Overview
ISO 797:1973 is an international standard published by the International Organization for Standardization (ISO) that specifies a gravimetric method for determining silicon content in aluminium and aluminium alloys. This method is primarily designed for aluminium materials with a silicon content greater than or equal to 0.30%. The standard provides a reliable laboratory procedure to accurately quantify silicon by isolating it in the form of silica, which is then weighed after careful preparation.
The document also highlights limitations for certain alloys, including those with high silicon or magnesium content (see Annex A) and those containing tin or antimony (see Annex B), where modifications to the standard method are required to ensure accuracy.
Key Topics
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Scope of Application
Applicable for silicon determination in aluminium and aluminium alloys where silicon content is ≥ 0.30%. Special caution and adapted procedures are prescribed for:
- Aluminium-silicon alloys with silicon > 3%
- Aluminium-magnesium alloys
- Alloys containing tin or antimony
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Analytical Principle
The silicon is determined gravimetrically by:
- Dissolution of the sample using alkaline and acid treatments
- Insolubilization of silicon as silica through hydrolysis and acidification
- Filtration, drying, calcination, and weighing of silica
- Use of controlled volatilization of silica with hydrofluoric acid to ensure accurate weight determination by difference
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Reagents and Preparation
The method involves precise preparation of reagents such as:
- Hydrochloric acid, nitiric acid, sulfuric acid, fluorhydric acid (hydrofluoric acid), perchloric acid, and sodium hydroxide solutions of specified normalities and concentrations
- Use of saturated bromine water and hydrogen peroxide to address interference and enhance accuracy
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Sample Handling and Testing
Defined procedures for:
- Sampling: Use of chips ≤ 1 mm thick obtained by drilling or milling
- Weighing samples with ±0.001 g precision depending on expected silicon content
- Parallel blank testing to ensure reagent purity and method control
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Special Cases Modifications
- Annex A: Modified attack procedure for aluminium-silicon and aluminium-magnesium alloys using two-step alkaline attack and controls for hyper-eutectic compositions.
- Annex B: Adjusted method for alloys containing tin and antimony to prevent interference via bromination and controlled evaporation steps.
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Computation of Results
Silicon content expressed as mass percentage using a formula based on the difference in masses of residue before and after silica volatilization, employing a correction factor to convert silica mass to silicon mass.
Applications
ISO 797:1973 is critical for:
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Quality Control in Metallurgy
Ensuring aluminium and aluminium alloy products conform to specified silicon content, impacting mechanical properties like strength, castability, and thermal conductivity.
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Research and Development
Assisting in materials characterization for alloy development where silicon content control is essential.
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Compliance and Certification
Supporting certification procedures where documented accuracy in silicon measurement is required according to international norms.
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Failure Analysis and Forensics
Identifying causes of material failure related to compositional anomalies in aluminium products.
Related Standards
- Other ISO standards on aluminium and aluminium alloys cover sampling methods, chemical analysis techniques for different elements, and testing procedures.
- Standards related to chemical analysis and gravimetric methods offer complementary approaches for other impurities in metals.
- International standards on metallurgical testing techniques that specify procedures for thermodynamic property assessments and microstructural analysis may be used alongside ISO 797.
Keywords: ISO 797, aluminium alloys, silicon determination, gravimetric method, chemical analysis, metallurgical testing, aluminium-silicon alloys, aluminium-magnesium alloys, tin, antimony, sample preparation, laboratory procedure, international standard, silicon content measurement.
