**10.8. Phosphate conversion coatings**

Conversion coatings provide the resistance to corrosive environments. Phosphate conver‐ sion coatings (PCC) bring about the transformation of metal substrates into new surfaces having non-metallic and non-conducting properties. The transformations occur in phosphat‐ ing solution containing divalent metal phosphates and, in some cases, in solutions contain‐ ing monovalent metal phosphates. Generally, the solutions are prepared from liquid concentrates containing one or more divalent metals (zinc, magnesium, calcium, etc., phos‐ phates), free phosphoric acid and an accelerator. Three types of phosphate conversion coatings are currently being used [103],[104]:


The addition of metal ions, such as cupric ions, to a conversion bath greatly reduces the formation time and the size and nonuniformity of coating crystals. Copper, which is catho‐ dic to dissolving metals, deposits on the base metalto form many local cells and thus to increase the potential difference between local anode and cathode site. Nickel ions behave differently than cupric ions, and their benefit results from the catalytic action associated with the release of molecular hydrogen. Furthermore, the addition of Ni2+ and Mn2+ into the treating solution refines the grain size and reduces the porosity of phosphate conversion coatings on electro‐ galvanized steels. While Ni exists in both the zero-valance state and the two-valance state, Mn is mainly present in the two-valance state in the phosphate conversion coating [103],[105].

presence of base (ethanolic ammonia solution, tetramethylethylenediamine, pyridine, …),

 ºº

Although this reaction is known for a long time, the mechanism is still under the discussion. It is possible to catalyze the Glaser-Hay reaction under heterogeneous conditions using Cumodified hydroxyapatite (Cu-HAp). With several para-substituted phenyl-acetylenes and alkynols, where Cu-HAp acts as a catalyst for single-bond coupling reactions leading to diyne derivatives in high yields without using auxiliary chelating molecules and organic bases. These heterogeneous conditions allow easy recovery of the catalyst and simplify the purification

The apatite catalyst was utilized for the catalysis of the synthesis of n-butanol, 1,3-butadiene and high octane fuel from bioethanol. The process requires relatively low temperature. The synthesis shows significantly lower cost compared to n-butanol derived from petroleum-based processes. The technology offers a closed-loop system with no waste or emissions [102].

Conversion coatings provide the resistance to corrosive environments. Phosphate conver‐ sion coatings (PCC) bring about the transformation of metal substrates into new surfaces having non-metallic and non-conducting properties. The transformations occur in phosphat‐ ing solution containing divalent metal phosphates and, in some cases, in solutions contain‐ ing monovalent metal phosphates. Generally, the solutions are prepared from liquid concentrates containing one or more divalent metals (zinc, magnesium, calcium, etc., phos‐ phates), free phosphoric acid and an accelerator. Three types of phosphate conversion coatings

**1. Zinc phosphate coatings** are often used as a pretreatment for painted parts. They are also used to impart the corrosion resistance and to aid in cold-forming operations.

**2. Iron phosphate coatings** are primarily used to form a passive substrate under paints.

**3. Manganese phosphate coatings** are primarily on machined parts such as gears and internal combustion engine components as an anti-scuff film for the break-in wear. The addition of metal ions, such as cupric ions, to a conversion bath greatly reduces the formation time and the size and nonuniformity of coating crystals. Copper, which is catho‐ dic to dissolving metals, deposits on the base metalto form many local cells and thus to increase the potential difference between local anode and cathode site. Nickel ions behave differently than cupric ions, and their benefit results from the catalytic action associated with the release of molecular hydrogen. Furthermore, the addition of Ni2+ and Mn2+ into the treating solution

(12)

which can bind to copper ions, an organic base and dioxygen [100],[101]:

º

476 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

work-up [100].

