Phosphating

Phosphating During phosphating, porous, absorbent and poorly soluble metal phosphate layers form on metal surfaces that are firmly fixed to the base metal. Phosphate layers consist of a mixture of secondary and tertiary phosphates of the metal contained in the phosphating solution (zinc, manganese, iron) and the processed base metal (iron, zinc, cadmium, aluminium). Phosphating occurs most frequently on steel. The crystal structure of phosphates approximately corresponds to that of Hopeite, phosphophilite and vivianite minerals. Because phosphating is not possible due to resistance to phosphoric acid, stainless steels are oxalated (metal oxalates are formed through the bonding process of oxalic acid). Both conventional processes and special processes are used in surface engineering (Figure 1). Conventional processes are widely used, such as layer-forming processes using zinc, manganese or iron phosphates and non-layer-forming processes in which alkali phosphates are produced. Phosphate coatings improve the service properties of workpieces. They provide temporary corrosion protection, reduce the coefficient of friction, create a good adhesion base for organic coatings and have an electrically insulating effect. For temporary corrosion protection of components during storage or transport, layer-forming phosphate coatings with passivation are used.   The coating weight on steel corresponds to 15 g/m2 (approximately 5.5 µm thickness) and on electrodeposited zinc or cadmium coatings is up to 4 g/m2 (approximately 1.6 µm). Non-layer-forming shaping processes are used only for coatings that require very short storage times and must be applied immediately. Phosphate coatings with a coating weight of 40 g/m2 (thickness approximately 15 µm) and post-process treatment with special anti-rust oil or wax provide corrosion protection properties similar to chromated zinc coatings. Both layer-forming and non-layer-forming processes are used to improve the corrosion resistance and adhesion of paint layers and plastic films. The porosity and rough surface of the phosphate layer provide good mechanical anchorage for the organic layer. The layer weight ranges between 0.3 and 6.0 g/m2. An overview of phosphating processes for pre-treatment before painting and use as a sliding layer is shown in Table 1. Phosphate coatings are also used as lubricant carriers to facilitate continuous deformation that is not severed during, for example, the drawing of sheet metal parts, wire drawing, tube drawing or extrusion. After phosphating, the surface to be formed is immersed in a soap solution and metal soaps are formed as a result of chemical reaction. The firmly adhering lubricant film reduces interfacial friction between the tool and workpiece surfaces and maintains its effect throughout the entire forming process. This provides higher forming degree, faster forming, longer tool life and better surface quality. Coating weights range between 2 and 20 g/m2 (corresponding to approximately 0.8 to 7 µm).   On cutting tools, wear can be significantly reduced by phosphating the surface. The effect is presumably due to improved chip flow. The insulation properties of the coatings are mainly applied to transformer sheets phosphated in a continuous process.

