28 декабря, 2015 
Pokraskin As mentioned in Section 5.2.5.1, zinc phosphate, while having many desirable properties as an anticorrosive pigment, does not demonstrate the degree of corrosion protection offered by lead and chromate pigments [5.55]. Therefore the pigment industry has concentrated on developing phosphate-based pigments with improved properties.
By controlled chemical modifications under consideration of different points of view with suitable elements and compounds connected with the optimization of the manufacturing processes, it has become possible to improve the effectiveness of zinc phosphate for many applications [5.68, 5.70].
An overview ofmodified orthophosphate pigments that have gained economic importance is given in Table 5.6. When looking at Table 5.6, it becomes obvious that the pigment industry has developed several variations of zinc phosphate to improve
the performance properties. These developments have been possible by taking account the effect of synergistic values [5.54]. For example, the development of basic zinc phosphates, based on the knowledge that when the hydroxyl ion concentration is increased, the equilibrium of the local cathodic reaction will be stabilized, resulting in prevention or inhibition of the emission of electrons [5.78]. In addition, a pH-stabilizing effect in the coating due to the presence of basic compounds in the pigment is also discussed [5.54].
The aim of phosphate borate combinations was to accelerate the readiness to hydrolyze [5.78] because, e as discussed in Section 5.2.5.1, the hydrolyzation process is one prerequisite for the effectiveness of zinc phosphates but these pigments need a certain time for activation. The improved anticorrosive activity of zinc phosphate molybdates is attributed to the inhibitive effect of water-soluble molybdate ions [5.55].
Tab. 5.6: Anticorrosive pigments based on orthophosphates [5.71]
|
Product |
Modification |
Reference |
|
Zinc aluminum |
with aluminum phosphate |
[5.72-5.74] |
|
phosphates Basic zinc phosphates |
containing basic components |
[5.72,5.73] |
|
(e. g. zinc hydroxide) partly with differently treated |
[5.72,5.75] |
|
|
organic compounds partly with zinc molybdate and/or |
[5.76-5.78] |
|
|
calcium molybdate partly with basic zinc borate |
[5.75,5.78] |
|
|
partly with iron phosphate |
[5.79] |
|
|
partly with calcium phosphate |
[5.77] |
|
|
partly with potassium phosphate |
[5.73] |
|
|
partly with barium phosphate |
[5.77] |
|
|
partly with aluminum phosphate |
[5.80] |
|
|
and zinc molybdate partly with treated inorganic |
[5.75] |
|
|
Zinc-free phosphates |
compounds variation ofthe cations e. g. with |
[5.54, 5.81] |
|
Zinc phosphate silicates |
calcium phosphate and/or magnesium phosphate partly with calcium silicate and |
[5.54, 5.80, 5.82] |
|
strontium phosphate partly with barium phosphate |
[5.83] |
|
|
partly with calcium carbonate |
[5.84] |
|
|
partly with differently treated |
[5.54, 5.83] |
|
|
Zinc-free phosphate |
organic compounds variation ofthe cations, e. g. with |
[5.83] |
|
silicates |
calcium phosphate silicate |
|
|
partly with barium phosphate |
[5.83] |
|
|
partly with strontium phosphate |
[5.83] |
An aluminum zinc phosphate is produced by coprecipitation of primary aluminum phosphate (Al(H2PO4)3) with zinc oxide. Such pigments have a higher phosphate content than standard zinc phosphate. The improved performance properties are attributed to this higher phosphate content [5.85].
In the case ofthe organically treated modified phosphate pigments, a closer bonding between pigment and binder, and also between coating, and substrate are discussed [5.86].
As a further example, phosphate silicates that are often termed as mixed phase or core pigments, may also be mentioned. By fixing the active components onto the surface of wollastonite (calcium silicate-core) and adjusting the pH value near neutral, these pigments show a relatively universal applicability [5.86].
5.2.5.3
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