• What is the purpose of surface treatment?
  • What are the advantages of surface treatment?
  • What is degreasing?
  • What is phosphating?
  • Iron phosphate and its applications
  • Zinc phosphate and its applications
  • What is passivation?
  • Manganese phosphate and its applications
  • Aluminium chromate and its applications
What is the purpose of surface treatment?
The main reason to use surface treatment is to improve the efficiency of dry or powder paint coatings. A Proper surface treatment provides corrosion resistance and good adhesion between surface and paint. Pretreatment of metals much more important than subsequent 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 form negative effects on the process.

What are the advantages of surface treatment?
  • Surface is cleared of grease, scale and rust.
  • Conditions are obtained for good adhesion by providing uneven surface. Total surface area is larger than untreated metal surface.
  • Scratch based continuing rustings are reduced.
  • Metal becomes suitable to form.
  • Corrosion resistance can be increased by waxes and lubricants.
  • Friction effect is reduced for continuous moving separe parts.
  • Dilation based sudden changes are minimized.

What is degreasing?
Degreasing is an important stage in pretreatment. There are mineral greases on metal surfaces which are prevent oxidation by cutting the direct air diffusion. These minerals bring on difficulties to phosphate and chromate conversion coatings.

Degreasing application can be applied at 50 – 90 ° C , 5 – 10 minute by wiping, 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 observe the workpieces in rinsing bath. If there are accumulated water droplets on workpieces , it means that grease is not completely cleared.

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 for coating iron and 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.

Iron phosphate and its applications
This is the conventional 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 must be under control.

Iron phosphate conversion coating can be applied in three different ways:

Wipe Iron Phosphate
Conversion coating products are used with a sponge or brush and makes degreasing and phosphating at the same time. High temperature of treatment solution has possitive effect. Phosphated pieces must be dried after treatment. Simple and the cheapest method of all.

Immersion Iron Phosphate
First stage is degreasing. Degreased workpieces pass to second stage; rinsing. It is recommended to use two seperate rinsing bath to prevent chemical transfer from degreasing bath to phosphating bath. Third stage is iron phosphating. Workpieces 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 production and low consumption. It is possible to provide a proper 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.

Zinc phosphate and its applications
Generally zinc phosphate conversion coating is used to provide long lasting corrosion protection. Almost all automotive industries use this type of conversion coating. It is suitable for the products come against hard weather conditions. Coating quality is better than iron phosphate coating. 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 other methods and can be applied by immersion or spray.

Organic compounds like nickel and manganese are added to the bath to increase the coating performance. Also activation can be used to form small phosphate crystals on the metal surface before zinc phosphating.

Zinc phosphate reaction happens in amorphous shape with gray - black color.

pH optimizers are added to accelerate the reaction. Temperature, application time, concentration, pH, total acid and free acid values are the parameters which must be under control.

Zinc phosphates, coating range between 7 - 15 gr/m², are used in wire drawing, tube drawing and cold forming industries. Phosphated metal workpieces 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 the addition of acid and activation baths. Acid bath has to be in line if there is rust on the metal surface. Quality will improve with acid pickling. Rinsing bath must be refreshed frequently.

Activation is not needed if zinc phosphate crystals are thin. If crystals are thick, activation baths are used and they must be ventilated while working. Activation makes homogeneous and thin crystalled layer and reaction times reduce.

In wire drawing and tube drawing operations, zinc phosphated and reactive soap coated workpieces become ready for drawing.

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.

What is passivation?
Passivation is used at the last stage of phosphating process. There is two type of passivation; chromiom passivation and chrome free passivation. It can be used at room temperature with %0.1 - 0.3 concentration. Phosphate coated metal surface is not in homogeneous form. Passivation prevents air difussion to base metal. Leaking chromic acid provides extra corrosion resistance. Passivation also neutralizes anions and cations coming along with water.

Phosphated parts must not treated in passivation bath. Chromic acid has corrosive effect when applied long time.

Manganese phosphate and its applications
Hence manganese phosphate coating has absorbing ability of lubricants it is mostly 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 minutes immersion time. Manganese phosphate residues a coating 4-40 gr/m² on the surface. It does not have a usage as under paint coating.

Aluminium chromate and its applications
What we call as phosphating to steel an iron is called 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 must not pass over 65 º C for yellow chromate and 85 º C for green and transparent chromate coatings.

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 pickling. On the other hand, acidic degreasing baths have pickling ability with itselves. Chromating and paint adhesion will be much better on pickled and degreased aluminium surface.

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