Introduction
Pitting defects, often observed on the surface of investment castings, have long been a topic of debate among researchers and practitioners. Various handbooks and technical papers offer differing explanations—from surface oxidation to chemical interactions between metal and mold materials. This article synthesizes insights from multiple authoritative sources to clarify the mechanisms behind pitting defects and provide a clearer understanding for foundry professionals.

Diverse Perspectives on the Causes of Pitting Defects

Chen Bing & Wang Yihu in Analysis and Countermeasures of Investment Casting Defects attribute pitting to:

1. Surface oxidation of the casting.

2. High impurity content in the refractory powder of the face coat.

3. High pouring and mold temperatures, leading to slow cooling.

Yōki Yamaya in Precision Casting Practical Technology describes pitting (referred to as black spots) as a result of oxidation caused by air inside or outside the mold penetrating the shell and reacting with the casting surface.

The Atlas of Casting Defects by the American Investment Casting Institute explains that oxygen reacts with chromium in the metal, with possible causes including:

1. Oxidation of high-chromium iron alloy surfaces.

2. Lack of carbonaceous materials in the shell.

3. Slow cooling of the casting.

Chen Guozhen, Xiao Keze, & Jiang Buju in Casting Defects and Countermeasures Manual suggest that pitting forms due to chemical reactions between oxides in the metal and oxides in the shell material.

In a paper by Tang Bin et al. titled Defect Analysis and Quality Control of Stainless Steel Investment Casting, the primary cause is identified as substandard face coat materials. As impurity content increases, zircon sand’s refractoriness and decomposition temperature decrease. The amorphous silica released during decomposition is highly reactive and can chemically react with elements such as chromium, nickel, titanium, manganese, and aluminum at high temperatures, leading to pitting.

Che Shunqiang & Jing Zongliang in Precision Casting Practice note that zircon (a binary mineral of zirconia and silica) decomposes at high temperatures to release amorphous silica, often called “silica precipitation.” This reactive oxide can chemically interact with certain heavy metal elements in the molten metal, contributing to pitting defects.

Synthesizing the Theories

The first three sources emphasize oxidation—whether surface oxidation or chromium oxidation. The latter two sources and the paper highlight chemical reactions, either between metal oxides and shell oxides or between silica and specific metal elements.

While these descriptions alone do not provide a definitive conclusion, examining the slag composition within pitting defects offers further insight.

Analysis of Slag Composition in Pitting Defects

Chen Bing and Wang Yihu’s book includes metallographic analysis showing that slag-like substances in defect areas contain silicon, manganese, chromium, and other elements. X-ray diffraction reveals a chemical composition of α-Fe, chromium oxide, and spinel-type compounds (FeO·Cr₂O₃). Thus, pitting likely results from high-temperature reactions between basic metal oxides (e.g., Cr₂O₃, MnO) from surface oxidation and oxides in the shell face coat (e.g., SiO₂, Fe₂O₃, FeO).

Similarly, Yamaya’s Precision Casting Practical Technology (p. 153) includes XMA and X-ray analysis of pitted areas, confirming the presence of α-iron, chromium oxides, and spinel oxides—though the slag itself is not described.

The Casting Defects and Countermeasures Manual aligns with this view, stating that pitting forms due to chemical reactions between oxides in the molten metal and shell materials. Spectral analysis shows increased silicon and minimal manganese in defect areas. Slag metallographic analysis indicates compounds such as iron silicate, manganese silicate, and cobalt silicate, while X-ray diffraction confirms that black pits consist of magnetite (Fe₃O₄) and iron-chromium spinel.

Conclusion on the Mechanism

Based on these analyses, pitting defects can be reliably attributed to high-temperature reactions between basic metal oxides (from casting surface oxidation) and oxides in the shell face coat. Therefore, the common understanding that pitting is related to surface oxidation is correct. The detailed discussion here aims to deepen awareness of the defect’s nature.

Additional Considerations

1. Alloy Sensitivity: Chen Bing’s book notes that pitting often occurs in low-chromium alloy steels or stainless steels, particularly those with 5%–10% chromium. In practice, martensitic stainless steels (∼15% chromium) and even carbon steels are also prone to pitting. The mechanism remains consistent, though in carbon steels, preventing decarburization is an additional concern. Whether carbon oxidation contributes to pitting in carbon steels is uncertain, but gas involvement is likely, as slag and gas often coexist in molten metal.

2. Defect Location: Four sources (Atlas of Casting Defects, Analysis and Countermeasures of Investment Casting Defects, Casting Defects and Countermeasures Manual, and Precision Casting Practice) identify pitting in thick, poorly cooled sections of castings. Yamaya’s book, however, associates severe oxidation with areas of high permeability. Given the weight of evidence, pitting is more consistently linked to thick, slow-cooling regions. For practical insights, the article on improving pitting defects in Precision Casting Practice is highly recommended.

3. Note on 304 Stainless Steel: Pitting can also occur in 304 stainless steel, as noted in the Casting Defects and Countermeasures Manual and Professor Xu Yunxiang’s research, though this is beyond the scope of the current discussion.

Key Takeaways

• Pitting defects stem from high-temperature reactions between metal oxides and shell oxides.

• Material quality, cooling rate, and alloy composition critically influence defect formation.

• Thick, slow-cooling casting sections are most susceptible.

By integrating classical handbooks with modern analysis, this overview provides a clearer, actionable understanding of pitting defects in investment casting.