Circulating salt spray corrosion test

From about 1914 the salt spray method was first used to test the corrosion resistance of materials. In 1939, the neutral salt spray test was included in ASTM B117. This traditional salt spray standard requires samples to be continuously exposed to 5% salt spray at 35°C. Although the ASTM B117 test standard has been revised and improved many times during its 80 years of application, it has long been believed that the test results of the salt spray method do not correlate well with the actual corrosion effects of outdoor exposure of samples. Even so, ASTM B117 is currently the main standard for salt spray corrosion testing and is widely used for corrosion resistance testing of coatings, military components, and electronic components.

With the improvement of people's anticorrosion requirements for materials, engineers and researchers are constantly trying to develop testing procedures that can more accurately simulate outdoor corrosion effects. In the 1960s and 1970s, Harrison and Timmons2,3 in the UK developed the Prohesion test, especially for corrosion testing of industrial protective coatings. In recent years, SAE and AISI have started research and development of automotive cyclic corrosion test. The current results are satisfactory, and some articles have been published, and Japanese researchers have also developed many cyclic corrosion test methods

What is a Cyclic Corrosion Test?

The cyclic corrosion test is a more realistic salt spray test than the traditional constant state exposure. Because actual outdoor exposure typically involves both wet and dry environments, it is only meaningful to simulate these natural, cyclical conditions for accelerated laboratory testing. The research shows that after the cyclic corrosion test, the relative corrosion rate, structure, and morphology of the samples are very similar to the outdoor corrosion results. Therefore, the cyclic corrosion test is closer to real outdoor exposure than the traditional salt spray method. They can effectively evaluate many corrosion mechanisms, such as general corrosion, galvanic corrosion and crevice corrosion.

The purpose of the cyclic corrosion test is to reproduce the type of corrosion in an outdoor corrosive environment. CCT testing exposes samples to a series of cycling environments under different conditions. Simple exposure cycles, such as the Prohesion test, expose samples to cycles consisting of salt spray and dry conditions. More complex automotive testing methods require immersion, humidity and condensation cycles in addition to salt spray and dry cycles. Initially these test cycles were done manually, with lab operators moving samples from a salt spray cabinet to a Humidity Chamber to a drying unit and so on. More recently, microprocessor-controlled Test Chambers have automated these test steps, reducing experimental uncertainty.

exposure conditions

One or all of the following conditions are applied in the cyclic corrosion test:

Room temperature conditions:

In CCT testing, ambient refers to laboratory room temperature conditions. Room temperature conditions can often change the properties of test samples very slowly. For example, samples after a salt spray spray were left at room temperature for two hours. The sample actually undergoes a slow drying process under specific temperature and humidity conditions.

Generally speaking, there are no corrosive vapors and gases in "room temperature conditions". With little to no gas flow, temperature typically 25 ± 5°C, and relative humidity 50% or less, laboratory conditions should be monitored and recorded for each test.

Conditions inside the box:

Non-room temperature conditions generally refer to exposure conditions inside the Test Chamber. The transition between different non-room temperature conditions can be achieved by manually moving the test sample from one Test Chamber to another Test Chamber, or in a fully automatic Test Chamber, to realize the cycle from one condition to another.

Temperature and relative humidity are monitored for each test. Automatic control systems should be used if possible. The temperature deviation should be accurate to ±3°C or less.

Salt spray (spray) conditions:

Salt spray conditions can be achieved in a B117 type Test Chamber, or manually under laboratory conditions. The nozzle can spray a spray of saline solution. In general, electrolytes containing other chemicals besides NaCl (sodium chloride) can be used to simulate acid rain or other industrial corrosion. Figure 1 shows the salt spray conditions.

Humid conditions :

CCT testing procedures often require high humidity conditions. The relative humidity requirement is 95-100%. This requirement is included in the ASTM D2247 11 test standard. Sometimes it can also be achieved by using the B117 Test Chamber to spray pure water mist. Figure 2 shows the Q-Fog in a wet condition.

Drying conditions:

Dry conditions can be achieved in an open laboratory or in a Test Chamber. The space has sufficient air circulation to avoid stratification and to dry the samples. The definition of "dry" is complex, and there is debate as to whether it refers to surface dryness or effective drying of the sample. As the corrosion of the product penetrates, the time required for the sample to dry effectively may increase. Figure 3 shows Q-Fog in dry condition.

Immersion Corrosion Conditions:

These conditions usually involve a specific concentration of electrolyte, typically 5%, a pH of 4 to 8, and usually a specific temperature. During use, the solution may be contaminated and should be replaced periodically.

