Metals in dip coating baths References
Phosphatizing of metal surfaces using an atline analysis system

With an estimated worldwide turnover of more than $ 500 million, phosphatizing is the most important metal pretreatment process. ProcessLab is a customizable modular atline analysis system, that records, controls and documents all important analytical bath parameters of the entire phosphatizing process.
Frank Portala, Alfred Steinbach, Franz Müller, Michael Feige and Gerhard Kirner
The phosphatizing process produces a hard, electrically non-conducting surface coating that adheres tightly to the underlying metal. This layer protects the metal from corrosion and improves the adhesion of paints and organic finishes to be subsequently applied.

The basic phosphatizing process consists of the etching reaction and the formation of the surface coating. After thorough degreasing and rinsing of the metal workpieces, the phosphoric acid removes interfering surface-bound metal oxides and increases surface roughness. Subsequently, the alkali phosphates react with the previously generated metal ions at the surface of the workpiece, forming a layer of insoluble tertiary metal phosphates. With a coating thickness smaller than 1 μm, iron phosphate coatings provide a basic corrosion protection and are intended for interior use under controlled environments. In contrast, the addition of metal cations such as Zn2+, Mn2+ and Ca2+ to the phosphatizing bath results in the formation of very resistant mixed-metal phosphates with a coating thickness between 7 and 15 μm. Due to a modified crystal structure these layers are perfectly suited for outdoor use or rough interior environments. Both iron and zinc phosphatizing occur by the same mechanism. However, the latter process is characterized by the preferential incorporation of zinc cations. The iron cations at the metal surface hardly contribute to the metal phosphate formation. A first-class corrosion protection is achieved by the supplementary addition of further cations (e.g. Ni2+) to the phosphatizing bath, resulting in even better surface properties.

Phosphatizing of metal surfaces

The work pieces and car bodies to be phosphatized run through different baths where they are sequentially degreased, rinsed, activated, phosphatized and
again cleaned. A single analysis system — ProcessLab — specially adapted to the particular requirements of the phophatizing process, controls all bath-relevant parameters.

Atline analysis system

In an atline system the analytical unit is positioned directly at the process line allowing for on-site determination of the parameters required. Manual sampling occurs at different sampling points of the production line. This proximity combined with the modular concept — all analytical modules are accommodated in a single housing impervious to dust and splashes — allows efficient process control through the determination of all bath-relevant parameters. The user-friendly software and the TFT screen provide straightforward and easy operation. Additionally, a barcode reader guarantees unambiguous sample identification and makes laborious manual data entering obsolete. Automation of analytical tasks allows to easily manage high sample throughputs, at the same time enhancing repeatability. The presented ProcessLab atline analysis system records, controls and documents the important process parameters. The analytical data set is stored in a database and can be processed internally or transferred to a process control system.

Conclusions

The metal surfaces are treated using strictly defined process steps in different degreasing, cleaning, rinsing, activation and phosphatizing baths. The various bath parameters have to be closely monitored as they determine to a large extend the quality of the coating produced. The parameters determined in the cleaning, degreasing and rinsing baths are pH value, conductivity plus free and total alkalinity, while the phosphatizing bath is analyzed for free and total acids, accelerators, zinc and fluoride. The described ProcessLab atline analysis system monitors, records and documents the important analytical parameters of the entire phosphatizing process. The combination of the analytical methods involved as well as the intuitive operation via the user-friendly interface allow for complete process control. The analytical functions are supplemented by the integrated software that offers numerous possibilities for data processing and documentation of the measured values.

The atline analysis system described meets all requirements regarding process monitoring and documentation.n

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