The determination of the chemical stability of materials can be carried out from the following aspects:
**I. Theoretical Analysis**
1. **Analysis of Chemical Composition**
– Understanding the chemical composition of materials is the basis for judging their chemical stability. For example, for metallic materials, the chemical stability of pure metals is usually related to their position in the metal activity series. Precious metals such as gold (Au) and platinum (Pt) have relatively stable chemical properties because they are located at the rear of the metal activity series and are not prone to react with common acids, alkalis, and salts. Metals such as iron (Fe) and zinc (Zn) are relatively active and have slightly poorer chemical stability.
– For polymeric materials, their chemical stability is related to the structure and composition of the molecular chains. Polymeric materials containing more unsaturated bonds (such as carbon-carbon double bonds) may have poorer chemical stability because unsaturated bonds are prone to addition, oxidation, and other reactions. For example, natural rubber contains a large number of carbon-carbon double bonds and is easily oxidized by oxygen, leading to rubber aging.
2. **Analysis of Crystal Structure (for Crystalline Materials)**
– The crystal structure of materials can affect their chemical stability. For example, in metal crystals, closely packed crystal structures (such as face-centered cubic packing and hexagonal close-packed packing) are usually more stable than metal crystals with body-centered cubic packing structures. This is because the closely packed structure makes the bonds between atoms closer and more difficult for external substances to penetrate and react.
– For ionic crystals, the magnitude of the lattice energy can also reflect their chemical stability. Ionic crystals with high lattice energy (such as magnesium oxide MgO) have relatively high chemical stability because the ionic bonds are strong and it requires a relatively high amount of energy to break these ionic bonds, making the crystals less likely to undergo chemical reactions under normal conditions.
**II. Experimental Tests**
1. **Corrosion Resistance Tests** – **
Salt Spray Test**: This is a widely used testing method for metallic materials and materials with protective coatings. The material samples are placed in a salt spray test chamber, and a sodium chloride solution is sprayed (for example, in a neutral salt spray test, a sodium chloride brine with a concentration of 50 g/L and a pH value between 6.5 and 7.5 is used) to simulate a saline environment such as that of the ocean or coastal areas. Observe whether rusting, corrosion, blistering, and other phenomena occur on the surface of the material within a certain period of time (such as 24 hours, 48 hours, 72 hours, etc.). If the material shows obvious corrosion in a relatively short time, it indicates that its chemical stability is poor.
– **Immersion Test**: Select the corresponding immersion solution according to the usage environment of the material. For example, for materials that may be used in an acidic environment, they can be immersed in a certain concentration of acid solution (such as sulfuric acid, hydrochloric acid, etc.); for materials used in an alkaline environment, they are immersed in an alkaline solution (such as sodium hydroxide solution). Observe the mass change and surface morphology change of the material during the immersion process. If the material experiences a large mass loss and the appearance of corrosion pits on the surface during the immersion process, it indicates that its chemical stability is not good.
2. **Thermal Stability Tests** – **
Thermogravimetric Analysis (TGA)**: Under a programmed temperature control, the relationship between the mass of the material and the temperature is measured. When the material is heated, if there is an obvious mass loss at a relatively low temperature, it may be because the material has undergone decomposition, oxidation, and other chemical reactions. For example, some organic polymeric materials will undergo thermal decomposition at high temperatures, and through TGA, the thermal decomposition temperature can be determined to evaluate their chemical stability in a high-temperature environment.
– **Differential Scanning Calorimetry (DSC)**: It can measure the heat change of the material during the heating or cooling process. If the material shows endothermic or exothermic peaks during the heating process, it may be because of phase transitions, chemical reactions, etc. By analyzing the position and size of these peaks, the chemical stability of the material can be judged. For example, certain alloys will undergo phase transitions at specific temperatures, and this phase transition may affect the chemical stability of the material.
3. **Oxidation Stability Tests** – **Accelerated Oxidation Test**: For materials that are prone to oxidation (such as metals, fats, etc.), the oxidation stability can be evaluated through an accelerated oxidation test. For example, in an environment with high temperature and high oxygen content, observe the oxidation rate of the material. For metal materials, the growth thickness of the oxide film and the mass increase can be measured to judge their oxidation stability. For fats, the degree of oxidation can be measured by detecting indicators such as the peroxide value. If the material has a fast oxidation rate in the accelerated oxidation test, it indicates that its chemical stability is poor. 4. **Reactivity Tests with Other Substances** – The material can be subjected to a contact test with other substances that it may come into contact with (such as solvents, other materials, etc.). For example, for packaging materials, the reactivity with food components (such as fats, acids, alkalis, etc.) needs to be tested. The material is brought into contact with food simulants, and by detecting whether there is substance migration and whether the material has changed, its chemical stability can be judged. For composite materials, it is necessary to test whether chemical reactions will occur between different materials, which may affect the overall performance of the material.
