How to choose a corrosion test for product qualification?

The ISO/TR 16335 Technical Report provides excellent suggestions for choosing the most suitable corrosion test to qualify products made from metallic materials, with or without protective coatings.

Table 1 summarizes the main test methods used. The most common laboratory method is the constant neutral salt spray test (category A). This type of test is useful for detecting discontinuities such as pores and other defects in some metallic coatings, conversion layers (e.g., anodic oxidation), and organic coatings. However, although it is widely used for qualification testing, the results of constant salt spray tests rarely correlate with the performance of components in service.

One way to improve this correlation is to combine salt spray exposure with humidity cycles, varying between a condensation condition and a low humidity level (category C), thereby introducing a drying phase into the corrosion test cycle.

Some atmospheric pollutants, such as sulfur dioxide (SO₂), nitrogen dioxide (NO₂), hydrogen sulfide (H₂S), and chlorine (Cl₂), present in trace amounts in the air, promote the corrosion of metals under high humidity conditions. Therefore, high humidity exposure tests in the presence of these atmospheric pollutants are often used in the qualification of electronic products (category D).

To evaluate the corrosion resistance of certain products, tests that combine intermittent salt spray with exposure to gases that promote corrosion (category E) have also been introduced.

Table 1 – Categories of accelerated atmospheric corrosion tests.

Tests that involve exposing samples to high humidity and condensate water are also considered corrosion tests (category F). Such tests can produce corrosion effects on metal parts if surface contaminants in the form of salts are present. These tests are also used to test organic coatings because they can induce damage caused by swelling and the leaching of additives. For electronic device testing, high humidity tests are used to verify hermeticity and water infiltration in equipment. A particular test method involves testing the corrosion protection capability of a semipermeable enclosure of an electrical device by subjecting the enclosure to rapid cooling. If the cooling effect is sufficiently high, it causes moist air to enter the enclosure and the subsequent condensation of water vapor.

Corrosion tests based on the field of application

Table 2 outlines four fields of application characterized by three different corrosivity levels of the environment in which a given component operates. For each combination, the table suggests the most suitable category of corrosion test for product qualification (P = preferred, U = useful for comparing similar products, N = useful only for quality control of identical products).

Table 2 – Category of corrosion test and fields of application.

Comparison of various test methods

For many test methods, data on the extent of corrosion of standard metallic samples subjected to these tests are available within the standards themselves. To illustrate how the corrosion process varies among some standardized tests, Table 3 shows the estimated average test times to achieve a mass loss of carbon steel equal to 670 g/m². This corresponds to approximately 5 years of atmospheric exposure in the medium corrosivity category C3, as defined by the ISO 9224 standard.

Table 3 – Comparison of average time required to reach a weight loss of carbon steel equal to 670 g/m² depending on the test method.

Cyclic test vs. neutral salt spray

The following figures compare two standard plates coated with the same painting process, subjected for 1008 hours to two different types of tests: on the left, the plate subjected to a cyclic corrosion test; on the right, the plate exposed to a constant neutral salt spray.

The extent of corrosion starting from the scribe mark is clearly visible on both coated samples but is much more severe in the case of cyclic corrosion.

Similar observations can be made for the cover image, where two identical brackets are illustrated.

It is always important to remember that corrosion test results cannot be used to predict the long-term behavior of the coating, as the environmental conditions encountered in practice are much more complex and therefore difficult to simulate.

The data presented here should be understood as an expression of the industrial know-how accumulated over the years.

Do you need specialistic support?

At MaterialScan, we understand the importance of ensuring your products meet the highest standards. Our unique corrosion testing service in Italy, provided at no additional cost, supports customers in selecting the most appropriate testing method based on specific product requirements.

Refer to our test catologue, calla us at: +39 351 723 9441 or email us at: material@materialscan.it.