Chromium alloys are widely used in industrial applications due to their outstanding corrosion resistance, particularly in aggressive environments. However, the presence of non-metallic inclusions in these alloys can significantly compromise their electrochemical and mechanical behavior. These discontinuities, generated during the manufacturing and solidification processes, have been identified as critical points in the initiation of localized corrosive phenomena. Recent research has further investigated the classification, formation, and effect of the different types of inclusions on the stability of the passive film that gives chromium its protective behavior.
What are inclusions and how do they affect chromium?
Inclusions are solid non-metallic particles that are incorporated into the metal during manufacturing or processing. In chromium alloys, these inclusions can act as preferential sites for corrosion initiation, especially in environments that favor the breakdown of the passive layer.
Nature and origin of inclusions
Inclusions in chromium alloys can be classified according to their chemical composition, morphology and distribution. In general, they are formed during melting and solidification of the metal, when residual elements or impurities react with oxygen, sulfur or nitrogen. These reactions produce compounds such as oxides, sulfides or nitrides that segregate into the metal matrix. Often, these non-metallic particles concentrate at grain boundaries, which affects the microstructural homogeneity and weakens the natural passive barrier that protects the metal from corrosion.
Types of inclusions in chromium alloys
Among the most common types of inclusions in these alloys are chromium oxides, complex inclusions such as mixed silicate and spinel oxides, and manganese sulfides. Each type exhibits a different interaction with the metal matrix and corrosive environment; oxides tend to be chemically more stable and less harmful, while sulfide inclusions often act as pitting initiators due to their lower electrochemical compatibility with the matrix.
Oxides, such as aluminum and silicon, tend to be more stable but can generate discontinuities in the protective chromium film. On the other hand, sulfides, such as MnS, tend to dissolve easily in acidic media, creating voids that accelerate localized corrosion.
Distribution and morphology of inclusions
The shape, size and distribution of inclusions are also determining factors in their impact on corrosion resistance. Current research using electron microscopy and X-ray spectroscopy techniques has shown that elongated inclusions close to the surface of the material significantly increase the susceptibility to pitting and intergranular corrosion.
Impact of inclusions on corrosion mechanisms
Although oxides are usually associated with protection, some types of complex oxide inclusions act as impurity traps and weaken the passive barrier. In certain studies, it has been observed that these oxides, when interacting with chlorides in marine environments, promote crack initiation and chromium loss in localized areas.
Pitting initiation and localized corrosion
Inclusions interrupt the continuity of the chromium-rich passive film, generating micro-areas of electrochemical imbalance. These regions act as local galvanic cells, where the metal matrix behaves as an anode and the inclusion as a cathode. This effect is especially pronounced when the inclusions have sharp edges or internal fissures, which allow the penetration of aggressive ions such as chlorides. Localized rupture of the passive film leads to pitting initiation and propagation of stress corrosion cracking, even under conditions where metal without inclusions would show high resistance.
Influence of oxides on surface protection
Not all inclusions are detrimental in equal measure. Some recent research has shown that certain complex oxides can partially integrate into the passive film, contributing to its thickness or adhesion. However, when these oxides are poorly adhered or porous, they act as moisture and contaminant traps, accelerating corrosion mechanisms. The key is to control their morphology and composition during the manufacturing process.
New research and mitigation approaches
Recent research on chromium alloys has revealed that non-metallic inclusions, especially oxides and sulfides, play a determining role in the initiation and propagation of localized corrosion. Advanced studies have shown that not only the type of inclusion, but also its morphology, distribution and chemical affinity to the metal matrix influence corrosion resistance.
Faced with this challenge, mitigation approaches focus on design techniques, use of inclusion modifiers, post-fabrication heat treatments and engineering of resistant microstructures, using advanced microscopy characterization techniques. In addition, emerging technologies such as directional solidification and nanometer-controlled internal coatings are showing promising results in reducing the electrochemical activity of inclusions in service.
