Résumé : |
Looking into the history of anticorrosive pigments (ACO-pigments), the driving innovation factors were always performance and legislation in combination with costs. The toxic behaviour of red lead or chromates already opened the door to non-toxic alternatives 30 years ago. The stepwise ongoing stricter handling of for example the Cr6 ended up in an almost complete ban in Europe for the benefit of all consumers : Chrome-based pigments are SVHCs (substances of very high concern) in the European Union and currently the sunset date for eg Strontiumchromate and Zinctetraoxychromate is set for January 2019 with the possibility to apply for an authorisation until July 2017.
These pigments show an excellent cost/performance ratio. Nevertheless, due to their toxicity, for more than 30 years, alternatives like zinc phosphates have made inroads. Standard products with universel applicability significantly reduced the consumption of chromates, however they did net always reach the required performance due to a different working mechanism. All phosphate pigments have in common the fact that they build an anodic passivation via precipitation of phosphate complex layers in combination with metal hydroxide barriers (see Figure 1), white Crelons additionally act cathodically.
Therefore, standard zinc phosphate has been improved by two major steps : The first step was the modification of standard zinc phosphate changing, for example, pH, solubility, adding synergistic values etc. Second was the introduction of polyphosphates for higher sophisticated applications, such as coil coatings, aircraft primers etc, which can also be supported by the use of silica-based ion-exchange pigments.
All improved products are highly linked to specific binder systems, therefore more universel zinc-containing pigments have been developed as 'wide-spectrum-anticorrosives', combining the different improvement factors into a single pigment or even developing different modifications. |