Crystal Structure and Properties of Zinc Phosphate Layers on Aluminum and Steel Alloy Surfaces

Abstract

Many studies have been carried out on the phosphating of steel and aluminum alloys used in automotive engineering, but characterization of the properties of the phosphate layers formed by the co-phosphating of these alloys in the presence of different base metals is still lacking. In this study, the crystal structure and properties of the phosphate conversion layers formed on the surface of the aluminum alloys important in vehicle manufacturing (cast and forged AlSi1MgMn, and AA6014 panel) and the CRS SAE 1008/1010 reference steel plate by co-deposition prior to painting were investigated. On a process line set up for the phosphating of typical iron and steel alloys, the phosphate coating was formed using nitrite and nitroguanidine accelerators under identical technological parameters. The microstructure of the formed phosphate layers was examined using scanning electron microscopy (SEM), its phase composition using X-ray diffraction (XRD), and its elemental composition using energy-dispersive X-ray analysis (EDX). The suggested main crystalline phase (Zn2.3(Ni0.1Mn0.6)(PO4)2·4H2O) in the surface phosphate layer of both aluminum alloys studied was similar to hopeite, whereas in the steel plate, a minor hopeite phase were identified in addition to the main crystalline phosphophyllite phase (~95%). It can be concluded that, during the combined phosphating treatments, the surfaces of different aluminum and steel alloys behaved similarly to the individual treatments and did not impede the coating reactions of the other metal. To obtain an adequate coating of aluminum and steel alloys, fluoride should always be present in the production line. Comparing the effects of accelerators, we found that the use of nitrite accelerator with the same amount of fluoride resulted in a higher coverage and better quality of the surface protective layer of the aluminum alloys. However, for the steel plate, there was no significant difference between the phosphate coatings prepared with the two different accelerators.

Effect of Adhesion and Corrosion Performance of Geomet Basecoat (321)- Topcoat (ML Black) Applications on Cataphoretic Coating

“…Activation pre-treatment affects phosphate crystal structure and size [30]. The crystal grains in the coating are thick, especially in zinc phosphate baths.…”

The Effect of Different Accelerators on the Corrosion Protection of a Surface Coating on Spheroidal Graphite Cast Iron (Ductile Iron)

“…The blasted ductile-iron sample plates were pretreated together with other sample plates as well as pieces made of steel and aluminum alloys in the same technological step simultaneously under industrial conditions according to a previously published process and its parameters [30].…”

“…Important parameters of phosphate baths are the free acid value (FA) which refers to the free H + ions present and the total acid value (TA) which represents the total phosphate content of the phosphate bath [8]. In our experiments, these parameters were as follows: FA=1.5 points; TA=22 points [30].…”

“…The samples were degreased, rinsed, activated, phosphated and rinsed again in successive steps using different baths. In the degreasing bath, any residual surface contamination was removed from the metal surface; in the rinsing bath(s), the degreasing chemicals were removed with water; in the subsequent bath, the metal surface was activated; in the phosphating bath, insoluble heavy-metal tertiary phosphates formed on the surface; and finally in the rinsing bath, acid residues, soluble salts and non-adhesive particles were removed from the metal surface [30]. Gardobond 2600 tricationic (Mn 2+ , Zn 2+ , Ni 2+ ) zinc phosphating solution (BASF, Chemetall Ltd.) was used for the experiments, which can be used for phosphating iron and steel by a dipping and spraying process.…”

“…In order to remove any residual salt content from the surface, a cascade rinse system using deionized water was employed. Finally, the pretreated samples were air-dried [30]. The composition of the bath and labeling of the samples are shown in Tables 3 and 4, respectively.…”