Advanced microscopy and electrochemical characterization
Advances in transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) techniques have made it possible to accurately map the composition of inclusions in chromium alloys. These methods, together with electrochemical techniques such as electrochemical impedance spectroscopy (EIS), have made it possible to correlate the presence of specific inclusions with degradation of passive behavior. It has been identified, for example, that chromium-rich inclusions form more stable interfaces than sulfide-type inclusions.
Alloy design and heat treatment
As a mitigation strategy, some current developments focus on modifying alloy composition to reduce the formation of harmful inclusions. Alloys with a higher proportion of deoxidizing elements such as titanium or cerium have shown better results in reducing the number of harmful inclusions. Also, controlled heat treatments can induce partial dissolution or homogeneous redistribution of inclusions, minimizing their corrosive effect.
Methods to minimize the impact of inclusions
The control of inclusions in chromium alloys is fundamental to ensure efficiency in terms of resistance in corrosive environments. Methods to minimize their impact range from early stages of the manufacturing process to post-processing treatments, combining physical metallurgy, materials engineering and quality control.
One of the most effective procedures is the optimization of liquid steel cleanliness, which is achieved by controlled deoxidation and secondary ladle furnace refining processes. These techniques reduce the dissolved oxygen content, control the formation of oxide inclusions and improve particle flotation to the slag. In particular, the use of synthetic slags and additives such as CaSi helps to modify the morphology of detrimental inclusions (e.g., transforming elongated MnS inclusions into low-impact spherical ones).
Another key line of action is the application of directional or controlled solidification technologies, which allow modifying the cooling rate and promoting the uniform distribution of inclusions in the metal matrix, avoiding accumulation at grain boundaries or in segregation zones, substantially improving the intergranular corrosion resistance and the structural integrity of the material.
In addition, post-fabrication heat treatments, such as annealing and isothermal treatment, have been developed that serve multiple functions: they redistribute fine inclusions, relax internal stresses generated during mechanical forming and promote a more homogeneous microstructure. Such treatments can also reduce anisotropy in corrosion resistance and improve performance against phenomena such as pitting or stress corrosion cracking.
Innovative simulation tools and in-situ inspection
In recent years, the use of thermo-mechanical simulation tools and multi-scale solidification models has made it possible to predict the formation, morphology and distribution of inclusions more accurately. This facilitates the design of cleaner metallurgical routes, adapted to the requirements of specific applications such as chemical plants, nuclear power plants or process equipment with high hygienic demands.
Finally, the incorporation of advanced in situ inspection techniques, such as optical emission spectroscopy (OES) and scanning electron microscopy (SEM) coupled to EDS, has significantly improved the ability to monitor steel quality and the nature of inclusions in real time, allowing finer control of the production process and full traceability of the metallurgical composition.
These combined techniques represent a significant evolution in materials engineering, allowing the manufacture of cleaner, stronger and more reliable chromium alloys, especially in environments where corrosion represents a limiting factor for safety and operational durability.
Conclusion
Inclusions play an impacting role in the durability of chromium alloys, as they directly influence their corrosion resistance. Control of their formation and distribution, together with intelligent alloy design and optimized manufacturing processes, are essential to maintain the integrity of components in harsh environments. Ongoing research in this field offers new perspectives to improve the performance of these materials in sectors such as the chemical, energy and aerospace industries.
References
- Kumar, R., & Li, J. (2021). Effect of non-metallic inclusions on the pitting resistance of stainless steels. Corrosion Science, 182, 109253. https://doi.org/10.1016/j.corsci.2020.109253
- Zhou, Y., Chen, L., & Zhang, H. (2020). Characterization of oxide inclusions in Cr-based alloys and their influence on corrosion behavior. Materials Chemistry and Physics, 250, 123115. https://doi.org/10.1016/j.matchemphys.2020.123115
- González, M., & Ruiz, D. (2022). Avances en microscopía aplicada al estudio de inclusiones en aceros inoxidables. Revista Iberoamericana de Corrosión y Protección, 13(2), 88–99.
