Product category:
Liquid Analysis: Titration, HPLC, IC
News Release from: Applikon Analytical | Subject: Phosphating analysis at-line
Edited by the Processingtalk Editorial
Team on 06 December 2007
At-line analysis for Phosphating process
control
Metrohm demonstrates how to monitor a complete process line for the phosphatising (or phosphating) of metal surfaces using an Applikon Analytical at-line analysis system
With an estimated worldwide turnover of more than 500 million US dollars, phosphating is probably the most important and widely used metal pre-treatment process The phosphating (phosphatizing or phosphatising) process produces a hard, electrically non-conducting surface coating that adheres tightly to the underlying metal
This article was originally published on Processingtalk on 29 Nov 2007 at 8.00am (UK)
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This layer protects the metal from corrosion and improves the adhesion of paints and organic finishes to be subsequently applied.
The basic phosphating process consists of the etching reaction and the formation of the surface coating.
After the thorough degreasing and rinsing of the metal work pieces, the phosphoric acid removes interfering surface-bound metal oxides and increases the surface roughness.
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Subsequently the alkali phosphates react with the previously generated metal ions at the surface of the work piece forming a layer of insoluble tertiary metal phosphates.
With a coating thickness smaller than 1 micron, 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 phosphating bath results in the formation of very resistant zinc phosphates with a coating thickness between 7 and 15microns.
Due to a modified crystal structure these layers are perfectly suited for outdoor use under hostile environments.
Both iron and zinc phosphating occur by the same mechanism.
However, the latter process is characterised 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 phosphating bath, resulting in even better surface properties.
* Phosphating of metal surfaces.
The work pieces, such as car bodies, to be phosphated, run through different baths where they are sequentially degreased, rinsed, activated, phosphated and again cleaned.
A single analysis system - ProcessLab - specially adapted to the particular requirements of the phophating process, controls all bath-relevant parameters.
ProcessLab is an at-line analysis system: in an at-line system the analytical unit is positioned directly at the process line allowing for an on-site determination of the parameters required.
Manual sampling occurs at various 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 an efficient process control through the determination of all bath-relevant parameters.
The user-friendly integrated operation software and the TFT screen offer straightforward and easy handling.
Additionally, a barcode reader guarantees unambiguous sample identification and makes laborious manual data entry obsolete.
Automation of analytical tasks allows to easily manage high sample throughputs, at the same time enhancing repeatability.
The presented ProcessLab at-line 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.
* Analytical parameters.
The at-line analysis system controls and documents the important bath-relevant parameters.
The different baths in the Phosphating process are listed below, with the parameters that are required to be monitored in each bath:.
1) Degreasing 1: Conductivity; pH value; Free alkalinity; Total alkalinity.
2) Degreasing 2: Conductivity; pH value; Free alkalinity; Total alkalinity.
3) Rinsing 1: Conductivity; pH value; Total alkalinity.
4) Rinsing 2: Conductivity; pH value; Total alkalinity.
5) Activation: Conductivity; pH value; Total alkalinity.
6) Phosphating: Free acid; Total acid; Nitrite or H2O2; Zinc; Fluoride.
7) Rinsing 3: Conductivity; pH value; Total acid.
8) Rinsing 4: Conductivity; pH value; Total acid.
9) Final Rinsing: Conductivity; pH value.
The parameters monitored are important indicators as follows:.
(a) Conductivity (baths 1, 2, 3, 4, 5, 6, 7, 8 and 9).
* allows to draw important conclusions regarding the ionic strength in the activation bath and the extent of contamination in the rinsing baths.
* monitors the cleaning process in the rinsing baths.
* is determined directly in the sample vessel immediately after sampling.
(b) pH value (baths 1, 2, 3, 4, 5, 7, 8 and 9).
* is a crucial parameter in the degreasing, rinsing and activation baths.
* is determined in the sample vessel prior to the determination of other analysis parameters.
(c) Free and total alkalinity (baths 1, 2, 3, 4 and 5).
* are determined by titration with hydrochloric acid; the first EP (pH between 3.7 and 4.0) to the total alkalinity, respectively.
(d) Free and total acid (baths 6, 7 and 8).
* provide information concerning the progress of the phosphating process and the total amount of metal phosphate formed on the surface.
* are determined by titration with NaOH: the first EP (pH between 4.5 and 4.7) corresponds to the free acid and the second EP (pH between 8.7 and 9.0) to the total acid content.
(e) Nitrite or hydrogen peroxide as accelerators (bath 6).
* accelerate the process of phosphating and thus increase sample throughput.
* are determined by redox titration.
(f) Zinc (bath 6).
* is crucial for the formation of metal phosphates (phosphophyllite, Zn2Fe(PO4)2; hopeite Zn3(PO4)2) on the surface.
* is quantified by titration with EDTA using an ion sensitive indicator electrode.
(g) Fluoride (bath 6).
* increases the reactivity of the phosphating process and masks interfering aluminium ions by forming [AIF6]3- complexes.
* is determined in a separate reaction vessel to increase sample throughput.
CONCLUSIONS.
The metal surfaces are treated using strictly defined process steps in different degreasing, cleaning, rinsing, activation and phosphating 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 phosphating bath is analySed for free and total acids, nitrite or hydrogen peroxide, zinc and fluoride.
The described ProcessLab at-line analysis system controls, records and documents the important analytical parameters of the entire phophating process.
The combination of the analytical methods involved as well as the intuitive handling via the well-arranged user interface allow for complete process control.
The analytical functions are supplemented by the integrated operation software that offers numerous possibilities for data processing and documentation of the measured values.
The at-line analysis system described meets all requirements regarding process monitoring and documentation.
For further information on the Metrohm ProcessLab please contact Nick Prince at the Metrohm UK office. Request a free brochure from Applikon Analytical ...
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