Correlation Between Natural Weathering Conditions and Laboratory Tests

This article has been prepared to explain how natural environmental conditions to which various coatings applied to steel are exposed correlate with accelerated corrosion tests performed in laboratory environments, and how product service-life predictions are made in this manner. Every manufacturer produces different products using various materials and sells these products. The most important demands of manufacturers are that their produced products find a market place, demonstrate successful performance, and that their brands gain recognition. Successful performance is evaluated according to 2 important parameters. The first is that the product contains differences compared to its equivalents, providing functional ease of use to the consumer and remaining sound for long years, thus having a long service life. The second is that the product demonstrates this long-term durability. While functional practicality, price, and aesthetic appeal proceed according to the manufacturer's own plans, producing sound products over long years emerges as a separate challenge. Durability here should be understood as the degree of performance that products manufactured from different materials such as metal, plastic, wood, etc., and especially coated with organic, inorganic, metallic or chemical conversion coatings to resist natural corrosion conditions, will display against these conditions. For example, if a product coated with wet or powder paint rusts in a short time, this would emerge as a situation that would satisfy neither the consumer nor the manufacturer. This article will provide information about which natural conditions are considered when subjecting coated products manufactured from steel in different sectors to tests, particularly regarding corrosion or rusting, how tests represent natural conditions, and the correlation between hours in laboratory tests and real-life service periods.

When we think about natural conditions, the first things that come to mind could be hot-cold, humid environments, sea effects, sun effects, rain, wind, and dust.

Besides this, corrosion other conditions that increase its effect include industrial environments emitting excessive polluting gases, contact of various chemicals with products, and interaction with soil and water minerals in underground systems. Outside of this, mechanical strength tests examining mechanical deformations and environmental tests within the scope of weight, such as hot-cold, thermal shock, and humid environment resistance tests, could be subjects of separate articles. As can be seen, there are quite a few parameters on the list, so we wonder which parameters are more priority in terms of corrosion in our product. Here the manufacturer can consider 2 points. The first is to determine in which conditions (closed environment, open atmosphere, industrial zone, seaside, underground, rainy region, humid region, or all) the product it manufactures will operate during the R&D phase, and to obtain information about the necessary tests by considering these environments or based on international standards (ASTM, ISO, EN ISO, SAE, MIL, QUALICOAT, TS, IEC, etc.) that specify the parameters that the relevant product used worldwide must meet. These product standards are easily available from the relevant standard websites online in exchange for a fee. The tests mentioned in these standards are again carried out based on similar international test standards. One point to note here is that test durations are not specified in test standards. International test standards are only specifications that explain under which conditions (temperature, humidity, concentration, pressure, flow rate, etc.) a test should be conducted, which types of test equipment should be used, which parameters should be recorded, what reproducibility and suitability values should be specified, and how results should be reported. In short, there is both a product standard and a test standard. After explaining these, the next step will be laboratory selection.

Which laboratories should these tests be performed in? The method followed worldwide is as follows: The arbitrator between manufacturer and consumer is test laboratories with international accreditation. Laboratory accreditation is defined according to EN ISO/IEC 17025-2017 version.

To hold this accreditation, which is granted for 4 years and checked with interim inspections each year, a genuine quality system must be established, appropriate laboratory conditions must be provided, test operators with necessary knowledge and training, and principles of independence and confidentiality are required, and accreditation is given to legal entities that implement all of these. Indeed, accreditation carries a very intensive work pace and challenges within it, including the establishment and operation of a quality system in compliance with it, training, maintenance of test equipment, interim inspections, calibration requirements, and measurement uncertainty studies. However, this is the requirement of this profession, what needs to be done, and what is also needed for the manufacturer. Most manufacturing companies carry out similar tests in the laboratories within their own quality departments. However, to be sure of the results and to evaluate their own test laboratory, all manufacturers are recommended to work in coordination with accredited laboratories. Because the aim here should be to detect problems that may arise during the product R&D phase, preventing the brand and company from suffering economic loss when the product reaches the market. Although accelerated corrosion tests performed in laboratories cannot be considered 100% accurate, it is accepted by authorities that they produce results very close to reality. Particularly in recent years, developed test equipment has successfully fulfilled compatibility with natural life. Problems that could arise in nature 5-10 years later are being guided to manufacturers through these devices and accelerated corrosion aging tests.

Some corrosion tests that should be performed on products in the laboratory can be specified as shown below:

1. Salt fog environment resistance test, 2. Cyclic corrosion tests (containing humidity, rain, hot environments), 3. Humid environment resistance tests, 4. SO2-containing humid environment resistance tests, 5. Water and other chemical resistance tests, 6. Hot-cold, thermal shock resistance tests, 7. Hot-cold humid environment resistance tests, 8. Sunlight (rain, humid environment) resistance tests, 9. Dust-water impermeability tests, IP tests. The corrosion aging test durations written here and performed in laboratories do not have a one-to-one equivalence with natural conditions. However, we can mention some unwritten comparisons accepted on a sector basis. For example, in the automotive sector, there is a common understanding that corrosion tests performed over 1000 hours provide real-life durability of 5-15 years. The reason the range is this wide is, as mentioned earlier in the article, related to the fact that the risks that will be created in terms of corrosion by the environments in which the product will be found in its real life are different. The degree of corrosion to which an automobile located in Ankara will be exposed is certainly different from the degree of corrosion to which an automobile located in Istanbul or Antalya will be exposed. It is an undeniable fact that in geographical areas near the sea, corrosion will accelerate due to chlorine present in the environment, high humidity, and rain. Similarly, in sun effect tests, the geography and climate conditions where the product is located can extend or shorten the harsh conditions to which the product will be exposed. Regarding corrosion classifications, I can share the following table in terms of providing information to manufacturers in EN ISO 12944-2 standard. The tests suitable for the categories mentioned in this table are given in EN ISO 12944-6 standard.

(EN ISO 12944-2 Standard) Table 1 — Atmospheric-corrosivity categories and examples of typical environments

Table 2 — Categories for water and soil

Paints and coatings manufacturers can classify their products after they pass the given tests by applying them over a steel substrate, according to the table provided on the next page. (EN ISO 12944-6 Standard) Table 1 — Test procedures for paint systems applied to carbon steel, hot dip galvanized steel or steel with thermal-sprayed metallic coating

Table 2 — Test procedures for paint systems applied to carbon steel, hot dip galvanized steel or steel with thermal-sprayed metallic coating

With the information provided in this article, I have tried to provide some brief but necessary information about how the vast world of laboratory work and real-life parameters run in parallel. I hope this information is beneficial to our readers.     Author: Tuncay Katırcı Chemical Engineer Laboratory Director Metaltek Teknoloji Laboratuvarı     Translation: Umut Ergöz Translator Metaltek Teknoloji Laboratuvarı

Sources / References: 1-EN ISO 12944-2 -2017 Standard 2-EN ISO 12944-6-2017 Standard