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Infrared spectroscopy as a means of assessing the phosphate coating on grain oriented silicon steels
Journal of Magnetism and Magnetic Materials 19 (1980) 257-259 © North-Holland Publishing Company
INFRARED SPECTROSCOPY AS A MEANS OF ASSESSING THE PHOSPHATE COATING ON GRAIN ORIENTED SILICON STEELS C. JONES and P. BECKLEY British Steel Corporation, Welsh Division, UK
The magnesium metaphosphate content of coating material from grain-oriented silicon steels has been measured using infrared spectroscopy. A non-destructive technique for direct measurement of this compound on the sheet has been examined. The effect of heat treatment on the magnesium metaphosphate content of the coating has been investigated.
Technique (a) was calibrated for magnesium metaphosphate using discs prepared from this compound and magnesium orthosilicate.
1. Introduction During the processing of grain-orientated silicon steel, a forsterite (magnesium orthosilicate) "glass" is produced on its surface. This often forms the basis of the final insulating coating. One method of sealing this layer, to improve its insulating properties, is to treat it with a phosphate-containing solution prior to the final heat treatment [ 1]. Control of the composition of such coatings is important and this project investigates the possibility of determining, quantitatively, the condensed phosphate component of the coating, using infrared spectroscopy.
3. Chemical analysis The need to correlate the data from the two infrared techniques led to the development of a total phosphorus procedure based on a single reagent molybdenum blue finish [2]. As this method determined orthophosphate, it was necessary to dissolve and hydrolyse the condensed phosphates present in the coatings. The phosphate layer was dissolved from the sheet in 10% (m/v) sodium hydroxide solution and from the coating powder in hot concentrated sulphuric acid. Hydrolysis to orthophosphate was completed in dilute sulphuric acid.
2. Infrared spectroscopy A Perkin Elmer 377 spectrometer was employed for the examination of insulating coatings using two techniques: (a) a conventional transmission technique in which sample powder was dispersed in a compressed potasSium bromide disc; and (b) a reflection technique in which a coated sample of Epstein strip width was examined, using a Pye Unicam Specular Reflectance assessory. Laboratory-prepared samples of possible coating compounds were employed in technique (a) to give standard spectra to aid the identification of the compounds in the coating and to calibrate the technique for the quantitative determination of magnesium metaphosphate in the coatings.
4. Sample selection and preparation One sample of Epstein strip dimensions was cut from each at a series of production coils. Selected strip was cut in half and the unwanted coating removed from the reverse side of the retained half. A 30 X 25 mm 2 coupon was cut from each strip and the coating was removed from the remainder by tightly coiling the strip, previously wetted with methanol, around a 5 mm diameter mandrel. The powder was washed from the loosened coil into methanol before separation by centrifugation. 257
258
C. Jones and P. Beckley / A ssessment of phosphate coatings
5. Heat t r e a t m e n t
1.2 0
Strips were cut in half and one half of each sample was retained while the remaining halves were split into two sets of five. Each set was heat treated at 800°C for 30 min, one set in air and the other in 4% hydrogen in nitrogen. The samples were allowed to cool in their respective atmospheres before being removed from the furnace. The original and the heat treated coatings were analysed by the above methods.
1.0
v
r-
0.8 J~
<¢
0.6 v
v v
0.4
v
6. Results and discussion v
The transmission spectrum of a typical finished coating is shown in fig. 1. Some changes in band intensities and slight drifts in band wavelengths occur in the reflection spectrum. Two metaphosphate bands between 700 and 800 cm - I lie in a low absorbance region of the forsterite spectrum and are therefore suitable for quantitative analysis. One of them, the 742 cin - I band, was chosen. The magnesium metaphosphate content of a coating powder, its total phosphorus and that of the corresponding sheet were used to calculate the metaphosphate value for the sheet coating. Fig. 2 shows these values plotted against their respective absorbance values as measured using the reflection technique. It can be seen that a relationship exists between metaphosphate absorption and actual content for the reflection technique although there is some scatter about the line.
g
I-
1500
1000 Wavenurnber,
500 cm -I
Fig. 1. Transmission spectrum for a typical coating.
v
v
v
v v
v
v
0.2
40 80 120 Metaphosphate pg P/cm 2
Fig. 2. Plot of absorbence against metaphosphate content.
In the coatings examined between 15 and 97% of the total phosphorus content was present as magnesium metaphosphate. Attempt~ were made to identify bands which appear in the low metaphosphate coating spectra but no firm identification could be made. Table 1 shows the results of heat treatment in two different gas mixtures. In all cases a decrease in
Table 1 The effect of heat treatment in different gas mixtures Sample identity
10(:]
/
v
1 2 3 4 5 6 7 8 9 10
Heat treatment atmosphere
air air air air air 4% H2 in N2 4% H2 in N2 4% H2 in N2 4% H2 in N2 4% H2 in N2
% Phosphorus as metaphosphate before heat treatment
after heat treatment
90 85 88 57 74 75 90 84 82 83
65 72 76 23 71 72 74 71 52 11
C. Jones and P. Beckley / Assessment o f phosphate coatings
metaphosphate content of the coating occurred. In addition some loss of phosphorus took place.
sion of European Communities, European Coal and Steel Community. The authors wish to thank the Director of B.S.C. Associated Products Group and the Commission for permission to publish this paper.
7. Outlook The non-destructive infrared method described shows some promise for the commercial estimation of the coating density of magnesium metaphosphate on steel sheet.
Acknowledgements This work was carried out as part of Research Contract 6210.KL/8/801 supported by the C ommis-
259
References [1] United States Patent, No. 3 840 3"18(1974). [2] BS 1016: Part 9: 1971, British Standards Institution (1971).
