What effect does the chromium content have on the coloring effect?

1. Coloring in Ceramic/Enamel Glazes

• Low chromium content (0.5%–2%): The glaze shows a light green or grass green color with high transparency. Chromium ions are uniformly dispersed in the glaze glass phase without forming a large number of colored crystals, resulting in a soft color but weak hiding power. It is suitable for thin-walled porcelain and decorative transparent glazes.
• Medium chromium content (2%–5%): Stable chromium-aluminum spinel (\(Cr_2O_3·Al_2O_3\)) crystals are formed, and the glaze presents a deep green or olive green color. This content range achieves the best balance between color saturation and glaze stability, with excellent acid and alkali resistance and weather resistance, making it the most commonly used ratio for daily ceramics and architectural tiles.
• High chromium content (>5%): The glaze color turns dark green or blackish green, and even “burnt green” defects may occur. Excessive chromium ions cause excessive crystallization of the glaze, reducing the glaze gloss and easily leading to color spots and streaks. At the same time, the melting difficulty of the glaze increases, which may cause pinholes and bubbles.

Note: The valence state of chromium also affects coloring—\(Cr^{3+}\) is the source of stable green, while toxic \(Cr^{6+}\) shows yellow. Therefore, the firing atmosphere needs to be controlled during production to avoid oxidation of \(Cr^{3+}\).

2. Coloring in Inorganic Pigments

• Chrome green (\(Cr_2O_3\)-based):
  • Low \(Cr_2O_3\) content (<80%): The pigment is light green with low color intensity, suitable for light-colored coatings and inks with low cost but poor hiding power.
  • High \(Cr_2O_3\) content (>95%): The pigment is deep green with high color purity and excellent light resistance/weather resistance, suitable for outdoor coatings and building materials coloring.
• Chrome yellow (chromate-based):
  • Low chromium content: The color is lemon yellow, bright but poor light resistance, easy to fade under sunlight.
  • High chromium content: The color turns deep yellow or orange yellow, with improved color fastness, but the heavy metal content increases, and its application is limited in some regions.

3. Surface Coloring of Metals (e.g., Stainless Steel)

• Low chromium content (<12%): The passivation film on the metal surface is thin and unstable. During electrochemical coloring, it is difficult to form a uniform oxide film, resulting in color difference, mottling, and poor color fastness. The color is mostly light yellow or light blue, which is easy to peel off.
• High chromium content (≥18%, e.g., 304/316 stainless steel): A dense passivation film is formed on the surface. The thickness of the oxide film can be precisely controlled during coloring, stably presenting blue, green, red, black and other colors. The colored layer has high adhesion and corrosion resistance, which is the ideal material for metal coloring.

Key Summary Table