(1) The coatings
Phosphating (previously also called bonderising or parkerising) is a surface treatment most specifically used for steel and iron components. It can also be carried out on e.g. zinc and, in special cases, other non-ferrous metals. A highly insoluble metal phosphate coating is formed on the surface during phosphating. There are different types of phosphating processes, such as iron phosphating, zinc phosphating and manganese phosphating in particular. There are also special finishing variants such as zinc-calcium-phosphating or tri-cationic phosphating. The iron phosphating process is the only non-coating-building process that creates an amorphous conversion layer on the iron base through the influence of free phosphoric acid and acid alkali phosphate. It is mainly used as an intermediate step prior to other coating processes such as e.g. paints. The other phosphating processes are classic coating processes. In the iron phosphating process, cations for the layer formation originate from the base material and the aqueous process solution only contains the phosphate anions. The aqueous solution used for other phosphate coatings contains both phosphate anions and metal cations. Metal cations from the base material may also be included in the coating. We always implement the most frequently requested phosphate coating processes using state-of-the-art production technology and the immersion process or we develop processes adapted specifically to the product as required by the customer.
(1a) Zinc phosphate / ZnPh
Corrosion protection is the main aim of zinc phosphating and this protection is increased by the zinc content. The pores that form between the crystals are perfectly suited to increase the take-up of various corrosion protection oils. Zinc phosphate is also suitable as a sliding layer. It is more commonly used for non-cutting shaping processes in preference to manganese phosphate as the alkaline soaps in those processes form friction-reducing zinc soaps with the zinc phosphate. The coating is pale to mid grey and matt. It forms from coarse to fine crystals and can be controlled, even down to low layer thicknesses of a few micrometres. The layer thicknesses usually lie within the range of 3 to 10 µm. The fine graininess or crystal size of the layer can be easily controlled through the correct selection of the process parameters and by adding special accelerator systems or calcium (zinc calcium phosphate) for fine graininess to the phosphating solution. Special pre-activation systems are also frequently used to apply specific influences on the layer formation.
(1b) Manganese phosphate / MnPh
Durable sliding properties are the focus for manganese phosphate. The layer reduces friction and is highly absorbent for oil so that it can be used, for instance, for slide bearings and bushes. It is preferred to the other phosphating processes for highly loaded sliding partners. It also has very good corrosion protection, dependent on the oil used, coming close to that of zinc phosphate. The corrosion protection of both layers is almost identical in the phosphating systems used today. Manganese phosphate forms finer crystals and can produce layers in the range of 2 to 3 µm with modern phosphating systems. Uniform thicknesses below approx. 5 μm were difficult to achieve on previously used substrates. Pre-activation (pre-rinsing solution for nuclei deposition) is almost essential here in contrast to zinc phosphating. The deposition of thicker manganese phosphate layers is also possible through special adjustment of the phosphating solution, dependent on the pieces being phosphated. The sliding properties do not however improve with thicker layers, therefore a range between 3 and 10 µm is most practical, as is the case for zinc phosphate. The coating is dark grey to black and matt.
