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Corrosion testing in atmospheres conducive to corrosion

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CORROSION TESTING IN ATMOSPHERES CONDUCIVE TO CORROSION

Corrosion tests aim to illustrate the effects of corrosion in nature in a shorter  period of time. Materials or components are exposed to atmospheres conducive to corrosion in test chambers and sprayed with an electrolyte (e.g. sodium chloride solution).

Corrosion tests measure how long a component, coated or uncoated, is resistant to the onset of corrosion. Although these tests do not completely represent the diverse climate conditions worldwide, they nonetheless take account of important factors which are conducive to corrosion. Above all others, the primary conditions which lead to corrosion are high salt concentrations, high humidity levels, and high temperatures. Due to these factors, maritime climates can be especially corrosive.

In climate corrosion tests a distinction is made between constant and alternating climate conditions.

Salt spray tests are classic corrosion investigations carried out under constant conditions. In the test chamber, materials and coatings are maintained at a temperature of 35 °C and sprayed with a 5% salt solution.  Depending upon the coating or material, the duration of testing may exceed well over 1,000 hours. The pH-​value of the salt solution is in the range of 6.5 – 7.2 and the relative humidity in the chamber is nearly100 %. The salt spray test is also known as NSS, which stands for neutral salt spray. The International Organization for Standardization (ISO) has specified regulations to be followed in these tests in its ASTM B117 standard.   As a general rule of thumb, 720 hours of salt spray testing corresponds to ten years of corrosion exposure.

There are different versions of salt spray tests. In acetic acid salt spray testing, acetic acid is added to the NaCl solution. This lowers the pH level to 3.1 – 3.3. In the copper accelerated salt spray test (i.e. CASS), copper chloride (CuCl2) is added to the acetic acid NaCl solution. This variation in testing is used to determine how effective aluminum-​oxide coatings and decorative surface finishes consisting of copper, nickel, and chromium (Cu- Ni-Cr) or nickel and chromium (Ni-Cr) are in protecting against corrosion.

In addition to the methods developed by national and international standards organizations, the automotive industry also has developed alternating climate tests for its products, which are marketed worldwide. These tests are characterized by salt spray steps with intervals at higher or lower temperatures, higher and lower levels of humidity, as well as alternating resting periods. The variations in salt concentration, humidity, and temperature leads to frequent changes in the corrosion environment for the component.

  • The Swedish vehicle manufacturer Volvo utilizes the ACT I Test.  In the ACT I test, components are sprayed with a 1% NaCl solution having a pH of 4.2.  The spray cycle has a defined duration lasting several hours.  Afterward, the temperature in the chamber is increased from 35°C to 50°C and relative humidity is adjusted between 40% - 95%.  In the ACT I test, one week is required to complete a cycle. 
  • Ford utilizes the L-467 cyclical corrosion test.  In the L-467 test, components are sprayed with a 0.5% NaCl solution and exposed to temperatures between 25°C - 50°C and relative humidity in the range of 70% - 95% for defined intervals.  In the L-467 test, one week is required to complete a cycle.

The results of the different alternating climate tests cannot be compared to one another, as the specified temperatures, humidity levels, and salt concentrations vary too greatly. However, these tests do permit the corrosion resistance of materials or components to be compared to one another in reproducible conditions. They also demonstrate the accelerated corrosion which results from exposure to high temperatures and condensed water.

Glossar

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Technical terms cannot always be avoided. As corrosion experts, we not only want to give you comprehensive advice, we are also interested in making you a corrosion expert yourself.

The variety around the topic of corrosion and corrosion protection is also in our glossary at home: explanations from A as in Adhesion to T as in Thread tolerance. Have fun clicking through!