelerated acetic acid salt spray test (CASS); Alternating salt spray test;
The evaluation methods of salt spray test results include: rating method; Corrosion occurrence assessment method; Weighing method.
The rating method is to divide the percentage of the ratio of corrosion area to total area into several levels according to a certain method, and take a certain level as the qualification judgment basis. It is suitable for the evaluation of flat plate samples.
Calculation method of RP and RA values:
RP - protection rating value;
Ra - appearance rating value;
A - when calculating RP, it is the percentage of base metal corrosion in the total area; When calculating RA, it is the percentage of the corrosion part of the protective layer in the total area.
Overburden classification and subjective evaluation
The expression method of protection rating is: RA / --
For example, if slight rust exceeds 1% of the surface and is less than 2.5% of the surface, it is expressed as: 5 / -
The expression method of appearance rating is - / Ra value + subjective evaluation + damage grade of overburden
For example, moderate spots with an area of more than 20% are expressed as - / 2mA
The expression method of performance rating is: Ra value + subjective evaluation + overburden damage grade
For example, there is no base metal corrosion in the sample, but there is slight corrosion of anodic coating less than 1% of the total area, which is expressed as 10 / 6sc
Photo of the coating that is cathodic to the base metal: (image source: ISO 10289:1999)
The appearance evaluation method of corrosives is a qualitative judgment method, which is based on whether the product has corrosion after salt spray corrosion test.
Corrosion characteristics of common electroplated parts after salt spray test:
Weighing method is a method of weighing the weight of samples before and after corrosion test to calculate the weight of corrosion loss to evaluate the corrosion resistance quality of samples. It is especially suitable for assessing the corrosion resistance quality of a certain metal.
Calculation method of corrosion rate:
V - metal corrosion rate, g / m2 · h;
M0 - mass of test piece before corrosion, G;
M1 - mass of test piece before corrosion, G;
S - area of test piece, m2;
T - corrosion time of test piece, H
Five factors affecting salt spray test 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 to form an electrolyte with good conductivity. When the relative humidity decreases, the concentration of salt solution will increase until crystalline salt is precipitated, and the corrosion rate will decrease accordingly. With the increase of temperature, the molecular motion intensifies, and the higher the corrosion rate of high salt spray. The International Electrotechnical Commission 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%. For neutral salt spray test, it is generally considered that the test temperature is 35 ℃.
2. Concentration of solution
When the concentration is below 5%, the corrosion rate of steel, nickel and brass increases with the increase of concentration; When the concentration is more than 5%, the corrosion rate of these metals decreases with the increase of concentration. This is because the oxygen content increases with the increase of salt concentration in the low concentration range; 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 decreases. For zinc, cadmium, copper and other metals, the corrosion rate always increases with the increase of salt solution concentration.
3. Sample placement angle
The sedimentation direction of the salt spray is close to the vertical direction. When the sample is placed horizontally, its projection area is the largest, and the sample surface bears the most salt spray, so the corrosion is the most serious. The results show that when the steel plate is at an angle of 45 degrees 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. GB / T 2423.17-93 standard stipulates that the placement method of flat sample shall make the test surface form an angle of 30 degrees with the vertical direction.
4. PH value
The lower the pH value, the higher the H2 ion concentration in the solution, the stronger the acidity and the stronger the corrosivity. The pH value of neutral salt spray test (NSS) is 6.5 ~ 7.2. Due to the influence of environmental factors, the pH value of salt solution will change. Therefore, the pH value range of salt solution is specified in the salt spray test standards at home and abroad, and the method of stabilizing the pH value of salt solution in the test process is put forward to improve the reproducibility of salt spray test results.
5. sedimentation and spray method of salt spray
The finer the salt spray particles, the larger the surface area formed, the more oxygen adsorbed and the stronger the corrosivity. The traditional spray methods include air pressure injection method and spray tower method. The most obvious drawback is that the uniformity of salt spray deposition is poor and the diameter of salt spray particles is larger. Different spraying methods also affect the pH value of salt solution.
Standards related to salt spray test:
Salt spray test can be divided into two categories, one is natural environment exposure test, the other is artificial accelerated simulated salt spray environment test. Artificial simulated salt spray environment test is to use a test equipment with a certain volume space - salt spray test chamber to create a salt spray environment by artificial method in its volume space to evaluate the salt spray corrosion resistance and quality of products. Compared with the natural environment, the salt concentration of chloride in the salt spray environment can be several times or dozens of times of the salt spray content in the general natural environment, which greatly improves the corrosion speed. The time to obtain the results of salt spray test on products is also greatly shortened. If a product sample is tested in the natural exposure environment, it may take 1 year to be corroded, while similar results can be obtained in the artificial salt spray environment for only 24 hours.
Artificial simulated salt spray test also includes 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) is one of the earliest accelerated corrosion test methods in the field of application. It uses 5% NaCl solution, and the pH of the solution is in neutral range (6~7) as a spray solution. The test temperature is 35 ℃, and the sedimentation rate of salt spray is required to be 1 ~ 2ml / 80cm H.
(2) Acetate spray test (ASS test) is developed on the basis of 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, turn the solution into acid, and finally form a salt mist from neutral salt mist to acid. Its corrosion rate is about 3 times faster than that of NSS test.
(3) Copper salt accelerated acetate spray test (CASS test) is a newly developed rapid salt spray corrosion test 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 about 8 times that of NSS test.
The specific time is converted into:
Neutral salt spray test 24h ⇌ natural environment 1 year
Acetate fog test 24h ⇌ natural environment for 3 years
Copper salt accelerated acetate spray test for 24h ⇌ natural environment for 8 years