MANGANESE PHOSPHATING
Manganese phosphate coating has the highest hardness and superior corrosion and wear resistances of general phosphate coatings. Manganese phosphating is extensively employed to improve the sliding properties of engine, gear, and power transmission systems. The use of manganese phosphated coatings for improved corrosion resistance can be found in virtually all branches of the metal working-industry. Typical examples mentioned here include motor vehicle components in brake and clutch assemblies, engine components, leaf or coil springs, drill bits, screws, nuts and bolts, washers, anti-vibration washers, tools, magnet cores, casting interiors and many other small items. Manganese phosphate coatings for conferment of good corrosion resistance, whether a post-treatment such as oil application is to be used or not, are invariably applied by the immersion method. The processing sequence can be summarized as follows:
Degreasing and cleaning Water rinse Pickling in mineral acid (where necessary) Water rinse (only after pickling) Activation Manganese phosphating Water rinse Final oven drying (optional) Lubricating with special oils or emulsions.
The degreasing and cleaning are usually done with strongly alkaline cleaners at concentrations of 1-5% and temperatures 65-95C. Treatment times range from 5-15 minutes. In recent years, a highly effective activating pre-rinse has been developed for manganese phosphates which permits alkaline cleaning and pickling of the work, without the penalty of coarse-crystalline phosphate formation. This is based on a finelydispersed manganese phosphate at concentrations 1-2 g/l. Manganese phosphating is mainly by immersion. Treatment times range from 5-20 minutes, the optimum time depending on the surface condition. The bath operating temperature is around 95C and only in special cases can satisfactory coatings be formed at temperatures around 80C. The phosphated components, after drying, are immersed in the oil or lubricant baths for 0.5-2 minutes, allowed to drain. The thickness of the resulting oil film depends on the oil used and its concentration. Manganese phosphating as a wear protection measure is widely used in the auto industry. Gearwheels in the gearbox, crown and pinion gears in the differential, camshafts, valves and valve-steams as well as pistons in larger diesel engines are frequently treated in this way. In other industries, the process is used to treat components in refrigerator compressors or oil pumps and their associated hydraulic rams for vehicle assembly plants.
- Operating parameters for some thick phosphating processes -
Phosphating System
Accelerator
Concentration (total acid points)
Temperature (C)
Treatment Time (min.)
Coating Weight 2 (g/m )
Zinc Phosphate Zinc Phosphate Manganese Phosphate Manganese Phosphate
None
20-50
90-98
30-60
20-45
Nitrate
50-80
70-98
5-15
10-35
None
20-50
90-98
30-60
20-45
Nitrate
30-60
90-98
5-15
8-30
The coating weight and crystal size of the manganese phosphate coatings are influenced to an even greater extent than in zinc phosphating by the mechanical, thermal, and chemical pretreatment of the workpiece surface prior to phosphating. For example, cleaning in alkaline aqueous cleaning agents or pickling in acids produces coatings with a much coarser texture. Even after such treatments, however, fine crystalline phosphate coatings are still obtainable if the workpieces are prerinsed in an activation rinse prior to the phosphating.
ZINC PHOSPHATING
In most operations where the corrosion resistance of finished workpieces must be especially high, conversion coatings are applied using zinc phosphate . This approach is widely used in the automotive industry and in certain sectors of the appliance and electronics industries. Similarly, zinc phosphating is often specified by the armed services, especially for equipment that may be exposed to severe environments. Moreover, many operations using electrocoating or powder coatings, particularly when a one-coat finish will be exposed to the weather, pretreat workpieces with zinc phosphate. TECHNICAL APPLICATIONS OF ZINC PHOSPHATING; Zinc Phosphating Prior to Powder Coating Zinc Phosphating Prior to Oiling / Lubricating Zinc Phosphating Prior to Wire Drawing Zinc Phosphating Prior to Tube Drawing IMMERSION PROCESS In the immersion mode, zinc and alkali metal phosphating systems do not greatly differ from one other. The individual steps, as normally carried out are ;
1. Cleaning stage 2. Water rinse 3. Activation 4. Phosphating 5. Water rinse 6. Post-rinse ; ; ; ; ; ; Temperature (C): 55-95 Temperature (C): 15-30 Temperature (C): 20-40 Temperature (C): 40-60 Temperature (C): 15-30 Temperature (C): 20-40 Time (min) : 5-10 Time (min) : 0.5-1.5 Time (min) : 0.5-1.5 Time (min) : 3-10 Time (min) : 0.5-1.5 Time (min) : 0.5-1.5
Cleaning is usually based on alkaline or strongly alkaline solutions in the concentration range 1-5%, and pH 10-13. Compared to the cleaners used in the spray mode, the cleaners used for immersion contain larger amounts of silicate and sodium hydroxide. The surfactants used often include the strongly-foaming anionic types, usually mixed with non-ionic types. Activation is mandatory for zinc phosphating and in its absence, because of the relatively high pH values involved, a thick and coarse-crystalline coating is formed which is quite unsuited of subsequent painting. Conditions for the rinse, post-rinse and drying stages differ little from spray phosphating.
