3.4 Analysis of substance function and technical requirements on surface structure of
3.4.1.2 Process and reactor design
3.4.1.2.1 Mechanistic explanation of the Pretex® process
The uniqueness of the Pretex® coating and the corresponding process is based on the generation of the surface structure in form of hemispherical caps in combination of the adjustability of their size and distribution density on the surface. Importantly, the hemispherical caps grow from the chrome deposition itself which is achieved by massively intervening in the electro-crystallization of the chrome layer. The underlying effects were studied for years and are unique to the Pretex® process using chromium trioxide.
Sophisticated knowledge of the process’ electrochemistry and material technology enables a high degree of reproducibility and adjustability, allowing the production of adjustable hemispherical surface structures.
For applying the structure layer on top of the base chrome layer, the deposition potential, and correspondingly, the applied current density, must be increased stepwise. Depth (Δt) and height (Δu) can be adjusted separately. This decoupling allows the separate setting of the count (distribution density) and the size of surface structure elements. This kind of manipulation of the deposition mechanism of the chrome layer is unique to the Pretex®
process. Galvanically produced chrome layers crystallize as α-chrome and therefore in a body-centered cubic lattice. Process parameters applied in decorative and hard chrome plating allows chrome layers from hexavalent chromium electrolytes to form the preferred field-oriented texture type (5). In case of Pretex® depositions, this kind of crystallization occurs in the base layer, the top layer, and in the first part of the structuring phase. Due to the continuous stepwise increases in the deposition potential, the electro-crystallization switches during the application of the structured layer. The switch to another crystal form – the preferred non-oriented dispersion type – rapidly occurs at a specific point of the current slope and is dependent on the remaining parameters. Those crystalline effects were verified by X-ray diffraction in the course of the development of topocrom Systems AG.
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45 Figure 18: Strongly etched metallographic cross-section polish of the Pretex® multi-layer system (SEM) (left) and top view of the hemispherical Pretex® topography (right)
Figure 18 (left) shows the sharp-edged transition of the crystal orientation. The preferred non-oriented form grows, in contrast to the field-oriented texture type, in the form of hemispherical elements. The current density, in relation to other process parameters during the structuring phase, significantly exceed the values normally applied in electroplating. This is the only way to induce the aforementioned electro-crystallization effects and to produce the Pretex® specific texture. The required peaks and leaps of the deposition potential can only be induced using hexavalent chromium electrolytes, since they are insensitive to hydroxidic precipitations.
Please note that at such current densities, hydroxidic precipitations form in other electrolytes based e.g. on nickel or chromium(III) salts, due to the high amounts of hydrogen evolving at the cathode. The hydrogen evolvement shifts the pH inside the cathode film towards basic conditions and causes formation of precipitation products and incorporation into the chrome layer. These unwanted by-products impair the chrome coating and make it unusable as performance requirements cannot be fulfilled. Any attempt to reach current densities required for the Pretex® structure forming by using Cr(VI)-free electrolytes failed at the very early stage due to the aforementioned reasons.
This holds true not only for state-of-the-art commercialized electrolytes, but as well for specialized approaches. Therefore, the application of Cr(VI)-based electrolytes is, as of today, integral to the production of Pretex® coatings from a mechanistic point of view.
The forming of the hemispherical structures depends on the electric field. The topographic parameters are set by the current slope. Since Salzgitter requires the Pretex®
topographies to be homogenous, a distribution of the electric field on the respective working roll needs to be as homogeneous as possible. This is not possible with commonly applied chrome plating technologies (e.g. immersion bath), thus making the specific reactor technology mandatory for Pretex® chrome coatings. Furthermore, the homogeneity of the electric field enables the application of a homogenous layer thickness distribution and, correspondingly, an accurate to finished size coating. Accurate to finished size coating is required as mechanical post-treatment by grinding would destroy the topography and therefore is not possible. In turn, this is beneficial for material consumption and reduction of waste such as grinding sludge, etc.
3.4.1.2.2 Plant design and reactor technology
The Pretex® functional chrome plating of Salzgitter’s working rolls is conducted using a proprietary reactor technology, which is a development of the topocrom Systems AG (topocrom). It is the same plating system to apply the ‘conventional’ chrome coating (see
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46 Use 1). Although the Pretex® functional chrome coating is a multilayer system (base, structure and top layer), xxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx
The plating unit is a large-scale facility operating at multiple levels. The whole plating process is operated within closed reactors which are connected to external electrolyte storage tanks. The specific reactor design enables flexible adjustments to different requirements in geometry and size of the working rolls to be chrome plated. The process parameters are controlled automatically and adjusted for the individual topography, i.e.
