Automotive Sector: Online Measurement and Process Monitoring in Zinc Phosphating Baths

Metals are fundamental building blocks in production across many industries and are now used on a large scale in sectors such as automotive and construction. Despite their advantageous properties—including strength, ease of processing, and cost-effectiveness—metals are susceptible to corrosion, which can cause significant economic losses in production. Chemical conversion coating is among the most widely used techniques to prevent corrosion in this context. In the automotive sector, the chemical conversion coating process is a prerequisite before painting automotive bodies. Zinc phosphating is the accepted process in most automotive paint shops worldwide. In phosphating processes, continuous monitoring of relevant parameters and rapid feedback are critical to maintaining high-quality coated surface layers, ensuring homogeneous and corrosion-resistant coatings.

Phosphating Processes in the Automotive Sector

The phosphating process produces a hard, electrically non-conductive surface coating that bonds strongly to the underlying metal. This layer protects the metal from corrosion and improves the adhesion performance of paints and organic finishes applied in subsequent stages. A basic phosphating process consists of an etching reaction and surface coating formation steps. Following thorough degreasing and washing of the metal parts to be processed, metal oxides bonded to the surface are cleaned using phosphoric acid and surface roughness is increased. Subsequently, alkali phosphates react with metal ions previously formed on the surface of the parts to be processed, creating a layer composed of insoluble tertiary metal phosphates. Iron phosphate coatings with a thickness thinner than 1 µm provide basic corrosion protection and are typically used indoors in controlled environmental conditions. Conversely, the addition of metal cations such as Zn²⁺, Mn²⁺, and Ca²⁺ to the phosphating bath leads to the formation of highly resistant mixed metal phosphates with coating thicknesses ranging from 7–15 µm. Due to their modified crystal structures, these layers offer excellent compatibility for outdoor and/or harsh indoor environmental use. Iron and zinc phosphating essentially operate on the same mechanism, though zinc phosphating is characterized by the specific inclusion of zinc cations in the process. In this process, the contribution of iron cations on the metal surface to metal phosphate formation is minimal. With the addition of complementary cations (e.g., Ni²⁺) to the phosphating bath, first-class corrosion protection with significantly advanced surface properties can be achieved. [caption id="attachment_106935" align="aligncenter"] Figure 1. Phosphating process[/caption]  

Phosphating Process Steps

In a typical phosphating process, the parts to be phosphated or automobile bodies pass through different baths containing degreasing, washing, activation, phosphating, and rinsing and cleaning steps. Although the sequence, number, and use preference of these steps in the phosphating process may vary depending on process conditions, the general workflow is as follows: [caption id="attachment_106938" align="aligncenter"] Figure 2. Phosphating process operation steps[/caption]   • Degreasing baths (1,2): Oil, grease, and wax residues are removed from the metal surface. • Rinsing baths (3,4): Degreasing chemicals are removed with water. • Activation bath (5): The metal surface is activated. • Phosphating bath (6): A surface layer composed of insoluble heavy metal tertiary phosphates is formed. • Rinsing baths (7,8, and 9): Acid residues, soluble salts, and all non-adhering particles on the metal are removed.

Analytical Parameters

Close monitoring of key analytical parameters in phosphating process baths is critical for the sustainability of coating quality. In this context, close monitoring of these analytical parameters, which provide specific indicators for each stage of operation, is essential: [caption id="attachment_106941" align="aligncenter"] Figure 3. Phosphating process analytical parameters[/caption]  

Online Analysis in Phosphating Processes

Analyses performed in baths during phosphating processes can cause significant production quality and time losses due to manual sampling before direct measurements or titrations and errors occurring in manually conducted analysis processes. When considering all factors that may be sources of potential errors—such as operator variation, experience level, sample volume taken, equipment/accessory cleanliness, analysis frequency, etc.—the advantages that an online analysis platform capable of enabling automatic sampling, fully automatic analysis, and result reporting become evident. In online phosphating line analysis systems, bath parameter measurements are performed through automatic sampling of the sample and completion of all subsequent analytical processes without requiring any manual intervention. Through automatic transfer of the obtained analysis results to PLC control systems, rapid intervention in the process is made possible, providing significant advantages particularly in the automotive industry. As Metrohm, based on the experience we have gained to date regarding phosphating bath analyses, our accumulated knowledge, our rich application range that forms the foundation of our global leadership position in the titration field, and within our commitment to the automotive sector, we present our fully automatic 2035 Zinc-Phosphate Bath Online Analyzer system solution, designed specifically for the automotive sector and compliant with Industry 4.0 generation requirements. [caption id="attachment_106944" align="aligncenter"] Figure 4. 2035 Zinc-Phosphate Bath Online Analyzer system[/caption]  