1. Conventional Methods 1.1 Layer-Forming Phosphating

Layer-forming phosphating solutions generally contain a soluble primary metal phosphate (zinc, manganese or iron phosphates), free phosphoric acid and usually an oxidizing substance as an accelerator. During phosphating, primary dihydrogen phosphates, secondary hydrogen phosphates and finally practically insoluble tertiary phosphates become more difficult to dissolve:     The reaction is triggered by the initial acidifying effect of phosphoric acid on iron in steel sheets or zinc in galvanized sheets:     Phosphoric acid consumption shifts the balance to the right and leads to the formation of insoluble phosphates. These form phosphate crystals at crystallization nuclei on the surface oriented according to the metal's crystal lattice. With further crystal growth and surface coverage, a phosphate layer forms that protects the base metal from further acid attack when sufficient layer thickness is reached. Phosphated zinc surfaces appear light to dark grey in colour. Zinc-based phosphating solutions contain zinc orthophosphates and free phosphoric acid. Typical zinc contents range between 5 and 20 g/l. Process times for immersion are 30 to 60 minutes and achievable coating weights are 30 to 50 g/m2. Working temperature ranges between 70 and 90°C. The result of the layer-forming phosphating process depends on (Figure 2):   The rate of formation of phosphate layers increases with an increase in solution concentration, a decrease in free phosphoric acid content, an increase in solution temperature and the addition of oxidizing substances. Layer thickness and crystal size depend on the surface condition of the base material prior to phosphating. On rough surfaces, thicker and coarser crystal layers form compared to smooth metal surfaces. Non-uniform phosphate layers may result from base material impurities or rolling defects. Another source of defects is insufficient cleaning of the parts to be phosphated. The largest source of defects comes from the phosphating process itself. For this reason, monitoring of process parameters is very important. The higher the total dissolved salt concentration in the solution, the more insoluble phosphate forms, that is, the faster the phosphate layer forms. The free phosphoric acid content is decisive for the dissolution reaction. The higher the free acid ratio, the greater the acidifying effect. On the one hand, the acid effect creates a bright metallic surface while on the other hand the surface becomes rough, which improves the adhesion of phosphate coatings. For this reason, continuous monitoring is required. Impurities introduced into the phosphating solution through inadequate pre-treatment have a harmful effect on phosphating. Foreign substances enriched in the solution, such as sodium, calcium and magnesium, are incorporated into phosphate layers. Excessive grease/oil, chloride and sludge content can also damage layer structure. The process temperature for phosphating is approximately 90°C. When oxidizing substances are used, the temperature of the phosphating solution can be reduced. The use of accelerators or oxidizing substances leads to a shift in chemical equilibrium and an increase in reaction rate. This process results in more uniform phosphate layers and the working temperature can be reduced to 35 to 60°C. The phosphating result depends significantly on the choice of oxidizing substance. Additives also contribute to reducing acid consumption. In this process, divalent iron is oxidized to trivalent, which results in very poorly soluble ferric phosphate that precipitates as sludge. The most important oxidizing substances are: • Alkali nitrates at NO3- ratio: P2O5 = 0.3 to 0.5 • Alkali nitrite, content 0.1 to 0.5 g/l NO2- • Alkali chlorate at ClO4- ratio: P2O5 = 1 to 4 • Hydrogen peroxide, content 0.05 to 0.15 g/l H2O2 Other oxidizing substances also include borates and organic nitro compounds. The type of accelerator determines the growth and structure of the phosphate layer. For example, phosphate layers accumulated from phosphating solutions accelerated with nitrite are fine-crystalline and show good coating coverage even at low thicknesses. When selecting a phosphating substance, these relationships should be considered to ensure that quality requirements for phosphate layers are met. Phosphating can be carried out by immersion, spraying or roll coating, that is, by applying liquid in measured quantities. The result of the phosphating process depends on the changing conditions in the solution in the immediate vicinity of the surface, that is, in the boundary layer. The change can be improved by intensifying circulation in the immersion process. Care should be taken to prevent oversaturation; therefore, a method appropriate to the process should be found between mixing the solution in application and concentration. Fine-grained layers in particular are produced using vibration and ultrasound. Spray technology provides improvement in reaction time. The surface to be coated is continuously fed with fresh phosphating compound, which results in the formation of closed phosphate layers in relatively short time. The acceleration of the process results from the increased diffusion rate arising from irregular mass transfer in the boundary layer. In the process called cold-rolled strip, roll application or roll coating, continuous phosphating is possible via spraying or by application with a rotating roll. In the roll application process, selective, one-sided coating is possible. Compared to the spraying process, this method requires less space and offers advantages in terms of maintenance, process control and preparation of the phosphating solution. In conventional zinc phosphating, the base weight of the applied coating is typically 5 to 20 g/m2. The phosphating compound contains approximately 12.2% zinc, 22.5% phosphorus pentoxide and 15.6% nitrate. The solution density is 1.6 g/cm3.

1.2. Non-Layer-Forming Phosphating Process

In non-layer-forming processes, phosphating solutions contain alkali phosphates that react with the base metal. Very thin layers form (layer weights range between 0.2 and 2.0 g/m2). In the phosphating reaction, the anion PO43- is used in the form of alkali hydrogen phosphate and the cation is supplied by the metal to be coated. Because of its iron phosphate content, this is also called iron phosphating and is of great practical importance for achieving optimum adhesion of paints or powder coatings. At pH 3.5 to 6, the coating reaction proceeds much more slowly than zinc phosphating: The layers formed contain a mixture of tertiary ferric phosphate and ferric hydroxide. The crystal structure of the layer has the same tendency as the base material. Iron-phosphated metal surfaces have a bluish bright colour. Solution processes for non-layer-forming phosphating operate at temperatures around 70°C with NaH2PO4 concentrations of 2 to 20 g/l. The pH is adjusted by adding phosphoric acid, acidic alkyl phosphate ester or acidic alkyl phosphonium salts. The addition of oxidizing substances accelerates film formation, increases adhesion and improves functional properties. Alkali chlorates, nitrates, nitrites or bromates are used as accelerators.

2. Special Processes

Special processes include phosphating involving high-pressure or steam spray treatment and solvent-based phosphating performed at low temperature. In high-pressure and steam spray processes, cleaning and phosphating are carried out in a single stage. This process is easy to apply, particularly in plant engineering operations involving large-area parts or small parts phosphating. Since the specified application times for pre-treatment and phosphating must be observed, neither process provides any special advantage over conventional techniques. In organic phosphating, the protective layer consists of a passive and amorphous phosphate layer coated with a thin phosphate polymer film. Process conditions must be strictly observed. Sources • Die Phosphatierung von Metallen, Eugen G. Leuze. • Technologie der Galvanotechnik, Eugen G. Leuze. • Phosphatieren, Oxalatieren, Merkblatt 166,Stahlanwendung. İzzet Aydın General Manager Hillebrand Chemicals Kimyasal Pazarlama Ltd. Şti.