(2) The layer thickness and the surface weight
The surface weight, and not the layer thickness in micrometres, is preferably specified in phosphate coating processes. This is to do with the fact that the layers are relatively soft and porous. The pressure applied by a mechanical measuring gauge will reduce the thickness being measured. The thickness also changes if a load is applied. Therefore, surface weights are more applicable for verifiable specifications (see DIN EN 12 476). We stated as a point of reference that the standard and most commonly used coatings have a layer thickness that usually ranges between 5 and 15 μm. In comparison, primers usually lie significantly below 10 µm (generally between 0.5 and 5 µm) and oil-absorbent anticorrosion coatings lie around approx. 10 µm. The following applies to surface weights:
- Manganese phosphate, optionally between 5–30 g/m², as a thin layer from 2 g/m²
- Zinc phosphate, optionally between 5–30 g/m², as a thin layer from 2 g/m²
- In both cases, it is possible to produce higher surface weights on request for special applications
- Zinc calcium phosphate, approx. 2–20 g/m²
For comparison:
- Iron phosphate, approx. 0.3–0.8 g/m², conversion layer thickness below 0.8 µm
(3) Production
We offer all types of phosphating using the immersion process and not by spraying. This is the standard method for general coating tasks. Spraying is used for special cases such as e.g. vehicle chassis coating before painting, etc. The steel, cast or iron components to be phosphated are immersed sequentially in a series of various baths. These are purely chemical processes without any use of external power. First of all, the surfaces are degreased and cleaned, pickled where necessary, then subject to nucleation in a separate activation dependent on the process, coated in the phosphating bath, rinsed, dried where necessary and then conserved. All phosphating processes use acids as it is necessary for the reaction sequence to generate a pickling attack with loosening of iron or metal cations from the surface. This leads to some roughening of the surface under the coating, but our long-term process optimisation means this can be appropriately influenced and kept to a minimum. The phosphating concentrate used must be matched in type and concentration to the components being phosphated and to the coating aims. The production of hydrogen on the interface between metal surface and phosphating substrate caused by the pickling attack (initial reaction step of multi-step equilibrium reaction for coating formation) means that it may be necessary to subject hardened, critical components to a thermal post-treatment. This will drive out any hydrogen penetrating the structure and prevent possible hydrogen embrittlement of the components. The times and temperatures required for this process are usually specified and can be found as normative regulations in special standards (e.g. applicable standards, specified in DIN EN 12476 and individual company standards). The maximum working temperature for manganese phosphating is just below 100 °C and far lower for zinc phosphating, so it is well below standard annealing temperatures. The delivery condition of the phosphated articles (phosphated workpieces) occurs as required either conserved with an oil emulsion or passivated using aqueous amine-based systems. It is also possible to provide variants such as waxing, greasing or special oiling treatments upon request. The phosphating treatments mentioned above are carried out in our house using automatic plants and computer-based program controllers to completely exclude any deviations and non-uniformity, such as those occurring in hand-operated systems. This guarantees that the customer receives only high-quality, reliable and economic results.
(4) Workpieces/Components
As workpieces are immersed, coating formation is continuous without any partial phosphating. It is however possible to apply maskings made of removable paints or covers so that specific areas remain blank. This may be necessary if the dimensions and tolerances of individual areas must not be changed or if phosphating of these partial areas would not be technically beneficial. Such masking is applied and removed manually, often leading to substantial added work. However, an absolutely linear delimitation cannot be guaranteed. It is not possible to completely prevent the substrate from running under the edges of numerous masking types. The components can be treated as drum goods in a cost-effective bulk goods process. Permissible friction and pressure values on the components in the drum are precisely defined for this purpose, and the permissible drop heights are determined and complied with. Baskets or frames are used when immersing larger and more sensitive parts. These racked goods are carefully loaded and unloaded by trained personnel – with gloves of course – so as to avoid any mechanical damage. We can rapidly provide you with a precise quote once we know the dimensions, geometry, weight, material type and surface quality of your workpieces. The knowledge of the precise workpiece geometry (drawing, where possible) allows us to optimise workpiece positioning and correctly take account of any possible scooping or air inclusions. It is important, in all cases, that the components consist of an iron-based material that can be phosphated, that they are as clean as possible (with the minimum required preservation agents) and that they are delivered without being treated with any silicone oil/grease or wax-based media. Hardened or unhardened steel surfaces that have been freshly mechanically processed are particularly suitable for appropriate phosphating. Workpieces with passive layers, rust or scale must also be treated by pickling; this is yet another service that we offer. In comparison to black oxidation, the acidic phosphating process is more tolerant against interfering surface influences. As some alloying elements in steel can affect the process and some materials act as bath toxins, the precise indication of the steel type is extremely useful to help achieve the required technical properties. Phosphating is normally applied to unalloyed or low to at most medium-alloyed steels. Stainless steels cannot therefore be phosphated in a classic phosphating process due to their high alloy content. However, we can offer other methods, as we do for other metals.
(5) Durability and protective value of phosphate coating processes
Phosphate coating processes are resistant in particular to lubricants such as greases, oils and most oil additives, and are therefore also suitable for numerous technical applications. Contact with various glycols can lead to coating attacks in some cases and must be tested empirically. It is possible to attack and remove the coating using highly alkaline solutions and special pickles. In interaction with a suitable oiling or sealing, the steel is shielded quite well from external environmental influences and particularly against corrosion. This rust protection meets far higher requirements than black oxidation can offer.
