Testing

Outdoor corrosion of metal parts is often affected by a combination of many factors, which often do not exist in isolation. Therefore, it is difficult to simulate an artificial environment test that can fully comply with the actual outdoor environment. Judging from the results of the previous salt spray test, although the constant salt spray test has been universally recognized and has always been regarded as the basic test of the corrosion test, it has no correlation with the actual natural corrosion test. The Cyclic Corrosion Test (CCT) is mainly used in the automobile industry. On the basis of the constant salt spray test, parameters such as high temperature, humidity, low temperature, and dryness are introduced, and many condition factors in the natural environment are considered as much as possible. The purpose is to obtain test results that are more relevant to the natural environment.

1 Exposure conditions for cyclic corrosion test

（1) Room temperature conditions

In the CCT test, the room temperature environment refers to the room temperature conditions in the laboratory. Room temperature conditions can usually change the performance of the test sample very slowly. For example, the sample after the salt spray is left at room temperature for two hours. The sample actually undergoes a slow drying process under certain temperature and humidity conditions. Generally speaking, there are no corrosive vapors and gases in "room temperature conditions". There is almost no gas flow. The temperature is usually 25±5°C, and the relative humidity is 50% or lower. The laboratory conditions should be monitored and recorded for each test.

（2) In-box conditions

Non-room temperature conditions usually refer to the exposure conditions in the test box. In the conversion between different non-room temperature conditions, the test sample can be manually moved from one test box to another test box, or in a fully automatic test box, to realize the cycle from one condition to another. The temperature and relative humidity need to be monitored for each test. If possible, an automatic control system should be used. The temperature deviation should be accurate to ±3°C or less.

（3) Salt spray (spray) conditions

The salt spray conditions can be realized in the salt spray test box, or manually operated under laboratory conditions. The nozzle can spray a mist of salt solution. Generally speaking, except for NaCl (sodium chloride), electrolytes containing other chemicals can also be used to simulate acid rain or other industrial corrosion. The figure below shows the salt spray test process.

（4) Humid conditions

CCT test procedures usually require high humidity conditions. The relative humidity requirement is 95-100%. The ASTM D224711 test standard has this requirement. It can be achieved by using a constant temperature and humidity test box, or using an automatic circulation test box.

（5) Drying conditions

Drying conditions can be achieved in an open laboratory or in a test box. There is sufficient air circulation in the space to avoid stratification and to dry the sample. The definition of "dry" is more complicated, and people still have disputes about whether the surface of the sample is dry or completely dry. As product corrosion penetrates, the time required for the sample to completely dry may increase.

（6) Soaking corrosion conditions

Such conditions usually include a specific concentration of electrolyte, generally 5% concentration, pH 4 to 8, and temperature is usually specific. In the using process, the solution may be contaminated, and the solution should be replaced regularly.

（7) Water immersion conditions

Distilled or deionized water must be used. For water quality requirements, please refer to the ASTM D1193 standard. The soaking container should be made of plastic or other inert materials. The pH value of the soaking solution is between 6 and 8, the temperature is 24±3°C, and the conductivity should be less than 50 mohm/cm at 25°C.

2. Sample preparation

The preparation of samples in the color-coated board salt spray test mainly includes flat samples, scratched samples, and notched samples, etc. The test purposes of different samples are shown in the following table:

3. Precautions for exposure experiment

Except for the general constant salt spray test precautions, the multiple test conditions of CCT exposure bring more potential problems to the repeatability and reproducibility of test results.

1) Test box load

A full-load test box usually takes longer to achieve temperature transitions than a light-load test box. In order to ensure air circulation during the test, the load of the test box should be even.

2) Conversion (ramp) time

Whether it is a manual operation or a fully automatic test box, the conversion time is a factor that affects the test results. In manual operation, the conversion time refers to the time required to transfer a sample from one environment or exposure condition to another environment or exposure condition. In the fully automatic test box, the conversion time refers to the time required for the equipment to change the exposure conditions in the box. Automatical exposure gives more predictable and repeatable transitions than manual exposure. The impact of conversion time on test results still needs further reaserch. Therefore, monitor and record the conversion time as much as possible. The conversion time will change with changes in the following conditions: changes in room temperature conditions, changes in manual operating procedures, the type of equipment used, and the load of the test box.

3) Deposition and uniformity of salt spray

In the traditional salt spray test, the uniformity of the spray is usually tested by collecting salt spray at different locations in the tank. Unlike the constant salt spray test, the detection of CCT salt spray deposition rate cannot be completed during the test operation. This is because the vast majority of CCT exposures stipulate that the salt spray cycle time is relatively short. Therefore, in order to measure the uniformity of the salt spray deposition in a CCT test device, we must collect the salt spray deposition during continuous spraying for 16 hours. Check the method of the constant salt spray test to get a detailed description of the collection of salt spray deposition.

4) Test interruption

When the test must be interrupted, the tested sample should be placed in a condition that is not corroded as much as possible. Record all interruptions and processing of templates.