Analysis of The Causes of Surface Rusting in Stainless Steel Castings

Abstract: During storage, the surface of stainless steel castings produced by a certain foundry exhibited rusting. To analyze the causes of surface rusting in stainless steel castings, chemical composition analysis, energy dispersive spectroscopy (EDS) analysis of rusting products, and comprehensive analysis were conducted. This was done to identify the causes of surface rusting in stainless steel castings and to implement targeted measures, with the aim of providing reference for relevant personnel.

Introduction: As a commonly used metal product, stainless steel castings have widespread applications in industrial production and daily life. Therefore, conducting an in-depth analysis of the causes of surface rusting in stainless steel castings through this study is expected to contribute to mitigating surface rusting issues, improving quality and reliability, and promoting sustainable development in related industries.

1. Experimental Analysis

1.1 Visual Analysis

The surface of the samples is uniformly covered with bright red rust stains, with only localized areas remaining unaffected by corrosion. The corrosion products exhibit a loose structure with insufficiently tight bonds, resulting in microscopic pores on the surface. As shown in Figure 1, the appearance is as follows.

1.2 Chemical Composition

The client did not specify the required material, so it was not possible to test the compound composition based on a specific sample. According to experimental data, 1Cr13Mn6NiN stainless steel has material properties similar to nickel-based austenitic stainless steel but belongs to the ferritic structure. This material achieves nickel resource conservation by replacing nickel with manganese. The copper content accounts for as much as 0.57%, with particular attention to the relevant specific designation.

1.3 Surface Rust Corrosion Product Analysis

1.3.1 Severely Corroded Areas

Observation of the surface layer using a scanning electron microscope reveals that the structure of the oxidation products is relatively loose and protrudes from the surface layer, exhibiting a texture resembling mud. Energy dispersive spectroscopy (EDS) analysis revealed that the metal matrix region not only contains the material's inherent elements but also primarily includes oxygen, sodium, and calcium; through energy spectroscopy analysis, the metal matrix region, in addition to the original material composition, primarily contains oxygen, sodium, and calcium components; the corrosion region is mainly composed of oxygen, sodium, and calcium atoms, with a significantly increased proportion of oxygen atoms.

1.3.2 Non-corroded regions

In the non-corroded regions, corrosion traces are barely observable. In addition to analyzing the material's intrinsic elements, energy spectrum analysis also focuses on the study of oxygen, sodium, and calcium atoms. Additionally, a large amount of clay-like material appears on the surface layer. Spectroscopic analysis identifies it as primarily composed of silicon and oxygen atoms.

2. Discussion

2.1 Material Aspects

The material generally aligns with the 1Cr13Mn6NiN type of austenitic stainless steel, where manganese replaces nickel, achieving the effect of conserving nickel resources. In terms of preventing general corrosion and resisting pitting and other localized damage, nickel-saving stainless steel performs inferior to 443 ferritic and 304 austenitic materials. It is typically used only in cases of mild corrosion.

2.2 Corrosion Resistance

The corrosion resistance of stainless steel depends on the proportion of its components, with chromium playing a crucial role in enhancing corrosion resistance [2]. When the chromium content in steel reaches approximately 12%, chromium reacts with oxygen in an oxidizing medium to form a thin oxide layer on the steel surface, effectively preventing further corrosion.

2.3 Casting shrinkage defects

Macroscopic and microscopic shrinkage defects are present in the castings, particularly numerous shrinkage issues in the lower layers beneath the surface. Typically, alloys with a wider solidification temperature range are prone to forming porous structures at grain boundaries. When such metallic materials cool and solidify in sand molds, their solidification zones expand widely and rapidly into the core regions of the castings, causing the entire molten metal to enter the solidification phase. This process completes the transition from liquid to solid in a grid-like porous or viscous manner. The original crystals rapidly fill the mold interior through their dendritic structures or homogeneous polycrystalline grains, forming a robust solid framework. As the crystal nuclei gradually expand, they divide the remaining liquid material in the grid-like framework into several isolated small molten zones. In these small melt pools, the transformation from liquid to solid or the reduction in solid volume cannot be compensated for, leading to the formation of pores and resulting in intergranular shrinkage porosity. The process of lattice gap reduction is complex, with the relatively high hydrogen content in the metal melt being a key factor of concern.