(6) Tribology and technical suitability
Phosphated components have a series of different properties that are individually significant, depending on the application case. The extent of the required property can be supported by the special process:
- With regards to appearance, the matt-grey to almost matt black surface is preferred for components where light reflections are not wanted. The surface (micro-porous, crystalline structure) "absorbs" the incoming light and does not emit any reflections. The even darker manganese phosphate is of particular interest here.
- Phosphating is used most frequently as corrosion protection. In this case, the phosphate coating is frequently used in combination with appropriate oiling or sealing. The hollows, pores and connecting capillaries between the crystals (micro-porous structure) in the phosphate coatings are highly oil absorbent.
This property is often used in combination with the excellent adhesion to the base material property, so that the phosphate coating (unoiled) is used as the primer for a wet paint or powder coating. This micro-porous/crystalline structure thus offers a superb intermeshing adhesive substrate for paints. In a dry, unoiled condition, the pores between the crystals and their connecting passages (micro-porous structure), which can also reach up to the surface, lead to the opposite effect. This can lead more rapidly to the absorption of moisture and other media that promote corrosion.
- Another essential property of a phosphate coating is electrical insulation, which can be used for particular applications such as electrical steel sheets, magnetic cores, transformers, generators and also for non-conductive ball and roller bearings.
Phosphated components also evidence particular technical properties with regards to their relative movements against each other. In general, components designed for sliding are made of hardened or wear-resistant steels, have a very low surface roughness due to machining processes and are then phosphated:
- The advantages include improvement of running-in and emergency running properties, reduction in sliding resistance and reduced wear during sliding movements. The tendency towards scuffing between the contact surfaces is also reduced.
- The improvement in sliding properties can also be achieved in interaction with lubricants that are held in the micro-porous structure and which can then be extruded as required under pressure.
This is a basis today for the machining processes used in chipless cold forming (punch parts, bending parts, deep drawing). Semi-finished products such as sheets are often only zinc or iron-phosphated so that they can be shaped and processed using more demanding processes. The workpiece service lives can also be increased. The field of tribological applications for phosphating coating processes is so wide that we can only provide a basic outline here. With our well-equipped technical centre, we are happy to offer appropriate sample processing so that we can adapt the process parameters during tests and in future serial production to obtain the results required.
(7) A special DEWE Brünofix skill
Phosphating can be implemented using diverse methods and with diverse quality, even if the results initially appear almost similar at first. The layer growth, crystal size and anchoring on the surface can be positively influenced through our specific process management. We also take care to avoid damaging the base material with too many or larger pits due to pickling. The reliable maintenance of the layer thickness presupposes very tight bath management tolerances. We can precisely match the chemicals, concentrations, temperatures, times and accurate process control to the required result. An obvious statement is that many classic phosphating recipes contain environmentally damaging substances such as e.g. nickel, which have special positive properties with regards to layer formation. Due to the innovative developments in phosphating systems, the choice can now be made for a modern and environmentally friendlier recipe. We constantly work on creating chemical products that are as environmentally compatible and user-friendly as possible. This means that we could offer our customers such environmentally-friendly products for use in their own systems and also use such state of the art recipes for contract treatment in-house. We can offer particularly comprehensive knowledge about all aspects of the phosphating process as we are not just a contract coater; we also supply complete phosphating plants and produce the phosphating chemicals ourselves. We carry out bath tests rapidly and in detail in our well-equipped in-house laboratory. As developers and manufacturers of the chemicals in question, our engineers are capable of analysing the composition of the baths and precisely adjusting or modifying it to specific requirements as necessary. We set up the optimal process baths and sequences for the requirements and special features of the workpieces, so that this, in combination with our modern plant engineering, allows us to offer customers a high and uniform quality level and ensure that the required technical and optical product properties are maintained. A lot of experience and know-how are necessary to select the right parameters, and to optimally and reproducibly ensure the desired effects. This know-how is available in house and can be used by our customers to further their aims. Reprints, including excerpts, are prohibited. (CB/KN/TS/12.10.2011)