**10.8. Phosphate conversion coatings**

are currently being used [103],[104]:

CuCl base R C CH R C C C R - ®- - -

In fact, all of the chemical reactions of phosphating process are based on mutual interactions of metal immersed in phosphate bath and redox of the accelerators. Generally, phosphating proceeds in the acidic solution containing Zn2+, Mn2+, Ca2+, Na+ , Fe2+ and Mg2+. Phosphating with different metal substrates and types is not of the same reaction mechanism. For exam‐ ple, when a pure iron is immersed in the phosphating solution, iron dissolves on the microanodes through the following reaction [106]:

$$\text{Fe} + 2\text{ H}^\* \rightarrow \text{Fe}^{2\*} + \text{H}\_2 \tag{13}$$

The hydrogen evolution occurs at the micro-cathodic sites resulting in an increase of pH value at the metal-solution interface. This change in pH alters the dissociation equilibrium, which leads to the formation of PO4 3−:

$$\rm H\_3PO\_4 \rightarrow H\_2PO\_4^- + H^+ \rightarrow HPO\_4^{2-} + 2\ H^+ \rightarrow PO\_4^{3-} + 3\ H^+ \tag{14}$$

When PO4 3− and Me2+ (metal ion, e.g. Zn2+, Mn2+, Ca2+ and Fe2+ ) in the solution reach the saturation, the deposition of insoluble phosphate will be achieved. Then, it can crystallize in PCC coating, as shown in **Fig. 12**.

**Fig. 12.** Schematic representation of the deposition process of PCC coating on the surface of pure iron [106].

In the zinc phosphating solution, the addition of Zn2+ supports the formation of crystals of zinc phosphate. Zinc ions combine with phosphate ions to form an insoluble film. The formation of hopeite is described by the reaction [106]:

$$\begin{aligned} \text{3 Zn}^{2+} &+ 2\text{ H}\_2\text{PO}\_4^- + 2\text{ H}^+ + 4\text{ H}\_2\text{O} + 6\text{ e}^- \rightarrow\\ \text{Zn}\_3\text{(PO}\_4\text{)}\_2 \cdot 4\text{H}\_2\text{O} + 3\text{ H}\_2 \end{aligned} \tag{15}$$

In some cases, Zn and ZnO were found in the phosphating process in reactions:

$$\text{Zn} + \text{2} \text{ e}^- \rightarrow \text{Zn} \tag{16}$$

$$\text{Zn} + 2\text{ H}\_2\text{O} \rightarrow \text{Zn(OH)}\_2 + \text{H}\_2\tag{17}$$

$$\text{Zn(OH)}\_{2} \rightarrow \text{ZnO} + \text{H}\_{2}\text{O} \tag{18}$$

In the phosphate solution of coexisting Zn2+ and Ca2+, zinc calcium phosphate can be formed by the reactions:

$$\text{Ca}^{2+} + \text{HPO}\_4^{2-} \rightarrow \text{CaHPO}\_4 \tag{19}$$

$$\text{Ca}^{2+} + 2\text{ Zn}^{2+} + 2\text{ H}\_2\text{PO}\_4^- + 2\text{ H}\_2\text{O} \rightarrow \text{CaZn}\_2\text{(PO}\_4\text{)}\_2 \cdot 2\text{H}\_2\text{O} + 4\text{ H}^+\tag{20}$$

It is notable that the phosphating mechanism varies in different phosphating systems and materials [106].

The formation of conversion coating on zinc-coated samples under cathodic conditions was studied by PERRIN et al [107] in a chromating bath containing phosphate (phosphate-chro‐ mate solution). Thick chromium phosphate (Cr-P) coating has two distinct layers: an outer porous layer and an inner and thinner pore-free adherent one. Both layers contain chromi‐ um phosphate as the main constituent and, to a lower extent, zinc phosphate species, the concentrations of which decrease from the metal-coating interface outwards. Formed zinc phosphates have a general formula of *x*CrPO4·*y*Zn3(PO4)2·*z* H2O where *x* > (*y*, *z*).
