Lead chrome pigments are isomorphous blends of lead chromate with lead sulphate, the composition of which corresponds to the formula PbCrO4- n PbSO4l, where n can be within the range from 0 to 2.
Additionally, the chrome pigments can contain modifying additives [А1(ОН)3, Ti(OH)4, surfactants, etc.].
The color of the chrome pigments varies from yellow to light lemon yellow; under otherwise equal conditions, the more is the РbСrО4 content, the more yellow are chrome pigments.
The chrome pigments can crystallize in both rhombic and monoclinic systems, the latter being more stable. While having the same chemical composition, lead chromates and sulphochromates crystallizing in rhombic system are considerably lighter and finer grained than those crystallizing in monoclinic system.
Specific surface: 16–30 m2/g, refraction index: 2.7, particle size: 0.2 to 2–3 μm, particle shape: granular or needle-shaped, aqueous extract рН: 5.0-7.0.
The chrome pigments are hydrophobic, insoluble in water and organic acids, but soluble in hydrochloric and nitric acids.
They redden under the action of alkalis (basic lead chromates are formed) and darken under the action of H2S (PbS is formed); therefore the chrome pigments must not be mixed with sulphur-containing pigments.
The thermal stability of the chrome pigments is up to 90°C; their light resistance is rather low.
In oily coatings, the chrome pigments change their ground tint with the course of time: they become green (Sg6+–► Sg3+) and dark due to formation of lead peroxide compounds.
The chrome pigments are weather and corrosion resistant, and have high coverage rates.
The chrome pigments of monoclinic modification have better light resistance and covering power and relatively lower dispersion and coloring power than the chrome pigments of rhombic modification.
The most common methods of production of lead chrome pigments are the followings: sedimentation from the basic lead acetous and nitric compounds, continuous method of producing the chrome pigments from Рb metal as well as from yellow lead monoxide and chromic anhydride.
Although lead sulphate crystallizes in the same two systems as lead chromate does, its stable form is the rhombic modification. Therefore, introduction of lead sulphate into the composition of the chrome pigment increases the stability of rhombic modification of lead chromate in isomorphous blend.
In addition, the following additives are recommended to introduce when synthesizing the chrome pigments to stabilize the rhombic modification of lead chromates and sulphochromates: lead phthalate, aluminum hydroxide, titanium hydroxide and some surfactants.