Equipment and Software Used

The 2035 Zinc-Phosphate Bath Online Analyzer, with features such as fully automatic analysis, automatic cleaning procedures, precise sampling, electrode calibration, and transfer of results to the PLC system specifically for sequential analysis and control of parameters monitored during the phosphating step, creates a complete platform. [caption id="attachment_106945" align="aligncenter"] Table 1. 2035 Zinc-Phosphate Bath Online Analyzer system components[/caption]  

Sampling

In the 2035 Zinc-Phosphate Bath Online Analyzer, the bath sample is brought directly to the reaction vessel via a closed sampling line. Thus, unlike manual sampling, a representative sample taken from the correct sampling point that does not contact the external environment reaches the analyzer. With the 2035 system, sampling can be performed from a single point, or the system can be designed with a multi-channel option to enable sampling from multiple points.

Online Potentiometric Titration

The titration methods applied after automatic sampling in the 2035 Zinc-Phosphate Bath Online Analyzer are compatible with methods from our laboratory-type automatic titrators that have been used by our customers in their facilities for phosphating bath analysis purposes over many years. Each analysis step applied in the customer laboratory is programmed with the same care and integrated into the online system, making it possible to compare results obtained in the laboratory environment. The system enables monitoring of the following parameters in the zinc-phosphate bath: • pH and temperature, • Free acid/total acid, • Fluoride/SiF₆, • Zinc content. [caption id="attachment_106965" align="aligncenter"] Figure 5. Typical zinc-phosphate bath titration curve[/caption]  

Analysis Results

The accurate and repeatable results observed as a result of online analyses performed using the 2035 Zinc-Phosphate Bath Online Analyzer demonstrate that the system has been successfully integrated into the phosphating bath. The system's fully automated nature brings numerous advantages. [caption id="attachment_106967" align="aligncenter"] Figure 6. Comparison of manual monitoring and online analysis in zinc-phosphate baths[/caption]   In fully automatic online potentiometric analyses performed compared to manual analyses, errors such as faulty sampling, failure to detect equivalence points in titration-based analyses, and incorrect and non-repeatable results can be prevented. Since all procedures—such as sample withdrawal from the bath, sampling, chemical additions, sample analysis, and result recording—are automatically provided under the 2035 system framework, measurement errors, differences between operators, and analysis time inconsistencies are eliminated. Hands-free continuous analysis results in reduced labor requirements while increasing operator safety. As bath measurements become more accurate, precise, and efficient, process disruptions can be monitored in real time, quality improvements are achieved, and production rework problems are minimized. [caption id="attachment_106970" align="aligncenter"] Figure 7. Advantages of online bath monitoring[/caption]   The 2035 Zinc-Phosphate Bath Online Analyzer system transfers all data—including ongoing analyses, obtained results, alarms, etc.—to PLC or other requested data systems via data transmission infrastructures such as 4–20 mA, Modbus TCP, Profinet, Profibus, and Teamviewer that are integrated into its hardware, enabling remote continuous process monitoring and immediate intervention in the event of any disruption. In addition, if needed, a remote connection option is available to receive quick support from the Metrohm Process Analytics Team.

Complete Phosphating Line Monitoring

The 2035 Zinc-Phosphate Bath Online Analyzer system generally establishes a multi-parameter baseline level, particularly for the zinc-phosphate bath. When simultaneous measurement of other parameters in other baths on the phosphating line—such as degreasing, rinsing, and activation—alongside the zinc-phosphate bath is requested, or when increased analysis frequency is desired, the 2035 Zinc-Phosphate Analyzer system can be expanded in hardware and software, or if needed, a higher-level model or a custom-designed online analyzer system capable of reaching the entire line level can be developed.

Conclusion

Metrohm Process Analytics has over 40 years of global experience in online process analysis. In the automotive sector alone, with more than 70 installed analyzer system units in zinc-phosphate line processes in the EMEA region, we continue to serve all our customers—particularly leading automotive giants—with confidence and to present our global experience in the sector. As the Metrohm Turkey Process Analytics Team, we can provide comprehensive services spanning a wide range—from project consulting, engineering, and sales phases; to analyzer installation, commissioning and training, system maintenance, accessory and spare parts supply, support, service, and repair processes. Working in integration with the Metrohm Process Analytics EMEA Regional Center, our local operations ensure we are by your side for all your online analysis needs. [caption id="attachment_106974" align="aligncenter"] Figure 8. 2045 Full Phosphate Line Analyzer[/caption]

Sources

1. Metrohm Application Poster 80006082—Monitoring of a complete process line for the phosphatizing of metal surfaces using an at-line analysis system. 2. Metrohm Application Bulletin No. 289e Monitoring parameters in a phosphatizing process (pH, conductivity, acidity, alkalinity, fluoride and zinc). 3. Metrohm Technical Article TA-003EN Metals in dip coating baths – phosphatizing of metal surfaces using an at-line analysis system.