2.4 Comprehensive Analysis

Although stainless steel is the raw material for castings, its low chromium and nickel content inherently limits its corrosion resistance. The presence of intergranular porosity not only reduces the ductility and recovery ability of the metal alloy but also significantly weakens the corrosion resistance of the casting. In particular, numerous porosity defects are concealed beneath the casting's surface, forming a connected state with the surface. In humid environments, if the surrounding oxygen is abundant, the surface will be subjected to oxidative damage. The 1Cr13Mn6NiN alloy is primarily used for indoor decoration but has poor corrosion resistance. Warehouses are generally unfavorable for maintaining the rust-resistant properties of items, especially in humid and poorly ventilated environments. Even if the items themselves have some rust-resistant capabilities, these unfavorable external conditions may still lead to rusting.

3. Countermeasures for addressing surface rust on stainless steel castings

3.1 Selecting appropriate stainless steel materials

During the manufacturing process of stainless steel castings, selecting appropriate stainless steel materials is a critical step in preventing surface oxidation. Various types of stainless steel possess distinct corrosion-resistant characteristics, and selection should be based on specific application scenarios and regulatory requirements to choose the appropriate stainless steel grade. Commonly used metal materials include austenitic, ferritic, and martensitic stainless steels, among others. Austenitic stainless steel is renowned for its excellent corrosion resistance, particularly in acidic or alkaline media, where its corrosion-resistant properties are especially prominent. As a result, it is widely used in various environments, playing an important role and delivering significant value. Ferritic stainless steel is favored for its excellent wear resistance; however, its performance is somewhat lacking when it comes to resisting corrosive media such as acidic substances and chloride solutions. Therefore, this material is more commonly used in environments with lower corrosion risks. Additionally, martensitic stainless steel combines hardness and toughness, but its corrosion resistance is somewhat lacking in special corrosive environments. Therefore, when selecting durable metal materials, it is essential to comprehensively consider the application environment, the characteristics of the chemicals involved, and the inherent properties of the metal material to ensure that the selected material possesses excellent corrosion resistance, thereby effectively preventing rusting of metal products.

3.2 Adding Alloy Elements

By incorporating specific alloy components, the corrosion resistance of stainless steel is optimized, thereby enhancing its rust resistance. By combining metal elements such as chromium, nickel, and molybdenum, the corrosion resistance of stainless steel can be improved, enhancing its ability to resist oxidation reactions. Among these, chromium, as an important alloy component of stainless steel, plays a role in forming a dense oxide layer that effectively prevents further oxidation reactions, thereby significantly enhancing the corrosion resistance of stainless steel. Increasing the chromium content can significantly improve the corrosion resistance of stainless steel, particularly in oxidizing environments, where it effectively enhances its antioxidant properties. The introduction of nickel can effectively enhance the rust-proofing functionality and oxidation resistance of stainless steel, thereby making its structure more robust. Additionally, nickel improves the ductility and impact resistance of stainless steel, enhancing its production characteristics.

Furthermore, molybdenum, as an important component of the alloy, can effectively enhance the corrosion resistance of stainless steel in high-temperature and corrosive environments, particularly in the presence of chloride ions, where its resistance to chloride ion corrosion is particularly pronounced. Therefore, increasing the proportion of molybdenum in steel can effectively enhance the rust-proofing capability of the steel and reduce the reaction rate when it comes into contact with corrosive factors. It is thus evident that introducing alloy elements can significantly improve the rust-proofing performance of carbon steel castings, enhance their oxidation resistance, effectively prevent rusting, extend their service life, and ensure stable operation in harsh environments.

3.3 Controlling Process Parameters

Reasonably controlling various parameters during the casting process can effectively reduce the risk of corrosion on the surface of stainless steel castings while enhancing their resistance to acids and alkalis, thereby improving surface quality. To this end, staff should control the casting temperature to ensure complete melting of the metal components, thereby reducing the occurrence of internal defects and pores, minimizing oxidation levels and the formation of oxide layers, and alleviating surface corrosion. Additionally, controlling the cooling rate is equally critical. This is because an appropriate cooling speed can precisely control the crystallization and microstructural development of the casting, reduce impurity accumulation and oxide formation at grain boundaries, and enhance the corrosion resistance of chromium-nickel alloy castings. Thus, controlling the metal pouring speed and pressure during the shaping process plays a crucial role in the surface quality of stainless steel castings. By appropriately adjusting the fluidity and pressure of the metal, it is possible to reduce porosity and impurities on the casting surface, lower surface roughness, thereby reducing the likelihood of rusting and improving surface quality.

Conclusion: In summary, to effectively address surface rusting in stainless steel castings, reduce the likelihood of rusting, and maximize the surface quality of stainless steel castings, it is necessary to comprehensively consider various measures and specific circumstances. Only by integrating these measures can the likelihood of rusting in stainless steel castings be reduced, thereby improving appearance quality, enhancing corrosion resistance, and enabling them to perform their intended functions.