SPRAY PROCESS
In case of spray phosphating, major differences are found in the process sequences for zinc phosphating and alkali metal phosphating systems. Zinc phosphating lines are normally based on 5-stage to 6-stage plants. They differ little in the pre-phosphating stages. The 5stage type includes a rinse between cleaning and phosphating steps. In the case of the 6-stage type, there are either two cleaning and one rinse stages or one cleaning and two rinsing steps.
1. Cleaning ; Temperature (C): 40-60 Pressure (bar): 1-2.5 2. Water rinse ; Temperature (C): 15-30 Pressure (bar): 0.7-1.5 3. Activation ; Temperature (C): 15-30 Pressure (bar): 0.7-1.5 4. Phosphating ; Temperature (C): 40-60 Pressure (bar): 1-2 5. Water rinse ; Temperature (C): 15-30 Pressure (bar): 0.7-1.5 6. Passivation ; Temperature (C): 20-40 Pressure (bar): 0.7-1.5 Time (min): 2-2.5 Time (min): 0.5-0.7 Time (min): 0.5-0.7 Time (min): 2-2.5 Time (min): 0.5-0.7 Time (min): 0.5-0.7
The cleaning stage is usually based on weakly alkaline products based on alkali phosphates, alkali carbonates or borates, low-foaming surfactants and alkali silicates. To these, further compounds such as titanium orthophosphates may be added to increase the rate of the phosphating process and facilitate formation of uniform finely-crystalline coatings. It is normal to dry phosphated work before the application of conventional paints and in the case of anodic electro coating paint, it is highly recommended. Circulating air ovens, operating at 120-180 C with a drying time of 5-15 minutes are typically used for such drying. In 4-stage plants the concentrations in the first bath run from 3-10 g/l, somewhat higher (5-15 g/l) in the second bath. The rinse and post-rinse stages are operated on the same basis as normal zinc phosphating systems.
ZINC PHOSPHATING PRIOR TO POWDER COATING
There are countless applications of powder coating coupled with phosphate pretreatment. They include the finishing of refrigerators, pressure vessels for fire extinguishers, garden furniture, electric panels, steel fencing, car wheels and other car accessories. The degree of success in the devolopment of powder coating is largely governed by the outcome of advances in other environmentally-friendly processes such as high solids containing, water based paints or two-pack systems in the market as a whole. The principle of powder coating is to apply the organic coating, in finely-devided powder form to the surface of the work to be
coated either at ambient or somewhat elevated temperatures. The film is then formed by melting the powder on the surface of the substrate. Powder coating is being increasingly used not least because of the absence of volatile solvents involved thus eleminating virtually all problems of airborne effluent discharge. The corrosion resistance of powder coating is many times better than of the equivalent thickness of conventional paints. Against this, it must be said that maintaining a given colour value is, by the nature of the process, more difficult for powder coating. In well-run plants using spray powder coating, the over-sprayed powder is collected and re-cycled which, if colour values are to be held, calls for high standarts of cleanliness. Introduction of zinc phosphates and the manganese-modified zinc phosphates coupled with powder coating, allowed still further improvements as manifested in the atmospheric corrosion test, the cyclic salt spray water condensate test, and salt spray test or water condensate test. The beneficial effects of the manganese modified process are specially in evidence for galvanized steel. Even for these more recently introduced processes, the best results are found where the Cr (VI) post rinse has been used to passivate the phosphate. In certain cases, post-rinse solutions based Cr (III) or chromium-free solutions are used, the performance of which is intermediate between a simple water rinse and the Cr (VI) containin water rinse.
