ZOG seminar on corrosion from different perspectives

For surface coaters, it is clear that corrosion is more than just "rust". The annual damage caused by corrosion amounts to 3-4% of the gross domestic product. In Germany, the annual damage amounts to EUR 110-140 billion and worldwide to EUR 2.9 trillion [1, 2]. EUR [1, 2]. The interest was great and the seminar in the platinum hall of the fem (Research Institute for Precious Metals and Metal Chemistry) in Schwäbisch Gmünd was fully booked under the applicable Corona rules. Over two days, the participants were able to find out about the causes and consequences of corrosion as well as corrosion prevention, corrosion testing and corrosion protection. The fem opened its laboratories for an insight into the practice of corrosion testing.

Corrosion is derived from the Latin word "corrodere" and means to decompose, eat away, gnaw [3]. The definition in DIN ISO 8044:2015 is: "Corrosion is the physicochemical interaction between a metal and its environment that leads to a change in the properties of the metal and that can lead to significant impairment of the function of the metal, the environment or the technical system of which it forms a part." Behind this complicated description lies a simple process:

• the process = corrosion reaction

• the result = corrosion phenomenon

• the damage = corrosion damage

The cause of corrosion is a law of nature, according to which all elements strive towards the most energetically favorable state. Base metals occur naturally in ores, mostly as oxides. They also strive for this state after processing. Unprotected metals therefore react with their environment, with water and oxygen, in order to reach their energetically more favorable state. The tendency to corrode can be seen from the electrochemical voltage series. Negative potential stands for a higher tendency to corrode. It is therefore logical that precious metals are found on the positive side of the voltage series.

The issue of corrosion came to the fore again when the use of chromium VI was banned. Alternative coating systems had to be found and the structure of the coatings had to be reconsidered. In the seminar, the causes of corrosion and the possibilities of measuring and preventing corrosion were examined using many examples. Table 1 lists the known types of corrosion.

Fig. 1: Density of field lines and layer thickness distribution [4]The first goal is to avoid corrosion. The possibility of manufacturing products directly from corrosion-resistant materials is limited for technical and economic reasons, so that a corrosion-protective layer is required. Various electroplating methods are used to create a layer structure that provides the best corrosion protection. In addition to the deposition of pure elements, the deposition of alloys is used for this purpose. Table 2 lists a selection of pure elements and alloys that are electroplated to suit the base materials and product requirements. In addition to the layer structure, an electroplating-compatible design and production leads to more corrosion protection. Depending on the density of the electric field lines, there are areas of high and low current density on a component. The layer structure is shown schematically in Figure 1 as a function of the field line density. Taking this physical principle into account, a uniform coating can be achieved in the design of components by avoiding undercuts, blind holes or holes in the geometry, for example, or at least reducing them to a minimum.

Fig. 2: Influencing variables from the coating process [4]

Fig. 3: Mass-produced parts: optimized part geometry [4]

Further influencing factors result from the coating process itself, see Figure 2, and the contacting of the parts. Larger parts are electroplated on suitable racks, on which they are guided through the process from pre-treatment to drying. A good coating result is achieved when the parts are firmly contacted, a uniform field line distribution is ensured, bubbles can escape unhindered and the rack space is optimally utilized from an economic point of view. Bulk and small parts are usually coated in drums. The coating thickness distribution can be optimized with simple considerations during part design, examples of which are shown in Figure 3.

Table 2: Metals and metal alloys that are electroplated [4]

The corrosion protection of steel is still of great importance. Coatings of zinc, zinc-nickel and now also chromium from Cr (III) processes play a role, which are given topcoats, sealed or passivated depending on the requirements.

• Topcoats: Coating thicknesses > 3 µm; e.g. thin-film or stove enamels, CDP coatings

• Sealers: < 2 µm; e.g. inorganic or organic or mixed system of both

• Passivations: 0.05-1 µm; conversion coatings made from e.g. Cr (III) or Co (II) solutions

Sealants and passivations are usually applied in a wet-on-wet process directly after the coating process. They are selected depending on the application of the component and the coating applied and they

• improve the optical properties of the surfaces

• increase corrosion protection and resistance in the salt spray test

• improve scratch resistance and anti-fingerprint properties

• protect against mechanical stress

• enable the targeted adjustment of friction and sliding properties.

Fig. 4: Example of a test chamber and possible test conditions [4]An important component in the prevention of corrosion are test methods that can be used to quickly obtain information about the corrosion properties of products, components and surfaces from standardized laboratory setups. They are also used for quality monitoring in ongoing processes or after process changeovers and for quality control during the development or optimization of protective systems for their release.

Free weathering provides a realistic statement. Its main disadvantages are the duration of 3-10 years and the limited validity, which relates to the area and the conditions of the free weathering. Nevertheless, outdoor weathering tests are carried out for long-term studies in order to investigate corrosion behavior in a natural environment. Laboratory tests lead to results much faster and take place under standardized conditions, which ensures comparability and thus qualifies them for quality control.

In laboratory tests, a distinction is made between

• Corrosion and corrosion cycle tests

• Corrosion climate change tests and special tests

• Climate and climate change tests

There are a number of DIN regulations for all test procedures, from which the appropriate ones for the problem are selected if they are not specified, for example for automotive parts.

