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Corrosion testing atmospheres conducive corrosion

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Glossary

CORROSION TESTING IN ATMOSPHERES CONDUCIVE TO CORROSION

Corrosion tests aim to illustrate the effects of corrosion in nature in a short space of time. Materials or components are exposed to atmospheres conducive to corrosion in test chambers and sprayed with saline solution, for example.

Corrosion tests examine how long a component – coated or uncoated – resists corrosion. Although these tests do not illustrate the diverse climate conditions worldwide, they may nonetheless take account of important factors conducive to corrosion. These are above all high salt exposure, high moisture levels and a high temperature. A maritime climate can be especially corrosive, as high salt exposure, high moisture levels and high temperatures are combined.

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

Salt spray tests are classic investigations carried out under constant conditions. These involve materials and coatings being sprayed in spraying chambers at 35 °C with a 5% salt solution, sometimes for over 1,000 hours. Sodium chloride (NaCl) is used as salt, with the pH-value at 6.5 – 7.2, relative humidity at almost 100 %. This test is also known as NSS, which stands for neutral salt spray. As a rule of thumb, 720 hours of spraying corresponds to ten years of corrosion exposure. The International Organization for Standardization (ISO) has specified regulations to be followed in these tests in its ISO 9227 standard. The full name of the standard is EN ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests.

There are different versions of salt spray tests. In acetic acid salt spray testing – ASS for short – acetic acid is added to the NaCl solution. This lowers the pH level to 3.1 to 3.3. In the copper accelerated salt spray test – CASS for short – the acetic acid NaCl solution receives the addition of copper chloride (CuCl2). This test variant is used to examine how well aluminium-oxide coats and decorative coatings consisting of copper, nickel and chrome (Cu- Ni-Cr) or nickel and chrome (Ni-Cr) protect.

Tests can be made more realistic by exposing components to multiple climates. For example, the Deutsches Institut für Bautechnik (DIBT), Berlin, employs different tests to examine the corrosion stability of surfaces. Corrosion tests in the laboratory are combined with open-air weathering in the urban climate of Berlin and on the island of Sylt.

The car industry has developed alternating climate tests for its products, which are marketed worldwide. These tests are characterised by salt spray stages with stages at higher or lower temperatures, higher and lower levels of humidity as well as alternating rest stages. These variants of salt concentration, humidity and temperature mean that the corrosion environment on the component is changing frequently. Examples: a test conducted by the German Association of the Automotive Industry (VDA) and one by the car manufacturer Volkswagen take account of frost.

In the VDA test 233-102 components are sprayed with a 1% NaCl solution for a number of hours and then exposed to temperatures of -15 to 50 °C and humidity of 50 to 95 % in other test stages. One test cycle takes a week. In the VW PV 1209 test components are sprayed with a 4 % NaCl solution and 1 % calcium chloride (CaCl2) for a number of hours and then exposed to temperatures of -40 to 80 °C and humidity of 30 to 95 % in other test stages. One test cycle also takes a week.
In two alternating climate tests of the Swedish vehicle manufacturer Volvo components are not sprayed, but sprinkled from above in stages. These tests are known as accelerated corrosion tests (ACTs).

  • In the ACT I test components are sprinkled with a 1% NaCl solution with a pH level of 4.2 for a number of hours and then exposed to temperatures of 35 to 50 °C and humidity of 40 to 95 % in other test stages. One test cycle takes a week.
  • In the ACT II test components are sprinkled with a 0.5% NaCl solution and then exposed to temperatures of 25 to 50 °C and humidity of 70 to 95 % in other test stages. One test cycle takes a week.
     

The results of the different alternating climate tests cannot be compared to one another, as the specified temperatures, humidity levels and salt content levels are too different. However, they do permit the corrosion resistance of materials or components to be compared to one another in reproducible conditions. The tests therefore show that high temperatures and condensed water accelerate corrosion.