7.12.1P hosphate Coating
When ametal surfaceistreated with aweak phosphoric acid solution of iron,
zinc,o rm anganese phosphate, phosphate layers aref ormed. These
phosphate coatings area pplied to iron and steel,z inc, aluminum, and
theiralloys.
Phosphate films are formed by the dissolution of base metal and the
precipitation of phosphate films. The metal surfacemust be free of greases,
oils, and other carbonaceous material before immersioninthe phosphating
solution or before spray application. Baths operating at 1208 F/508 Chave pH
values of approximately 2, whereas, those operating below 1208 F/508 Chave
pH values of approximately 3.
The zinc phosphate coating is basically the result of ac orrosion process.
Reactions of irona nd steel in az inc phosphate solution area sf ollows:
1. The dissolution of base metal at the anodic sites:
Fe C 2H 3 PO 4 / Feð H 2 PO 4 Þ 2 C H 2
(7.23)
Promotion by the activator:
2Fe C 2H 2 PO
K
4 C 2H
C C 3NO 2 / 2FePO 4 C 3H 2 O C 3NO (7.24)
4Fe C 3Zn
2 C C 6H 2 PO
K
4 C 6NO 2 / 4FePO 4 C Zn 3 ð PO 4 Þ 2
C 6H 2 O C 6NO
(7.25)
2. Precipitation of phosphate films at the cathodic sites:
2Znð H 2 PO 4 Þ 2 C Feð H 2 PO 4 Þ 2 C 4H 2 O
/ Zn 2 Feð PO 4 Þ 2 $ 4H 2 O
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Phosphophyllite
C 4H 3 PO 4
(7.26)
3Znð H 2 PO 4 Þ 2 C 4H 2 O / Zn 3 ð PO 4 Þ 2 $ 4H 2 O
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Hopeite
C 4H 3 PO 4
(7.27)
With these reactions, the phosphate film consists of phosphophyllite and
hopeite. Phosphate solubilities are lowest in the pH range of 6–8. They are
stable in neutral environments and are nonelectric conductive compounds.
Phosphate film deposits on cathodic areas and anodic sites remain in
the form of pinholes. Consequently,the continuity of phosphate films is not
as good as those of anodicoxide and chromate films.
Since the barrier action of aconversion film is dependentonits solubility
and continuity,i ti se vident that the phosphate films provide only limited
protection. However, they do provide an excellent base for paint, plastic, and
rubber coatings.
Corrosion of Linings and Coatings
454
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
When ametal surfaceistreated with aweak phosphoric acid solution of iron,
zinc,o rm anganese phosphate, phosphate layers aref ormed. These
phosphate coatings area pplied to iron and steel,z inc, aluminum, and
theiralloys.
Phosphate films are formed by the dissolution of base metal and the
precipitation of phosphate films. The metal surfacemust be free of greases,
oils, and other carbonaceous material before immersioninthe phosphating
solution or before spray application. Baths operating at 1208 F/508 Chave pH
values of approximately 2, whereas, those operating below 1208 F/508 Chave
pH values of approximately 3.
The zinc phosphate coating is basically the result of ac orrosion process.
Reactions of irona nd steel in az inc phosphate solution area sf ollows:
1. The dissolution of base metal at the anodic sites:
Fe C 2H 3 PO 4 / Feð H 2 PO 4 Þ 2 C H 2
(7.23)
Promotion by the activator:
2Fe C 2H 2 PO
K
4 C 2H
C C 3NO 2 / 2FePO 4 C 3H 2 O C 3NO (7.24)
4Fe C 3Zn
2 C C 6H 2 PO
K
4 C 6NO 2 / 4FePO 4 C Zn 3 ð PO 4 Þ 2
C 6H 2 O C 6NO
(7.25)
2. Precipitation of phosphate films at the cathodic sites:
2Znð H 2 PO 4 Þ 2 C Feð H 2 PO 4 Þ 2 C 4H 2 O
/ Zn 2 Feð PO 4 Þ 2 $ 4H 2 O
|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl ffl {zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl ffl}
Phosphophyllite
C 4H 3 PO 4
(7.26)
3Znð H 2 PO 4 Þ 2 C 4H 2 O / Zn 3 ð PO 4 Þ 2 $ 4H 2 O
|fflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflffl}
Hopeite
C 4H 3 PO 4
(7.27)
With these reactions, the phosphate film consists of phosphophyllite and
hopeite. Phosphate solubilities are lowest in the pH range of 6–8. They are
stable in neutral environments and are nonelectric conductive compounds.
Phosphate film deposits on cathodic areas and anodic sites remain in
the form of pinholes. Consequently,the continuity of phosphate films is not
as good as those of anodicoxide and chromate films.
Since the barrier action of aconversion film is dependentonits solubility
and continuity,i ti se vident that the phosphate films provide only limited
protection. However, they do provide an excellent base for paint, plastic, and
rubber coatings.
Corrosion of Linings and Coatings
454
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485