Лако-красочные материалы — производство

Chromates have long been utilized as anticorrosives. The anticorrosive action of chromates is based on the content of a certain amount of water-soluble chromate within these pigments (Table 5.3).

Chromates have been widely successful because most paint films are permeable to water. If the rate of moisture permeability is matched to the solubility of the chromate pigment used then enough chromate ions may migrate to the metal surface to initiate and sustain the formation of passive layers [5.51]. The effectiveness of chromates in both the cathodic and anodic areas is simplified and illustrated in Figure 5.11 [5.58].

Fe—Fe2++2 e Fe(OH)2 H20+ 1/2 02 + 2e—2 OH’

-► Cathode

n[3]Cr? n . r^r————- ;—і. Cr6+ + 3e—Cr3+

0 0 —| Chromate I-* Qr3+ + зе_^сг

oYr«o

Fe(OH)2 • 2 CrOOH *—

Passivation

Fig. 5.11 Atmospheric corrosion of iron and inhibitive effects of chromate pigments [5.58].

The anticorrosive properties of chromate pigments are related to

• the content of water-soluble chromate ions.

• the neutralizing and pH-stabilizing effect of zinc hydroxide present in zinc — based chromate pigments.

ISO 1249 — Zinc chromate pigments — Basic zinc potassium chromate pigments and zinc tetrahydroxychromate pigments — specifies the requirements, and corresponding test methods for zinc chromate pigments suitable for use in paints and/or corrosion — inhibiting coatings [5.59].

The requirements on strontium chromate pigments are specified in ISO 2040 — Strontium chromate pigments for paints [5.60]. Required characteristics for zinc potassium chromate, zinc tetrahydroxychromate and strontium chromate are sum­marized in Table 5.4.