Corrosion of Metallic Coatings
291
Conversion coatings belonging in this group are phosphate, chromate,
oxide, and anodized coatings. These coatings are composed of corrosion
products that have been formed artificially by chemical or electrochemical
reactions in selected solutions. The corrosion products thus formed build
a barrier protection for the substrate metal. This barrier reduces the active
surface area on the base metal, thereby delaying the transport of oxidizers
and aggressive species. By so doing, the coating inhibits the formation of corrosion cells. The degree of secondary barrier action depends on the compactness, continuity, and stability of the corrosion product layer.
Each conversion coating protects the base metal against corrosion with
two or three of the following protective abilities:
1. Secondary barrier action of corrosion products
2. Inhibiting action of soluble compounds contained in the corrosion
products
3. Improvement in paint adhesion by the formation of a uniform corrosion product layer
8.5.1 Phosphate Coatings
When a metal surface is treated with a weak phosphoric acid solution of
iron, zinc, or manganese phosphate, phosphate layers are formed. These
phosphate coatings are applied to iron and steel, zinc, aluminum, and
their alloys.
Phosphate films are formed by the dissolution of base metal and the precipitation of phosphate films. The metal surface must be free of greases, oils,
and other carbonaceous materials before immersion in the phosphating
solution or before spray application. Baths operating at 120°F (50°C) have pH
values of approximately 2 while those operating below 120°F (50°C) have pH
values of approximately 3.
The zinc phosphate coating is basically the result of a corrosion process.
Reactions of iron and steel in a zinc phosphate solution are as follows:
1. The dissolution of base metal at the anodic sites:
Fe 2H PO
Fe H PO
H
3
4
2
4
+
→ (
) +
2
2
Promotion by the activator:
2
2
3
3
3
4
Fe H PO 2H
NO
2FePO 3H O NO
4Fe Zn
2
4
–
2
2
2
+
+
+
→
+
+
+
2 2+
2
4
–
2
4
3
4
2
6H PO
NO
4FePO Zn PO
6H O NO
+
+
→
+
( ) +
+
6
6
2
291
Conversion coatings belonging in this group are phosphate, chromate,
oxide, and anodized coatings. These coatings are composed of corrosion
products that have been formed artificially by chemical or electrochemical
reactions in selected solutions. The corrosion products thus formed build
a barrier protection for the substrate metal. This barrier reduces the active
surface area on the base metal, thereby delaying the transport of oxidizers
and aggressive species. By so doing, the coating inhibits the formation of corrosion cells. The degree of secondary barrier action depends on the compactness, continuity, and stability of the corrosion product layer.
Each conversion coating protects the base metal against corrosion with
two or three of the following protective abilities:
1. Secondary barrier action of corrosion products
2. Inhibiting action of soluble compounds contained in the corrosion
products
3. Improvement in paint adhesion by the formation of a uniform corrosion product layer
8.5.1 Phosphate Coatings
When a metal surface is treated with a weak phosphoric acid solution of
iron, zinc, or manganese phosphate, phosphate layers are formed. These
phosphate coatings are applied to iron and steel, zinc, aluminum, and
their alloys.
Phosphate films are formed by the dissolution of base metal and the precipitation of phosphate films. The metal surface must be free of greases, oils,
and other carbonaceous materials before immersion in the phosphating
solution or before spray application. Baths operating at 120°F (50°C) have pH
values of approximately 2 while those operating below 120°F (50°C) have pH
values of approximately 3.
The zinc phosphate coating is basically the result of a corrosion process.
Reactions of iron and steel in a zinc phosphate solution are as follows:
1. The dissolution of base metal at the anodic sites:
Fe 2H PO
Fe H PO
H
3
4
2
4
+
→ (
) +
2
2
Promotion by the activator:
2
2
3
3
3
4
Fe H PO 2H
NO
2FePO 3H O NO
4Fe Zn
2
4
–
2
2
2
+
+
+
→
+
+
+
2 2+
2
4
–
2
4
3
4
2
6H PO
NO
4FePO Zn PO
6H O NO
+
+
→
+
( ) +
+
6
6
2