‘conventional’ or Pretex® chrome coating. The reactor design is a well-proven technology, showing economic, ecological, and occupational health and safety advantages. The latter because the plating process is performed in an entirely closed system reducing the Cr(VI) exposure to workers to a minimum (see CSR for handling).
Furthermore, the Pretex® functional chrome plating process performed inside the reactor system ccccccccccccccccccccccccccccccccccccccccccccc. Therefore, the usage of this highly developed plating system contributes to the overall reduction of Cr(VI) used by Salzgitter in functional chrome coating of rolls compared to 'traditional' immersion bath processes.
In principle, the reactor technology is based on the circulation of the chromium electrolyte and electrolytic deposition of metallic chrome layers. The Cr(VI)-containing electrolyte is kept in separate containers and only pumped into the reactor when the plating process is initiated and the reactor is closed (see Figure 19 for illustration).
Figure 19: Illustration of reactor technology as closed-circuit system
Please note that Salzgitter uses the reactor technology also for the application of
‘conventional’ chrome coatings on their rolls by applying different process parameters. For further information on this please refer to Use 1.
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47 3.4.1.2.3 Description of Pretex® functional chrome plating process
xxxxxxxxxxx pre-treatment
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxx. However, the worn-down working rolls need to be prepared mechanically before re-application of the Pretex® functional chrome coating is possible.
At first, the old chrome coating is removed from the working roll by grinding leading to a rough surface finish. For further preparation, a xxxxxxxxxxx fine grinding step is applied for surface smoothening, removal of impurities and surface activation. Finally, the working roll is cleaned and checked xxxxxxxxxxxxxx for defects such as scratches, roundness, etc.
The xxxxxxx system for fine grinding, cleaning and surface testing is illustrated in Figure 20.
Figure 20: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Cr(VI)-based main plaiting process
The Pretex® functional chrome plating is a wet-in-wet process, xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxx at any time in the process chain. Before initiation, the part to be plated is brought into the reactor which, at this stage, does not contain Cr(VI)-containing plaiting solution. Afterwards, the reactor is closed and the plating process is initiated by automatic filling of the reactor chamber with Cr(VI)-containing electrolyte from the bottom inlets and application of a current. During the plating process the Cr(VI)-based electrolyte circulates continuously inside the reactor. The automated recirculation of the electrolyte from the reactor back into the electrolyte containers takes place via the spill-ways in the upper section of the reactor (see Figure 19). xxxxxxxxxxxxxxXxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxx
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48 For the plating process, an electrolyte containing xxxx g chromium trioxide per liter is used at a temperature of XX °C. As catalyst xxxxxxxxxxxx is added to the electrolyte in a concentration of xxxxxxxxx %. The process parameters, deposition potential and resulting current density, of the Pretex® plating procedure are adjusted to yield a custom surface topography required for the respective working roll application.
The metallic chrome coating layer is applied by electroplating based on the principle of electrolysis. During the electroplating process, the hexavalent chromium cations are reduced and build-up a metallic chrome coating layer (electrodeposition). The plating procedure forms a coherent metal coating on the part to be plated (either the direct substrate or the substrate with already chrome plated intermediate layers) by using the substrate as cathode and an inert anode (xxxxxxxxxxxxxxxxxxxxxxxxxxxx) and inducing an electrical current.
After the plating process is finished, the electrolyte is completely returned to the storage container. The chrome plated working roll is automatically rinsed inside the reactor with demineralized water, which directly flows to the storage container to balance evaporation losses. Until that step the process is entirely automated and no worker comes into contact with Cr(VI)-containing solutions. After opening the reactor, the pre-cleaned part is removed with the support of a lift. xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx. To ensure surface quality, the chrome coating on the working roll is inspected by different proprietary methods.
An illustration of the closed-circuit plating unit (reactor) is shown in Figure 19. Figure 21 shows the insertion of a working roll into the reactor. The reactor contains the anode, which is necessary for plating of a specific part, and the part to be plated itself. The cap, which functions as cathode, is attached to the part to be plated.
Figure 21: Closed-circuit plating unit used for Pretex® process
A working roll is sub-divided in three parts: two chrome free pins for the attachment in the roll stand and roll drive, and the chrome plated working surface (see Figure 22). Since only the working surface requires Pretex@ functional chrome plating, the reactor technology provides further beneficial attributes for Salzgitter because the capping mechanism (protective sleeves) for both mounting pins is incorporated in the system (see Figure 19).
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49 Figure 22: Working roll with functionalities: pins for roll stand mount and Cr-plated roll surface