Principle of salt spray test chamber!

The cyclic corrosion test (CCT) is mainly used in the automotive industry. Based on the constant salt spray test, high temperature, humidity, low temperature, drying and other parameters have been introduced, taking into account as much as possible the many condition factors in the natural environment, with the aim of obtaining test results with a higher correlation with the natural environment.

Room temperature conditions

In CCT testing, room temperature environment refers to laboratory room temperature conditions. Room temperature conditions can often change the performance of a test specimen very slowly. For example, a sample after a salt spray is left at room temperature for two hours. The sample actually undergoes a slow drying process under specific temperature and humidity conditions. In general,“room temperature conditions”are free of corrosive vapors and gases. There is virtually no gas flow, the temperature is usually 25±5°C, the relative humidity is 50% or less, and the laboratory conditions should be monitored and recorded for each test.

Chamber Conditions

Non-room temperature conditions usually refer to the exposure conditions inside the test chamber. Transitioning between different non-room temperature conditions can be accomplished by manually moving the test specimen from one chamber to another, or by cycling from one condition to another in a fully automated chamber. Temperature and relative humidity should be monitored for each test. If possible, an automatic control system should be used. Temperature deviations should be accurate to±3°C or less.

Salt Spray Conditions

Salt spray conditions can be achieved in a salt spray test chamber or manually under laboratory conditions. Nozzles can spray a misty salt solution. Generally, electrolytic solutions containing chemicals other than NaCl (sodium chloride) can be used to simulate acid rain or other industrial corrosion. The following diagram represents the salt spray test process.

Humid Conditions

CCT test procedures typically require high humidity conditions. The relative humidity requirement is 95-100% as specified in the ASTM D224711 test standard. This can be accomplished using a constant temperature and humidity chamber, or an automatic cycling chamber. The figure below shows the wetting process.

Drying Conditions

Drying conditions can be achieved in an open laboratory or in a test chamber. The space is well ventilated so that stratification is avoided and the sample can be dried. As product corrosion penetrates, the time required for complete drying of the sample may increase.

Immersion corrosion conditions

These conditions usually consist of a specific concentration of electrolyte, typically 5%, with a pH of 4 to 8, and usually at a specific temperature. The solution may become contaminated during use and should be replaced periodically.

Immersion conditions

Distilled or deionized water must be used. The ASTM D1193 standard can be consulted for water quality requirements. Containers for immersion should be made of plastic or other inert materials. The pH value of the immersion solution should be between 6 and 8, the temperature should be 24±3°C, and the conductivity should be less than 50 mhm/cm at 25°C.

Sample preparation

Color coated plate salt spray test in the preparation of specimens are mainly flat plate specimen, scratch specimen, incision specimen, and so on. The test purpose of different specimens is shown in the table below.

In addition to the general constant salt spray test precautions CCT, the exposure to a variety of test conditions brings more potential problems to the test results repeatability and reproducibility.

Test Chamber Loading

Fully loaded test chambers typically require a longer period of time for the temperature transition than lightly loaded chambers. To ensure air circulation during testing, the chamber should be evenly loaded.

Salt Spray Deposition and Uniformity

In conventional salt spray testing, the uniformity of the spray is usually checked by collecting salt spray at different locations within the chamber. Unlike the constant salt spray test, the CCT salt spray deposition rate cannot be checked during the test operation. This is due to the fact that most CCT exposures specify a relatively short salt spray cycle time. Therefore, in order to measure the uniformity of salt spray deposition in a CCT chamber, we must collect salt spray deposition over a continuous spray period of up to 16 hours.

Test Interruption

When a test must be interrupted, the sample under test is placed in a non-corrosive condition as far as possible. All interruptions and sample handling are recorded.

The automotive industry is technologically advanced in cyclic corrosion testing methods and the majority of CCT tests are used in automotive testing.