INFRARED SPECTROSCOPY AS A MEANS OF ASSESSING THE PHOSPHATE COATING ON GRAIN ORIENTED SILICON STEELS C. JONES and P. BECKLEY British Steel Corporation, Welsh Division, UK
The magnesium metaphosphate content of coating material from grain-oriented silicon steels has been measured using infrared spectroscopy. A non-destructive technique for direct measurement of this compound on the sheet has been examined. The effect of heat treatment on the magnesium metaphosphate content of the coating has been investigated.
Technique (a) was calibrated for magnesium metaphosphate using discs prepared from this compound and magnesium orthosilicate.
1. Introduction During the processing of grain-orientated silicon steel, a forsterite (magnesium orthosilicate) "glass" is produced on its surface. This often forms the basis of the final insulating coating. One method of sealing this layer, to improve its insulating properties, is to treat it with a phosphate-containing solution prior to the final heat treatment [ 1]. Control of the composition of such coatings is important and this project investigates the possibility of determining, quantitatively, the condensed phosphate component of the coating, using infrared spectroscopy.
3. Chemical analysis The need to correlate the data from the two infrared techniques led to the development of a total phosphorus procedure based on a single reagent molybdenum blue finish [2]. As this method determined orthophosphate, it was necessary to dissolve and hydrolyse the condensed phosphates present in the coatings. The phosphate layer was dissolved from the sheet in 10% (m/v) sodium hydroxide solution and from the coating powder in hot concentrated sulphuric acid. Hydrolysis to orthophosphate was completed in dilute sulphuric acid.
2. Infrared spectroscopy A Perkin Elmer 377 spectrometer was employed for the examination of insulating coatings using two techniques: (a) a conventional transmission technique in which sample powder was dispersed in a compressed potasSium bromide disc; and (b) a reflection technique in which a coated sample of Epstein strip width was examined, using a Pye Unicam Specular Reflectance assessory. Laboratory-prepared samples of possible coating compounds were employed in technique (a) to give standard spectra to aid the identification of the compounds in the coating and to calibrate the technique for the quantitative determination of magnesium metaphosphate in the coatings.
4. Sample selection and preparation One sample of Epstein strip dimensions was cut from each at a series of production coils. Selected strip was cut in half and the unwanted coating removed from the reverse side of the retained half. A 30 X 25 mm 2 coupon was cut from each strip and the coating was removed from the remainder by tightly coiling the strip, previously wetted with methanol, around a 5 mm diameter mandrel. The powder was washed from the loosened coil into methanol before separation by centrifugation. 257
258
C. Jones and P. Beckley / A ssessment of phosphate coatings
5. Heat t r e a t m e n t
1.2 0
Strips were cut in half and one half of each sample was retained while the remaining halves were split into two sets of five. Each set was heat treated at 800°C for 30 min, one set in air and the other in 4% hydrogen in nitrogen. The samples were allowed to cool in their respective atmospheres before being removed from the furnace. The original and the heat treated coatings were analysed by the above methods.
1.0
v
r-
0.8 J~
<¢
0.6 v
v v
0.4
v
6. Results and discussion v
The transmission spectrum of a typical finished coating is shown in fig. 1. Some changes in band intensities and slight drifts in band wavelengths occur in the reflection spectrum. Two metaphosphate bands between 700 and 800 cm - I lie in a low absorbance region of the forsterite spectrum and are therefore suitable for quantitative analysis. One of them, the 742 cin - I band, was chosen. The magnesium metaphosphate content of a coating powder, its total phosphorus and that of the corresponding sheet were used to calculate the metaphosphate value for the sheet coating. Fig. 2 shows these values plotted against their respective absorbance values as measured using the reflection technique. It can be seen that a relationship exists between metaphosphate absorption and actual content for the reflection technique although there is some scatter about the line.
g
I-
1500
1000 Wavenurnber,
500 cm -I
Fig. 1. Transmission spectrum for a typical coating.
v
v
v
v v
v
v
0.2
40 80 120 Metaphosphate pg P/cm 2
Fig. 2. Plot of absorbence against metaphosphate content.
In the coatings examined between 15 and 97% of the total phosphorus content was present as magnesium metaphosphate. Attempt~ were made to identify bands which appear in the low metaphosphate coating spectra but no firm identification could be made. Table 1 shows the results of heat treatment in two different gas mixtures. In all cases a decrease in
Table 1 The effect of heat treatment in different gas mixtures Sample identity
10(:]
/
v
1 2 3 4 5 6 7 8 9 10
Heat treatment atmosphere
air air air air air 4% H2 in N2 4% H2 in N2 4% H2 in N2 4% H2 in N2 4% H2 in N2
% Phosphorus as metaphosphate before heat treatment
after heat treatment
90 85 88 57 74 75 90 84 82 83
65 72 76 23 71 72 74 71 52 11
C. Jones and P. Beckley / Assessment o f phosphate coatings
metaphosphate content of the coating occurred. In addition some loss of phosphorus took place.
sion of European Communities, European Coal and Steel Community. The authors wish to thank the Director of B.S.C. Associated Products Group and the Commission for permission to publish this paper.
7. Outlook The non-destructive infrared method described shows some promise for the commercial estimation of the coating density of magnesium metaphosphate on steel sheet.
Acknowledgements This work was carried out as part of Research Contract 6210.KL/8/801 supported by the C ommis-
259
References [1] United States Patent, No. 3 840 3"18(1974). [2] BS 1016: Part 9: 1971, British Standards Institution (1971).