Low-temperature solutions for crystalline phosphating
In order to reduce temperature and toxisity of phosphating solutions used for adhesion and protective treatment nickel ions were replaced by cerium ones and hydroxyl amine was added as an accelerator. Adhesion phosphate layers with maximum protective ability (90 sec) were shown to form at cerium concentration 30 to 50 mg/l (Fig.1). In “KF-7” protective phosphating solution the layers of 6 g/m <sup>2</sup> are formed over the concentration range of cerium 40 to 90 mg/l and Akimov's protective ability (ЗСА) is tree times higher than in nickel-containing solution (160 sec against 45 sec). An appreciable worsening of protective ability is observed at lower temperature (30 degree C) in both solutions, while the mass of the phosphate layer is still within acceptable limits and is about 5 g/m <sup>2</sup> in the protective solution and about 2 g/m <sup>2</sup> in the adhesion solution (Fig.3). Layers of low quality with bad protective power are formed at room temperature - 5 and 14 sec. respectively. As it follows from the kinetic curves, formation of the phosphate layer (3 min for adhesion pbps hating and 10 min for protective pups-pushing) within the prescribed time is completed only at temperatures above 40 deg. C and it is confirmed by the stabilization of the mass of the phosphate layer in the course of the process (Fig. 4). At lower temperatures the formation of the layer is not completed within the time shown above. Electron microscope photos show that in the presence of hydroxyl amine tightly packed layers should form with smaller grain size and densier packing and this effect manifests itself in the increasing protective ability (Fig. 5). Cyclic corrosion tests have shown (Fig. 6) that with respect to the protective ability all phosphate coatings tested meet the re-quirements since the penetration width of corrosion in all cases was less than 2.0 mm after 240 hr tests.