Vol.5 No.1 2012

Research paper : An analysis method for oxygen impurity in magnesium and its alloys (A. Tsuge et al.)−30−Synthesiology - English edition Vol.5 No.1 (2012) For international standardization, it is necessary to show that the method to be standardized reach a level required by experts in various countries, in addition to the establishment of the element technologies described above. Specifically, 1) an analysis value should represent the precise oxygen content of the object, 2) the range of the alloys to which the analysis method can be applied should be clear, and 3) results with a good reproducibility can be obtained by this method using a commercially available inert gas fusion device. In the following, we examine whether our proposed method meets these requirements or not.3.1 Verification of the analysis valueWhen a new analysis method is developed, it is necessary to examine whether the analysis value accurately represents the content of the analyte in the measured specimen. However, the higher the novelty of developed analysis method is, the more difficult the examination is. For example, if a certified reference material with known content of the analyte is widely available, an analysis value of the new method can be verified by comparison with the certified value by analyzing the reference material. However, in an analysis method for a substance considered difficult to measure, a certified reference material does not naturally exist. In such a case, the widely used practice is to check the validity by comparing the analysis value with that obtained by another applicable measurement method, in compliance to the concept of traceability, validation, and data evaluation described in “ISO Guide 34: General requirements for the competence of reference material producers”Note 2) issued by ISO.Here, the “phenol dissolution method” was selected as a standard for comparison. This is a method for quantifying the magnesium oxide in Mg, based on the fact that magnesium oxide in Mg is insoluble in phenol while Mg reacts with phenol and generates phenoxide as it dissolves. The outline of this operation is shown in Fig. 9. This method is applicable only to chip samples which dissolve readily in phenol. In the cases where the phenol or methanol used for dissolving or dilution contains water, the generated phenoxide should be hydrolyzed to magnesium hydroxide. This will cause an overestimate of the oxygen content, and hence it can be asserted that the phenol dissolution method is sensitive to humidity. The phenol dissolution method is considered inappropriate as a standard analysis method to be employed at production fields of Mg components because it requires skills in complex procedures of wet analysis to eliminate the effect of humidity.Figure 10 shows the comparison between the analysis results of various Mg materials by the IGF-IRA method and those by the phenol dissolution method. While there is a correlation between both analyses, the results of phenol dissolution method are somewhat higher than those of IGF-IRA method. This is attributed to the aforementioned moisture effect. Next, another standard for comparison was the charged particle activation analysis (CPAA). In this method, the sample is irradiated with helium atom ( particle). The oxygen in the sample is activated as the radioactive fluoride atom, and the oxygen amount in the sample is calculated from the amount of radiation of the fluoride atom. Since an accelerator is necessary in the CPAA, it cannot be used as a routine on-site analysis. It is, however, used relatively frequently in the measurement of oxygen content in Mg in the development stage of alloys, and there are many analysis laboratories experienced in this analysis. The outline of the CPAA is shown in Fig. 11 and the results in Table 1. The samples were extruded pure Mg (no.1), the tablet formed from chips of 0.2 mm thickness by turning of extruded material in the atmosphere (no.2), and the tablet from the chips which were left for 3 days in the desiccators with 100 % humidity to enhance oxidation (no.3). Sample no.1 had oxygen concentration that was close to the lower limit of the IGF-IRA, and therefore had high standard deviation against the average value, while, for samples no.1 and no.2, the analysis values agreed within the standard deviations for Fig. 9 Conceptual diagram of phenol dissolution methodFig. 10 Relationship between the analysis values by inert gas fusion method and phenol dissolution method Dissolve with hydrochloric acidTeflon filterFilter (0.2 micron Teflon filter)Dilute at constant volume and measure Mg content by ICP-OESBoil with phenol(boiling point 182°C)MgOMgInert gas fusion method (wt%) dissolution method (wt%)A1A2ArbAox1Aox2B1B2C1C2


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