ZINC PHOSPHATING PRIOR TO WIRE DRAWING
The primary benefit in phosphating of wires and sections for drawing and profiling lies in its superb adhesion and the survival of the coating even after several passes through a die. The crystalline nature of the phosphate coating offers an excellent substrate for the lubricant used, inhibiting film rupture of these. Use phosphated steel reduces wear of tools and dies and, in practice, significant increases in the working life of the diesis found. The phosphate film after drawing imparts a high surface finish to the wire at the same time affording a certain degree of corrosion protection during storage and transport. In case of hot dip galvanized iron or steel wires which are then further drawn, phosphating has advantages to offer. With a heavy zinc coating of 80-150 g/m, there is a tendency for such wires to buil up debris in the drawing die which results in increased wear rates of the die, a reduction in the quality of the drawn wire surface and the failure of brakes in the drawing operation. By phosphating in conjunction with galvanizing, all these problems are largely eliminated. The wire product has a somewhat darker appearance, but is ductile with a smooth, corrosion resistant surface. In the production of thin, low carbon wires in a drawing machine, phosphating can increase the production rate while at the same time providing corrosion protection during storage and transport. In drawing of phosphated steel wires, care must be taken to use correct phosphate coating thickness. If it is too great, enhanced friction will occur at the first drawing, resulting in bloom on the wire. On the other hand, though, the phosphate coating must be sufficiently which that its benefits persists even at the last of a number of drawings. As a general rule, a residual phosphate film after the final drawing of 0.5 - 1.0 g/m is desirable. Exceptions to this are those steel wires where a phosphate film is useful in some subsequent forming process. Thus wires for cold extrusion require phosphate coatings of 5-15 g/m as measured after the first calibrating pass. Immediately before immersion phosphating of wire coils, an
alkaline pre-rinse is often installed. This serves to neutralise residual acid traces on the incoming material. Such alkaline prerinses with added activating agents such as titanium salts have proved especially valuable in their promotion of very fine dense zinc phosphate coatings. Such finely crystalline coatings are less liable to lead to bloom on drawn wire than their coarsercrystalline forms. Phosphating of wires is usually based on the immersion method in which coils of wire are lowered into individual baths. The phosphating tank is thus incorporated together with other posttreatment stages downstream of the pickling plant. A typical treatment sequence for wire bundles can be schematically shown as follows ;
Pickling, in several stages, with sulfuric acid or hydrochloric acid. Water rinse, for example with two dip tanks and a spray rinse. Activation. Phosphating. Water rinse. Post treatment. Drying.
ZINC PHOSPHATING PRIOR TO TUBE DRAWING Where phosphating is used for the cold drawing of seamless tube, the requirements are somewhat different from those applicable to welded steel tube. Until about 30 years ago, phosphate coating weights of 20-40 g/m were used allowing the largest possible number of successive draws. Such coatings were formed with nitrate-accelerated systems at 90-95C. Over the years, in search for increased operational efficiency, the number of draws for a given reduction of seamless tubing, has decreased. In order to maintain the dimensional tolerances and quality of surface finish, tubes in the mid-range of available sizes are now reduced in a single or, at most, two draws. At the same time, drawing rates have been increased. For these reasons phosphate coating for drawing of seamless tubing are now formed with weights of 4-10 g/m. This has improved the efficiency of the surface treatment and, at the same time,
avoided the adverse effects which act in the firs drawing stage where coarser-crystalline phosphate coating are found. The most suitable coating is based on nitrate/nitrite accelerated zinc phosphate, formed at 40-75C. At the upper end of this temperature range, the option exists to use self dosing nitrate type systems. Chlorate accelerated zinc phosphate baths are also found. In all cases, the preferred form of the phosphate for cold drawing of tube and section is strongly adherent but soft structured. In the drawing of welded tubing, the seam must first be ground down. In the case of smaller diameter tubing, this is not possible inside the welding machine. In some cases, there may be a deformation to give a particular cross-section. Since, as a rule, less severe deformations can be tolerated by welded, as opposed to seamless tubing, the use of phosphating is widespread, coating weights being of the order 1.5 - 5 g/m. These are mostly based on zinc phosphate baths operated between 50 and 75 C with additives used to promote thinner coatings. Phosphating is also used for tubing of un-alloyed or low-alloyed steel with chromium content up to 4-6%. Such coatings offer a number of advantages, all arising from reduced metal-to-metal contact between tube and die. Thus, cold welding damage, leading to grooving or crack formation, is minimised, tool and die life is extended and higher drawing rates may be used. Zinc phosphate coating also allows a greater degree of reduction per pass and an increased number of passes without intermediate heat-treatment. Phosphating is carried out by immersion along the following lines: 1. Alkaline degreasing. 2. Water rinse. 3. Pickling in sulphuric or hydrochloric acid. 4. Water rinse. 5. Neutralising pre-rinse. 6. Phosphating. 7. Water rinse 8. Neutralising rinse. 9. Lubrication. 10. Drying and storage. In earlier times, the surface treatment of tubes took place using bundled tubes lifted by a manually-operated hoist from one tank to the next. More recently, automatic conveyors have been introduced and, while retaining the sequence of operations
listed above, the whole operation takes place automatically.