Fig. 5: Corrosion test procedure in the laboratory [4]To ensure that the tests lead to the desired comparable results, the DIN regulations must be adhered to exactly. In particular, care must be taken to ensure that the parts in the test chamber do not influence each other and that the time sequences, temperature or additives in the test solutions are precisely adhered to. Some tests require the introduction of an artificial defect beforehand. A test chamber with possible test conditions is shown in Figure 4. In addition, the chambers must be subjected to functional tests at regular intervals using standardized test panels and predetermined evaluation. To evaluate the test specimens, the condition before and after the test is documented and evaluated using tabulated specifications, e.g. on the degree of blistering, degree of rust, delamination, e.g. on the artificial defects, degree of flaking or according to customer specifications. Figure 5 shows a schematic diagram of the laboratory test procedure. After this theoretical insight into laboratory testing procedures, the participants had the opportunity to view test chambers in the fem's laboratories and receive answers to any questions that remained unanswered.

After so much theory, it was interesting to learn about some examples from fem's day-to-day damage analysis work. In addition to steel, aluminium is a widely used material that is anodized to protect it from corrosion and thus receives an anodized layer (ELOXAL = electrolytic oxidationof aluminium) or is coated with a layer of paint. Figures 6 and 7 show examples where the base material has pores and cracks, as a result of which the anodized layer cannot provide the necessary protection, or corrosion occurs on a damaged lacquer layer.

Fig. 6: Case study of corrosion on window profiles, pitting corrosion on an anodized layer [4]

Fig. 7: Corrosion caused by road salt [4]

In the course of the process to ban Cr (VI), the PVD process (PVD = "Physical Vapor Deposition") has come to the fore as an alternative process for both corrosion protection and decorative purposes. Against this background, a comparison of the PVD process with the electroplating process is a good idea. The main differences are the aggregate states in which the processes are carried out. Electroplating is carried out in aqueous electrolyte solutions or, in less common cases, in aprotic solutions. PVD takes place in the gas phase under reduced pressure. This is reflected in the range of materials that can be deposited in both processes (see Table 3) and the associated coating properties.

Tab. 3: Typical coating systems [4]

In the galvanic process, dense and crystalline layers of 0.1-1000 µm are deposited. The layers mainly cause tensile stresses, but compressive stresses or stress-free layers are also possible. The leveling can be specifically adjusted, but only the respective metal colors and black are possible. Color and hardness can be influenced with alloying elements. Metals that are resistant in the electrolyte as well as plastics, ceramics and, to a limited extent, textiles can be coated after the conductivity has been adjusted. Extensive pre-treatment - degreasing, pickling and activation - is required. The electroplating process is relatively simple, enables selective coatings and, depending on the geometry of the parts, fabric or electrolyte movement, more or less uniform coating thicknesses are deposited. Waste water treatment is required.

In the PVD process, layer thicknesses of 0.1 - approx. 10 µm are achieved, hard material layers fall into the range of approx. 3 µm. Crystalline and amorphous layers with mostly high compressive stress are achieved, which are characterized by high hardness. A wide range of colors can be set, but no leveling is possible and defects often occur in the layers. Materials that do not gas in a vacuum can be coated: Metals, hard metals, ceramics, many plastics, glasses and, to a limited extent, textiles. Passive layers must be removed before coating, which can usually be carried out in the same process. With relatively sophisticated plant technology and an energy-intensive vacuum process, hardly any waste is produced. No substrate conductivity is required, and fabric movement, usually by rotation, is necessary for uniform coating.

Finally, a comparison of the advantages and disadvantages of the two processes shows that they exist side by side. As shown in Table 4, the advantages of one process are the disadvantages of the other and vice versa.

Table 4: Electroplating technology - PVD / advantages and disadvantages compared [4]

In terms of corrosion protection, the very hard and decorative coatings are corrosion-resistant in themselves. Due to their other properties, they do not protect the base material from corrosion. Coating systems consisting of galvanic and PVD coatings also provide good corrosion protection. Only in the decorative area is the PVD process seen as a competitor to the electroplating process.

[1] Report of the Interdisciplinary Research of Dechema, 22.04.2020, Dr. Kathrin Rübberdt, idw - Informationsdienst Wissenschaft, https://www.innovations-report.de/fachgebiete/interdisziplinaere-forschung/weit-mehr-als-rost-korrosion-geht-alle-an/
[2] "Korrosion und die Folgen", Dessauer Korrosionsschutz GmbH, Zum Gänsewall 9, 06844 Dessau-Roßlau, https://www.korrosionsschutz-dessau.de/korrosionsschutz/korrosion-folgen.html
[3] Wikipedia, https://de.wikipedia.org/wiki/Korrosion
[4] ZOG seminar documents

Speakers:

Erich Arnet, Z.O.G. Managing Director
Oliver Daub, Dr.-Ing.Max Schlötter GmbH & Co. KG
Stefan Funk, fem Forschungsinstitut Edelmetalle + Metallchemie
Herbert Kappel, fem Research Institute Precious Metals + Metal Chemistry
Ralph Krauß, Dr.-Ing.Max Schlötter GmbH & Co. KG
Alexander Pfund, fem Research Institute for Precious Metals + Metal Chemistry
Dr. Christof Langer, fem Research Institute Precious Metals + Metal Chemistry
Robert Mayerhofer, Mercedes Benz
Dr. Detlef Stoeckert, Dörken Coatings GmbH & Co. KG