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D6280-98 – Standard Test Method Technical Guide
🧪 Classification and Composition of Zinc Phosphates
ASTM D6280 − 98 (Reapproved 2020) provides the standard specification for three distinct types of zinc phosphate pigments. The classification is based on the hydration state of the zinc phosphate, which is verified by measuring the loss on ignition (LOI) of the dried pigment at 600°C. The standard strictly requires that all types be free from extenders, diluents, and other pigments to ensure performance in corrosion-inhibiting formulations.
| 🟦 Type | 📐 Predominant Composition | 🔥 Loss on Ignition (600 °C wt%) |
|---|---|---|
| Type I | Zinc Phosphate Dihydrate (Zn3(PO4)2·2H2O) | 8.5 to 10.0 |
| Type II | Mixture of Dihydrate and Tetrahydrate | 10.0 to 14.0 |
| Type III | Zinc Phosphate Tetrahydrate (Zn3(PO4)2·4H2O) | 14.0 to 18.0 |
⚠️ Compositional Integrity: Per Section 4.2, the chemically prepared pigment must be free of extenders, modifiers, diluents, carbonaceous material, co-reacted precipitates, and any alteration of the stoichiometric chemical structure.
🔬 Significance and Testing Methods
Zinc phosphate serves a unique dual role as both a functional chemical and a pigment, primarily utilized in corrosion-inhibiting paints (Section 3.1). Conformance to the standard requires verification using several ASTM standard test methods to determine physical and chemical properties.
| ⚡ Test Requirement | 📏 Standard Method |
|---|---|
| Specific Gravity | D153 |
| Coarse Particles (63 µm Sieve) | D185 |
| Hygroscopic Moisture | D280 |
| Oil Absorption (Spatula Rub-out) | D281 |
| Water-Soluble Salts (Resistivity) | D2448 |
| Fineness of Dispersion | D1210 |
💡 Reference Sample Matching: For properties not explicitly defined in the standard tables, Section 4.3 emphasizes that requirements must be subject to mutual agreement between interested parties. This agreement is based upon a satisfactory match between the submitted sample and a previously agreed-upon reference standard.
❓ Frequently Asked Questions
🔍 What distinguishes Type I, Type II, and Type III zinc phosphate pigments?
The types are distinguished by their hydration state and the resulting crystalline structure. This is verified by the loss on ignition (LOI) at 600°C. Type I is dihydrate predominant (8.5-10.0% LOI), Type II is a defined mixture of dihydrate and tetrahydrate (10.0-14.0% LOI), and Type III is tetrahydrate predominant (14.0-18.0% LOI).
⚡ What is the primary application for these zinc phosphate pigments?
According to Section 3.1 of ASTM D6280, zinc phosphate functions as both a chemical and a pigment. Its primary application is in the formulation of corrosion-inhibiting paints, where it provides active anti-corrosive properties.
📌 Are there specific purity requirements outlined in the standard?
Yes. Section 4.2 explicitly mandates that the pigment must be free from extenders, diluents, other pigments, carbonaceous material, co-reacted precipitates, and any alteration of the stoichiometric chemical structure.
💡 How is oil absorption determined for compliance with this specification?
Oil absorption is determined using the spatula rub-out method, conducted in strict accordance with ASTM D281, which is specifically referenced in Section 2.1 of the standard for this purpose.