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>Why surface treatment is used? >What are the advantages of surface treatment? >Why degreasing application is used? >What is phosphating? >Iron phosphate and its applications >Zinc phosphate and its applications >Why passivation is used? >Manganese phosphate and its applications >Aluminium chromate and its applications
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Why surface treatment is used?
The main reason to use surface treatment is to improve the efficiency of the dr yor powder paint cover. Proper surface treatment application provides corrosion resistance and good adhesion between surface and paint patricals. We can easily say that pretreatment much more important than last applications.
It is not to be expected to have long lasting high quality products unless the surface is not clear of grease, rust or oxidation even painting system is proper and well working.Unfamiliar elements between surface and paint, will prevent the adhesion and make negative effects to your process.
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What are the advantages of surface treatment?
- - Surface is cleared of grease, rust and dirtiness. - - Conditions are reached for good adhesion by providing smooth and spreaded surface. - Scratch based continuing corruptions are reduced. - Metal is suitable to form. - Corrosion resistance is increased by waxes and lubricants. - F- Friction effect is reduced for continuous moving separe parts. - - Dilation based sudden changes are minimized.
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Why degreasing application is used?
Degreasing is an important stage in pretreatment. There are mineral greases on metal surfaces which are prevent oxidation by cuttin the direct air connection. These minerals also make difficult phosphate and chromate coating.
Degreasing application can be applied at 50 – 90 ° C , 5 – 10 minute by wipe, immersion or spray. Temperature, time and concentration are the important parameters. If concentration or temperature is low, it can be compensated by long time application. Generally, it is prefered to use soft water in degreasing baths.
The easiest way to define grease on surfce is to look the pieces in rinsing bath. If there are accumulated water droplets on pieces , it means grease is not cleared. |
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What is phosphating?
Mostly used petreatment method is known as “phosphating”. Phosphate coating is the layer form on surface which includes iron, zinc or manganese crystals.
There are three types of phosphate coating ; iron phosphate , zinc phosphate and manganese phosphate. Generally it is used to cover iron, steel surfaces. Immersion and spraying are the most common application methods.
Phosphate coating reaction is simply known as the reaction between phosphoric acid and metal (acid – base reaction). Metalic phosphate becomes insoluble on the surface and forms a layer with metalic colour.
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Iron Phosphate and Its Applications This is the oldest and easiest method of phosphating. Corrosion resistance is lower than other phosphating methods. Generally, it leaves blue coloured layer on the surface. Coating range is between 0,2 - 1,0 gr/m² . Total acid, pH, temperature and apllication time should be under control. Iron phosphate can be applied in three different ways : - Wipe Iron Phosphate The product used with sponge or brush and makes degreasing and phosphating at the same time. High temperatures effect positively. Phosphated pieces must be dried after treatment. Simple and the cheapest method of all. - Immersion Iron Phosphate First stage is degreasing bath. Degreased pieces pass to second stage; rinsing. It is recommended to use two seperate rinsing bath to prevent chemical transport from degreasing bath to phosphating bath. Third stage is iron phosphating. Pieces are treated 5 - 10 min. at 45 - 55 º C . After passivation treatment for 30 - 45 sec. process ends with drying at maximum 130 º C. There is oxidation risk in 24 - 48 hours because of thin layer of iron phosphate coating. - Spray Iron Phosphate This is the newest method of conversion coating. It compansates first investment by high quality prodution and low consumption. It is possible to provide a quality phosphate layer at 45 - 55 º C , 1.5 - 1.8 bar presure and minumum 1,5 min. application time. If there will be no alkaline cleaning, proper degreasing chemicals can be added to phosphating bath. Bath options and cleaning method can vary according to working conditions. After passivation treatment process ends with drying at maximum 130 º C. |
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Zinc Phosphate and Its Applications Generally it is used for providing long lasting corrosion resistance. Almost all automotive industries use this type of conversion coating. It is suitable for the products come against bad weather conditions. Coating quality is better than iron phosphate. It forms 2 - 5 gr/m² coating on the metal surface when used as under paint. Application, set up and control of this process are more difficult than others. Can be applied by immersion or spray. Organic compounds like nickel and manganese are added to the bath to increase the performance. Also activation can be used to make small the phosphate crystals before zinc phosphating. Zinc phosphate reaction happens in amorphous shape with gray - black color. |
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pH optimizers are added to accelerate the reaction. Temperature, application time, concentration, ph, total acid, free acid are the parameters should be under control. Zinc phosphates, coating range between 7 - 15 gr/m², are used in wire drawing, tube drawing and cold forming. Phosphated metal parts are prepared to the next stage by application of protective lubricans and soaps.
- Immersion Zinc Phosphate Three operation baths are needed at least and can reach to 7 - 10 baths with acid and activation baths. Acid bath has to be in line if there is rust on the surface. Quality will improve with acid pickling. Rinsing bath should be refreshed frequently. Activation is not needed if zinc phosphate crystals are thin. If crystals are thick, activation baths are used and baths must be ventilated. Activation makes homogeneous and thin crystalled layer. Reaction times reduce. In wire drawing and tube drawing operations, zinc phosphated and soap treated pieces become ready for drawing. |
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- Spray Zinc Phosphate Application time and concentration are lower than immersion process. Approximately, 1.5 - 1.8 bar presure is applicated. Generally alkaline degreasing is used before phosphating.Pieces are treated minumum 1.5 minute after activation. After that process ends with passivation and drying. To prevent sludge accumulation, cone shaped bath should be used.
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Why Passivation is Used? Passivation is used at the last stage of phosphating process.There is two type of passivation ; chrome including and chrome free. Can be used at room temperature with %0.1 - 0.3 concentration. The surface coming after phosphating is not in homogeneous form. Passivation prevents air accumulation under paint cover. Leaking chromic acid provides extra corrosion resistance.Also passivation neutralizes the anions and cations that come with water. Phosphated parts should not be holded in passivation bath. Chromic acid has corrosive effect when applied long time. Manganese Phosphate and Its Applications Manganese phosphate absorbes and holds lubricants that's why used in piston and weapon industries. It makes easy the sliding of moving parts which are running in lubricant by reducing the friction between metal parts. This type of phosphate coatings have more stable structure than zinc phosphate under high presure and high temperature. After degreasing activation is needed. Temperature, application time, concentration, free and total acid points are the parameters to be cared. The process generally works at 70-98ºC , %15-20 concentration and with 5-20 minute immersion time. Manganese phosphate residues a coating 4-40 gr/m² on the surface. It cannot be used as under paint coverage. |
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Aluminium Chromate and Its Applications What we do as "phosphating" to steel an iron is "chromating" for aluminium surfaces.Also it is knows as "alodine coating". There are yellow, green and transparent chromating types. Yellow chromate coats Cr+6, green chromate coats Cr+3. Coating weight can vary according to application time and coating type. Drying temperature shouldn't pass over 65 º C for yellow chromate and 85 º C for green and transparent chromate coatings. |
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It is importat to provide clean, grease free surface before chromate application. If hot degreasing bath is prepared, caustic bath and following nitric acid bath can be used for wearing effect. On the other hand, acidic degreasing baths have wearing function with itselves. Chromating and paint adhesion will be much better on weared and degreased aluminium surface. |
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