Immersion conditions:

  Distilled or deionized water is required. For water quality requirements, please refer to ASTM D1193 12 standard. Containers for immersion should be made of plastic or other inert material. 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 50mohm/cm at 25°C.

CCT Salt Spray Chamber Testing Guide

Because CCT testing is often complex and requires multiple exposure cycles, it can be confusing. The following testing guidelines can help operators understand the possible step-to-step differences in CCT exposure testing conditions and help to reach a consensus on test results among different laboratories.

Use a reference sample

If possible, the reference sample (tested, known performance sample) should be tested simultaneously with the sample to be tested, and it is better to use multiple reference samples if possible. The reference sample helps to judge the test performance of the tested sample. The use of reference samples can help to standardize the parameters of the test conditions for repeated tests and can also guide the comparison of the results of different repeated tests.

Preparation of test samples

It is common practice to streak coated samples prior to performing CCT testing. This can damage the coating to accelerate the corrosion process. When using a stone impact Tester, it is recommended to operate according to the ASTM D3170 standard.

There is increasing evidence that differences in scratch depth can significantly affect CCT salt spray test results. This has the greatest impact on galvanized substrates. In most cases, scratches should penetrate deep into the metal substrate. Because the geometry of the scratch can also affect the results, the scribing tool used needs to be documented. Scratch damage can be observed using a microscope and the scratch method is described in ASTM D1654.

Precautions for Exposure Experiments

In addition to the caveats mentioned in B117, the variety of test conditions for CCT exposure poses additional potential problems for the repeatability and reproducibility of test results.

Test chamber load:

A fully loaded Test Chamber usually takes longer to achieve a temperature transition than a lightly loaded Test Chamber. In order to ensure air circulation during the test, the load of the Test Chamber should be uniform.

Conversion (ramp) time:

Whether it is a manual operation or a fully automatic Test Chamber, the changeover time is a factor that affects the test results. When performed manually, transition time is the time required to transfer a sample from one environment or exposure condition to another. In fully automatic Test Chambers, transition time refers to the time required for the equipment to change the exposure conditions in the chamber. Automatic gives more predictable and repeatable transitions than manual exposure. The effect of switching time on test results still needs further study. Therefore, monitor and record the transition time as much as possible, and the transition time will change as the following conditions change:

Changes in room temperature conditions

Changes in manual procedures

type of instrument used

Load of the Test Chamber

Salt mist deposition and uniformity:

In a traditional salt spray test, the uniformity of the spray is usually tested by collecting the salt spray at different locations in the chamber. Unlike B117, the detection of CCT salt mist deposition rate cannot be done during the test operation. This is because the vast majority of CCT exposures dictate relatively short salt spray cycle times. Therefore, in order to measure the uniformity of the salt mist deposition in a CCT test device, we had to collect the salt mist deposition during continuous spraying up to 16 hours. See Method B117 for detailed instructions on collecting salt spray deposits.

Test break:

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

Report

In addition to all test conditions that need to be recorded as required by traditional salt spray testing, the CCT salt spray test report should also include:

• Changeover times for fully automatic salt spray chambers

• Loading of the chamber (eg, number of samples)

• Controlled temperature in the laboratory, average values ​​of relative humidity, and daily fluctuations in these parameters during manual testing.

Advantages of Automated CCT Salt Spray Testing

Cyclic corrosion testing began as a labor-intensive manual testing method. Now, automated multifunctional salt spray chambers are available and it can complete all CCT tests in one chamber. Here are some of its advantages:

• Eliminates the need to manually move test samples from one chamber to another

• Effortless spraying of test samples

• Minimizes variation in measurement results due to sample handling

• Allows for more accurate prediction of transition times

Q-fog CCT Cyclic Salt Spray Corrosion Test Chamber

Commonly used cyclic corrosion test methods

The following are some commonly used test methods. The following test conditions are brief descriptions of various standards, test methods and practices. For more detailed information, please refer to the relevant standards. Other test cycles may better suit your requirements, SAE J1563 is useful as a source of cycle test guidance.

Currently, there are many salt spray corrosion testing methods to choose from, each method has its own advantages and disadvantages. Some researchers believe that salt spray conditions are superior to immersion conditions. Some use special electrolytes to simulate acid rain. But most of them choose automatic salt spray box. The setting of different exposure temperatures, durations, and exposure sequences is still debated, so research into modification of cycle times and improvement of corrosion solutions will no doubt continue. However, for most materials, the cyclic corrosion test results are more realistic than traditional salt spray, which is the consensus of most people.